Myosin light-chain phosphatase, also called myosin phosphatase (EC 3.1.3.53; systematic name yosin-light-chainphosphate phosphohydrolase), is an

enzyme Enzymes () are proteins that act as biological catalysts 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 products ...

(specifically a serine/threonine-specific protein phosphatase) that dephosphorylates the regulatory light chain of

myosin II Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility. The first myosin (M2 ...

: : yosin light-chainphosphate + H₂O = yosin light-chain+ phosphate This dephosphorylation reaction occurs in

smooth muscle tissue Smooth muscle is an involuntary non- striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit ...

and initiates the relaxation process of the muscle cells. Thus, myosin phosphatase undoes the

muscle contraction Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as ...

process initiated by

myosin light-chain kinase 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. General structural features While there ar ...

. The enzyme is composed of three subunits: the catalytic region (

protein phosphatase 1 Protein phosphatase 1 (PP1) belongs to a certain class of phosphatases known as protein serine/threonine phosphatases. This type of phosphatase includes metal-dependent protein phosphatases (PPMs) and aspartate-based phosphatases. PP1 has been fo ...

, or PP1), the myosin binding subunit (MYPT1), and a third subunit (M20) of unknown function. The catalytic region uses two

manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...

ions as catalysts to dephosphorylate the light-chains on myosin, which causes a conformational change in the myosin and relaxes the muscle. The enzyme is highly conserved and is found in all organisms’ smooth muscle tissue. While it is known that myosin phosphatase is regulated by

rho-associated protein kinase Rho-associated protein kinase (ROCK) is a kinase belonging to the AGC (PKA/ PKG/PKC) family of serine-threonine specific protein kinases. It is involved mainly in regulating the shape and movement of cells by acting on the cytoskeleton. ROCKs ...

s, there is current debate about whether other molecules, such as arachidonic acid and

cAMP Camp may refer to: Outdoor accommodation and recreation * Campsite or campground, a recreational outdoor sleeping and eating site * a temporary settlement for nomads * Camp, a term used in New England, Northern Ontario and New Brunswick to descri ...

, also regulate the enzyme.

Function

Smooth muscle tissue is mostly made of

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ov ...

and myosin,Page 174
in: ''The vascular smooth muscle cell: molecular and biological responses to the extracellular matrix''. Authors: Stephen M. Schwartz, Robert P. Mecham. Editors: Stephen M. Schwartz, Robert P. Mecham. Contributors: Stephen M. Schwartz, Robert P. Mecham. Publisher: Academic Press, 1995. , two proteins that interact together to produce muscle contraction and relaxation. Myosin II, also known as conventional myosin, has two heavy chains that consist of the head and tail domains and four light chains (two per head) that bind to the heavy chains in the “neck” region. When the muscle needs to contract,

calcium Calcium is a chemical element with the 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 t ...

ions flow into the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

from the

sarcoplasmic reticulum The sarcoplasmic reticulum (SR) is a membrane-bound structure found within muscle cells that is similar to the smooth endoplasmic reticulum in other cells. The main function of the SR is to store calcium ions (Ca2+). Calcium ion levels are ke ...

, where they activate calmodulin, which in turn activates

(MLC kinase). MLC kinase phosphorylates the myosin light chain (MLC₂₀) at the Ser-19 residue. This phosphorylation causes a conformational change in the myosin, activating crossbridge cycling and causing the muscle to contract. Because myosin undergoes a conformational change, the muscle will stay contracted even if calcium and activated MLC kinase concentrations are brought to normal levels. The conformational change must be undone to relax the muscle. When myosin phosphatase binds to myosin, it removes the

phosphate group In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...

. Without the group, the myosin reverts to its original conformation, in which it cannot interact with the actin and hold the muscle tense, so the muscle relaxes. The muscle will remain in this relaxed position until myosin is phosphorylated by MLC kinase and undergoes a conformational change.

Structure

PP1 and a portion of MYPT1 with the Manganese ion catalysts 3D

Myosin phosphatase is made of three subunits. The catalytic subunit, PP1, is one of the more important Ser/Thr phosphatases in

eukaryotic cells Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bact ...

, as it plays a role in glycogen metabolism, intracellular transport, protein synthesis, and

cell division Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ar ...

as well as smooth muscle contraction. Because it is so important to basic cellular functions, and because there are far fewer protein phosphatases than kinases in cells, PP1’s structure and function is highly conserved (though the specific isoform used in myosin phosphatase is the δ isoform, PP1δ). PP1 works by using two manganese ions as catalysts for the dephosphorylation (see below). Surrounding these ions is a Y-shaped cleft with three grooves: a hydrophobic, an acidic, and a C-terminal groove. When PP1 is not bonded to any other subunit, it is not particularly specific. However, when it bonds to the second subunit of myosin phosphatase, MYPT1 (MW ~130 kDa), this catalytic cleft changes configuration. This results in a dramatic increase in myosin specificity. Thus, it is clear that MYPT1 has great regulatory power over PP1 and myosin phosphatase, even without the presence of other activators or inhibitors. The third subunit, M20 (not to be confused with MLC₂₀, the critical regulatory subunit of myosin), is the smallest and most mysterious subunit. Currently little is known about M20, except that it is not necessary for catalysis, as removing the subunit does not affect turnover or selectivity. While some believe it could have regulatory function, nothing has been determined yet.

Mechanism

The mechanism of removing the phosphate from Ser-19 is very similar to other dephosphorylation reactions in the cell, such as the activation of glycogen synthase. Myosin's regulatory subunit MLC₂₀ binds to both the hydrophobic and acid grooves of PP1 and MYPT1, the regulatory site on myosin phosphatase. Once in the proper configuration, both the phyosphorylated serine and a free water molecule are stabilized by the hydrogen-bonding residues in the active site, as well as the positively charged ions (which interact strongly with the negative phosphate group). His-125 (on myosin phosphatase) donates a proton to Ser-19 MLC₂₀), and the water molecule attacks the phosphorus atom. After shuffling protons to stabilize (which happens rapidly compared to the attack on phosphorus), the phosphate and alcohol are formed, and both leave the active site. PP1 mechanism for myosin phosphatase

Regulation and Human Health

The regulatory pathways of MLC kinase have been well-established, but until the late 1980s, it was assumed that myosin phosphatase was not regulated, and contraction/relaxation was entirely dependent on MLC kinase activity. However, since the 1980s, the inhibiting effect of rho-associated protein kinase has been discovered and thoroughly investigated.

RhoA Transforming protein RhoA, also known as Ras homolog family member A (RhoA), is a small GTPase protein in the Rho family of GTPases that in humans is encoded by the ''RHOA'' gene. While the effects of RhoA activity are not all well known, it is ...

GTP activates Rho-kinase, which phosphorylates the MYPT1 at two major inhibitory sites, Thr-696 and Thr-866. This fully demonstrates the value of the MYPT1, not only to increase reaction rate and specificity, but also to greatly slow down the reaction. However, when

telokin Telokin (also known as kinase-related protein or KRP) is an abundant protein found in smooth-muscle. It is identical to the C-terminus of myosin light-chain kinase. Telokin may play a role in the stabilization of unphosphorylated smooth-muscl ...

is added, it effectively undoes the effect of Rho-kinase, even though it does not dephosphorylate MYPT1. One other proposed regulatory strategy involves arachidonic acid. When arachidonic acid is added to tensed muscle tissue, the acid decreases the rate of dephosphorylation (and thus relaxation) of myosin. However, it is unclear how arachidonic acid functions as an inhibitor. Two competing theories are that either arachidonic acid acts as a co-messenger in the rho-kinase cascade mentioned above, or that it binds to the c-terminal of MYPT1. When the regulatory systems of myosin phosphatase begin to fail, there can be major health consequences. Since smooth muscle is found in the respiratory, circulatory, and reproductive systems of humans (as well as other places), if the smooth muscle can no longer relax because of faulty regulation, then a wide number of problems ranging from

asthma Asthma is a long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wheezing, co ...

, hypertension, and erectile dysfunction can result.

Function

Structure

Mechanism

Regulation and Human Health

See also

References

Further reading