Template:Short description Template:Use dmy dates Template:Cs1 config

File:Myosine.gif
Part of the myosin II structure. Atoms in the heavy chain are colored pink (on the left-hand side); atoms in the light chains are colored faded-orange and faded-yellow (also on the left-hand side).

Myosins (Template:IPAc-enTemplate:RefnTemplate:Refn) are a family of motor proteins (though most often protein complexes) 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) to be discovered was in 1864 by Wilhelm Kühne. Kühne had extracted a viscous protein from skeletal muscle that he held responsible for keeping the tension state in muscle. He called this protein myosin.<ref name="JCS">Template:Cite journal</ref><ref name="JGP">Template:Cite journal</ref> The term has been extended to include a group of similar ATPases found in the cells of both striated muscle tissue and smooth muscle tissue.

Following the discovery in 1973 of enzymes with myosin-like function in Acanthamoeba castellanii, a global range of divergent myosin genes have been discovered throughout the realm of eukaryotes.<ref name="Pollard">Template:Cite journal</ref>

Although myosin was originally thought to be restricted to muscle cells (hence myo-(s) + -in), there is no single "myosin"; rather it is a very large superfamily of genes whose protein products share the basic properties of actin binding, ATP hydrolysis (ATPase enzyme activity), and force transduction. Virtually all eukaryotic cells contain myosin isoforms. Some isoforms have specialized functions in certain cell types (such as muscle), while other isoforms are ubiquitous. The structure and function of myosin is globally conserved across species, to the extent that rabbit muscle myosin II will bind to actin from an amoeba.<ref name="McMahon">McMahon, T. A. 1984. Muscles, Reflexes and Locomotion. 1st Edition. Princeton University Press. Template:ISBN</ref><ref>Template:Cite journal</ref>

Template:TOC limit

Structure and functionsEdit

DomainsEdit

Most myosin molecules are composed of a head, neck, and tail domain.

  • The head domain binds the filamentous actin, and uses ATP hydrolysis to generate force and to "walk" along the filament towards the barbed (+) end (with the exception of myosin VI, which moves towards the pointed (-) end).
  • the neck domain acts as a linker and as a lever arm for transducing force generated by the catalytic motor domain. The neck domain can also serve as a binding site for myosin light chains which are distinct proteins that form part of a macromolecular complex and generally have regulatory functions.
  • The tail domain generally mediates interaction with cargo molecules and/or other myosin subunits. In some cases, the tail domain may play a role in regulating motor activity.

Power strokeEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}

Multiple myosin II molecules generate force in skeletal muscle through a power stroke mechanism fuelled by the energy released from ATP hydrolysis.<ref>Template:Cite journal</ref> The power stroke occurs at the release of phosphate from the myosin molecule after the ATP hydrolysis while myosin is tightly bound to actin. The effect of this release is a conformational change in the molecule that pulls against the actin. The release of the ADP molecule leads to the so-called rigor state of myosin.<ref>Template:Cite journal</ref> The binding of a new ATP molecule will release myosin from actin. ATP hydrolysis within the myosin will cause it to bind to actin again to repeat the cycle. The combined effect of the myriad power strokes causes the muscle to contract.

Nomenclature, evolution, and the family treeEdit

The wide variety of myosin genes found throughout the eukaryotic phyla were named according to different schemes as they were discovered. The nomenclature can therefore be somewhat confusing when attempting to compare the functions of myosin proteins within and between organisms.

Skeletal muscle myosin, the most conspicuous of the myosin superfamily due to its abundance in muscle fibers, was the first to be discovered. This protein makes up part of the sarcomere and forms macromolecular filaments composed of multiple myosin subunits. Similar filament-forming myosin proteins were found in cardiac muscle, smooth muscle, and nonmuscle cells. However, beginning in the 1970s, researchers began to discover new myosin genes in simple eukaryotes<ref name="Pollard"/> encoding proteins that acted as monomers and were therefore entitled Class I myosins. These new myosins were collectively termed "unconventional myosins"<ref>Template:Cite journal</ref> and have been found in many tissues other than muscle. These new superfamily members have been grouped according to phylogenetic relationships derived from a comparison of the amino acid sequences of their head domains, with each class being assigned a Roman numeral<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> (see phylogenetic tree). The unconventional myosins also have divergent tail domains, suggesting unique functions.<ref>Template:Cite journal</ref> The now diverse array of myosins likely evolved from an ancestral precursor (see picture).

Analysis of the amino acid sequences of different myosins shows great variability among the tail domains, but strong conservation of head domain sequences. Presumably this is so the myosins may interact, via their tails, with a large number of different cargoes, while the goal in each case – to move along actin filaments – remains the same and therefore requires the same machinery in the motor. For example, the human genome contains over 40 different myosin genes.

These differences in shape also determine the speed at which myosins can move along actin filaments. The hydrolysis of ATP and the subsequent release of the phosphate group causes the "power stroke", in which the "lever arm" or "neck" region of the heavy chain is dragged forward. Since the power stroke always moves the lever arm by the same angle, the length of the lever arm determines the displacement of the cargo relative to the actin filament. A longer lever arm will cause the cargo to traverse a greater distance even though the lever arm undergoes the same angular displacement – just as a person with longer legs can move farther with each individual step. The velocity of a myosin motor depends upon the rate at which it passes through a complete kinetic cycle of ATP binding to the release of ADP.

Myosin classesEdit

Myosin IEdit

Myosin I, a ubiquitous cellular protein, functions as monomer and functions in vesicle transport.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> It has a step size of 10 nm and has been implicated as being responsible for the adaptation response of the stereocilia in the inner ear.<ref>Template:Cite journal</ref>

Myosin IIEdit

File:Cardiac sarcomere structure.png
Cardiac sarcomere structure featuring myosin

Myosin II (also known as conventional myosin) is the myosin type responsible for producing muscle contraction in muscle cells in most animal cell types. It is also found in non-muscle cells in contractile bundles called stress fibers.<ref>Template:Cite journal</ref>

  • Myosin II contains two heavy chains, each about 2000 amino acids in length, which constitute the head and tail domains. Each of these heavy chains contains the N-terminal head domain, while the C-terminal tails take on a coiled-coil morphology, holding the two heavy chains together (imagine two snakes wrapped around each other, as in a caduceus). Thus, myosin II has two heads. The intermediate neck domain is the region creating the angle between the head and tail.<ref name=aguilar2010>Template:Cite journal</ref> In smooth muscle, a single gene (MYH11)<ref>Template:Cite journal</ref>) codes for the heavy chains myosin II, but splice variants of this gene result in four distinct isoforms.<ref name=aguilar2010/>
  • It also contains 4 myosin light chains (MLC), resulting in 2 per head, weighing 20 (MLC20) and 17 (MLC17) kDa.<ref name=aguilar2010/> These bind the heavy chains in the "neck" region between the head and tail.

In muscle cells, the long coiled-coil tails of the individual myosin molecules can auto-inhibit active function in the 10S conformation or upon phosphorylation, change to the 6S conformation and join, forming the thick filaments of the sarcomere.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The force-producing head domains stick out from the side of the thick filament, ready to walk along the adjacent actin-based thin filaments in response to the proper chemical signals and may be in either auto-inhibited or active conformation. The balance/transition between active and inactive states is subject to extensive chemical regulation. Template:Further Template:Further

Myosin IIIEdit

Myosin III is a poorly understood member of the myosin family. It has been studied in vivo in the eyes of Drosophila, where it is thought to play a role in phototransduction.<ref>{{#invoke:citation/CS1|citation |CitationClass=web

}}</ref> A human homologue gene for myosin III, MYO3A, has been uncovered through the Human Genome Project and is expressed in the retina and cochlea.<ref>Template:EntrezGene</ref>

File:Myosin V.png
Crystal structure of myosin V motor with essential light chain – nucleotide-free

Myosin IVEdit

Myosin IV has a single IQ motif and a tail that lacks any coiled-coil forming sequence. It has homology similar to the tail domains of Myosin VII and XV.<ref>Template:Cite journal</ref>

Myosin VEdit

Myosin V is an unconventional myosin motor, which is processive as a dimer and has a step size of 36 nm.<ref name="Warshaw">Template:Cite journal</ref> It translocates (walks) along actin filaments traveling towards the barbed end (+ end) of the filaments. Myosin V is involved in the transport of cargo (e.g. RNA, vesicles, organelles, mitochondria) from the center of the cell to the periphery, but has been furthermore shown to act like a dynamic tether, retaining vesicles and organelles in the actin-rich periphery of cells.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> A recent single molecule in vitro reconstitution study on assembling actin filaments suggests that Myosin V travels farther on newly assembling (ADP-Pi rich) F-actin, while processive runlengths are shorter on older (ADP-rich) F-actin.<ref>Template:Cite journal</ref>

File:Image animated.gif
A ribbon diagram of the Myosin V molecular motor<ref name=":0" /> pseudo-colored to illustrate major subdomains. In the interest of visual clarity, important loops (which are often labeled separately in the literature) are not singled out. This perspective highlights the nucleotide-binding site and the separation of the U50 and L50 subdomains which form the actin-binding site cleft.

The Myosin V motor head can be subdivided into the following functional regions:<ref name=":0">Template:Cite journal</ref>

  • Nucleotide-binding site - These elements together coordinate di-valent metal cations (usually magnesium) and catalyze hydrolysis:
    • Switch I - This contains a highly conserved SSR motif. Isomerizes in the presence of ATP.
    • Switch II - This is the Kinase-GTPase version of the Walker B motif DxxG. Isomerizes in the presence of ATP.
    • P-loop - This contains the Walker A motif GxxxxGK(S,T). This is the primary ATP binding site.
  • Transducer - The seven β-strands that underpin the motor head's structure.<ref>Template:Cite journal</ref>
  • U50 and L50 - The Upper (U50) and Lower (L50) domains are each around 50kDa. Their spatial separation<ref name=":1">Template:Cite journal</ref> forms a cleft critical for binding to actin and some regulatory compounds.
  • SH1 helix and Relay - These elements together provide an essential mechanism for coupling the enzymatic state of the motor domain to the powerstroke-producing region (converter domain, lever arm, and light chains).<ref name=":2">Template:Cite journal</ref><ref name=":3">Template:Cite journal</ref>
  • Converter - This converts a change of conformation in the motor head to an angular displacement of the lever arm (in most cases reinforced with light chains).<ref name=":3" />

Myosin VIEdit

File:MyosinVI 2V26.png
State of myosin VI from PDB 2V26 before the power stroke <ref>Template:Cite journal</ref>

Myosin VI is an unconventional myosin motor, which is primarily processive as a dimer, but also acts as a nonprocessive monomer. It walks along actin filaments, travelling towards the pointed end (- end) of the filaments.<ref>Template:Cite journal</ref> Myosin VI is thought to transport endocytic vesicles into the cell.<ref>Template:Cite journal</ref>

Myosin VIIEdit

Myosin VII is an unconventional myosin with two FERM domains in the tail region. It has an extended lever arm consisting of five calmodulin binding IQ motifs followed by a single alpha helix (SAH)<ref name="pmid28262393">Template:Cite journal</ref> Myosin VII is required for phagocytosis in Dictyostelium discoideum, spermatogenesis in C. elegans and stereocilia formation in mice and zebrafish.<ref>Template:Cite book</ref>

Myosin VIIIEdit

Myosin VIII is a plant-specific myosin linked to cell division;<ref>Template:Cite journal</ref> specifically, it is involved in regulating the flow of cytoplasm between cells<ref>Template:Cite journal</ref> and in the localization of vesicles to the phragmoplast.<ref>Template:Cite journal</ref>

Myosin IXEdit

Myosin IX is a group of single-headed motor proteins. It was first shown to be minus-end directed,<ref>Template:Cite journal</ref> but a later study showed that it is plus-end directed.<ref>Template:Cite journal</ref> The movement mechanism for this myosin is poorly understood.

Myosin XEdit

Myosin X is an unconventional myosin motor, which is functional as a dimer. The dimerization of myosin X is thought to be antiparallel.<ref>Template:Cite journal</ref> This behavior has not been observed in other myosins. In mammalian cells, the motor is found to localize to filopodia. Myosin X walks towards the barbed ends of filaments. Some research suggests it preferentially walks on bundles of actin, rather than single filaments.<ref>Template:Cite journal</ref> It is the first myosin motor found to exhibit this behavior.

Myosin XIEdit

Myosin XI directs the movement of organelles such as plastids and mitochondria in plant cells.<ref>Template:Cite journal</ref> It is responsible for the light-directed movement of chloroplasts according to light intensity and the formation of stromules interconnecting different plastids. Myosin XI also plays a key role in polar root tip growth and is necessary for proper root hair elongation.<ref>Template:Cite journal</ref> A specific Myosin XI found in Nicotiana tabacum was discovered to be the fastest known processive molecular motor, moving at 7μm/s in 35 nm steps along the actin filament.<ref>Template:Cite journal</ref>

Myosin XIIEdit

Template:Empty section

Myosin XIIIEdit

Template:Empty section

Myosin XIVEdit

This myosin group has been found in the Apicomplexa phylum.<ref>Template:Cite journal</ref> The myosins localize to plasma membranes of the intracellular parasites and may then be involved in the cell invasion process.<ref>Template:Cite journal</ref>

This myosin is also found in the ciliated protozoan Tetrahymena thermophila. Known functions include: transporting phagosomes to the nucleus and perturbing the developmentally regulated elimination of the macronucleus during conjugation.

Myosin XVEdit

Myosin XV is necessary for the development of the actin core structure of the non-motile stereocilia located in the inner ear. It is thought to be functional as a monomer.

Myosin XVIEdit

Template:Empty section

Myosin XVIIEdit

Template:Empty section

Myosin XVIIIEdit

MYO18A A gene on chromosome 17q11.2 that encodes actin-based motor molecules with ATPase activity, which may be involved in maintaining stromal cell scaffolding required for maintaining intercellular contact.

Myosin XIXEdit

Unconventional myosin XIX (Myo19) is a mitochondrial associated myosin motor.<ref name="uniprot2">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Genes in humansEdit

Note that not all of these genes are active.

Myosin light chains are distinct and have their own properties. They are not considered "myosins" but are components of the macromolecular complexes that make up the functional myosin enzymes.

ParamyosinEdit

Paramyosin is a large, 93-115kDa muscle protein that has been described in a number of diverse invertebrate phyla.<ref name="Winkelman, L. 1976">Template:Cite journal</ref> Invertebrate thick filaments are thought to be composed of an inner paramyosin core surrounded by myosin. The myosin interacts with actin, resulting in fibre contraction.<ref>Template:Cite journal</ref> Paramyosin is found in many different invertebrate species, for example, Brachiopoda, Sipunculidea, Nematoda, Annelida, Mollusca, Arachnida, and Insecta.<ref name="Winkelman, L. 1976"/> Paramyosin is responsible for the "catch" mechanism that enables sustained contraction of muscles with very little energy expenditure, such that a clam can remain closed for extended periods.

Paramyosins can be found in seafood. A recent computational study showed that following human intestinal digestion, paramyosins of common octopus, Humboldt squid, Japanese abalone, Japanese scallop, Mediterranean mussel, Pacific oyster, sea cucumber, and Whiteleg shrimp could release short peptides that inhibit the enzymatic activities of angiotensin converting enzyme and dipeptidyl peptidase.<ref>Template:Cite journal</ref>

ReferencesEdit

Template:Reflist

Further readingEdit

Template:Refbegin

Template:Refend

Additional imagesEdit

External linksEdit

Template:Sister project

Template:Cytoskeletal Proteins Template:ATPases Template:Muscle tissue