Phosphorylation of smooth muscle myosin light chain by five different kinases

Phosphorylation of the 20 kDa myosin light chain from smooth muscle by five different kinases was investigated. Three of the kinases (myosin light chain kinase, phosphorylase kinase, and cAMP-dependent protein kinase) phosphorylate serine residues, the fourth (casein-kinase-2) mainly threonine, and the fifth (glycogen synthase (casein) kinase-1) both serine and threonine. Isoelectric focusing analyses of 32P-labelled chymotryptic peptides indicate that phosphorylase kinase and cAMP-dependent protein kinase phosphorylate the same site as myosin light chain kinase. However, both casein kinase-2 and glycogen synthase (casein) kinase-1 phosphorylate different sites.