Phosphorylation of smooth muscle myosin light chains by cAMP-dependent protein kinase

The 20,000-dalton light chain of chicken gizzard myosin was phosphorylated in vitro by the cAMP-dependent protein kinase (ATP: protein phosphotransferase, EC 2.7.1.37) from bovine heart. The enzyme catalyzed incorporation of 1 mol of Pi/mol of light chain in a reaction that was completely dependent upon cAMP and independent of Ca2+. Two-dimensional peptide mapping of alpha-chymotryptic digests, as well as phosphoamino acid analysis of acid hydrolysates, were used to compare the site phosphorylated by cAMP-dependent protein kinase to that phosphorylated by turkey gizzard myosin light chain kinase. The results indicate that both enzymes phosphorylate the same serine residue. However, the light chains were a better in vitro substrate for myosin light chain kinase than for cAMP-dependent protein kinase. The amino acid sequence around the phosphorylated serine is characteristic of substrates of cAMP-dependent protein kinases.