Structure-Function Relationships in Smooth Muscle Myosin Light Chain Kinase
Regulation of the contractile apparatus in vertebrate smooth muscle is thought to involve the phosphorylation-dephosphorylation of the two 20,000-dalton light chains of myosin (Hartshorne, 1987). It has been known for many years that contraction in smooth muscle, as in skeletal muscle, is determined by the concentration of intracellular Ca2+. Contractile activity is coupled to the Ca2+ transients via the formation of the Ca2+-calmodulin (CaM) complex and subsequent activation of the CaM-dependent enzyme, myosin light chain kinase (MLCK). Phosphorylation of the myosin light chain increases the actin-activated ATPase activity of myosin and this event is thought to be reflected, under physiological conditions, by an increased rate of cross-bridge cycling and the development of tension. As long as the intracellular Ca2+ concentration remains above the threshold determined by the affinity of Ca2+ binding to the Ca2+-CaM-MLCK complex (i.e. ∼ 0.5 μM) myosin remains phosphorylated and contraction persists. Reduction in the Ca2+ level leads to a commensurate decrease in kinase activity and varying extents of myosin dephosphorylation. It is assumed (in the absence of conflicting data) that the myosin phosphatase is unregulated and dephosphorylation occurs at the same rate both in the presence and absence of Ca2+. However, the identity of the phosphatase involved in myosin dephosphorylation has not been established and one of the intriguing questions to be answered is whether phosphatase activity is regulated in vivo? Thus, by decreasing the MLCK activity the balance of phosphorylation to dephosphorylation is altered. In the simplest scenario the role of myosin phosphorylation is to initiate contraction and dephosphorylation leads to relaxation. While this simple relationship is not obvious under physiological conditions, and several papers in this volume attest to the complexity of the situation, it is clear that two key regulatory enzymes in smooth muscle are MLCK and myosin phosphatase.
KeywordsMyosin Light Chain Kinase Inhibitory Domain Inhibitory Sequence Myosin Phosphatase Myosin Light Chain Kinase Activity
Unable to display preview. Download preview PDF.
- Hartshorne, D. J., 1987, Biochemistry of the contractile process in smooth muscle, in: “Physiology of the Gastrointestinal tract”, L. R. Johnson, ed., Raven Press, New York, p. 423.Google Scholar
- Ikebe, M., Stepinska, M., Kemp, B. E., Means, A. R., and Hartshorne, D. J., 1987, Proteolysis of smooth muscle myosin light chain kinase. Formation of inactive and calmodulin-independent fragments, J. Biol. Chem., 260: 13828.Google Scholar
- Ito, M., Dabrowska, R., Guerriero Jr., V., and Hartshorne, D. J., 1989, Identification in turkey gizzard of an acidic protein related to the C-terminal portion of smooth muscle myosin light chain kinase, J. itBiol. Chem., 264: 13971.Google Scholar
- Ito, M., Hartshorne, D. J., Pearson, R., and Kemp, B. E., 1989, Proteolysis of smooth muscle myosin light chain kinase, Biophys. J., 55: 494a.Google Scholar