Myosin light-chain kinase
Encyclopedia
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.
. 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).
Serine/threonine-specific protein kinase
Serine/threonine protein kinases phosphorylate the OH group of serine or threonine .At least 125 of the 500+ human protein kinases are serine/threonine kinases .-Regulation:...
that phosphorylates the regulatory light chain of myosin II.
Isoforms
Four different MLCK isoforms exist:- MYLKMYLKMyosin light chain kinase, smooth muscle also known as kinase-related protein or telokin is an enzyme that in humans is encoded by the MYLK gene.- Function :...
– smooth muscle - MYLK2MYLK2Myosin light chain kinase 2 also known as MYLK2 is an enzyme which in humans is encoded by the MYLK2 gene.- Function :This gene encodes a myosin light chain kinase, a calcium / calmodulin dependent enzyme, that is exclusively expressed in adult skeletal muscle....
– skeletal - MYLK3MYLK3Myosin light chain kinase 3 also known as MYLK3, is an enzyme which in humans is encoded by the MYLK3 gene.- Function :Phosphorylation of cardiac myosin heavy chains and light chains by a kinase, such as MYLK3, potentiates the force and rate of cross-bridge recruitment in cardiac...
– cardiac - MYLK4 – novel
Function
These enzymes are important in the mechanism of contraction in muscleMuscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
. Once there is an influx of calcium
Calcium in biology
Calcium plays a pivotal role in the physiology and biochemistry of organisms and the cell. It plays an important role in signal transduction pathways, where it acts as a second messenger, in neurotransmitter release from neurons, contraction of all muscle cell types, and fertilization...
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
Calmodulin
Calmodulin is a calcium-binding protein expressed in all eukaryotic cells...
. This binding will activate MLCK, which will go on to phosphorylate the myosin light chain
Myosin
Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...
at serine
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...
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
Troponin
400px|thumb|right|alt = Colored dice with checkered background|Ribbon representation of the human cardiac troponin core complex in the calcium-saturated form...
complex, as striated muscle
Striated muscle
Striated muscle tissue is a form of fibers that are combined into parallel fibers. More specifically, it can refer to:* Cardiac muscle .* Skeletal muscle* Branchiomeric muscles...
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).