Phosphorylation of a second site for myosin light chain kinase on platelet myosin.
The 20,000-dalton light chain of bovine platelet myosin is phosphorylated at two sites by myosin light chain kinase. The first and second phosphorylation sites are at a serine and a threonine residue, respectively. The location of the phosphorylation sites was determined by using limited proteolysis. The N-terminal sequence of the 17,000-dalton tryptic fragment of platelet myosin 20,000-dalton light chain was found to be identical with that of gizzard 20,000-dalton light chain from Ala-17 to Phe-33. On the basis of these results and the distribution of 32P among the proteolytic fragments, it was concluded that serine-19 and threonine-18 were the two phosphorylation sites. Phosphorylation at the threonine residue markedly increases the actin-activated ATPase activity of myosin. It was found that platelet myosin forms 10S and 6S conformations and its Mg2+-ATPase activity parallels the transition from the 6S to the 10S conformation. The conformational transition was influenced by phosphorylation at both sites, and the phosphorylation at the threonine residue further shifted the equilibrium toward the 6S conformation. The phosphorylation at the threonine residue also induced thick filament formation in the presence of ATP. These results suggest that the phosphorylation at the threonine residue as well as at the serine residue may play an important role in the contractility of nonmuscle cells.
Ikebe
M
M
Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, Ohio 44106.
D000251:Adenosine Triphosphatases / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D001792:Blood Platelets / Q000201:enzymology*; D002417:Cattle; D004591:Electrophoresis, Polyacrylamide Gel; D008969:Molecular Sequence Data; D009219:Myosin-Light-Chain Kinase / Q000378:metabolism*; D009218:Myosins / Q000378:metabolism*; D010766:Phosphorylation
D000255:Adenosine Triphosphate; D009219:Myosin-Light-Chain Kinase; D000251:Adenosine Triphosphatases; D009218:Myosins