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Myosins () are a
superfamily SUPERFAMILY is a database and search platform of structural and functional annotation for all proteins and genomes. It classifies amino acid sequences into known structural domains, especially into SCOP superfamilies. Domains are functional, str ...
of
motor protein Motor proteins are a class of molecular motors that can move along the cytoplasm of cells. They convert chemical energy into mechanical work by the hydrolysis of ATP. Flagellar rotation, however, is powered by a proton pump. Cellular function ...
s best known for their roles in
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 a ...
and in a wide range of other
motility Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...
processes in
eukaryote 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. Bacter ...
s. They are ATP-dependent and responsible for
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 ...
-based motility. The first myosin (M2) to be discovered was in 1864 by Wilhelm Kühne. Kühne had extracted a viscous protein from
skeletal muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of m ...
that he held responsible for keeping the tension state in muscle. He called this protein ''myosin''. The term has been extended to include a group of similar
ATPase ATPases (, Adenosine 5'-TriPhosphatase, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, SV40 T-antigen, ATP hydrolase, complex V (mitochondrial electron transport), (Ca2+ + Mg2+)-ATPase, HCO3−-ATPase, adenosine triphosphatase) are ...
s found in the
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
of both striated muscle tissue and smooth muscle tissue. Following the discovery in 1973 of enzymes with myosin-like function in ''
Acanthamoeba castellanii ''Acanthamoeba'' is a genus of amoebae that are commonly recovered from soil, fresh water, and other habitats. ''Acanthamoeba'' has two evolutive forms, the metabolically active trophozoite and a dormant, stress-resistant cyst. Trophozoites are ...
'', a global range of divergent myosin
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
s have been discovered throughout the realm of eukaryotes. Although myosin was originally thought to be restricted to
muscle cell A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a mus ...
s (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 Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysi ...
(ATPase enzyme activity), and force transduction. Virtually all eukaryotic cells contain myosin
isoforms A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some iso ...
. 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.McMahon, T. A. 1984. Muscles, Reflexes and Locomotion. 1st Edition. Princeton University Press.


Structure and functions


Domains

Most myosin molecules are composed of a
head A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple animals ...
, neck, and tail domain. * The '' head domain'' binds the filamentous
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 uses ATP
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysi ...
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
subunit Subunit may refer to: * Subunit HIV vaccine, a class of HIV vaccine *Protein subunit, a protein molecule that assembles with other protein molecules *Monomer, a molecule that may bind chemically to other molecules to form a polymer * Sub-subunit, ...
s. In some cases, the tail domain may play a role in regulating motor activity.


Power stroke

Multiple myosin II molecules generate force in
skeletal muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of m ...
through a power stroke mechanism fuelled by the energy released from ATP hydrolysis. 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. 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 tree

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 encoding proteins that acted as monomers and were therefore entitled Class I myosins. These new myosins were collectively termed "unconventional myosins" 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 Roman numerals are a numeral system that originated in ancient Rome and remained the usual way of writing numbers throughout Europe well into the Late Middle Ages. Numbers are written with combinations of letters from the Latin alphabet, ea ...
(see phylogenetic tree). The unconventional myosins also have divergent tail domains, suggesting unique functions. The now diverse array of myosins likely evolved from an ancestral
precursor Precursor or Precursors may refer to: * Precursor (religion), a forerunner, predecessor ** The Precursor, John the Baptist Science and technology * Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of u ...
(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 In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
. 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 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 ...
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 classes


Myosin I

Myosin I, a ubiquitous cellular protein, functions as monomer and functions in vesicle transport. 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.


Myosin II

Myosin II (also known as conventional myosin) is the myosin type responsible for producing
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 a ...
in
muscle cell A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a mus ...
s in most animal cell types. It is also found in non-muscle cells in contractile bundles called stress fibers. * 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 The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...
head domain, while the C-terminal tails take on a
coiled-coil A coiled coil is a structural motif in proteins in which 2–7 alpha-helices are coiled together like the strands of a rope. (Dimers and trimers are the most common types.) Many coiled coil-type proteins are involved in important biological f ...
morphology, holding the two heavy chains together (imagine two snakes wrapped around each other, as in a
caduceus The caduceus (☤; ; la, cādūceus, from grc-gre, κηρύκειον "herald's wand, or staff") is the staff carried by Hermes in Greek mythology and consequently by Hermes Trismegistus in Greco-Egyptian mythology. The same staff was also ...
). Thus, myosin II has two heads. The intermediate ''neck'' domain is the region creating the angle between the head and tail. In smooth muscle, a single gene (''
MYH11 Myosin-11 is a protein that in humans is encoded by the ''MYH11'' gene. Function Myosin-11 is a smooth muscle myosin belonging to the myosin heavy chain family. Myosin-11 is a subunit of a hexameric protein that consists of two heavy chain s ...
'')) codes for the heavy chains myosin II, but splice variants of this gene result in four distinct isoforms. * It also contains 4 myosin light chains (MLC), resulting in 2 per head, weighing 20 (MLC20) and 17 (MLC17) kDa. These bind the heavy chains in the "neck" region between the head and tail. **The MLC20 is also known as the ''regulatory light chain'' and actively participates in
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 a ...
. **The MLC17 is also known as the ''essential light chain''. Its exact function is unclear, but is believed to contribute to the structural stability of the myosin head along with MLC20. Two variants of MLC17 (MLC17a/b) exist as a result of alternative splicing at the MLC17 gene. In muscle cells, the long
coiled-coil A coiled coil is a structural motif in proteins in which 2–7 alpha-helices are coiled together like the strands of a rope. (Dimers and trimers are the most common types.) Many coiled coil-type proteins are involved in important biological f ...
tails of the individual myosin molecules join, forming the thick filaments of the sarcomere. 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.


Myosin III

Myosin III is a poorly understood member of the myosin family. It has been studied ''
in vivo Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, and p ...
'' in the eyes of ''
Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many speci ...
'', where it is thought to play a role in
phototransduction Visual phototransduction is the sensory transduction process of the visual system by which light is detected to yield nerve impulses in the rod cells and cone cells in the retina of the eye in humans and other vertebrates. It relies on the visu ...
. A human homologue gene for myosin III,
MYO3A Myosin-IIIa is a protein that in humans is encoded by the ''MYO3A'' gene. The protein encoded by this gene belongs to the myosin Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide ra ...
, has been uncovered through the
Human Genome Project The Human Genome Project (HGP) was an international scientific research project with the goal of determining the base pairs that make up human DNA, and of identifying, mapping and sequencing all of the genes of the human genome from both ...
and is expressed in the
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which the ...
and
cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory o ...
.


Myosin IV

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.


Myosin V

Myosin V is an unconventional myosin motor, which is processive as a
dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ( ...
and has a step size of 36 nm. 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. 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. The Myosin V motor head can be subdivided into the following functional regions: *
Nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecu ...
-binding site - These elements together coordinate di-valent metal cations (usually
magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...
) 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 The Walker A and Walker B motifs are protein sequence motifs, known to have highly conserved three-dimensional structures. These were first reported in ATP-binding proteins by Walker and co-workers in 1982. Of the two motifs, the A motif is t ...
DxxG. Isomerizes in the presence of ATP. ** P-loop - This contains the
Walker A motif The Walker A and Walker B motifs are protein sequence motifs, known to have highly conserved three-dimensional structures. These were first reported in ATP-binding proteins by Walker and co-workers in 1982. Of the two motifs, the A motif is ...
GxxxxGK(S,T). This is the primary ATP binding site. * Transducer - The seven β-strands that underpin the motor head's structure. * U50 and L50 - The Upper (U50) and Lower (L50) domains are each around 50 kDa. Their spatial separation forms a cleft critical for binding to
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 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). * 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).


Myosin VI

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. Myosin VI is thought to transport
endocytic Endocytosis is a cellular process in which substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a vesicle containing the ingested material. ...
vesicles into the cell.


Myosin VII

Myosin VII is an unconventional myosin with two
FERM domain In molecular biology, the FERM domain (F for 4.1 protein, E for ezrin, R for radixin and M for moesin) is a widespread protein module involved in localising proteins to the plasma membrane. FERM domains are found in a number of cytoskeletal-a ...
s in the tail region. It has an extended lever arm consisting of five calmodulin binding IQ motifs followed by a single alpha helix (SAH) Myosin VII is required for phagocytosis in '' Dictyostelium discoideum'',
spermatogenesis Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testis. This process starts with the mitotic division of the stem cells located close to the basement membrane of the tubu ...
in ''
C. elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' (r ...
'' and stereocilia formation in mice and zebrafish.


Myosin VIII

Myosin VIII is a plant-specific myosin linked to cell division; specifically, it is involved in regulating the flow of cytoplasm between cells and in the localization of vesicles to the
phragmoplast image:Phragmoplast.png, 300px, Phragmoplast and cell plate formation in a plant cell during cytokinesis. Left side: Phragmoplast forms and cell plate starts to assemble in the center of the cell. Towards the right: Phragmoplast enlarges in a donut- ...
.


Myosin IX

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


Myosin X

Myosin X is an unconventional myosin motor, which is functional as a
dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ( ...
. The dimerization of myosin X is thought to be antiparallel. This behavior has not been observed in other myosins. In mammalian cells, the motor is found to localize to
filopodia Filopodia (singular filopodium) are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. Within the lamellipodium, actin ribs are known as ''microspikes'', and when they extend beyond the lame ...
. Myosin X walks towards the barbed ends of filaments. Some research suggests it preferentially walks on bundles of actin, rather than single filaments. It is the first myosin motor found to exhibit this behavior.


Myosin XI

Myosin XI directs the movement of organelles such as plastids and
mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...
in plant cells. It is responsible for the light-directed movement of chloroplasts according to light intensity and the formation of
stromule A stromule is a microscopic structure found in plant cells. Stromules ( stroma-filled tubules) are highly dynamic structures extending from the surface of all plastid types, including proplastids, chloroplasts, etioplasts, leucoplasts, amyloplasts, ...
s interconnecting different plastids. Myosin XI also plays a key role in polar root tip growth and is necessary for proper root hair elongation. A specific Myosin XI found in ''
Nicotiana tabacum ''Nicotiana tabacum'', or cultivated tobacco, is an annually grown herbaceous plant of the ''Nicotiana'' genus. The plant is tropical in origin, is commonly grown throughout the world, and is often found in cultivation. It grows to heights be ...
'' was discovered to be the fastest known processive
molecular motor Molecular motors are natural (biological) or artificial molecular machines that are the essential agents of movement in living organisms. In general terms, a motor is a device that consumes energy in one form and converts it into motion or mec ...
, moving at 7μm/s in 35 nm steps along the
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 ...
filament.


Myosin XII


Myosin XIII


Myosin XIV

This myosin group has been found in the Apicomplexa phylum. The myosins localize to plasma membranes of the intracellular parasites and may then be involved in the cell invasion process. This myosin is also found in the ciliated protozoan ''Tetrahymena thermaphila''. Known functions include: transporting phagosomes to the nucleus and perturbing the developmentally regulated elimination of the macronucleus during conjugation.


Myosin XV

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 XVI


Myosin XVII


Myosin XVIII

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 XIX

Unconventional myosin XIX (Myo19) is a mitochondrial associated myosin motor.


Genes in humans

Note that not all of these genes are active. * Class I: MYO1A,
MYO1B Myosin-Ib is a protein that in humans is encoded by the ''MYO1B'' gene. References Further reading

* * * * * * * * {{gene-2-stub ...
,
MYO1C Myosin-Ic is a protein that in humans is encoded by the ''MYO1C'' gene. This gene encodes a member of the unconventional myosin protein family, which are actin-based molecular motors. The protein is found in the cytoplasm, and one isoform with a ...
, MYO1D,
MYO1E Myosin-Ie (Myo1e) is a protein that in humans is encoded by the ''MYO1E'' gene. Myosin-Ie is a long tailed myosin. It contains an N-terminal motor domain, an IQ motif, a TH1 domain containing a plecstrin homology (PH) domain, a proline rich TH2 ...
,
MYO1F Myosin-If is a protein that in humans is encoded by the ''MYO1F'' gene. It is expressed mainly in the immune system and might be involved in cell adhesion and motility. It is a candidate gene for (among other things) nonsyndromic deafness De ...
,
MYO1G Myosin IG, also known as myosin 1G and MYO1G, is a protein that in humans is encoded by the ''MYO1G'' gene. MYO1G is a member of class I unconventional myosins. Its expression is highly restricted to hematopoietic tissues and cells. It localise ...
, MYO1H * Class II:
MYH1 Myosin-1, also known as 'striated muscle myosin heavy chain 1', is a protein that in humans is encoded by the ''MYH1'' gene. This gene is most highly expressed in fast type IIX/D muscle fibres of vertebrates and encodes a protein found uniquely ...
,
MYH2 Myosin-2 (myosin heavy chain 2) is a protein that in humans is encoded by the ''MYH2'' gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generatio ...
,
MYH3 Myosin-3 is a protein that in humans is encoded by the ''MYH3'' gene. Function Myosin is a major contractile protein which converts chemical energy into mechanical energy through the hydrolysis of ATP. Myosin is a hexameric protein compose ...
,
MYH4 Myosin-4 also known as myosin, heavy chain 4 is a protein which in humans is encoded by the ''MYH4'' gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." me ...
, MYH6, MYH7,
MYH7B Myosin-7B also known as myosin, heavy chain 7B is a protein that in humans is encoded by the ''MYH7B'' gene. Function MYH7B is a slow-twitch myosin Myosins () are a superfamily of motor proteins best known for their roles in muscle cont ...
,
MYH8 Myosin-8 is a protein that in humans is encoded by the ''MYH8'' gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ' ...
,
MYH9 Myosin-9 also known as myosin, heavy chain 9, non-muscle or non-muscle myosin heavy chain IIa (NMMHC-IIA) is a protein which in humans is encoded by the ''MYH9'' gene. Non-muscle myosin IIA (NM IIA) is expressed in most cells and tissues where i ...
,
MYH10 Myosin-10 also known as myosin heavy chain 10 or non-muscle myosin IIB (NM-IIB) is a protein that in humans is encoded by the ''MYH10'' gene. Non-muscle myosins are expressed in a wide variety of tissues, but NM-IIB is the only non-muscle myosin ...
,
MYH11 Myosin-11 is a protein that in humans is encoded by the ''MYH11'' gene. Function Myosin-11 is a smooth muscle myosin belonging to the myosin heavy chain family. Myosin-11 is a subunit of a hexameric protein that consists of two heavy chain s ...
,
MYH13 Myosin-13 also known as myosin, heavy chain 13 is a protein which in humans is encoded by the ''MYH13'' gene. Function MYH13 is a myosin Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and ...
,
MYH14 Myosin-14 is a protein that in humans is encoded by the ''MYH14'' gene. This gene encodes a member of the myosin superfamily. Myosins are actin-dependent motor proteins with diverse functions, including regulation of cytokinesis, cell motility ...
,
MYH15 Myosin-15 also known as myosin, heavy chain 15 is a protein that in humans is encoded by the ''MYH15'' gene. Function MYH15 is a slow-twitch myosin Myosins () are a superfamily of motor proteins best known for their roles in muscle cont ...
,
MYH16 The MYH16 gene encodes a protein called myosin heavy chain 16, which is a muscle protein in mammals. At least in primates, it is a specialized muscle protein found only in the temporalis and masseter muscles of the jaw. Myosin heavy chain protein ...
* Class III:
MYO3A Myosin-IIIa is a protein that in humans is encoded by the ''MYO3A'' gene. The protein encoded by this gene belongs to the myosin Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide ra ...
, MYO3B * Class V: MYO5A, MYO5B, MYO5C * Class VI: MYO6 * Class VII:
MYO7A Myosin VIIA is protein that in humans is encoded by the ''MYO7A'' gene. Myosin VIIA is a member of the unconventional myosin superfamily of proteins. Myosins are actin binding molecular motors that use the enzymatic conversion of ATP - ADP + ino ...
, MYO7B * Class IX: MYO9A,
MYO9B ''MYO9B'' is a gene that encodes the Myosin-IXb protein. References Further reading * * * * * * * * * * * * * * * * * * * External links

* {{gene-19-stub ...
* Class X:
MYO10 Myosin X, also known as MYO10, is a protein that in humans is encoded by the ''MYO10'' gene. Myo10 is an actin-based motor protein that can localize to the tips of the finger-like cellular protrusions known as filopodia. Myo10 is broadly express ...
* Class XV:
MYO15A Myosin-XV is a protein that in humans is encoded by the ''MYO15A'' gene. Gene Read-through transcript containing an upstream gene and this gene have been identified, but they are not thought to encode a fusion protein. Several alternatively ...
* Class XVIII:
MYO18A Myosin-XVIIIa is a protein that in humans is encoded by the ''MYO18A'' gene. References Further reading

* * * * * * * * * * * * * * {{protein-stub ...
,
MYO18B Myosin-XVIIIb is a protein that in humans is encoded by the ''MYO18B'' gene. The protein encoded by this gene may regulate muscle-specific genes when in the nucleus and may influence intracellular trafficking when in the cytoplasm. The encoded ...
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. * Light chain: MYL1, MYL2, MYL3, MYL4, MYL5,
MYL6 Myosin light polypeptide 6 is a protein that in humans is encoded by the ''MYL6'' gene. Myosin is a hexameric ATPase ATPases (, Adenosine 5'-TriPhosphatase, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, SV40 T-antigen, ATP h ...
,
MYL6B Myosin light chain 6B is a protein that in humans is encoded by the ''MYL6B'' gene. Myosin is a hexameric ATPase cellular motor protein. It is composed of two heavy chains, two nonphosphorylatable alkali light chains, and two phosphorylatable re ...
,
MYL7 Atrial Light Chain-2 (ALC-2) also known as Myosin regulatory light chain 2, atrial isoform (MLC2a) is a protein that in humans is encoded by the ''MYL7'' gene. ALC-2 expression is restricted to cardiac muscle atria in healthy individuals, where i ...
,
MYL9 Myosin regulatory light polypeptide 9 is a protein that in humans is encoded by the ''MYL9'' gene. Function Myosin, a structural component of muscle, consists of two heavy chains and four light chains. The protein encoded by this gene is a my ...
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MYLIP Myosin regulatory light chain interacting protein, also known as MYLIP, is a protein that in humans is encoded by the ''MYLIP'' gene. MYLIP is also known as IDOL "Inducible Degrader of the LDL receptor" based on its involvement in cholesterol reg ...
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MYLK Myosin light chain kinase, smooth muscle also known as kinase-related protein (KRP) or telokin is an enzyme that in humans is encoded by the ''MYLK'' gene. Function This gene, a muscle member of the immunoglobulin superfamily, encodes a myosi ...
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MYLK2 Myosin 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 ...
, MYLL1


Paramyosin

Paramyosin is a large, 93-115kDa
muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of mus ...
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
that has been described in a number of diverse
invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chorda ...
phyla. Invertebrate thick filaments are thought to be composed of an inner paramyosin core surrounded by myosin. The myosin interacts with
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 ...
, resulting in fibre contraction. Paramyosin is found in many different invertebrate species, for example, Brachiopoda, Sipunculidea, Nematoda, Annelida,
Mollusca Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000  extant species of molluscs are recognized. The number of fossil species is es ...
,
Arachnida Arachnida () is a class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegar ...
, and Insecta. 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.


References


Further reading

* * * * *Molecular Biology of the Cell. Alberts, Johnson, Lewis, Raff, Roberts, and Walter. 4th Edition. 949–952.


Additional images

Image:Querbrückenzyklus 1.png, Phase 1 Image:Querbrückenzyklus 2.png, Phase 2 Image:Querbrückenzyklus 3.png, Phase 3 Image:Querbrückenzyklus 4.png, Phase 4


External links


MBInfo – Myosin IsoformsMBInfo – The Myosin Powerstroke
A video of a moving myosin motor protein. *

*http://cellimages.ascb.org/cdm4/item_viewer.php?CISOROOT=/p4041coll12&CISOPTR=101&CISOBOX=1&REC=2 Animation of a moving myosin motor protein *
3D macromolecular structures of myosin from the EM Data Bank(EMDB)
{{Muscle tissue Motor proteins Cytoskeleton proteins Protein superfamilies Skeletal muscle