The presence of actin and myosin in non-muscle cells suggests that they may be involved in a wide range of cellular contractile activities1. The generally accepted view2 is that interaction between actin and myosin in these cells3-5 and in vertebrate smooth muscle6,7, is regulated by the level of phosphorylation of the 20,000-molecular weight (MW) light chain. In the absence of calcium, this light chain is not phosphorylated and the myosin cannot interact with actin. Calcium activates a specific calmodulin-dependent kinase8-11 which phosphorylates the light chain, initiating actin-myosin interaction. Although most studies on the role of phosphorylation have concentrated on the regulation of actin-activated myosin Mg-ATPase activity, phosphorylation of the light chain also seems to control the assembly of smooth muscle myosin into filaments12,13. Using purified smooth muscle light chain kinase, we have confirmed this observation. We report here studies of myosins isolated from the two non-muscle sources, thymus cells and platelets. We observed that these myosins are assembled into filaments at physiological ionic strength and Mg-ATP concentrations, only when the 20,000-MW light chain is phosphorylated.
ASJC Scopus subject areas