The interaction of platelet actin, myosin and myosin light chain kinase.

Human blood platelets contain two proteins very similar in structure and function to the contractile proteins of muscle: actin and myosin. Platelet actin has a similar molecular weight (43 000 daltons) and amino acid composition to muscle actin. It polymerizes into long filaments which can form ‘arrowheads’ with skeletal muscle and platelet myosin subfragment-1 (S-1) as viewed in the electron microscope. These ‘arrowheads’ are dissociated by Mg-ATP. Platelet actin activates the ATPase activity of muscle heavy meromyosin to approximately the same extent as muscle actin. Platelet myosin (molecular weight 460 000), like muscle myosin, is composed of heavy chains (200 000 daltons) andlight chains (20000 and 16 000 daltons). The light chains resemble those found in other cytoplasmic (non-muscle) myosins and smooth muscle myosin in charge (at pH 8.4) and size and differ from the light chains of skeletal muscle and cardiac myosin. Human platelets contain a kinase that transfers the terminal phosphate from y-labelled AT32P to the 20 000 dalton light chain of platelet myosin. When platelet myosin is phosphorylated, its actin-activated ATPase activity is markedly increased. Moreover, if phosphorylated myosin is dephosphorylated with E. coli alkaline phosphatase, its actin-activated ATPase activity is decreased. These findings indicate that the phosphorylation-dephosphorylation of platelet myosin is a major controlling factor in platelet actin-myosin interaction. The ability of platelet myosin kinase to phosphorylate myosin from fibroblast and smooth muscle cells suggests that myosin phosphorylation may play a functional role in other cells. Human blood platelets contain proteins similar in structure and function to the muscle proteins actin and myosin (Bettex-Galland & Liischer 1965; Booyse et al. 1971; Adelstein & Conti 1972; Booyse et al. 1973). Indeed, it is now known that cytoplasmic actin and myosin are present in a large variety of non-muscle cells from human and other vertebrate and non-vertebrate systems (for a review on the subject of cytoplasmic actin and myosin, see Pollard &