The phosphorylation site for casein kinase II on 20,000-Da light chain of gizzard myosin.

The 20-kDa light chain isolated from gizzard myosin has recently been reported to be phosphorylated by casein kinase II at a site distinct from that phosphorylated by Ca2+- and calmodulin-dependent myosin light-chain kinase. In the present study, the site phosphorylated by casein kinase II has been analyzed through procedures including tryptic digestion of the radioactively phosphorylated light chain and CNBr cleavage of the purified tryptic phosphopeptide, followed by amino acid analysis of these phosphopeptides. Comparison of the amino acid compositions of these peptides with the previously reported sequence has indicated that the phosphorylation site is threonine-134 of the light chain. The significance of the phosphorylation of the light chain by casein kinase II, as well as the substrate specificity of the protein kinase, is discussed on the basis of the result.