Myosin light-chain kinase

Myosin light-chain kinase also known as MYLK or MLCK is a serine/threonine-specific protein kinase that phosphorylates the regulatory light chain of myosin II.

Isoforms
Four different MLCK isoforms exist:


 * MYLK – smooth muscle
 * MYLK2 – skeletal
 * MYLK3 – cardiac
 * MYLK4 – novel

Function
These enzymes are important in the mechanism of contraction in muscle. Once there is an influx of calcium cations (Ca++) into the muscle, either from the sarcoplasmic reticulum or, more important, from the extracellular space, contraction of smooth muscle fibres 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, as striated muscle does, this mechanism is the main pathway for regulating smooth muscle contraction. Reducing intracellular calcium concentration inactivates MLCK but does not stop smooth muscle contraction since the myosin light chain has been physically modified through phosphorylation. To stop smooth muscle contraction this change needs to be reversed. Dephosphorylation of the myosin light chain (and subsequent termination of muscle contraction) occurs through activity of a second enzyme known as MLCP (Myosin Light Chain Phosphatase).