Myosin is composed of 2 nonphosphorylatable alkali light chains， 2 heavy chains and 2 phosphorylatable regulatory light chains. MYL12B is a hexameric ATPase cellular motor protein which controls contraction in smooth muscle and non-muscle cells through phosphorylation by myosin light chain kinase （MLCK）. Phosphorylation of myosin regulatory light chains catalyzed by MLCK in the presence of calcium and calmodulin， rises Actin-activated myosin ATPase activity， thus regulating contractile activity.
