PHOSPHORYLATION OF HIGH-MR CALDESMON BY PROTEIN KINASE-C MODULATES THE REGULATORY FUNCTION OF THIS PROTEIN ON THE INTERACTION BETWEEN ACTIN AND MYOSIN

High‐Mr caldesmon, which is involved in smooth muscle contraction, was phosphorylated by protein kinase C. By chymotryptic digestion, actin‐ and calmodulin‐binding assays and immunoprecipitation with the antibody to the C‐terminal 35‐kDa fragment, we have identified that all phosphate groups are incorporated exclusively into this fragment, which is the functional domain for binding actin and calmodulin. Phosphorylation of high‐Mr caldesmon and its C‐terminal 35‐kDa fragment reduced their binding abilities to both F‐actin and calmodulin. Further, their inhibitory effects on the actin‐activated ATPase activity of gizzard myosin were also reversed in proportion to the degree of phosphorylation. These results suggest that phosphorylation of high‐Mr caldesmon by protein kinase C, which is restricted within the C‐terminal 35‐kDa domain, results in the modulation of its activity in the smooth muscle actin–myosin interaction. Copyright © 1990, Wiley Blackwell. All rights reserved