Myosin light-chain kinase also known as MYLK or MLCK is a

serine/threonine-specific protein kinase A serine/threonine protein kinase () is a kinase enzyme, in particular a protein kinase, that phosphorylation, phosphorylates the hydroxyl, OH group of the amino acid, amino-acid residues serine or threonine, which have similar side chains. ...

that

phosphorylate In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writt ...

s a specific

myosin light chain A myosin light chain is a light chain (small polypeptide subunit) of myosin. Myosin light chains were discovered by Chinese biochemist Cao Tianqin (Tien-chin Tsao) when he was a graduate student at the University of Cambridge in England. Struc ...

, namely, the regulatory light chain of myosin II.

General structural features

While there are numerous differing domains depending on the cell type, there are several characteristic domains common amongst all MYLK isoforms. MYLK’s contain a catalytic core domain with an ATP binding domain. On either sides of the catalytic core sit calcium ion/calmodulin binding sites. Binding of calcium ion to this domain increases the affinity of MYLK binding to myosin light chain. This myosin binding domain is located at the C-Terminus end of the kinase. On the other side of the kinase at the N-Terminus end, sits the actin-binding domain, which allows MYLK to form interactions with actin filaments, keeping it in place.

Isoforms

Four different MYLK isoforms exist: * MYLK1 – smooth muscle * MYLK2 – skeletal * MYLK3 – cardiac * MYLK4 – novel

Function

These

enzymes An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as pro ...

are important in the mechanism of contraction in

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

. Once there is an influx of

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

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 Calmodulin (CaM) (an abbreviation for calcium-modulated protein) is a multifunctional intermediate calcium-binding messenger protein expressed in all Eukaryote, eukaryotic cells. It is an intracellular target of the Second messenger system, sec ...

. After the influx of calcium ions and the binding to calmodulin, pp60 SRC (a protein kinase) causes a conformational change in MYLK, activating it and resulting in an increase in phosphorylation of

serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α- amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − ...

residue 19. The phosphorylation of MLC will enable the myosin crossbridge to bind to the

actin filament Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cells that form part of the cytoskeleton. They are primarily composed of polymers of actin, but are modified by and interact with numerous other p ...

and allow contraction to begin (through the crossbridge cycle). Since smooth muscle does not contain a

troponin Troponin, or the troponin complex, is a complex of three regulatory proteins (troponin C, troponin I, and troponin T) that are integral to muscle contraction in skeletal muscle and cardiac muscle, but not smooth muscle. Measurements of cardiac-spe ...

complex, as

striated muscle Striated muscle tissue is a muscle tissue that features repeating functional units called sarcomeres. Under the microscope, sarcomeres are visible along muscle fibers, giving a striated appearance to the tissue. The two types of striated muscle a ...

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).

Upstream Regulators

Protein kinase C In cell biology, protein kinase C, commonly abbreviated to PKC (EC 2.7.11.13), is a family of protein kinase enzymes that are involved in controlling the function of other proteins through the phosphorylation of hydroxyl groups of serine and t ...

and ROC Kinase are involved in regulating Calcium ion intake; these Calcium ions, in turn stimulate a MYLK, forcing a contraction. Rho kinase also modulates the activity of MYLK by downregulating the activity of MYLK's counterpart protein: Myosin Light Chain Phosphatase (MYLP). In addition to downregulation of MYLK, ROCK indirectly strengthens actin/myosin contraction through inhibiting Cofilin, a protein which depolymerizes actin stress fibers. Similar to ROCK, Protein Kinase C regulates MYLK via the CPI-17 protein, which downregulates MYLP.

Mutations and resulting diseases

Some pulmonary disorders have been found to arise due to an inability of MYLK to function properly in lung cells. Over-activity in MYLK creates an imbalance in mechanical forces between adjacent

endothelial The endothelium (: endothelia) is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the res ...

and lung tissue cells. An imbalance may result in

acute respiratory distress syndrome Acute respiratory distress syndrome (ARDS) is a type of respiratory failure characterized by rapid onset of widespread inflammation in the lungs. Symptoms include shortness of breath (dyspnea), rapid breathing (tachypnea), and bluish skin co ...

, in which fluid is able to pass into the alveoli. Within the cells, MYLK provides an inward pulling force, phosphorylating myosin light chain causing a contraction of the myosin/actin stress fiber complex. Conversely, cell-cell adhesion via tight and

adherens junction In cell biology, adherens junctions (or zonula adherens, intermediate junction, or "belt desmosome") are protein complexes that occur at cell–cell junctions and cell–matrix junctions in epithelial and endothelial tissues, usually more basa ...

s, along with anchoring to extra cellular matrix (ECM) via integrins and focal adhesion proteins results in an outward pulling force. Myosin light chain pulls the actin stress fiber attached to the cadherin, resisting the force of the adjacent cell's

cadherin Cadherins (named for "calcium-dependent adhesion") are cell adhesion molecules important in forming adherens junctions that let cells adhere to each other. Cadherins are a class of type-1 transmembrane proteins, and they depend on calcium (Ca2+) ...

. However, when the inward pulling force of the actin stress fiber becomes greater than the outward pulling force of the cell adhesion molecules due to an overactive MYLK, tissues can become slightly pulled apart and leaky, leading to passage of fluid into the lungs. Another source of smooth muscle disorders like ischemia–reperfusion,

hypertension Hypertension, also known as high blood pressure, is a Chronic condition, long-term Disease, medical condition in which the blood pressure in the artery, arteries is persistently elevated. High blood pressure usually does not cause symptoms i ...

, and

coronary artery disease Coronary artery disease (CAD), also called coronary heart disease (CHD), or ischemic heart disease (IHD), is a type of cardiovascular disease, heart disease involving Ischemia, the reduction of blood flow to the cardiac muscle due to a build-up ...

arise when mutations to

protein kinase C In cell biology, protein kinase C, commonly abbreviated to PKC (EC 2.7.11.13), is a family of protein kinase enzymes that are involved in controlling the function of other proteins through the phosphorylation of hydroxyl groups of serine and t ...

(PKC) result in excessive inhibition of MYLP, which counteracts the activity of MYLK by dephosphorylating myosin light chain. Because myosin light chain has no inherent phosphate cleaving property over active PKC prevents the dephosphorylation of myosin light protein leaving it in the activated conformation, causing an increase in smooth muscle contraction.

References

External links

* * {{Portal bar, Biology, border=no EC 2.7.11