Abstract
Myosin light chain phosphorylation is thought to play a central role in regulating the interaction of actin and myosin in smooth muscle, and even in non-muscle cells. Although myosin light chain phosphorylation is reportedly catalyzed mainly by Ca2+, calmodulin-dependent myosin light chain kinase (MLCK), myosin light chain was also phosphorylated by another species of Ca2+-dependent protein kinase (protein kinase C). Smooth muscle (chicken gizzard) myosin but not skeletal muscle was phosphorylated in vitro by protein kinase C from human platelets. The apparent Kms for ATP and 20,000-dalton light chain of chicken gizzard myosin were 7.5 uM and 93 uM, respectively. Two-dimensional peptide mapping of tryptic digests, of myosin light chain revealed that protein kinase C and MLCK catalyzed the incorporation of the same amount of phosphate into gizzard myosin light chain, at different sites. These results suggest that the protein kinase C phosphorylation of myosin light chain acts independently or in a complementary manner with the MLCK phosphorylation system, at least in smooth muscle.
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