Comprehensive Physiology Wiley Online Library

Biochemistry of the Contractile Proteins of Smooth Muscle

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Background on Contractile Mechanism
1.1 Aspects of Striated Muscle Biochemistry
2 Comparison of Striated and Smooth Muscle Biochemistry
3 Content of Contractile Proteins in Smooth Muscle
4 Actin
5 Tropomyosin
6 Myosin
6.1 Isolation
6.2 Physical Properties and Subunit Composition
6.3 Myosin Fragments
6.4 ATPase Activity
6.5 Ca2+ Binding
6.6 Thick Filament Formation
6.7 Immunochemical Properties
7 Actomyosin
7.1 ATPase Activities
8 Regulation
8.1 Phosphorylation of Myosin
8.2 Phosphatase Activity
8.3 Phosphorylation and the Regulatory Mechanism
8.4 Other Mechanisms
8.5 Role of Tropomyosin in the Regulatory Mechanism
8.6 Summary of the Regulatory Mechanism
8.7 Correlation of Actomyosin ATPase Activity With In Vivo Functioning
9 Other Protein Components
9.1 α‐Actinin
9.2 130,000 Component
9.3 100‐Å Filament Protein
Figure 1. Figure 1.

Diagrammatic representation of a model for the regulation of skeletal muscle activity. To simplify the figure the normal actin double helix (F‐actin) is shown as a linear array, and it should be remembered that the true arrangement of the actin, tropomyosin, and troponin molecules is helical. A: in the relaxed state the tropomyosin strand (Tm) blocks the interaction of the myosin head (shown in dashed line) with actin. This configuration is stabilized by the interaction of troponin I (TpI) with actin. TpT, troponin T. B: in the presence of Ca2+ a conformational change occurs on troponin C (TpC) that alters the affinity of troponin I for actin; the troponin I‐actin bond is released and the tropomyosin strand moves back to a position at which it no longer blocks the actin‐myosin interaction.

From Hartshorne
Figure 2. Figure 2.

Diagrammatic representation of the myosin molecule. Coiled‐coil α‐helical portion of the molecule is depicted as a ropelike structure. At the origin of the cross‐bridge, i.e., one end of the heavy meromyosin subfragment 2 (HMM S‐2) molecule, it is not known whether the α‐helical structure is retained (dashed line). The position of the four light chains (shown as the smaller circles in the globular head) is completely arbitrary. Conformation of the globular head differs from the rest of the molecule in that it is not predominantly an α‐helix, and this difference is presented diagrammatically. LMM, light meromyosin; HMM S‐1, heavy meromyosin subfragment 1.

Figure 3. Figure 3.

The pH dependence of the Mg2+‐ATPase activity of skeletal muscle myosin and gizzard myosin, and the pH dependence of actin activation. Gizzard myosin, 2.0 mg/assay (⊡); skeletal myosin, 1.14 mg/assay (⊙). Actin activation (x) is similar for both skeletal and smooth muscle myosin.

From Driska and Hartshorne
Figure 4. Figure 4.

Mg2+ dependence of ATPase activity of smooth and skeletal muscle actomyosins. Activities are shown in the presence (⊙) and absence (•) of Ca2+. Note difference in level of ATPase activity for the two actomyosins.

Figure 5. Figure 5.

Rationale for distinguishing between thin filament‐linked regulation and myosin‐linked regulation. M, myosin; MR, myosin with regulatory proteins attached; A, actin; A‐TM‐TN, actin plus tropomyosin and troponin; Ao, pure actin.

Figure 6. Figure 6.

Effect of varying actin concentrations on the phosphorylation and Mg2+‐ATPase activity of gizzard myosin. ATPase assays, in the presence of kinase, were done in the presence (•) and absence (○) of Ca2+. Phosphorylation of myosin in the presence of Ca2+ and kinase is also shown (□). In the absence of kinase (▵) there was no significant actin activation of ATPase activity.

From Gorecka et al.
Figure 7. Figure 7.

Scheme for the regulation of smooth muscle activity based on the phosphorylation of myosin. Pseudo‐ATPase is a reflection of the combined action of the kinase and phosphatase and does not involve ATP hydrolysis by myosin. Pseudo‐ATPase is usually low and does not contribute significantly to the overall extent of ATP hydrolysis. M, myosin; MP, phosphorylated myosin; A, actin; Pi, inorganic phosphate; AMP, phosphorylated actomyosin.



Figure 1.

Diagrammatic representation of a model for the regulation of skeletal muscle activity. To simplify the figure the normal actin double helix (F‐actin) is shown as a linear array, and it should be remembered that the true arrangement of the actin, tropomyosin, and troponin molecules is helical. A: in the relaxed state the tropomyosin strand (Tm) blocks the interaction of the myosin head (shown in dashed line) with actin. This configuration is stabilized by the interaction of troponin I (TpI) with actin. TpT, troponin T. B: in the presence of Ca2+ a conformational change occurs on troponin C (TpC) that alters the affinity of troponin I for actin; the troponin I‐actin bond is released and the tropomyosin strand moves back to a position at which it no longer blocks the actin‐myosin interaction.

From Hartshorne


Figure 2.

Diagrammatic representation of the myosin molecule. Coiled‐coil α‐helical portion of the molecule is depicted as a ropelike structure. At the origin of the cross‐bridge, i.e., one end of the heavy meromyosin subfragment 2 (HMM S‐2) molecule, it is not known whether the α‐helical structure is retained (dashed line). The position of the four light chains (shown as the smaller circles in the globular head) is completely arbitrary. Conformation of the globular head differs from the rest of the molecule in that it is not predominantly an α‐helix, and this difference is presented diagrammatically. LMM, light meromyosin; HMM S‐1, heavy meromyosin subfragment 1.



Figure 3.

The pH dependence of the Mg2+‐ATPase activity of skeletal muscle myosin and gizzard myosin, and the pH dependence of actin activation. Gizzard myosin, 2.0 mg/assay (⊡); skeletal myosin, 1.14 mg/assay (⊙). Actin activation (x) is similar for both skeletal and smooth muscle myosin.

From Driska and Hartshorne


Figure 4.

Mg2+ dependence of ATPase activity of smooth and skeletal muscle actomyosins. Activities are shown in the presence (⊙) and absence (•) of Ca2+. Note difference in level of ATPase activity for the two actomyosins.



Figure 5.

Rationale for distinguishing between thin filament‐linked regulation and myosin‐linked regulation. M, myosin; MR, myosin with regulatory proteins attached; A, actin; A‐TM‐TN, actin plus tropomyosin and troponin; Ao, pure actin.



Figure 6.

Effect of varying actin concentrations on the phosphorylation and Mg2+‐ATPase activity of gizzard myosin. ATPase assays, in the presence of kinase, were done in the presence (•) and absence (○) of Ca2+. Phosphorylation of myosin in the presence of Ca2+ and kinase is also shown (□). In the absence of kinase (▵) there was no significant actin activation of ATPase activity.

From Gorecka et al.


Figure 7.

Scheme for the regulation of smooth muscle activity based on the phosphorylation of myosin. Pseudo‐ATPase is a reflection of the combined action of the kinase and phosphatase and does not involve ATP hydrolysis by myosin. Pseudo‐ATPase is usually low and does not contribute significantly to the overall extent of ATP hydrolysis. M, myosin; MP, phosphorylated myosin; A, actin; Pi, inorganic phosphate; AMP, phosphorylated actomyosin.

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D. J. Hartshorne, A. Gorecka. Biochemistry of the Contractile Proteins of Smooth Muscle. Compr Physiol 2011, Supplement 7: Handbook of Physiology, The Cardiovascular System, Vascular Smooth Muscle: 93-120. First published in print 1980. doi: 10.1002/cphy.cp020204