Phosphorylation of smooth muscle myosin by type II Ca^(2+)/calmodulin-dependent protein kinase.
Brain type II Ca^(2+)/calmodulin-dependent protein kinase was found to phosphorylate smooth muscle myosin, incorporating maximally ~ 2 mol of phosphoryl per mol of myosin, exclusively on the 20,000 dalton light chain subunit. After maximal phosphorylation of myosin or the isolated 20,000 dalton light chain subunit by myosin light chain kinase, the addition of type II Ca^(2+)/calmodulin-dependent protein kinase led to no further incorporation indicating the two kinases phosphorylated a common site. This conclusion was supported by two dimensional mapping of tryptic digests of myosin phosphorylated by the two kinases. By phosphoamino acid analysis the phosphorylated residue was identified as a serine. The phosphorylation by type II Ca^(2+)/calmodulin-dependent protein kinase of myosin resulted in enhancement of its actin-activated Mg^(2+)-ATPase activity. Taken together, these data strongly support the conclusion that type II Ca^(2+)/calmodulin-dependent protein kinase phosphorylates the same amino acid residue on the 20,000 dalton light chain subunit of smooth muscle myosin as is phosphorylated by myosin light chain kinase and suggest an alternative mechanism for the regulation of actin-myosin interaction.
© 1990 Kluwer Academic Publisher. Received 19 October 1989; accepted 26 January 1990. We are grateful to Rob Saeli, Floyd Kennedy and Barbara Flug for technical assistance, Tecla Atkinson for illustrations, and Gerry Richtand for typing the manuscript. We also wish to thank Karen Polak for performing some of the phosphorylations of myosin. This work has been partially supported by GM 07145 and NS 24738 and by BNS 8617110 (NSF).