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Thin Filament Structure and the Steric Blocking Model

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ABSTRACT

By interacting with the troponin‐tropomyosin complex on myofibrillar thin filaments, Ca2+ and myosin govern the regulatory switching processes influencing contractile activity of mammalian cardiac and skeletal muscles. A possible explanation of the roles played by Ca2+ and myosin emerged in the early 1970s when a compelling “steric model” began to gain traction as a likely mechanism accounting for muscle regulation. In its most simple form, the model holds that, under the control of Ca2+ binding to troponin and myosin binding to actin, tropomyosin strands running along thin filaments either block myosin‐binding sites on actin when muscles are relaxed or move away from them when muscles are activated. Evidence for the steric model was initially based on interpretation of subtle changes observed in X‐ray fiber diffraction patterns of intact skeletal muscle preparations. Over the past 25 years, electron microscopy coupled with three‐dimensional reconstruction directly resolved thin filament organization under many experimental conditions and at increasingly higher resolution. At low‐Ca2+, tropomyosin was shown to occupy a “blocked‐state” position on the filament, and switched‐on in a two‐step process, involving first a movement of tropomyosin away from the majority of the myosin‐binding site as Ca2+ binds to troponin and then a further movement to fully expose the site when small numbers of myosin heads bind to actin. In this contribution, basic information on Ca2+‐regulation of muscle contraction is provided. A description is then given relating the voyage of discovery taken to arrive at the present understanding of the steric regulatory model. © 2016 American Physiological Society. Compr Physiol 6:1043‐1069, 2016.

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Figure 1. Figure 1. The Hanson‐Lowy model of F‐actin. Electron micrographs of negatively stained actin filaments (A, B) were the basis of the F‐actin model proposed by Hanson and Lowy () shown in (C). In (B), an F‐actin filament is magnified to illustrate individual actin subunits (dash marks) and points where the two strands of the filament crossover each other (arrows). In (C), the Hanson‐Lowy model is illustrated with actin subunits depicted as spheres that associate to form two intertwined “long‐pitch” helical strands crossing over approximately every 36 nm. The arrangement also yields a single‐start short‐pitch “genetic” helix with a repeat of 5.9 nm. Since each successive actin subunit is shifted axially by 27.5 Å (i.e., half staggered relative to the next one) and is rotated 166° to 167° along the short‐pitch helix (i.e., fairly close to a 180° value), the arrangement also yields two right handed long‐pitch helices repeating every 770 Å. Figure adapted, with permission, from (). Note that up until the late 1980s and early 1990s it was not widely recognized that actin subunits are not spherical in shape, viz., that even at low resolution they look rather closer to “book‐like”‐shaped rectangular solids. Thus, when actin subunits associate to form helices, we now know that no distinct grooves are evident (cf. Figs. ).
Figure 2. Figure 2. (A, B) The two‐state on‐off steric regulatory model showing tropomyosin movement on actin filaments as initially proposed by Huxley (). Versions of thin filament components are drawn schematically and viewed in transverse section, actin (Ac, purple) with tropomyosin's on‐ and off‐positions (Tm, green and red) influencing the S1 myosin head (yellow), blocking binding in low‐Ca2+ relaxed muscle while permitting interaction in high‐Ca2+ active muscle. Note that the steric‐blocking position of tropomyosin (red) overlaps with the S1‐binding site on actin, in low Ca2+. (A) Modeling based on X‐ray diffraction data of intact muscle and EM reconstructions of S1‐decorated F‐actin. Figure adapted, with permission, from () based on (). (B) A corresponding model of the thin filament by Potter and Gergely () based on additional in vitro studies suggesting binding interactions between troponin subunits, actin and tropomyosin and the influence of Ca2+ on the behavior of troponin‐tropomyosin. At low Ca2+ (a), troponin subunits (labeled C, I, T) clasp onto tropomyosin, trapping it over the myosin‐binding site, whereas in (b) Ca2+ binding to TnC breaks the actin‐TnI interaction releasing the constraint on tropomyosin. The cartoon shows movement of troponin‐tropomyosin away from the myosin‐binding site on actin following Ca2+ binding, so that myosin heads (S1) can now attach to actin. Figure adapted, with permission, from (), Copyright 1974 American Chemical Society. Hitchcock et al. () described a similar set of changing interactions based on related work. Note that the relative azimuthal positioning of tropomyosin and troponin was not known at the time and was drawn arbitrarily in the cartoons [cf. Yang et al. ()].
Figure 3. Figure 3. (A) Schematic diagram showing the arrangement of the t‐tubule and S.R. network in striated muscle. Striated muscle cells, also called muscle fibers, are composed of myofibrils (#1) containing thick and thin filaments. Myofibrils are surrounded by the S.R. (#2) and its terminal cisternae (#3). The t‐tubules, are extensions of the plasma membrane (#4), also called the sarcolemma, thus, they are part of a separate membrane system from the S.R. The juxtaposition of S.R. terminal cisternae on both sides of the t‐tubule forms a “triad.” Also shown are mitochondria and a basal lamina around the sarcolemma. Figure from () with permission. (B) Illustration comparing the control of Ca2+ release from the S.R. of skeletal (left) and cardiac muscle (right). In skeletal muscle, an action potential passing from the sarcolemma through the t‐tubule system is detected by dihydropyrindine receptors (DHPR) voltage sensors in the t‐tubule membrane, which directly open RyR1 ryanodine‐Ca2+ release channels in the adjacent SR. In cardiac muscle, action potentials open DHPR voltage‐dependent Ca2+ channels, and inflow of extracellular Ca2+ activates RyR2 Ca2+‐release channels. Figure adapted, with permission, from ().
Figure 4. Figure 4. Ebashi's model of troponin‐tropomyosin on thin filaments. (A) Tropomyosin is shown lying longitudinally along successive actin subunits of the thin filament with troponin distributed periodically on every seventh actin subunit. Figure adapted, with permission, from (). The troponin complex is now known to be considerably larger and is more asymmetric than depicted in (A). (B) The relative orientation and component organization of troponin and tropomyosin, along with end‐to‐end tropomyosin linkage, is more realistically drawn in the schematic of Flicker et al. (). Figure adapted, with permission, from (). T1 and T2 denote the N‐ and C‐terminal domains of TnT that can be separated by chymotryptic cleavage ().
Figure 5. Figure 5. (A) Immunoelectron microscope localization of troponin. Negatively stained thin filaments emerging from a Z‐disk fragment are shown labeled with antibodies to troponin‐C (white spots) and yielding a 40 nm periodicity. Figure adapted, with permission, from (). (B) A negatively stained Cohen‐Longley‐type tropomyosin paracrystal () with the arrangement of tropomyosin molecules indicated by arrows and giving rise to a 40 nm periodicity. (C) A tropomyosin paracrystal additionally containing troponin. Note the extra band of density due to the presence of troponin bound to tropomyosin and repeating at 40 nm intervals (arrows). Figures in (B, C) adapted, with permission, from (); cf. reference () for similar images. Scale bar = 50 nm.
Figure 6. Figure 6. (A) Low‐angle X‐ray fiber diffraction of skinned rabbit psoas muscle fibers, shown as split “half‐film” records comparing layer lines patterns of relaxed and Ca2+‐activated preparations; right panel, calcium absent; left panel, calcium present. To avoid effects of myosin cross‐bridge‐actin interactions for the pattern displayed, muscle fibers were stretched by Poole et al. () so that thick and thin filaments in sarcomeres did not overlap. Note the heavy central scattering that dominates the pattern, and the enhancement of the J4, second‐order layer line in calcium, whereas the 59 Å layer line, derived from the F‐actin short‐pitch genetic helix is constant, as was originally noted by Huxley (). (B, C) Difference patterns generated from the same data as in (A) by subtracting one pattern from the other. (B) In going from Ca2+ to EGTA containing solutions there an increase in the strength of the J2 term. (C) In going from EGTA to Ca2+ there is an increase in the J4 term. Figures taken, with permission, from ().
Figure 7. Figure 7. Possible angular shifts of tropomyosin (Tm, red, green, pink) around actin (Ac, purple), as first modeled by Haselgrove () and by Parry and Squire (). Here, F‐actin is illustrated in transverse section with actin subunits represented as spheres and tropomyosin as a cylinder. The fourfold symmetry of the filament is most evident when tropomyosin is positioned at a 90° azimuth (green), and least pronounced when tropomyosin is at a 0° azimuth (pink), since in the latter case the twist of the tropomyosin super‐helices (i.e., their coiled coiled‐coil configuration) and the actin helices are in phase. Similarly, the fourfoldedness enhances second‐order layer line intensity in corresponding diffraction diagrams most when tropomyosin lies closest to the 90° azimuth, and as depicted close to the “groove” between actin subunits. Red colored tropomyosin indicates a possible steric blocking location. The modeling suggests that radial distance of tropomyosin (r) to the central axis of the filament varies as tropomyosin repositions toward and away from the filament groove. It now is known that the tropomyosin radius is virtually constant during regulatory transitions, while current evidence indicates that tropomyosin's low‐ to high‐Ca2+ azimuthal movement over a relatively flat surface of actin is between 15° to 25° (). Illustration adapted and modified, with permission, from ().
Figure 8. Figure 8. Myosin arrowhead structure on F‐actin. (A) Electron micrograph of negatively stained actin filaments “decorated” with myosin S1. The polarity of S1 arrowheads defines the pointed and barbed ends of F‐actin filaments. (B) Model of myosin heads bound to F‐actin based on the first 3D reconstructions of the S1‐decorated F‐actin. Figure adapted, with permission, from ().
Figure 9. Figure 9. Actin structure solutions. (A) Ribbon representation of the first crystal structure of the actin molecule. Note the distinct domain organization, and the cleft between subdomains containing nucleotide and Ca2+; domains and some residues labeled. Figure of PDB ID:1ATN adapted, with permission, from (). (B,C) Orthogonal views of 5 actin subunits (variously colored) from a high resolution cryo‐EM structure of actin, constructed from PDB ID:3J8A (). The shape of actin subunits is obviously flat and not spherical as depicted in earlier cartoons. The global shape of the actin subunits can be appreciated by comparing the face on and side views of the red highlighted actin molecule of the 5 subunit structure. Thus, it was known as early as 1990 that the surface over which tropomyosin moves is also likely to be flat. The actin pointed end is facing up, a convention used in all subsequent figures.
Figure 10. Figure 10. The first cryo‐EM reconstructions showing the molecular structure of F‐actin and location of surface binding sites (). (A) Reconstruction of F‐actin tagged with an undecagold cluster label (gold) linked to the C‐terminal Cys374 residues of actin. (B) The undecagold attachment site is shown as a reference point establishing the actin subdomain organization in the reconstruction [subdomains numbered on one actin subunit, in (A)]. (C, D) Myosin S1 binding sites on actin are shown (yellow). Difference maps between reconstructions of F‐actin decorated with S1 and undecorated F‐actin localized the myosin‐binding sites for S1‐A2 (C) and S1‐A1 (D) (yellow) (myosin S1 density itself is not shown in this figure but is in Fig. 17B). The region defining the strong S1‐binding site on actin subdomain 1 is marked by an asterisk. Essential myosin light chains (either A1 or A2, a.k.a. alkali light chains) are low molecular subunits associated with the lever arm portion of skeletal muscle myosin S1. [Muscle myosin is a hexamer composed of two heavy chains, representing the head and rod of parts of myosin, and two nonphosphorylatable essential light chains and two phosphorylatable regulatory light chains located on the lever arms of myosin and thus positioned between the head and rod domains ().] S1‐A2 and S1‐A1 bind to actin with some differences (arrows) due to the presence of the different myosin light chain isoforms. (E, F) Tropomyosin and its binding site on S1‐decorated F‐actin containing troponin‐tropomyosin. Note that both tropomyosin and S1 interact on the same actin interface and that tropomyosin lies next to the S1‐binding site on such S1‐decorated F‐actin filaments. Figure adapted, with permission, from ().
Figure 11. Figure 11. Electron micrographs of negatively stained Ca2+ regulated thin filaments showing troponin and tropomyosin densities. (A) Filaments isolated directly from frog cardiac muscles. Note helically arranged tropomyosin cables (black arrows) seen extending along the filament in many micrographs. Figure adapted, with permission, from (). (B) Reconstituted thin filament here consisting of rabbit skeletal muscle F‐actin and bovine cardiac troponin‐tropomyosin. Note in this micrograph periodic troponin densities that are in register on opposite sides of the filament (marked by white arrow heads) partially masking tropomyosin cables (black arrow). Figure adapted, with permission, from (). Scale bar = 50 nm.
Figure 12. Figure 12. Ca2+‐induced tropomyosin movement on thin filaments revealed by 3D‐reconstruction. (A) Surface representation of EM reconstructions generated from negatively stained Limulus thin filaments following exposure to low‐ and high‐Ca2+ conditions. Note the movement of the tropomyosin cable (black arrows) from the outer domain (Ao) toward the inner domain of actin (Ai) that occurs in Ca2+. At low‐Ca2+, tropomyosin associates with subdomain 1 of actin and bridges over subdomain 2, whereas, at high‐Ca2+, its association is with subdomain 3 of actin and it arches over subdomain 4 (subdomains numbered on one actin subunit). (B) “Helical projections” of the same reconstructions made by projecting densities along their helical paths onto a plane perpendicular to the filament axis. Again, note the change in position of the tropomyosin (Tm); the center of the tropomyosin densities are highlighted in red and green for the low‐Ca2+ and high‐Ca2+ locations. The two projections are also superposed for comparison. Ca2+‐induced tropomyosin movement was first demonstrated on thin filaments isolated from striated muscles of the horseshoe crab Limulus and later to occur on filaments obtained from other invertebrate and vertebrate striated muscles. Figure adapted, with permission, from (). It should be noted that the two inner and two outer domains of neighboring actin subunits shown in projection already delineate what can be considered four long‐pitch “strand‐like” densities (two inner and two outer strands). In the off‐state, tropomyosin is positioned close to the mid‐point between inner and outer domains of actin, perhaps not adding significantly to the filament fourfoldedness. In contrast, in the on‐state when tropomyosin moves closer to the inner domain of actin, the appearance of fourfoldedness is clearly emphasized. Thus, the four‐strandedness defined in the Fourier transforms of thin filaments (see Fig. 13) may not be accurately described by the conventional meaning of the term, viz., two actin and two tropomyosin strands, but rather is due to contributions from four actin components (two inner and two outer domains), augmented differentially by tropomyosin in one or another position.
Figure 13. Figure 13. Fourier transforms of the Limulus filament EMs. The averaged layer line data used to produce the helical reconstructions in Figure 12 is plotted here. (A) low‐Ca2+ and (B) high‐Ca2+ data. Layer line amplitudes are represented as solid lines and phases as dotted lines. Bessel and layer line orders, (n) and (l), defining the actin‐helical symmetry are given. As in fiber diffraction patterns records of intact muscle (see Fig. 6), the amplitude along the second order J4 layer line (l = 2, n = 4) of the EM data is greater for the filaments associated with the high‐Ca2+ than for those at low‐Ca2+ (arrows). Figure adapted, with permission, from ().
Figure 14. Figure 14. Dual effect of tropomyosin and troponin‐tropomyosin on actomyosin subfragment 1 ATPase. Acto‐S1 ATPase assays show that troponin‐tropomyosin regulated thin filaments activate actomyosin ATPase in a Ca2+‐dependent manner at all S1 concentrations tested, while control F‐actin and F‐actin‐tropomyosin (no troponin) do not confer Ca2+ dependence. The activation of actomyosin S1 ATPase by control F‐actin is linear as a function of added S1, yet sigmoidal when either troponin‐free tropomyosin or troponin‐tropomyosin is present (plateaus at high S1 levels not shown). Thus, relative to control F‐actin values, tropomyosin, in the presence and absence of troponin and Ca2+, inhibits actomyosin ATPase at low S1:F‐actin ratios, and stimulates ATPase at high S1:F‐actin ratios. Inhibition and activation by tropomyosin are greatest when troponin is present. The observations of Lehrer and Morris (), like those of Weber and colleagues (), demonstrated that cooperative activation of thin filaments is dependent on S1 interactions with tropomyosin on actin. The studies suggested that, in addition to Ca2+ involvement in the activation process, myosin itself induces an “open‐state” of the thin filament. Figure adapted, with permission, from (). This figure was originally published in The Journal of Biological Chemistry by S.S. Lehrer and E.P. Morris in an article “Dual effects of tropomyosin and tropomyosin‐troponin on skeletal muscle subfragment 1 ATPase” J. Biol. Chem. 1982; 257:682‐693 © the American Society for Biochemistry and Molecular Biology.
Figure 15. Figure 15. Schematic diagrams of the McKillop‐Geeves three‐state model of the thin filament. (A) Regulatory units of the thin filament are illustrated by single lines depicting tropomyosin over seven circles representing actin; myosin is drawn as a triangle. Blocked, closed, open states are indicated. The dynamic equilibrium between states is illustrated as an azimuthal translocation of tropomyosin on actin and an isomerization of myosin into a strongly attached configuration. K B and K T are Ca2+‐sensitive steps. (B) The same transitions represented in different cartoon format. It should be recognized that for the weak state of myosin attachment to actin and also for actin‐bound myosin that has not yet released phosphate (not shown in the figure), any favorable or unfavorable tropomyosin‐myosin interactions are still beyond current knowledge. Moreover, the structural correlates of the model do not explain all aspects of myosin regulatory function, and were derived from experiments not including the presence of ATP. Figure in panel A adapted, with permission, from (), Copyright 1974 American Chemical Society. Figure in panel B adapted, with permission, from ().
Figure 16. Figure 16. The effect of troponin I trapping tropomyosin in the blocked B‐state thin‐filament position. Reconstructions of (A) F‐actin control filaments, subdomains numbered, and (B) TnI‐decorated F‐actin‐tropomyosin, where the C‐terminal regulatory fragment of TnI is present on every actin subunit. TnI densities are indicated by arrows. (C) Densities attributable to TnI (purple) seen in (B) trapping tropomyosin (red) in the blocking position on actin. (D) Reconstruction of low‐Ca2+ native cardiac thin filaments now containing tropomyosin and the entire troponin complex (hence there is one troponin complex on every seventh actin). Densities representing the troponin core domain consisting of the TnIT arm and the N and C lobes of TnC labeled. Black arrow marks tropomyosin in the blocking state. (E) Difference maps highlighting troponin (gold) made by subtracting the actin‐tropomyosin from the map in (D) and superposing the difference on actin‐tropomyosin. Note the position of density attributable to TnT (open arrow) and to the C‐terminal domain of TnI (double‐sided arrow). In (F), maps in (B) and (E) merged; the TnI density seen in (E) superposes on the corresponding TnI density found in (B). Figure adapted, with permission, from () (panels A, B, D‐F) and () (panel C).
Figure 17. Figure 17. Docking atomic structures of F‐actin into 3D reconstructions of native thin filaments. This procedure by Vibert et al. () provided the first structural evidence for the three‐state model of thin filament regulation. (A‐D) Helical reconstructions shown as a blue wire mesh density envelopes cut to about 6 nm to illustrate one actin subunit [actin subdomains numbered in (A)] and the changing position of tropomyosin (arrows) in response to Ca2+ and myosin binding. In each case, a high‐resolution actin subunit has been docked within the reconstruction and is shown as a yellow α‐carbon chain with several clusters of residues highlighted in various colors. (A) Reconstruction of low‐Ca2+ thin filaments. Note that charged residues 1 to 4 on the N‐terminus of actin and actin residues 92 to 95 (green) on the edge of the subdomain, which are thought to interact electrostatically with myosin prior to cross‐bridge cycling, are not blocked by the tropomyosin density. In contrast, actin residues involved in strong stereospecific binding to myosin (e.g., 24 to 28, 144 to 148, and 340 to 346) () (red) are obstructed by tropomyosin. In (C), a reconstruction of high‐Ca2+ thin filaments shows that tropomyosin has repositioned and that most highlighted myosin binding residues on actin are now exposed (green) with the exception of, for example, amino acids 332 to 336 (red) at the junction of actin subdomains 1 and 3, which remain occluded. (B) Reconstruction of S1 decorated thin filaments. Note that tropomyosin (arrows) has moved further away from the blocking position than it does in Ca2+ (cf. to C) and that all highlighted residues (magenta) are free to bind myosin. In (D), the reconstruction of S1 decorated thin filaments has been contoured to remove part of the S1 density to provide an outline of the S1‐binding site and a better view of the actin residues involved in S1 binding. The densities shown in the reconstruction of the S1 decorated filament and its corresponding actin‐binding site detected are similar to those shown by Milligan et al. (). The Lorenz et al. atomic model of F‐actin () was used for the docking shown. Figure adapted, with permission, from ().
Figure 18. Figure 18. Fitting atomic coordinates of actin, tropomyosin, and myosin to single particle reconstructions of reconstituted thin filaments. Filaments consisted of actin, troponin, and tropomyosin. Poole et al. () brought together the 3D‐EM of Pirani et al. () and the synchrotron X‐ray diffraction data from nonoverlapped muscle fibers, thereby synthesizing ideas about steric blocking into a consistent and comprehensive account. (A, B, C) Structural models of F‐actin (blue, cyan, white) [Holmes et al. ()] and tropomyosin (red, yellow, green) [Lorenz et al. ()] were fitted within EM density envelopes of reconstituted filaments (translucent surfaces). Reconstructions of (A) low‐Ca2+, (B) high‐Ca2+, and (C) S1‐decorated filaments fitted with actin and tropomyosin structures. Note the shifting position of tropomyosin in the three cases and the position of the fitted S1 [Rayment et al. ()] in (C). (D) Tropomyosin in its three average positions superposed for comparison; the locations of tropomyosin agree with earlier results of Vibert et al. (). (E) The S1 binding configuration in (C) is superposed on the low‐Ca2+ B‐state filament model from (A). Note the steric clashes between S1 and tropomyosin. While the superposition in (E) makes apparent that tropomyosin occludes the S1‐binding site on actin, it also shows that tropomyosin at low‐Ca2+ keeps the cleft between the upper and lower 50K domains of myosin (red) separated and prevents S1 cleft closure, thus interfering with actin‐myosin ATPase and cross‐bridge cycling. This can be best appreciated by comparing S1 heads and tropomyosin on the left and right side of the figure. Figure adapted, with permission, from ().
Figure 19. Figure 19. High‐resolution cryo‐EM reconstructions of F‐actin and F‐actin tropomyosin. (A) F‐actin at 3.7 Å resolution [constructed from PDB ID:3J8A ()]. Note the charged residues on actin [Lys 326, Lys 328, Arg 147, Arg 28 (blue), and Glu 25 (red)] that define the binding path assumed by tropomyosin in (B) () when unperturbed by troponin or myosin. Also note that residues near to Arg 28 and Glu 25 protrude from the surface of actin to form a barrier on subdomain 1 that limits the azimuthal rotation of tropomyosin toward the extreme outer edge of actin in the blocked B‐state. Similarly, the lateral edge of subdomain 4 bulges out, limiting the extent of azimuthal rotation of tropomyosin in the direction of the inner actin domain in the M‐state. Pro 333 (highlighted in pink) defines the boundary between the B‐ and C‐states, but as yet, there is no high resolution model for the C‐state position of tropomyosin. (B, C) The locations of tropomyosin in blocked (magenta) and open (green) positions, respectively, and superposed in (D). B‐ and M‐state tropomyosin constructed from PDBs given in () and (). (E) 8 Å resolution reconstruction of F‐actin‐tropomyosin decorated with Dictyostelium S1 () superposed on (D), defining the binding positions of tropomyosin relative to myosin interaction. Behrmann et al. (), Sousa et al. (), and von der Ecken et al. () were the first to resolve individual α‐helical chains of the tropomyosin coiled‐coil; however, at 8 Å resolution, tropomyosin's side chains are not defined and hence corresponding residues cannot be assigned explicitly to densities in the maps.
Figure 20. Figure 20. Atomic model of F‐actin‐tropomyosin on F‐actin. Li et al. () proposed a residue‐specific model for the actin‐tropomyosin structure by optimizing the electrostatic interaction energies between actin and tropomyosin structures. The model was later refined to include the head‐to‐tail tropomyosin overlapping domain by Orzechowski et al. (). In (A), likely contacts between tropomyosin pseudorepeats and actin subunits are displayed, and, in (B), a magnified view of tropomyosin pseudorepeat 4 is highlighted, illustrating acidic residues on tropomyosin in close proximity to basic Arg 147, Lys 326, and Lys 328 located on actin at the border between subdomain 1 and 3, while Glu 334, Arg 28, and Asp 25 higher on actin subdomain 1 position to associate with oppositely charged amino acids in tropomyosin (cf. Fig. 19). This pattern of contacts is repeated between all tropomyosin pseudorepeats and successive actin subunits along the filament. The position of tropomyosin in the model of troponin‐free actin‐tropomyosin () agrees with that observed for tropomyosin when pinned down on regulated filaments by TnI at low‐Ca2+ (). The model also agrees with predictions of actin‐tropomyosin interaction by Brown and Cohen () and Barua et al. (). Figure adapted, with permission, from () and is based on ().


Figure 1. The Hanson‐Lowy model of F‐actin. Electron micrographs of negatively stained actin filaments (A, B) were the basis of the F‐actin model proposed by Hanson and Lowy () shown in (C). In (B), an F‐actin filament is magnified to illustrate individual actin subunits (dash marks) and points where the two strands of the filament crossover each other (arrows). In (C), the Hanson‐Lowy model is illustrated with actin subunits depicted as spheres that associate to form two intertwined “long‐pitch” helical strands crossing over approximately every 36 nm. The arrangement also yields a single‐start short‐pitch “genetic” helix with a repeat of 5.9 nm. Since each successive actin subunit is shifted axially by 27.5 Å (i.e., half staggered relative to the next one) and is rotated 166° to 167° along the short‐pitch helix (i.e., fairly close to a 180° value), the arrangement also yields two right handed long‐pitch helices repeating every 770 Å. Figure adapted, with permission, from (). Note that up until the late 1980s and early 1990s it was not widely recognized that actin subunits are not spherical in shape, viz., that even at low resolution they look rather closer to “book‐like”‐shaped rectangular solids. Thus, when actin subunits associate to form helices, we now know that no distinct grooves are evident (cf. Figs. ).


Figure 2. (A, B) The two‐state on‐off steric regulatory model showing tropomyosin movement on actin filaments as initially proposed by Huxley (). Versions of thin filament components are drawn schematically and viewed in transverse section, actin (Ac, purple) with tropomyosin's on‐ and off‐positions (Tm, green and red) influencing the S1 myosin head (yellow), blocking binding in low‐Ca2+ relaxed muscle while permitting interaction in high‐Ca2+ active muscle. Note that the steric‐blocking position of tropomyosin (red) overlaps with the S1‐binding site on actin, in low Ca2+. (A) Modeling based on X‐ray diffraction data of intact muscle and EM reconstructions of S1‐decorated F‐actin. Figure adapted, with permission, from () based on (). (B) A corresponding model of the thin filament by Potter and Gergely () based on additional in vitro studies suggesting binding interactions between troponin subunits, actin and tropomyosin and the influence of Ca2+ on the behavior of troponin‐tropomyosin. At low Ca2+ (a), troponin subunits (labeled C, I, T) clasp onto tropomyosin, trapping it over the myosin‐binding site, whereas in (b) Ca2+ binding to TnC breaks the actin‐TnI interaction releasing the constraint on tropomyosin. The cartoon shows movement of troponin‐tropomyosin away from the myosin‐binding site on actin following Ca2+ binding, so that myosin heads (S1) can now attach to actin. Figure adapted, with permission, from (), Copyright 1974 American Chemical Society. Hitchcock et al. () described a similar set of changing interactions based on related work. Note that the relative azimuthal positioning of tropomyosin and troponin was not known at the time and was drawn arbitrarily in the cartoons [cf. Yang et al. ()].


Figure 3. (A) Schematic diagram showing the arrangement of the t‐tubule and S.R. network in striated muscle. Striated muscle cells, also called muscle fibers, are composed of myofibrils (#1) containing thick and thin filaments. Myofibrils are surrounded by the S.R. (#2) and its terminal cisternae (#3). The t‐tubules, are extensions of the plasma membrane (#4), also called the sarcolemma, thus, they are part of a separate membrane system from the S.R. The juxtaposition of S.R. terminal cisternae on both sides of the t‐tubule forms a “triad.” Also shown are mitochondria and a basal lamina around the sarcolemma. Figure from () with permission. (B) Illustration comparing the control of Ca2+ release from the S.R. of skeletal (left) and cardiac muscle (right). In skeletal muscle, an action potential passing from the sarcolemma through the t‐tubule system is detected by dihydropyrindine receptors (DHPR) voltage sensors in the t‐tubule membrane, which directly open RyR1 ryanodine‐Ca2+ release channels in the adjacent SR. In cardiac muscle, action potentials open DHPR voltage‐dependent Ca2+ channels, and inflow of extracellular Ca2+ activates RyR2 Ca2+‐release channels. Figure adapted, with permission, from ().


Figure 4. Ebashi's model of troponin‐tropomyosin on thin filaments. (A) Tropomyosin is shown lying longitudinally along successive actin subunits of the thin filament with troponin distributed periodically on every seventh actin subunit. Figure adapted, with permission, from (). The troponin complex is now known to be considerably larger and is more asymmetric than depicted in (A). (B) The relative orientation and component organization of troponin and tropomyosin, along with end‐to‐end tropomyosin linkage, is more realistically drawn in the schematic of Flicker et al. (). Figure adapted, with permission, from (). T1 and T2 denote the N‐ and C‐terminal domains of TnT that can be separated by chymotryptic cleavage ().


Figure 5. (A) Immunoelectron microscope localization of troponin. Negatively stained thin filaments emerging from a Z‐disk fragment are shown labeled with antibodies to troponin‐C (white spots) and yielding a 40 nm periodicity. Figure adapted, with permission, from (). (B) A negatively stained Cohen‐Longley‐type tropomyosin paracrystal () with the arrangement of tropomyosin molecules indicated by arrows and giving rise to a 40 nm periodicity. (C) A tropomyosin paracrystal additionally containing troponin. Note the extra band of density due to the presence of troponin bound to tropomyosin and repeating at 40 nm intervals (arrows). Figures in (B, C) adapted, with permission, from (); cf. reference () for similar images. Scale bar = 50 nm.


Figure 6. (A) Low‐angle X‐ray fiber diffraction of skinned rabbit psoas muscle fibers, shown as split “half‐film” records comparing layer lines patterns of relaxed and Ca2+‐activated preparations; right panel, calcium absent; left panel, calcium present. To avoid effects of myosin cross‐bridge‐actin interactions for the pattern displayed, muscle fibers were stretched by Poole et al. () so that thick and thin filaments in sarcomeres did not overlap. Note the heavy central scattering that dominates the pattern, and the enhancement of the J4, second‐order layer line in calcium, whereas the 59 Å layer line, derived from the F‐actin short‐pitch genetic helix is constant, as was originally noted by Huxley (). (B, C) Difference patterns generated from the same data as in (A) by subtracting one pattern from the other. (B) In going from Ca2+ to EGTA containing solutions there an increase in the strength of the J2 term. (C) In going from EGTA to Ca2+ there is an increase in the J4 term. Figures taken, with permission, from ().


Figure 7. Possible angular shifts of tropomyosin (Tm, red, green, pink) around actin (Ac, purple), as first modeled by Haselgrove () and by Parry and Squire (). Here, F‐actin is illustrated in transverse section with actin subunits represented as spheres and tropomyosin as a cylinder. The fourfold symmetry of the filament is most evident when tropomyosin is positioned at a 90° azimuth (green), and least pronounced when tropomyosin is at a 0° azimuth (pink), since in the latter case the twist of the tropomyosin super‐helices (i.e., their coiled coiled‐coil configuration) and the actin helices are in phase. Similarly, the fourfoldedness enhances second‐order layer line intensity in corresponding diffraction diagrams most when tropomyosin lies closest to the 90° azimuth, and as depicted close to the “groove” between actin subunits. Red colored tropomyosin indicates a possible steric blocking location. The modeling suggests that radial distance of tropomyosin (r) to the central axis of the filament varies as tropomyosin repositions toward and away from the filament groove. It now is known that the tropomyosin radius is virtually constant during regulatory transitions, while current evidence indicates that tropomyosin's low‐ to high‐Ca2+ azimuthal movement over a relatively flat surface of actin is between 15° to 25° (). Illustration adapted and modified, with permission, from ().


Figure 8. Myosin arrowhead structure on F‐actin. (A) Electron micrograph of negatively stained actin filaments “decorated” with myosin S1. The polarity of S1 arrowheads defines the pointed and barbed ends of F‐actin filaments. (B) Model of myosin heads bound to F‐actin based on the first 3D reconstructions of the S1‐decorated F‐actin. Figure adapted, with permission, from ().


Figure 9. Actin structure solutions. (A) Ribbon representation of the first crystal structure of the actin molecule. Note the distinct domain organization, and the cleft between subdomains containing nucleotide and Ca2+; domains and some residues labeled. Figure of PDB ID:1ATN adapted, with permission, from (). (B,C) Orthogonal views of 5 actin subunits (variously colored) from a high resolution cryo‐EM structure of actin, constructed from PDB ID:3J8A (). The shape of actin subunits is obviously flat and not spherical as depicted in earlier cartoons. The global shape of the actin subunits can be appreciated by comparing the face on and side views of the red highlighted actin molecule of the 5 subunit structure. Thus, it was known as early as 1990 that the surface over which tropomyosin moves is also likely to be flat. The actin pointed end is facing up, a convention used in all subsequent figures.


Figure 10. The first cryo‐EM reconstructions showing the molecular structure of F‐actin and location of surface binding sites (). (A) Reconstruction of F‐actin tagged with an undecagold cluster label (gold) linked to the C‐terminal Cys374 residues of actin. (B) The undecagold attachment site is shown as a reference point establishing the actin subdomain organization in the reconstruction [subdomains numbered on one actin subunit, in (A)]. (C, D) Myosin S1 binding sites on actin are shown (yellow). Difference maps between reconstructions of F‐actin decorated with S1 and undecorated F‐actin localized the myosin‐binding sites for S1‐A2 (C) and S1‐A1 (D) (yellow) (myosin S1 density itself is not shown in this figure but is in Fig. 17B). The region defining the strong S1‐binding site on actin subdomain 1 is marked by an asterisk. Essential myosin light chains (either A1 or A2, a.k.a. alkali light chains) are low molecular subunits associated with the lever arm portion of skeletal muscle myosin S1. [Muscle myosin is a hexamer composed of two heavy chains, representing the head and rod of parts of myosin, and two nonphosphorylatable essential light chains and two phosphorylatable regulatory light chains located on the lever arms of myosin and thus positioned between the head and rod domains ().] S1‐A2 and S1‐A1 bind to actin with some differences (arrows) due to the presence of the different myosin light chain isoforms. (E, F) Tropomyosin and its binding site on S1‐decorated F‐actin containing troponin‐tropomyosin. Note that both tropomyosin and S1 interact on the same actin interface and that tropomyosin lies next to the S1‐binding site on such S1‐decorated F‐actin filaments. Figure adapted, with permission, from ().


Figure 11. Electron micrographs of negatively stained Ca2+ regulated thin filaments showing troponin and tropomyosin densities. (A) Filaments isolated directly from frog cardiac muscles. Note helically arranged tropomyosin cables (black arrows) seen extending along the filament in many micrographs. Figure adapted, with permission, from (). (B) Reconstituted thin filament here consisting of rabbit skeletal muscle F‐actin and bovine cardiac troponin‐tropomyosin. Note in this micrograph periodic troponin densities that are in register on opposite sides of the filament (marked by white arrow heads) partially masking tropomyosin cables (black arrow). Figure adapted, with permission, from (). Scale bar = 50 nm.


Figure 12. Ca2+‐induced tropomyosin movement on thin filaments revealed by 3D‐reconstruction. (A) Surface representation of EM reconstructions generated from negatively stained Limulus thin filaments following exposure to low‐ and high‐Ca2+ conditions. Note the movement of the tropomyosin cable (black arrows) from the outer domain (Ao) toward the inner domain of actin (Ai) that occurs in Ca2+. At low‐Ca2+, tropomyosin associates with subdomain 1 of actin and bridges over subdomain 2, whereas, at high‐Ca2+, its association is with subdomain 3 of actin and it arches over subdomain 4 (subdomains numbered on one actin subunit). (B) “Helical projections” of the same reconstructions made by projecting densities along their helical paths onto a plane perpendicular to the filament axis. Again, note the change in position of the tropomyosin (Tm); the center of the tropomyosin densities are highlighted in red and green for the low‐Ca2+ and high‐Ca2+ locations. The two projections are also superposed for comparison. Ca2+‐induced tropomyosin movement was first demonstrated on thin filaments isolated from striated muscles of the horseshoe crab Limulus and later to occur on filaments obtained from other invertebrate and vertebrate striated muscles. Figure adapted, with permission, from (). It should be noted that the two inner and two outer domains of neighboring actin subunits shown in projection already delineate what can be considered four long‐pitch “strand‐like” densities (two inner and two outer strands). In the off‐state, tropomyosin is positioned close to the mid‐point between inner and outer domains of actin, perhaps not adding significantly to the filament fourfoldedness. In contrast, in the on‐state when tropomyosin moves closer to the inner domain of actin, the appearance of fourfoldedness is clearly emphasized. Thus, the four‐strandedness defined in the Fourier transforms of thin filaments (see Fig. 13) may not be accurately described by the conventional meaning of the term, viz., two actin and two tropomyosin strands, but rather is due to contributions from four actin components (two inner and two outer domains), augmented differentially by tropomyosin in one or another position.


Figure 13. Fourier transforms of the Limulus filament EMs. The averaged layer line data used to produce the helical reconstructions in Figure 12 is plotted here. (A) low‐Ca2+ and (B) high‐Ca2+ data. Layer line amplitudes are represented as solid lines and phases as dotted lines. Bessel and layer line orders, (n) and (l), defining the actin‐helical symmetry are given. As in fiber diffraction patterns records of intact muscle (see Fig. 6), the amplitude along the second order J4 layer line (l = 2, n = 4) of the EM data is greater for the filaments associated with the high‐Ca2+ than for those at low‐Ca2+ (arrows). Figure adapted, with permission, from ().


Figure 14. Dual effect of tropomyosin and troponin‐tropomyosin on actomyosin subfragment 1 ATPase. Acto‐S1 ATPase assays show that troponin‐tropomyosin regulated thin filaments activate actomyosin ATPase in a Ca2+‐dependent manner at all S1 concentrations tested, while control F‐actin and F‐actin‐tropomyosin (no troponin) do not confer Ca2+ dependence. The activation of actomyosin S1 ATPase by control F‐actin is linear as a function of added S1, yet sigmoidal when either troponin‐free tropomyosin or troponin‐tropomyosin is present (plateaus at high S1 levels not shown). Thus, relative to control F‐actin values, tropomyosin, in the presence and absence of troponin and Ca2+, inhibits actomyosin ATPase at low S1:F‐actin ratios, and stimulates ATPase at high S1:F‐actin ratios. Inhibition and activation by tropomyosin are greatest when troponin is present. The observations of Lehrer and Morris (), like those of Weber and colleagues (), demonstrated that cooperative activation of thin filaments is dependent on S1 interactions with tropomyosin on actin. The studies suggested that, in addition to Ca2+ involvement in the activation process, myosin itself induces an “open‐state” of the thin filament. Figure adapted, with permission, from (). This figure was originally published in The Journal of Biological Chemistry by S.S. Lehrer and E.P. Morris in an article “Dual effects of tropomyosin and tropomyosin‐troponin on skeletal muscle subfragment 1 ATPase” J. Biol. Chem. 1982; 257:682‐693 © the American Society for Biochemistry and Molecular Biology.


Figure 15. Schematic diagrams of the McKillop‐Geeves three‐state model of the thin filament. (A) Regulatory units of the thin filament are illustrated by single lines depicting tropomyosin over seven circles representing actin; myosin is drawn as a triangle. Blocked, closed, open states are indicated. The dynamic equilibrium between states is illustrated as an azimuthal translocation of tropomyosin on actin and an isomerization of myosin into a strongly attached configuration. K B and K T are Ca2+‐sensitive steps. (B) The same transitions represented in different cartoon format. It should be recognized that for the weak state of myosin attachment to actin and also for actin‐bound myosin that has not yet released phosphate (not shown in the figure), any favorable or unfavorable tropomyosin‐myosin interactions are still beyond current knowledge. Moreover, the structural correlates of the model do not explain all aspects of myosin regulatory function, and were derived from experiments not including the presence of ATP. Figure in panel A adapted, with permission, from (), Copyright 1974 American Chemical Society. Figure in panel B adapted, with permission, from ().


Figure 16. The effect of troponin I trapping tropomyosin in the blocked B‐state thin‐filament position. Reconstructions of (A) F‐actin control filaments, subdomains numbered, and (B) TnI‐decorated F‐actin‐tropomyosin, where the C‐terminal regulatory fragment of TnI is present on every actin subunit. TnI densities are indicated by arrows. (C) Densities attributable to TnI (purple) seen in (B) trapping tropomyosin (red) in the blocking position on actin. (D) Reconstruction of low‐Ca2+ native cardiac thin filaments now containing tropomyosin and the entire troponin complex (hence there is one troponin complex on every seventh actin). Densities representing the troponin core domain consisting of the TnIT arm and the N and C lobes of TnC labeled. Black arrow marks tropomyosin in the blocking state. (E) Difference maps highlighting troponin (gold) made by subtracting the actin‐tropomyosin from the map in (D) and superposing the difference on actin‐tropomyosin. Note the position of density attributable to TnT (open arrow) and to the C‐terminal domain of TnI (double‐sided arrow). In (F), maps in (B) and (E) merged; the TnI density seen in (E) superposes on the corresponding TnI density found in (B). Figure adapted, with permission, from () (panels A, B, D‐F) and () (panel C).


Figure 17. Docking atomic structures of F‐actin into 3D reconstructions of native thin filaments. This procedure by Vibert et al. () provided the first structural evidence for the three‐state model of thin filament regulation. (A‐D) Helical reconstructions shown as a blue wire mesh density envelopes cut to about 6 nm to illustrate one actin subunit [actin subdomains numbered in (A)] and the changing position of tropomyosin (arrows) in response to Ca2+ and myosin binding. In each case, a high‐resolution actin subunit has been docked within the reconstruction and is shown as a yellow α‐carbon chain with several clusters of residues highlighted in various colors. (A) Reconstruction of low‐Ca2+ thin filaments. Note that charged residues 1 to 4 on the N‐terminus of actin and actin residues 92 to 95 (green) on the edge of the subdomain, which are thought to interact electrostatically with myosin prior to cross‐bridge cycling, are not blocked by the tropomyosin density. In contrast, actin residues involved in strong stereospecific binding to myosin (e.g., 24 to 28, 144 to 148, and 340 to 346) () (red) are obstructed by tropomyosin. In (C), a reconstruction of high‐Ca2+ thin filaments shows that tropomyosin has repositioned and that most highlighted myosin binding residues on actin are now exposed (green) with the exception of, for example, amino acids 332 to 336 (red) at the junction of actin subdomains 1 and 3, which remain occluded. (B) Reconstruction of S1 decorated thin filaments. Note that tropomyosin (arrows) has moved further away from the blocking position than it does in Ca2+ (cf. to C) and that all highlighted residues (magenta) are free to bind myosin. In (D), the reconstruction of S1 decorated thin filaments has been contoured to remove part of the S1 density to provide an outline of the S1‐binding site and a better view of the actin residues involved in S1 binding. The densities shown in the reconstruction of the S1 decorated filament and its corresponding actin‐binding site detected are similar to those shown by Milligan et al. (). The Lorenz et al. atomic model of F‐actin () was used for the docking shown. Figure adapted, with permission, from ().


Figure 18. Fitting atomic coordinates of actin, tropomyosin, and myosin to single particle reconstructions of reconstituted thin filaments. Filaments consisted of actin, troponin, and tropomyosin. Poole et al. () brought together the 3D‐EM of Pirani et al. () and the synchrotron X‐ray diffraction data from nonoverlapped muscle fibers, thereby synthesizing ideas about steric blocking into a consistent and comprehensive account. (A, B, C) Structural models of F‐actin (blue, cyan, white) [Holmes et al. ()] and tropomyosin (red, yellow, green) [Lorenz et al. ()] were fitted within EM density envelopes of reconstituted filaments (translucent surfaces). Reconstructions of (A) low‐Ca2+, (B) high‐Ca2+, and (C) S1‐decorated filaments fitted with actin and tropomyosin structures. Note the shifting position of tropomyosin in the three cases and the position of the fitted S1 [Rayment et al. ()] in (C). (D) Tropomyosin in its three average positions superposed for comparison; the locations of tropomyosin agree with earlier results of Vibert et al. (). (E) The S1 binding configuration in (C) is superposed on the low‐Ca2+ B‐state filament model from (A). Note the steric clashes between S1 and tropomyosin. While the superposition in (E) makes apparent that tropomyosin occludes the S1‐binding site on actin, it also shows that tropomyosin at low‐Ca2+ keeps the cleft between the upper and lower 50K domains of myosin (red) separated and prevents S1 cleft closure, thus interfering with actin‐myosin ATPase and cross‐bridge cycling. This can be best appreciated by comparing S1 heads and tropomyosin on the left and right side of the figure. Figure adapted, with permission, from ().


Figure 19. High‐resolution cryo‐EM reconstructions of F‐actin and F‐actin tropomyosin. (A) F‐actin at 3.7 Å resolution [constructed from PDB ID:3J8A ()]. Note the charged residues on actin [Lys 326, Lys 328, Arg 147, Arg 28 (blue), and Glu 25 (red)] that define the binding path assumed by tropomyosin in (B) () when unperturbed by troponin or myosin. Also note that residues near to Arg 28 and Glu 25 protrude from the surface of actin to form a barrier on subdomain 1 that limits the azimuthal rotation of tropomyosin toward the extreme outer edge of actin in the blocked B‐state. Similarly, the lateral edge of subdomain 4 bulges out, limiting the extent of azimuthal rotation of tropomyosin in the direction of the inner actin domain in the M‐state. Pro 333 (highlighted in pink) defines the boundary between the B‐ and C‐states, but as yet, there is no high resolution model for the C‐state position of tropomyosin. (B, C) The locations of tropomyosin in blocked (magenta) and open (green) positions, respectively, and superposed in (D). B‐ and M‐state tropomyosin constructed from PDBs given in () and (). (E) 8 Å resolution reconstruction of F‐actin‐tropomyosin decorated with Dictyostelium S1 () superposed on (D), defining the binding positions of tropomyosin relative to myosin interaction. Behrmann et al. (), Sousa et al. (), and von der Ecken et al. () were the first to resolve individual α‐helical chains of the tropomyosin coiled‐coil; however, at 8 Å resolution, tropomyosin's side chains are not defined and hence corresponding residues cannot be assigned explicitly to densities in the maps.


Figure 20. Atomic model of F‐actin‐tropomyosin on F‐actin. Li et al. () proposed a residue‐specific model for the actin‐tropomyosin structure by optimizing the electrostatic interaction energies between actin and tropomyosin structures. The model was later refined to include the head‐to‐tail tropomyosin overlapping domain by Orzechowski et al. (). In (A), likely contacts between tropomyosin pseudorepeats and actin subunits are displayed, and, in (B), a magnified view of tropomyosin pseudorepeat 4 is highlighted, illustrating acidic residues on tropomyosin in close proximity to basic Arg 147, Lys 326, and Lys 328 located on actin at the border between subdomain 1 and 3, while Glu 334, Arg 28, and Asp 25 higher on actin subdomain 1 position to associate with oppositely charged amino acids in tropomyosin (cf. Fig. 19). This pattern of contacts is repeated between all tropomyosin pseudorepeats and successive actin subunits along the filament. The position of tropomyosin in the model of troponin‐free actin‐tropomyosin () agrees with that observed for tropomyosin when pinned down on regulated filaments by TnI at low‐Ca2+ (). The model also agrees with predictions of actin‐tropomyosin interaction by Brown and Cohen () and Barua et al. (). Figure adapted, with permission, from () and is based on ().
References
 1. Adelstein RS , Sellers JR , Conti MA , Pato MD , de Lanerolle P . Regulation of smooth muscle contractile proteins by calmodulin and cyclic AMP. Fed Proc 41: 2873‐2878, 1982.
 2. Aksoy MO , Williams D , Sharkey EM , Hartshorne DJ . A relationship between Ca2+ sensitivity and phosphorylation of gizzard actomyosin. Biochem Biophys Res Commun 69: 35‐41, 1976.
 3. Ashley CC , Ridgway EB . Simultaneous recording of membrane potential, calcium transient and tension in single muscle fibers. Nature 219: 1168‐1169, 1968.
 4. Bailey K . Myosin and adenosinetriphosphatase. Biochem J 36: 121‐139, 1942.
 5. Bailey K . Tropomyosin a new asymmetric protein of muscle. Nature 157: 368‐369, 1946.
 6. Bailey K . Tropomyosin: A new asymmetric protein component of the muscle fibril. Biochem J 43: 271‐273, 1948.
 7. Barua B , Fagnant PM , Winkelmann DA , Trybus KM , Hitchcock‐Degregori SE . A periodic pattern of evolutionarily conserved basic and acidic residues constitutes the binding interface of actin‐tropomyosin. J Biol Chem 288: 9602‐9609, 2013.
 8. Behrmann E , Müller M , Penczek PA , Mannherz HG , Manstein DJ , Raunser S . Structure of the rigor actin–tropomyosin–myosin complex. Cell 150: 327‐338, 2012.
 9. Bivin DB , Stone DB , Schneider DK , Mendelson RA . Cross‐helix separation of tropomyosin molecules in acto‐tropomyosin as determined by neutron scattering. Biophys J 59: 880‐888, 1991.
 10. Block SM , Blair DF , Berg HC . Compliance of bacterial flagella measured with optical tweezers. Nature 338: 514‐518, 1989.
 11. Bremel RD . Myosin linked calcium regulation in vertebrate smooth muscle. Nature 252: 405‐407, 1974.
 12. Bremel RD , Murray JM , Weber A . Manifestations of cooperative behaviour in the regulated actin filament during actin‐activated ATP hydrolysis in the presence of calcium. Cold Spring Harb Symp Quant Biol 37: 267‐275, 1972.
 13. Bremel RD , Weber A . Cooperation within actin filament in vertebrate skeletal muscle. Nat New Biol 238: 97‐101, 1972.
 14. Brenner B , Schoenberg M , Chalovich JM , Greene LE , Eisenberg E . Evidence for cross‐bridge attachment in relaxed muscle at low ionic strength. Proc Natl Acad Sci U S A 79: 7288‐7291, 1982.
 15. Brenner S , Horne RW . A negative staining method for high‐resolution electron microscopy of viruses. Biochim Biophys Acta 34: 103‐110, 1959.
 16. Brown JH , Cohen C . Regulation of muscle contraction by tropomyosin and troponin: How structure illuminates function. Adv Protein Chem 71: 121‐159, 2005.
 17. Brown JH , Kim KH , Jun G , Greenfield NJ , Dominguez R , Volkmann N , Hitchcock‐DeGregori SE , Cohen C . Deciphering the design of the tropomyosin molecule. Proc Natl Acad Sci U S A 98: 8496‐8501, 2001.
 18. Brown JH , Zhou Z , Reshetnikova L , Robinson H , Yammani RD , Tobacman LS , Cohen C . Structure of the mid‐region of tropomyosin: Bending and binding sites for actin. Proc Natl Acad Sci U S A 102: 18878‐18883, 2005.
 19. Burgess S , Walker M , Knight PJ , Sparrow J , Schmitz S , Offer G , Bullard B , Leonard K , Holt J , Trinick J . Structural studies of arthrin: Monoubiquitinated actin. J Mol Biol 341: 1161‐1173, 2004.
 20. Cammarato A , Hatch V , Saide J , Craig R , Sparrow JC , Tobacman LS , Lehman W . Drosophila muscle regulation characterized by electron microscopy and three‐dimensional reconstruction of thin filament mutants. Biophys J 86: 1618‐1624, 2004.
 21. Carlson JM , Doyle J . Complexity and robustness. Proc Natl Acad Sci U S A 99: 2538‐2545, 2002.
 22. Chacko S , Conti MA , Adelstein RS . Effect of phosphorylation of smooth muscle myosin on actin activation and Ca2+ regulation. Proc Natl Acad Sci U S A 74: 129‐133, 1977.
 23. Chalovich JM , Chock PB , Eisenberg E . Mechanism of action of troponin.tropomyosin. Inhibition of actomyosin ATPase activity without inhibition of myosin binding to actin. J Biol Chem 256: 575‐578, 1981.
 24. Chalovich JM , Eisenberg E . Inhibition of actomyosin ATPase activity by troponin‐tropomyosin without blocking the binding of myosin to actin. J Biol Chem 257: 2432‐2437, 1982.
 25. Cohen C , Caspar DLD , Johnson JP , Nauss K , Margossian SS , Parry DAD . Troponin‐tropomyosin assembly. Cold Spring Harb Symp Quant Biol 37: 287‐297, 1972.
 26. Cohen C , Longley W . Tropomyosin paracrystals formed by divalent cations. Science 152: 794‐796, 1966.
 27.Watson JD. Cold Spring Harbor Symposium on Quantitative Biology, The Mechanism of Muscle Contraction. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory, Vol. 37, pp. 1‐706, 1972.
 28. Constantin LL , Franzini‐Armstrong C , Podolsky RJ . Localization of calcium‐accumulating structures in striated muscle fibers. Science 147: 158‐160, 1965.
 29. Craig R , Lehman W . Crossbridge and tropomyosin positions observed in native, interacting thick and thin filaments. J Mol Biol 311: 1027‐1036, 2001.
 30. Crick FHC . The packing of α‐helices: Simple coiled‐coils. Acta Crystallogr 6: 689‐697, 1953.
 31. Davies RE . A molecular theory of muscle contraction: Calcium‐dependent contractions with hydrogen bond formation plus ATP‐dependent extensions of part of the myosin‐actin cross‐bridges. Nature 199: 1068‐1074, 1963.
 32. Dabrowska R , Aromatorio DK , Sherry JM , Hartshorne DJ . Composition of myosin light chain from chicken gizzard. Biochem Biophys Res Commun 78: 1263‐1272, 1977.
 33. DeRosier DJ , Klug A . Reconstruction of 3 dimensional structures from electron micrographs. Nature 217: 130‐134, 1968.
 34. DeRosier DJ , Moore PB . Reconstruction of three‐dimensional images from electron micrographs of structures with helical symmetry. J Mol Biol 52: 355‐369, 1970.
 35. Dominguez R . Tropomyosin: The gatekeeper's view of the actin filament revealed. Biophys J 100: 797‐798, 2011.
 36. Eaton BL , Kominz, DR , Eisenberg E . Correlation between the inhibition of the acto‐heavy meromyosin ATPase and the binding of tropomyosin to F‐actin: Effects of Mg2+, KCl, troponin I, and troponin C. Biochemistry 14: 2718‐2725, 1975.
 37. Ebashi S . Ca‐binding activity of vesicular relaxing factor. J Biochem 50: 236‐244, 1961.
 38. Ebashi S . Third component participating in the superprecipitation of “Natural Actomyosin”. Nature 200: 1010, 1963.
 39. Ebashi S , Ebashi F . A new protein component participating in superprecipitation of myosin B. J Biochem (Tokyo) 55: 604‐613, 1964.
 40. Ebashi S , Endo M . Calcium ion and muscle contraction. Prog Biophys Mol Biol 18: 125‐183, 1968.
 41. Ebashi S , Endo M , Ohtsuki I . Control of muscle contraction. Quart Rev Biophys 2: 351‐384, 1969.
 42. Ebashi S , Lipmann F . Adenosine triphosphate‐linked concentration of calcium ions in a particulate fraction of rabbit muscle. J Cell Biol 14: 389‐400, 1962.
 43. Ebashi S , Ohtsuli I , Mihashi K . Regulatory proteins with special reference to troponin. Cold Spring Harb Symp Quant Biol 37: 215‐223, 1972.
 44. Egelman EH . A robust algorithm for the reconstruction of helical filaments using single particle methods. Ultramicroscopy 85: 225‐234, 2000.
 45. Egelman EH . Single‐particle reconstruction from EM images of helical filaments. Curr Opin Struct Biol 17: 556‐561, 2007.
 46. Egelman EH , Orlova A . Two conformations of G‐actin related to two conformations of F‐actin. Results Probl Cell Differ 32: 95‐101, 2001.
 47. El‐Mezgueldi M . Tropomyosin dynamics. J Muscle Res Cell Motil 35: 203‐210, 2014.
 48. Eliason JL . Using paradoxes to teach critical thinking in science. J College Sci Teaching 15: 341‐344, 1996.
 49. Finer JT , Simmons RM , Spudich JA . Single myosin molecule mechanics: Piconewton forces and nanometre steps. Nature 368:113‐119, 1994.
 50. Flicker PF , Milligan RA , Applegate D . Cryo‐electron microscopy of S1‐decorated actin filaments. Adv Biophys 27: 185‐196, 1991.
 51. Flicker PF , Phillips GN Jr , Cohen. C . Troponin and its interactions with tropomyosin. An electron microscope study. J Mol Biol 162: 495‐501, 1982.
 52. Franzini‐Armstrong C . The sarcoplasmic reticulum and the control of muscle contraction. FASEB J 13(Suppl 2): S266‐S270, 1999.
 53. Fujii T , Iwane AH , Yanagida T , Namba K . Direct visualization of secondary structures of F‐actin by electron cryomicroscopy. Nature 467: 724‐728, 2010.
 54. Galkin VE , Orlova A , Vos MR , Schröder GF , Egelman EH . Near‐atomic resolution for one state of F‐actin. Structure 23: 173‐182, 2015.
 55. Galińska‐Rakoczy A , Engel P , Xu C , Jung H , Craig R , Tobacman LS , Lehman W . Structural basis for the regulation of muscle contraction by troponin and tropomyosin. J Mol Biol 379: 929‐935, 2008.
 56. Ganong WF . Review of Medical Physiology (21st ed). New York: Lange Medical Books‐McGraw Hill, 2003, pp. 66.
 57. Geeves MA . Thin filament regulation. In: Egelman EH , Goldman YE , Ostap EM , editors. Comprehensive Biophysics, Vol 4, Molecular Motors and Motility. Oxford: Academic Press, 2012, pp. 251‐267.
 58. Geeves MA , Chai M , Lehrer SS . Inhibition of actin‐myosin subfragment 1 ATPase activity by troponin I and IC: Relationship to the thin filament states of muscle. Biochemistry 39: 9345‐9350, 2000.
 59. Geeves M , Griffiths H , Mijailovich S , Smith D . Cooperative Ca2+‐dependent regulation of the rate of myosin binding to actin: Solution data and the tropomyosin chain model. Biophys J 100: 2679‐2687, 2011.
 60. Geeves MA , Holmes KC . The molecular mechanism of muscle contraction. Adv Protein Chem 71: 161‐193, 2005.
 61. Geeves MA , Lehrer SS . Modeling thin filament cooperativity. Biophys J 82: 1677‐1681, 2002.
 62. Gilles JM , O'Brien EJ . The effect of calcium on the structure of reconstituted thin filaments. J Mol Biol 99: 445‐459, 1975.
 63. Goldberg A , Lehman W . Troponin‐like proteins from muscles of the scallop, Aequipecten irradians . Biochem J 171: 413‐418, 1978.
 64. Golitsina NL , Lehrer SS . Smooth muscle alpha‐tropomyosin crosslinks to caldesmon, to actin and to myosin subfragment 1 on the muscle thin filament. FEBS Lett 463: 146‐150, 1999.
 65. Górecka A , Aksoy MO , Hartshorne DJ . The effect of phosphorylation of gizzard myosin on actin activation. Biochem Biophys Res Commun 71: 325‐331, 1976.
 66. Greaser ML , Gergely J . Purification and properties of the components from troponin. J Biol Chem 246: 4226‐4233, 1971.
 67. Greaser M , Yamaguchi M , Berkke C , Potter J , Gergeley, J . Troponin subunits and their interactions. Cold Spring Harb Symp Quant Biol 37: 235‐244, 1972.
 68. Greenfield NJ , Huang YJ , Swapna GV , Bhattacharya A , Rapp B , Singh A , Montelione GT , Hitchcock‐DeGregori SE . Solution NMR structure of the junction between tropomyosin molecules: Implications for actin binding and regulation. J Mol Biol 364: 80‐96, 2006.
 69. Hanson J . Axial period of actin filaments. Nature 213: 353‐356, 1967.
 70. Hanson J . Evidence from electron microscope studies on actin paracrystals concerning the origin of the cross‐striation in the thin filaments of vertebrate skeletal muscle. Proc R Soc Lond B 183: 39‐58, 1973.
 71. Hanson J , Huxley HE . Structural basis of cross‐striations in muscle. Nature 172: 530‐532, 1953.
 72. Hanson J , Huxley HE . The structural basis of contraction in striated muscle. Symp Soc Exp Biol 9: 228‐264, 1955.
 73. Hanson J , Lednev V , O'Brien EJ , Bennett PM . Structure of the actin‐containing filaments in vertebrate skeletal muscle. Cold Spring Harb Symp Quant Biol 37: 311‐318, 1972.
 74. Hanson J , Lowy J . The structure of F‐actin and of actin filaments isolated from muscle. J Mol Biol 6: 46‐60, 1963.
 75. Hanson J , Lowy J . The structure of actin filaments and the origin of the axial periodicity in the I‐substance of vertebrate striated muscle. Proc R Soc Lond B 160: 449‐460, 1964.
 76. Hartshorne DJ . Biochemical basis for contraction of vascular smooth muscle. Chest J 78(Suppl 1): 140‐149, 1980.
 77. Haselgrove JC . X‐ray evidence for a conformational change in actin‐containing filaments of vertebrate striated muscle. Cold Spring Harb Symp Quant Biol 37: 341‐352, 1972.
 78. Haselgrove JC . Structure of vertebrate striated muscle as determined by X‐ray‐diffraction studies. In: Peachy LD , Adrian RH , Geiger SR , editors. Handbook of Physiology, Section 10, Skeletal Muscle. Bethesda, MD: American Physiological Society, 1988 Chapter 5, pp. 143‐171.
 79. Hasselbach W . Relaxing factor and the relaxation of muscle. Prog Biophys Mol Biol 14: 169‐222, 1964.
 80. Heeley DH , Belknap B , White HD . Maximal activation of skeletal muscle thin filaments requires both rigor myosin S1 and calcium. J Biol Chem 281: 668‐676 2006.
 81. Heilbrunn LV , Wiercinsky FJ . The action of various cations on muscle protoplasm. J Cell Comp Physiol 29: 15‐32, 1947.
 82. Higashi S , Ooi T . Crystals of tropomyosin and native tropomyosin. J Mol Biol 34: 699‐701, 1986.
 83. Hill AV. Introduction by the leader of the discussion. Proc R Soc B 137: 40‐87, 1949.
 84. Hitchcock SE , Huxley HE , Szent‐Györgyi AG . Calcium sensitive binding of troponin to actin tropomyosin: A two‐site model for troponin action. J Mol Biol 80: 825‐836, 1973.
 85. Hitchcock‐DeGregori SE . Tropomyosin: Function follows form. Tropomyosin and the steric mechanism of muscle regulation. Adv Exp Med Biol 644: 60‐67, 2008.
 86. Holmes KC . The actomyosin interaction and its control by tropomyosin. Biophys J 68: 2s‐7s, 1995.
 87. Holmes KC , Angert I , Kull FJ , Jahn W , Schröder RR . Electron cryo‐microscopy shows how strong binding of myosin to actin releases nucleotide. Nature 424: 423‐427, 2003.
 88. Holmes KC , Blow DM . The Use of X‐ray Diffraction in the Study of Protein and Nucleic acid Structure. Olney, UK: Interscience, 1966.
 89. Holmes KC , Lehman W . Gestalt‐binding of tropomyosin to actin filaments. J Muscle Res Cell Motil 29: 213‐219, 2008.
 90. Holmes KC , Popp D , Gebhard W , Kabsch W . Atomic model of the actin filament. Nature 347:44‐47, 1990.
 91. Holmes KC , Tirion M , Popp D , Lorenz M , Kabsch W , Milligan RA . A comparison of the atomic model of F‐actin with cryo‐electron micrographs of actin and decorated actin. Adv Exp Med Biol 332: 15‐22, 1993.
 92. Huxley AF . Introductory remarks. Proc R Soc Lond B 160: 434‐437, 1964.
 93. Huxley AF , Niedergerke R . Structural changes in muscle during contraction. Nature 173: 971‐973, 1954.
 94. Huxley HE . Electron microscope studies on the organisation of the filaments in striated muscle. Biochim Biophys Acta 12: 387‐394, 1953.
 95. Huxley HE. Some observations on the structure of tobacco mosaic virus. In: Proceedings of the First European Regional Conference on Electron Microscopy, edited by Sjöstrand SP, Rhodin J., Stockholm, Almqvist and Wiksell, 1956, p. 260.
 96. Huxley HE . The double array of filaments in cross‐striated muscle. J Biophys Biochem Cytol 3: 631‐647, 1957.
 97. Huxley HE . Electron microscope studies on the structure of natural and synthetic protein filaments from striated muscle. J Mol Biol 7: 281‐308, 1963.
 98. Huxley HE . The mechanism of muscle contraction. Science 164: 1356‐1366, 1969.
 99. Huxley HE. 1970. Structural changes in muscle and muscle proteins during contraction. In: Gregory JG, editor. Proceedings of the 8th International Congress of Biochemistry, Interlaken, Vol. 4, p. 23, 1970.
 100. Huxley HE . Cross‐bridge movement and filament overlap. Biophys J 11: 235a, 1971.
 101. Huxley HE . Structural changes during muscle contraction. Biochem J 125: 85P, 1971.
 102. Huxley HE . Structural changes in actin‐ and myosin‐containing filaments during contraction. Cold Spring Harb Symp Quant Biol 37: 361‐376, 1972.
 103. Huxley HE . Memories of early work on muscle contraction and regulation in the 1950's and 1960's. Biochem Biophys Res Commun 369: 34‐42, 2008.
 104. Huxley HE , Hanson J . Changes in the cross‐striations of muscle during contraction and stretch and their structural interpretation. Nature 173: 973‐976, 1954.
 105. Huxley HE , Simmons RE , Faruqi AR , Kress M , Borda J . Millisecond time‐resolved changes in X‐ray reflections from contracting muscle during rapid mechanical transients, recorded using synchrotron radiation. Proc Natl Acad Sci U S A 78: 2297‐2301, 1981.
 106. Huxley HE , Zubay G . Electron microscope observations on the structure of microsomal particles from Escherichia coli . J Mol Biol 2: 10‐18, 1960.
 107. Huxley HE , Zubay G . The structure of the protein shell of turnip yellow mosaic virus. J Mol Biol 2: 189‐196, 1960.
 108. Janco M , Suphamungmee W , Li X , Lehman W , Lehrer S , Geeves M . Polymorphism in tropomyosin structure and function. J Muscle Res Cell Motil 34:177‐187, 2013.
 109. Jöbsis F , O'Connor MJ . Calcium release and reabsorption in the sartorius muscle of the toad. Biochem Biophys Res Commun 25: 246‐252, 1966.
 110. Jung HS , Craig R . Ca2+‐induced tropomyosin movement in scallop striated muscle thin filaments. J Mol Biol 383: 512‐519, 2008.
 111. Kabsch W , Mannherz HG , Suck D , Pai EF , Holmes KC . Atomic structure of actin: DNAase 1 complex. Nature 347: 37‐43, 1990.
 112. Kendrick‐Jones J , Lehman W , Szent‐Györgyi AG . Regulation in molluscan muscles. J Mol Biol 54: 313‐326, 1970.
 113. Klug A , DeRosier DJ . Optical filtering of electron micrographs: Reconstruction of one‐sided images. Nature 212: 29‐32, 1966.
 114. Kress M , Huxley HE , Faruqi AR , Hendrix J . Structural changes during activation of frog muscle studied by time‐resolved X‐ray diffraction. J Mol Biol 188: 325‐342, 1986.
 115. Kron SJ , Spudich JA . Fluorescent actin filaments move on myosin fixed to a glass surface. Proc Natl Acad Sci U S A 83: 6272‐6276, 1986.
 116. Kushmerick MJ . Energetics of muscle contraction. In: Peachy LD , Adrian RH , Geiger SR , editors. Handbook of Physiology, Section 10, Skeletal Muscle. Bethesda, MD: American Physiological Society, Chap. 7, pp. 189‐236, 1988.
 117. Lamb GD . Excitation‐contraction coupling in skeletal muscle: Comparisons with cardiac muscle. Clin Exptl Pharm Physiol 27: 216‐224, 2000.
 118. Lehman W . Phylogenetic diversity of the proteins regulating muscular contraction. Int Rev Cytol 44: 55‐92, 1976.
 119. Lehman W , Craig R . The structure of the vertebrate striated muscle thin filament: A tribute to the contributions of Jean Hanson. J Muscle Res Cell Motil 25: 455‐466, 2004.
 120. Lehman W , Craig R . Tropomyosin and the steric mechanism of muscle regulation. Adv Exp Med Biol 644: 95‐109, 2008.
 121. Lehman W , Craig R , Lui J , Moody C . Caldesmon and the structure of smooth muscle thin filaments: Immunolocalization of caldesmon on thin filaments. J Muscle Res Cell Motil 10: 101‐112, 1989.
 122. Lehman W , Craig R , Vibert P . Ca2+‐induced tropomyosin movement in Limulus thin filaments revealed by three‐dimensional reconstruction. Nature 368: 65‐67, 1994.
 123. Lehman W , Galińska‐Rakoczy A , Hatch V , Tobacman LS , Craig R . Structural basis for the activation of muscle contraction by troponin and tropomyosin. J Mol Biol 388:673‐68, 2009.
 124. Lehman W , Hatch V , Korman V , Rosol M , Thomas L , Maytum R , Geeves MA , Van Eyk JE , Tobacman LS , Craig R . Tropomyosin and actin isoforms modulate the localization of tropomyosin strands on actin filaments. J Mol Biol 302: 593‐606, 2000.
 125. Lehman W , Kendrick‐Jones J , Szent‐Györgyi AG . Myosin‐linked regulatory systems: Comparative studies. Cold Spring Harb Symp Quant Biol 37: 319‐330, 1972.
 126. Lehman W , Orzechowski M , Li XE , Fisher S , Raunser S . Gestalt‐binding of tropomyosin during muscle activation. J Muscle Res Cell Motil 34: 155‐163, 2013.
 127. Lehman W , Szent‐Györgyi AG . Regulation of muscular contraction. Distribution of actin control and myosin control in the animal kingdom. J Gen Physiol 66: 1‐30, 1975.
 128. Lehman W , Vibert P , Uman P , Craig R . Steric‐blocking by tropomyosin visualized in relaxed vertebrate muscle thin filaments. J Mol Biol 251: 191‐196, 1995.
 129. Lehrer SS . The 3‐state model of muscle regulation revisited: Is a fourth state involved? J Muscle Res Cell Motil 32: 203‐208, 2011.
 130. Lehrer SS , Geeves MA . The muscle thin filament as a classical cooperative/allosteric regulatory system. J Mol Biol 277: 1081‐1089, 1998.
 131. Lehrer SS , Morris EP . Dual effects of tropomyosin and troponin‐tropomyosin on actomyosin subfragment 1 ATPase. J Biol Chem 257: 8073‐8080, 1982.
 132. Lehrer SS , Morris EP . Comparison of the effects of smooth and skeletal tropomyosin on skeletal actomyosin subfragment 1 ATPase. J Biol Chem 259: 2070‐2072, 1984.
 133. Li KL , Rieck D , Solaro RJ , Dong W . In situ time‐resolved FRET reveals effects of sarcomere length on cardiac thin‐filament activation. Biophys J 107: 682‐693, 2014.
 134. Li XE , Holmes KC , Lehman W , Jung H‐S , Fischer S . The shape and flexibility of tropomyosin coiled‐coils: Implications for actin filament assembly and regulation. J Mol Biol 395: 327‐399, 2010.
 135. Li XE , Lehman W , Fischer S . The relationship between curvature, flexibility and persistence length in the tropomyosin coiled‐coil. J Struct Biol 107: 313‐318, 2010.
 136. Li XE , Lehman W , Fischer S , Holmes KC . Curvature variation along the tropomyosin molecule. J Struct Biol 170: 307‐312, 2010.
 137. Li XE , Suphamungmee W , Janco M , Geeves MA , Marston SB , Fischer S , Lehman W . The flexibility of two tropomyosin mutants, D175N and E180G, that cause hypertrophic cardiomyopathy. Biochem Biophys Res Commun 424: 493‐496, 2012.
 138. Li XE , Tobacman LS , Mun JY , Craig R , Fischer S , Lehman W . Tropomyosin position on F‐actin revealed by EM reconstruction and computational chemistry. Biophys J 100: 1005‐1013, 2011.
 139. Loong CK , Badr MA , Chase PB . Tropomyosin flexural rigidity and single Ca2+ regulatory unit dynamics: Implications for cooperative regulation of cardiac muscle contraction and cardiomyocyte hypertrophy. Front Physiol 3: 1‐10, 2012.
 140. Lorenz M , Poole KJV , Popp D , Rosenbaum G , Holmes KC . An atomic model of the unregulated thin filament obtained by X‐ray fiber diffraction on oriented actin–tropomyosin gels. J Mol Biol 246: 108‐119, 1995.
 141. Lorenz M , Popp D , Holmes KC . Refinement of the F‐actin model against X‐ray fiber diffraction data by use of a directed mutation algorithm. J Mol Biol 234: 826‐836, 1993.
 142. Lowey S , Slayter HS , Weeds AG , Baker H . Substructure of the myosin molecule I. Subfragments of myosin by enzymic degradation. J Mol Biol 42: 1‐20, 1969.
 143. Lowy J . X‐ray diffraction studies of striated and smooth muscles. Bollettino di Zoologia 39: 119‐138, 1972.
 144. Lowy J , Vibert P . Studies on the low angle X‐ray pattern of molluscan smooth muscle during tonic contraction and rigor. Cold Spring Harb Symp Quant Biol 37: 353‐359, 1972.
 145. Marston S , Gautel M . Introducing a special edition of the Journal of Muscle Research and Cell Motility on tropomyosin: Form and function. J Muscle Res Cell Motil 34: 151‐153, 2013.
 146. Maytum R , Hatch V , Konrad M , Lehman W , Geeves MA . Ultra short yeast tropomyosins show novel myosin regulation. J Biol Chem 283: 1902‐1910, 2008.
 147. Maytum R , Lehrer SS , Geeves MA . Cooperativity and switching within the three‐state model of muscle regulation. Biochemistry 38: 1102‐1110, 1999.
 148. McKillop DFA , Geeves MA . Regulation of the interaction between actin and myosin subfragment‐1: Evidence for three states of the thin filament. Biophys J 65: 693‐701, 1993.
 149. McLachlan AD , Stewart M . The 14‐fold periodicity in alpha‐ tropomyosin and the interaction with actin. J Mol Biol 103: 271‐298, 1976.
 150. Mijailovich SM , Kayser‐Herold O , Li X , Griffiths H , Geeves MA . Cooperative regulation of myosin‐S1 binding to actin filaments by a continuous flexible Tm‐Tn chain. Eur Biophys J 41: 1015‐1032, 2012.
 151. Mijailovich SM , Li X , Griffiths RH , Geeves MA . Resolution and uniqueness of estimated parameters of a model of thin filament regulation in solution. Comput Biol Chem 34: 19‐33, 2010.
 152. Milligan RA , Whittaker M , Safer D . Molecular structure of F‐actin and the location of surface binding sites. Nature 348: 217‐221, 1990.
 153. Moody C , Lehman W , Craig R . Caldesmon and the structure of smooth muscle thin filaments: Electron microscopy of isolated thin filaments. J Muscle Res Cell Motil 11: 176‐185, 1990.
 154. Moore PB , Huxley HE , DeRosier DJ . Three‐dimensional reconstruction of F‐actin, thin filaments and decorated thin filaments. J Mol Biol 50: 279‐295, 1970.
 155. Morris EP , Lehrer SS . Troponin‐tropomyosin interactions. Fluorescence studies of the binding of troponin, troponin T, and chymotryptic troponin T fragments to specifically labeled tropomyosin. Biochemistry 23: 2214‐2220, 1984.
 156. Mun JY , Previs MJ , Yu HY , Gulick J , Tobacman LS , Beck‐Previs S , Robbins J , Warshaw DM , Craig R . Myosin‐binding protein C displaces tropomyosin to activate cardiac thin filaments and governs their speed by an independent mechanism. Proc Natl Acad Sci U S A 111: 2170‐2175, 2014.
 157. Murray JM , Weber A . Molecular control mechanisms in muscle contraction. Physiol Rev 53: 612‐673, 1973.
 158. Narita A , Yasunaga T , Ishikawa T , Mayanagi K , Wakabayashi T . Ca2+‐induced switching of troponin and tropomyosin on actin filaments as revealed by electron cryo‐microscopy. J Mol Biol 308: 241‐261, 2001.
 159. Nevzorov IA , Levitsky DI . Tropomyosin: Double helix from the protein world. Biochemistry (Moscow) 76: 1507‐1527, 2011.
 160. Nonomura Y , Drabikowski W , Ebashi S . The localization of troponin in tropomyosin paracrystals. J Biochem (Toyko) 64: 419‐422, 1968.
 161. O'Brien EJ , Bennett PM , Hanson J . Optical diffraction studies of myofibrillar structure. Phil Trans R Soc Lond B Biol Sci 261: 201‐208, 1971.
 162. O'Brien EJ , Couch J , Johnson GRP , Morris EP . Structure of actin and the thin filament. In: dos Remedios CG , Barden JA , editors. Actin, Structure and Function in Muscle and Non‐muscle Cells. Sydney, Australia: Academic Press, 1983, pp. 3‐15.
 163. O'Brien EJ , Gillis JM , Couch J . Symmetry and molecular arrangement in paracrystals of reconstituted muscle thin filaments. J Mol Biol 99: 461‐475, 1975.
 164. Oda T , Iwasa M , Aihara T , Maéda Y , Narita A . The nature of the globular‐ to fibrous‐actin transition. Nature 457: 441‐445, 2009.
 165. Ohtsuki I . Localization of troponin in thin filaments and in tropomyosin paracrystals. J Biochem (Tokyo) 75: 753‐765, 1974.
 166. Ohtsuki I . Distribution of troponin components in the thin filament studied by immunoelectron microscopy. J Biochem (Tokyo) 77: 633‐639, 1975.
 167. Ohtsuki I , Masaki T , Nonomura Y , Ebashi S . Periodic distribution of troponin along the thin filament. J Biochem (Tokyo) 61: 817‐819, 1967.
 168. Ohtsuki I , Onoyama Y , Shiraishi F . Electron microscopic study of troponin. J Biochem (Tokyo) 103: 913‐919, 1988.
 169. Orzechowski M , Li XE , Fischer S , Lehman W . An atomic model of the tropomyosin cable on F‐actin. Biophys J 107: 694‐699, 2014.
 170. Orzechowski M , Moore JR , Fischer S , Lehman W . Tropomyosin movement on F‐actin during muscle activation explained by energy landscapes. Arch Biochem Biophys 545: 63‐68, 2014.
 171. Owen C , DeRosier DJ . A 13‐Å map of the actin‐scruin filament from the Limulus acrosomal process. J Cell Biol 123: 337‐344, 1993.
 172. Owen CH , Morgan DG , DeRosier DJ . Image analysis of helical objects: The Brandeis Helical Package. J Struct Biol 116: 167‐175, 1996.
 173. Parry DAD , Squire JM . Structural role of tropomyosin in muscle regulation, analysis of X‐ray patterns from relaxed and contracting muscles. J Mol Biol 75: 33‐55, 1973.
 174. Paul DM , Morris EP , Kensler RW , Squire JM . Structure and orientation of troponin in the thin filament. J Biol Chem 284: 15007‐15014, 2009.
 175. Paul D , Patwardhan A , Squire JM , Morris EP . Single particle analysis of filamentous and highly elongated macromolecular assemblies. J Struct Biol 148: 236‐250, 2004.
 176. Peachy LD , Franzini‐Armstrong C . Structure and function of membrane systems of skeletal muscle. In: Peachy LD , Adrian RH , Geiger SR , editors. Handbook of Physiology, Section 10, Skeletal Muscle. Bethesda, MD: American Physiological Society, 1988, Chap. 2, pp. 23‐71.
 177. Perry SV , Grey TC . A study of the effect of substance concentration and certain relaxing factors on the magnesium‐activated myofibrillar adenosine triphosphatase. Biochem J 64: 184‐192, 1956.
 178. Perz‐Edwards RJ , Irving TC , Baumann BA , Gore D , Hutchinson DC Kržič U , Porter RL , Ward AB , Reedy MK . X‐ray diffraction evidence for myosin‐troponin connections and tropomyosin movement during stretch activation of insect flight muscle. Proc Natl Acad Sci U S A 108: 120‐125, 2011.
 179. Phillips JGN , Fillers JP , Cohen C . Tropomyosin crystal structure and muscle regulation. J Mol Biol 192: 111‐131, 1986.
 180. Pirani A , Vinogradova MV , Curmi PM , King WA , Fletterick RJ , Craig R , Tobacman LS , Xu C , Hatch V , Lehman W . An atomic model of the thin filament in the relaxed and Ca2+‐activated states. J Mol Biol 357: 707‐717, 2006.
 181. Pirani A , Xu C , Hatch V , Craig R , Tobacman LS , Lehman W . Single particle analysis of relaxed and activated muscle thin filaments. J Mol Biol 346: 761‐772, 2005.
 182. Poincaré JH . La Science et l'Hypothèse. 1901, English translation: Science and Hypothesis (1905), Mineola, NY: Dover abridged edition, 1952, preface p. xxii.
 183. Poole KJ , Lorenz M , Evans G , Rosenbaum G , Pirani A , Tobacman LS , Lehman W , Holmes KC . A comparison of muscle thin filament models obtained from electron microscopy reconstructions and low‐angle X‐ray fibre diagrams from non‐overlap muscle. J Struct Biol 155: 273‐284, 2006.
 184. Potter JD . The content of troponin, tropomyosin, actin and myosin in rabbit skeletal myofibrils. Arch Biochem Biophys 162: 436‐441, 1974.
 185. Potter JD , Gergely J . Troponin, tropomyosin, and actin interactions in the Ca2+ regulation of muscle contraction. Biochemistry 13: 2697‐2703, 1974.
 186. Proust M. À la Recherche du Temps Perdu (Remembrance of Things Past). (1913–1927) Paris, France: Grassel, 1923, Vol 5, La Prisonnière (The Prisoner), Chap. 2.
 187. Rayment I , Holden HM , Whittaker M , Yohn CB , Lorenz M , Holmes KC , Milligan RA . Structure of the actin–myosin complex and its implications for muscle contraction. Science 261: 58‐65, 1993.
 188. Rayment I , Rypniewski WR , Schmidt‐Base K , Smith R , Tomchick DR , Benning MM , Winkelmann DA , Wesenberg G , Holden HM . Three‐dimensional structure of myosin subfragment‐1: A molecular motor. Science 261: 50‐58, 1993.
 189. Rosol M , Lehman W , Landis C , Butters C , Craig R , Tobacman LS . Three‐dimensional reconstruction of thin filaments containing mutant tropomyosin. Biophys J 78: 918‐926, 2000.
 190. Schoenberg M , Brenner B , Chalovich JM , Greene LE , Eisenberg E . Cross‐bridge attachment in relaxed muscle. Adv Exp Med Biol 170: 269‐284, 1984.
 191. Seymour J , O'Brien EJ . The position of tropomyosin in muscle thin filaments. Nature 283: 680‐681, 1980.
 192. Seymour J , O'Brien EJ . Structure of myosin decorated actin filaments and natural thin filaments. J Muscle Res Cell Motil 6: 725‐755, 1985.
 193. Small JV , Sobieszek A . Ca‐regulation of mammalian smooth muscle actomyosin via kinase‐phosphatase‐dependent phosphorylation and dephosphorylation of the 20000‐Mr light chain of myosin. FEBS J 76: 521‐530, 1977.
 194. Smith DA , Geeves MA . Cooperative regulation of myosin‐actin interactions by a continuous flexible chain II: Actin‐tropomyosin‐troponin and regulation by calcium. Biophys J 84: 3168‐3180, 2003.
 195. Smith DA , Maytum R , Geeves MA . Cooperative regulation of myosin‐actin interactions by a continuous flexible chain I: Actin‐tropomyosin systems. Biophys J 84: 3155‐3167, 2003.
 196. Sobieszek A , Bremel RD . Preparation and properties of vertebrate smooth‐muscle myofibrils and actomyosin. Eur J Biochem 55: 49‐60, 1975.
 197. Sobieszek A , Small JV . Myosin‐linked regulation in vertebrate smooth muscle. J Mol Biol 102: 75‐92, 1976.
 198. Sobieszek A , Small JV . Regulation of the actin‐myosin interaction in vertebrate smooth muscle: Activation via myosin light chain kinase and the effect of tropomyosin. J Mol Biol 112: 559‐576, 1977.
 199. Sousa D , Cammarato A , Jang K , Graceffa P , Tobacman LS , Li XE , Lehman W . Electron microscopy and persistence length analysis of semi‐rigid smooth muscle tropomyosin strands. Biophys J 99: 1‐7, 2010.
 200. Sousa DR , Stagg SM , Stroupe ME . Cryo‐EM structures of the actin:tropomyosin filament reveal the mechanism for the transition from C‐ to M‐state. J Mol Biol 425: 4544‐4555, 2013.
 201. Squire JM , Morris EP . A new look at thin filament regulation in vertebrate skeletal muscle. FASEB J 12: 761‐771, 1998.
 202. Spudich JA , Huxley HE , Finch JT . The regulation of skeletal muscle contraction. II. Structural studies of the interaction of the tropomyosin–troponin complex with actin. J Mol Biol 72: 619‐632, 1972.
 203. Spudich JA , Kron SJ , Sheetz MP . Movement of myosin‐coated beads on oriented filaments reconstituted from purified actin. Nature 315: 584‐586, 1985.
 204. Squire JM , AL‐Khayat HA , Yagi N . Muscle thin‐filament structure and regulation. Actin sub‐domain movements and tropomyosin shift modelled from low‐angle X‐ray diffraction. J Chem Soc Faraday Trans 89: 2717‐2726, 1993.
 205. Squire JM , Morris EP . A new look at thin filament regulation in vertebrate striated muscle. FASEB J 12: 61‐771, 1998.
 206. Stewart M , McLachlan AD . Fourteen actin‐binding sites on tropomyosin? Nature 257: 331‐333, 1975.
 207. Sun YB , Irving M . The molecular basis of the steep force‐calcium relation in heart muscle. J Mol Cell Cardiol 48: 859‐865, 2010.
 208. Tao T , Lamkin M . Crosslinking of tropomyosin to myosin subfragment‐1 in reconstituted rabbit skeletal thin filaments. FEBS Lett 168: 169‐173, 1984.
 209. Taylor KA , Amos LA . A new model for the geometry of the binding of myosin crossbridges to muscle thin filaments. J Mol Biol 147: 297‐324, 1981.
 210. Tobacman LS , Butters CA . A new model of cooperative myosin‐thin filament binding. J Biol Chem 275: 27587‐27593, 2000.
 211. Tobacman LS , Nihli M , Butters C , Heller M , Hatch V , Craig R , Lehman W , Homsher E . The troponin tail domain promotes a conformational state of the thin filament that suppresses myosin activity. J Biol Chem 277: 27636‐27642, 2002.
 212. Toyoshima C , Wakabayashi T . Three‐dimensional image analysis of the complex of thin filaments and myosin molecules from skeletal muscle. IV. Reconstitution from minimal‐ and high‐dose images of the actin‐tropomyosin‐myosin subfragment‐1 complex. J Biochem (Tokyo) 97: 219‐243, 1985.
 213. Toyoshima C , Wakabayashi T . Three‐dimensional image analysis of the complex of thin filaments and myosin molecules from skeletal muscle. V. Assignment of actin in the actin‐tropomyosin‐myosin subfragment‐1 complex. J Biochem (Tokyo) 97: 245‐263, 1985.
 214. Vibert P , Craig R , Lehman W . Three‐dimensional reconstruction of caldesmon‐containing smooth muscle thin filaments. J Cell Biol 123: 313‐321, 1993.
 215. Vibert P , Craig R , Lehman W . Steric‐model for activation of muscle thin filaments. J Mol Biol 266: 8‐14, 1997.
 216. Vibert PJ , Haselgrove JC , Lowy J , Poulsen FR . Structural changes in actin‐containing filaments of muscle. J Mol Biol 71: 757‐767, 1972.
 217. Vibert PJ , Lowy J , Haselgrove JC , Poulsen FR . Structural changes in actin filaments of muscle. Nature New Biol 236: 182‐183, 1972.
 218. Vilfan A . The binding dynamics of tropomyosin on actin. Biophys J 81: 3146‐3155, 2001.
 219.von der Ecken J , Müller M , Lehman W , Manstein DJ , Penczek PA , Raunser S . Structure of the F‐actin‐tropomyosin complex. Nature 519: 114‐117, 2014.
 220. Wakabayashi T , Huxley HE , Amos LA , Klug A . Three‐dimensional image reconstruction of actin‐tropomyosin complex and actin‐tropomyosin‐troponin T troponin I complex. J Mol Biol 93: 477‐497, 1975.
 221. Walker ML , Burgess SA , Sellers JR , Wang F , Hammer JA 3rd, Trinick J , Knight PJ . Two‐headed binding of a processive myosin to F‐actin. Nature 405: 804‐807, 2000.
 222. Weber A . Energized Calcium Transport and Relaxing Factors. I: Current Topics in Bioenergetics. edited by Sanadi DR . New York, Academic Press, 1966, vol. 1, pp. 203‐254.
 223. Weber A , Herz R . The binding of calcium to actomyosin systems in relation to their biological activity. J Biol Chem 238: 599‐605, 1963.
 224. Weber A , Winicur S . The role of calcium in the superprecipitation of actomyosin. J Biol Chem 236: 3198‐3202, 1961.
 225. Weeds A , Lowey S . Substructure of the myosin molecule II. The light chains of myosin. J Mol Biol 61: 701‐725, 1971.
 226. Wegner A . The interaction of alpha, alpha‐and alpha, beta‐tropomyosin with actin filaments. FEBS Lett 119: 245‐248, 1980.
 227. Wilkinson JM , Perry SV , Cole HA , Trayer IP . The regulatory proteins of the myofibril. Separation and biological activity of the components of inhibitory‐factor preparations. Biochem J 127: 215‐228, 1972.
 228. Xu C , Craig R , Tobacman L , Horowitz R , Lehman W . Tropomyosin positions in regulated thin filaments revealed by cryoelectron microscopy. Biophys J 77: 985‐992, 1999.
 229. Yang S , Barbu‐Tudoran L , Orzechowski M , Craig R , Trinick J , White H , Lehman W . Three‐dimensional organization of troponin on cardiac muscle thin filaments in the relaxed state. Biophys J 106: 855‐864, 2014.

Additional recommended reading includes:


The August 2013 "Special Issue" of the Journal of Muscle Research and Cell Motility edited by S.B. Marston and M. Gautel and devoted to "Tropomyosin form and function", J Muscle Res Cell Motility Volume 34, Issues 3,4.

  
The November 2008 issue of Advances in Experimental Medicine and Biology edited by P. Gunning and also devoted to "Tropomyosin", Adv Exper Med Biol Volume 644.


Two reviews on the function and properties of troponin and tropomyosin -
Tobacman LS. Thin filament regulation of cardiac contraction. Ann Rev Physiol 58: 447-481, 1996.
Gordon AM, Homsher E, Regnier M. Regulation of contraction in striated muscle. Physiol Rev 80: 853-924, 2000.


Reviews on tropomyosin regulation of non-muscle actin activity -
Wang CL, Coluccio LM. New insights into the regulation of the actin cytoskeleton by tropomyosin. Int Rev Cell Mol Biol 281: 91-128, 2010.
Gunning P, O'Neill G, Hardemann E. Tropomyosin-based regulation of the actin cytoskeleton in time and space. Physiol Rev 88: 1-35, 2008.


Ken Holmes' excellent "Introduction to Fiber Diffraction" on his homepage   <http://homes.mpimf-heidelberg.mpg.de/~holmes/> as well as description of theories of muscle contraction beginning with those considered in ancient times and then those proposed through the end of the last century.


John Squire's book "The Structural Basis of Muscle Contraction" is very comprehensive, Plenum Press, New York, 1981.


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How to Cite

William Lehman. Thin Filament Structure and the Steric Blocking Model. Compr Physiol 2016, 6: 1043-1069. doi: 10.1002/cphy.c150030