Regulation of non-muscle myosin assembly by calmodulin-dependent light chain kinase

Abstract

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 cells^3–5 and in vertebrate smooth muscle^6,7, 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 kinase^8–11 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 filaments^12,13. 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.