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Myosins () are a superfamily of motor proteins 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 as ...
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 eukaryotes. 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 Wilhelm Friedrich Kühne (28 March 183710 June 1900) was a German physiologist. Born in Hamburg, he is best known today for coining the word enzyme in 1878. Biography Kühne was born at Hamburg on 28 March 1837. After attending the gymnasium ...
. 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 muscl ...
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 ATPases found in the cells of both striated muscle tissue and
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 ...
. Following the discovery in 1973 of enzymes with myosin-like function in '' Acanthamoeba castellanii'', 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 ba ...
s have been discovered throughout the realm of eukaryotes. 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 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 hydrolys ...
(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 An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudop ...
.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, 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 hydrolys ...
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 A macromolecule is a very large molecule important to biophysical processes, such as a protein or nucleic acid. It is composed of thousands of covalently bonded atoms. Many macromolecules are polymers of smaller molecules called monomers. The ...
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 stroke

Multiple
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 ...
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 muscl ...
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 A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue. It is the repeating unit between two Z-lines. Skeletal muscles are composed of tubular muscle cells (called mus ...
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 The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual mitochondria. These are usually treated separately as the ...
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 phospho ...
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 Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry) In cell biology, a vesicle is a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer. Vesicles form nat ...
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 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 ...
(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 as ...
in muscle cells in most animal cell types. It is also found in non-muscle cells in contractile bundles called
stress fiber Stress fibers are contractile actin bundles found in non-muscle cells. They are composed of actin (microfilaments) and non-muscle myosin II (NMMII), and also contain various crosslinking proteins, such as α-actinin, to form a highly regulated ...
s. * 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 The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...
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. 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 sub ...
'')) codes for the heavy chains myosin II, but splice variants of this gene result in four distinct isoforms. * It also contains 4
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. Str ...
s (MLC), resulting in 2 per head, weighing 20 (MLC20) and 17 (MLC17)
kDa The dalton or unified atomic mass unit (symbols: Da or u) is a non-SI unit of mass widely used in physics and chemistry. It is defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at re ...
. 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 as ...
. **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 tails of the individual myosin molecules join, forming the thick filaments of the
sarcomere A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue. It is the repeating unit between two Z-lines. Skeletal muscles are composed of tubular muscle cells (called mus ...
. 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 ...
'' 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 transduction (physiology), sensory transduction process of the visual system by which light is detected to yield Action potential, nerve impulses in the rod cells and cone cells in the retina of the eye in humans and ...
. A human homologue gene for myosin III, MYO3A, has been uncovered through the Human Genome Project 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 then ...
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 or ...
.


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 biomolecule ...
-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 ta ...
) 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 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 The dalton or unified atomic mass unit (symbols: Da or u) is a non-SI unit of mass widely used in physics and chemistry. It is defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at re ...
. 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. E ...
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-ass ...
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 Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...
in ''
Dictyostelium discoideum ''Dictyostelium discoideum'' is a species of soil-dwelling amoeba belonging to the phylum Amoebozoa, infraphylum Mycetozoa. Commonly referred to as slime mold, ''D. discoideum'' is a eukaryote that transitions from a collection of unicellular ...
'', spermatogenesis 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 Stereocilia (or stereovilli or villi) are non-motile apical cell modifications. They are distinct from cilia and microvilli, but are closely related to microvilli. They form single "finger-like" projections that may be branched, with normal cell ...
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 The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracellular endosymbiotic cyanobac ...
and mitochondria in plant cells. It is responsible for the light-directed movement of
chloroplasts A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
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 Root hair, or absorbent hairs, are outgrowths of epidermal cells, specialized cells at the tip of a plant root. They are lateral extensions of a single cell and are only rarely branched. They are found in the region of maturation, of the root. Root ...
elongation. A specific Myosin XI found in ''
Nicotiana tabacum ''Nicotiana tabacum'', or cultivated tobacco, is an Annual plant, 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 of tobacco, ...
'' 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 mech ...
, 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 The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. T ...
phylum. The myosins localize to plasma membranes of the intracellular
parasites Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...
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 Stereocilia (or stereovilli or villi) are non-motile apical cell modifications. They are distinct from cilia and microvilli, but are closely related to microvilli. They form single "finger-like" projections that may be branched, with normal cell ...
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 Unconventional myosin-Ia is a protein that in humans is encoded by the ''MYO1A'' gene. The protein encoded by this gene belongs to the myosin superfamily. Myosins are molecular motors that, upon interaction with actin filaments, utilize energy fro ...
,
MYO1B Myosin-Ib is a protein that in humans is encoded by the ''MYO1B'' 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 ...
,
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 Dea ...
,
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 localis ...
, 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 i ...
,
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 composed o ...
,
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..." mean ...
,
MYH6 Myosin heavy chain, α isoform (MHC-α) is a protein that in humans is encoded by the ''MYH6'' gene. This isoform is distinct from the ventricular/slow myosin heavy chain isoform, MYH7, referred to as MHC-β. MHC-α isoform is expressed predominan ...
,
MYH7 MYH7 is a gene encoding a myosin heavy chain beta (MHC-β) isoform (slow twitch) expressed primarily in the heart, but also in skeletal muscles (type I fibers). This isoform is distinct from the fast isoform of cardiac myosin heavy chain, MYH6, re ...
,
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 contr ...
, MYH8,
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 it ...
,
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 I ...
,
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 sub ...
,
MYH13 Myosin-13 also known as myosin, heavy chain 13 is a protein which in humans is encoded by the ''MYH13'' gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." m ...
,
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 motili ...
,
MYH15 Myosin-15 also known as myosin, heavy chain 15 is a protein that in humans is encoded by the ''MYH15'' gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." me ...
, MYH16 * Class III: MYO3A, MYO3B * Class V:
MYO5A Unconventional myosin-Va is a motor protein in charge of the intracellular transport of vesicles, organelles and protein complexes along the actin filaments. In humans it is coded for by the ''MYO5A'' gene. Structure In the presence of cargo ada ...
,
MYO5B Myosin-Vb, a myosin V type protein, is encoded by the ''MYO5B'' gene in humans. Recent evidence suggests that Myosin-Vb is related to the creation of memories by actin-dependent trafficking of AMPA receptor containing recycling endosomes in den ...
, MYO5C * Class VI:
MYO6 Unconventional myosin-VI, is a protein that in humans is coded for by ''MYO6''. Unconventional myosin-VI is a myosin molecular motor involved in intracellular vesicle and organelle transport. Structure Human myosin-VI contains a N-terminal myos ...
* 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 + inor ...
, MYO7B * Class IX: MYO9A,
MYO9B ''MYO9B'' is a 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 ...
* 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 In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' ...
,
MYO18B Myosin-XVIIIb is a protein that in humans is encoded by the ''MYO18B'' gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' ...
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 Myosin light chain 3, skeletal muscle isoform is a protein that in humans is encoded by the ''MYL1'' gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meanin ...
,
MYL2 Myosin regulatory light chain 2, ventricular/cardiac muscle isoform (MLC-2) also known as the regulatory light chain of myosin (RLC) is a protein that in humans is encoded by the ''MYL2'' gene. This cardiac ventricular RLC isoform is distinct fro ...
,
MYL3 Myosin essential light chain (ELC), ventricular/cardiac isoform is a protein that in humans is encoded by the ''MYL3'' gene. This cardiac ventricular/slow skeletal ELC isoform is distinct from that expressed in fast skeletal muscle ( MYL1) and car ...
,
MYL4 Atrial Light Chain-1 (ALC-1), also known as Essential Light Chain, Atrial is a protein that in humans is encoded by the ''MYL4'' gene. ALC-1 is expressed in fetal cardiac ventricular and fetal skeletal muscle, as well as fetal and adult cardiac ...
, MYL5, MYL6,
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 r ...
,
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 it ...
, MYL9,
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 ...
,
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 ...
,
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 muscl ...
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, respo ...
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 chordate ...
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 Golfingiida, also known as the Golfingiiformes, is an order of peanut worms.Gibbs, P.E. (1977): ''British sipunculans''. Academic Press. London. 35p. Ditadi, A.S.F. & Migotto, A.E. (1982): ''O Filo Sipuncula''. Concelho Nacional de Desemvolviment ...
,
Nematoda The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broa ...
, Annelida, Mollusca, Arachnida, and
Insecta Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs o ...
. Paramyosin is responsible for the "catch" mechanism that enables sustained contraction of muscles with very little energy expenditure, such that a
clam Clam is a common name for several kinds of bivalve molluscs. The word is often applied only to those that are edible and live as infauna, spending most of their lives halfway buried in the sand of the seafloor or riverbeds. Clams have two shel ...
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