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
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. Bact ...
s. They are
ATP
ATP may refer to:
Companies and organizations
* Association of Tennis Professionals, men's professional tennis governing body
* American Technical Publishers, employee-owned publishing company
* ', a Danish pension
* Armenia Tree Project, non ...
-dependent and responsible for
actin
Actin is a protein family, family of Globular protein, globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in myofibril, muscle fibrils. It is found in essentially all Eukaryote, eukaryotic cel ...
-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 of both
striated muscle tissue
Striations means a series of ridges, furrows or linear marks, and is used in several ways:
* Glacial striation
* Striation (fatigue), in material
* Striation (geology), a ''striation'' as a result of a geological fault
* Striation Valley, in Anta ...
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 m ...
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 hydrolysis ...
(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 pseudo ...
.
[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 protein family, family of Globular protein, globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in myofibril, muscle fibrils. It is found in essentially all Eukaryote, eukaryotic cel ...
, and uses
ATP
ATP may refer to:
Companies and organizations
* Association of Tennis Professionals, men's professional tennis governing body
* American Technical Publishers, employee-owned publishing company
* ', a Danish pension
* Armenia Tree Project, non ...
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 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
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, a ...
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, eac ...
(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 (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 b ...
.
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 as ...
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 m ...
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 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
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'')) 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.
Struct ...
s (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 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. 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 s ...
'', where it is thought to play a role in phototransduction. A human homologue gene for myosin III, MYO3A
Myosin-IIIa is a protein that in humans is encoded by the ''MYO3A'' 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 ...
, 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 a ...
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 org ...
.
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 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 biomolecul ...
-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
ATP may refer to:
Companies and organizations
* Association of Tennis Professionals, men's professional tennis governing body
* American Technical Publishers, employee-owned publishing company
* ', a Danish pension
* Armenia Tree Project, non ...
.
** 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 ...
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.
* 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 protein family, family of Globular protein, globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in myofibril, muscle fibrils. It is found in essentially all Eukaryote, eukaryotic cel ...
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 vesicles into the cell.
Myosin VII
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) 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 i ...
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
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 t ...
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'' ( ...
'' 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.
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. 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 lam ...
. 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 cy ...
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 i ...
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, moving at 7μm/s in 35 nm steps along the actin
Actin is a protein family, family of Globular protein, globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in myofibril, muscle fibrils. It is found in essentially all Eukaryote, eukaryotic cel ...
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, MYO1C, MYO1D, MYO1E, MYO1F, MYO1G, MYO1H
* Class II: MYH1, MYH2, MYH3, MYH4, MYH6, MYH7, MYH7B, 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 ''ge ...
, MYH9, MYH10, MYH11, MYH13, MYH14, MYH15, 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
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ...
, 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 + inor ...
, MYO7B
* Class IX: MYO9A, MYO9B
* Class X: MYO10
* 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 alternativel ...
* Class XVIII: MYO18A, MYO18B
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 cellular motor protein. It is composed of two heavy chains, two nonphosphorylatable alkali light chains, and two phosphorylatabl ...
, MYL6B, MYL7, MYL9, MYLIP, MYLK, MYLK2, MYLL1
Paramyosin
Paramyosin is a large, 93-115kDa muscle
Skeletal muscles (commonly referred to as muscles) are Organ (biology), 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 ...
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, respon ...
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 protein family, family of Globular protein, globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in myofibril, muscle fibrils. It is found in essentially all Eukaryote, eukaryotic cel ...
, resulting in fibre contraction. Paramyosin is found in many different invertebrate species, for example, Brachiopoda, Sipunculidea, Nematoda, Annelida, Mollusca, Arachnida
Arachnida () is a Class (biology), class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, opiliones, harvestmen, Solifugae, came ...
, 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 ...
. 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 sh ...
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 Isoforms
MBInfo – 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