Myosin-light-chain kinase (MLCK) is a serine/threonine-specific protein kinase that phosphorylates the regulatory light chain of myosin II. Three different MLCK isoforms exist. There is a cardiac-MLCK encoded by mylk3, a skeletal-MLCK encoded by mylk2, and smooth muscle-MLCK encoded by mylk. Smooth muscle and non-muscle MLCK are identical and are the product of the same gene, mylk.
This protein is important in the mechanism of contraction in smooth muscle. Once there is an influx of calcium into the smooth muscle, either from the sarcoplasmic reticulum or, more important, from the extracellular space, contraction of smooth muscle fibers may begin. First, the calcium will bind to calmodulin. This binding will activate MLCK, which will go on to phosphorylate the myosin light chain at serine residue 19. This will enable the myosin crossbridge to bind to the actin filament and allow contraction to begin (through the crossbridge cycle). Since smooth muscle does not contain a troponin complex like striated muscle does, this mechanism is the main pathway for regulating smooth muscle contraction.
