Myosin Light Chain Kinase Phosphorylation: Regulation of the Ca2+ Sensitivity of Contractile Elements
Cyclic AMP-dependent protein kinase phosphorylates myosin light chain kinase purified from gizzard smooth muscle (Adelstein et al., 1978). In the absence of Ca2+/calmodulin, 2 sites (sites A and B) are phosphorylated and the concentration of Ca2+/calmodulin required for half-maximal activation (KcaM) increases 10 fold (Conti and Adelstein, 1981). In the presence of Ca2+/calmodulin, phosphate is incorporated into site B with no effect on myosin light chain kinase activity. As shown in Figure 1, site A is near the calmodulin binding domain whereas site B is more toward the c-terminus. Similar observations on kinase phosphorylation have been made with myosin light chain kinases purified from mammalian smooth muscles including trachea (Miller et al., 1983), stomach (Walsh et al., 1982), myometrium (Higashi et al., 1983), aorta (Vallet et al., 1981), and carotid artery (Bhalla et al., 1982). It has been proposed that phosphorylation of site A by cyclic AMP-dependent protein kinase could decrease the extent of myosin light chain kinase activation with a resultant decrease in myosin light chain phosphorylation and inhibition of contraction (Conti and Adelstein, 1981).
KeywordsMyosin Light Chain Myosin Light Chain Kinase Dependent Protein Kinase Contractile Element Tracheal Smooth Muscle
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