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

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Abstract
The presence of actin and myosin in non-muscle cells suggests that they may be involved in a wide range of cellular contractile activities. The generally accepted view is that interaction between actin and myosin in these cells and in vertebrate smooth muscle is regulated by the level of phosphorylation of the 20,000-MW L chain. In the absence of Ca, this L chain is not phosphorylated and the myosin cannot interact with actin. Ca activates a specific calmodulin-dependent kinase which phosphorylates the L 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 L chain seems to control the assembly of smooth muscle myosin into filaments. Using purified smooth muscle L chain kinase, this observation was confirmed. Studies of myosins isolated from the 2 non-muscle sources, calf thymus cells and pig platelets are reported. These myosins are assembled into filaments at physiological ionic strength and Mg-ATP concentrations only when the 20,000-MW L chain is phosphorylated.