Phosphorylation of the 20,000-dalton light chain of smooth muscle myosin by the calcium-activated, phospholipid-dependent protein kinase. Phosphorylation sites and effects of phosphorylation.

Smooth muscle heavy meromyosin (HMM) is phosphorylated by the Ca2+-activated phospholipid-dependent protein kinase, i.e. protein kinase C, at three sites on each 20,000-dalton light chain. Phosphorylation of three sites also is observed with isolated 20,000-dalton light chain and HMM subfragment 1. The phosphorylation sites are serine 1, serine 2, and threonine 9. Threonine is phosphorylated most rapidly followed by either serine 1 or 2. Phosphorylation of the third site occurs only on prolonged incubation. Phosphorylation is a random process. HMM phosphorylated at two sites per light chain by protein kinase C can be dephosphorylated, as shown using two phosphatase preparations. Increasing levels of phosphorylation of HMM by protein kinase C causes a progressive inhibition of the subsequent rate of phosphorylation of serine 19 by myosin light chain kinase and causes a progressive inhibition of actin-activated ATPase activity of HMM, prephosphorylated by myosin light chain kinase. Inhibition of ATPase activity is due to a decreased affinity of HMM for actin rather than a change in Vmax. Previous results with HMM and protein kinase C (Nishikawa, M., Sellers, J. R., Adelstein, R. S., and Hidaka, H. (1984) J. Biol. Chem. 259, 8808-8814) examined effects induced by phosphorylation of the threonine residues. Our results confirm these and consider also the influence of higher levels of phosphorylation by protein kinase C.