Myosin light-chain kinase

myosin light-chain kinase 2, skeletal muscle
Crystal structure of the S. dubia centrin / human skeletal muscle myosin light-chain complex.
Identifiers
Symbol	MYLK2
Entrez	85366
HUGO	16243
OMIM	606566
RefSeq	NM_033118
UniProt	Q9H1R3
Other data
Locus	Chr. 20 q13.31

Human Myosin Light-Chain Kinase
The Crystal Structure of the Human Myosin Light Chain Kinase Loc340156.
Identifiers
Symbol	MYLK4
Entrez	340156
HUGO	27972
RefSeq	NM_001012418
UniProt	Q86YV6

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

Four different MLCK isoforms exist:

These enzymes are important in the mechanism of contraction in muscle. Once there is an influx of calcium cations (Ca²⁺) into the muscle, either from the sarcoplasmic reticulum or 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(and not via ATPase activity). 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 myosin light-chain phosphatase (MLCP).