Myosin
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224676458
2008-07-09T22:42:03Z
Medical geneticist
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[[Image:Myosine.gif|thumb|Part of the myosin structure, atoms in the heavy chain are colored red on the left-hand side, and atoms in the light chains are colored orange and yellow.]]
'''Myosins''' are a large family of [[motor proteins]] found in [[eukaryotic]] [[Biological tissue|tissues]]. They are responsible for [[actin]]-based [[motility]].
:''The term “myosin” was originally used to describe a group of similar, but nonidentical, ATPases found in striated and smooth muscle cells.'' <ref>Pollard and Korn, 1973</ref>
== Structure and Function ==
[[image:Sarcomere.svg|thumb|350px|Sliding filament model of muscle contraction.]]
===Domains===
Most myosin molecules are composed of a head, neck, and tail domain.
* The ''head domain'' binds the filamentous [[actin]], and uses [[Adenosine triphosphate|ATP]] [[hydrolysis]] to generate force and to "walk" along the filament towards the (+) end (with the exception of one family member, myosin VI, which moves towards the (-) 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 [[protein subunits|subunit]]s. In some cases, the tail domain may play a role in regulating motor activity.
== 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 non-muscle cells. However, beginning in the 1970s researchers began to discover new myosin genes in simple eukaryotes <ref>Pollard and Korn, 1973</ref> encoding proteins that acted as monomers and were therefore entitled Class I myosins. These new myosins were collectively termed "unconventional myosins" <ref>Cheney and Mooseker, 1992</ref> and have been found in many tissues other than muscle. These new superfamily members have been grouped according to phylogenetic relationships derived from a comparison of the amino acid sequences of their head domains, with each class being assigned a [[Roman numeral]] <ref>Cheney ''et al.'', 1993</ref><ref>Goodson, 2004</ref><ref>Hodge and Cope, 2000</ref><ref>Berg ''et al.'', 2001</ref>(see phylogenetic tree). The unconventional myosins also have divergent tail domains, suggesting unique functions<ref>Oliver ''et al.'', 1999</ref>. The now diverse array of myosins likely evolved from an ancestral [[wiktionary:Precursor|precursor]] (see picture).
[[Image:MyosinUnrootedTree.jpg|thumb|right|300px|Myosin unrooted [[phylogenetic tree]] — click on image to see larger version (Source: http://www.mrc-lmb.cam.ac.uk/myosin/trees/trees.html)]]
Analysis of the amino acid sequences of different myosins shows great variability among the tail domains but strong conservation of head domain sequences. Presumably this is so the myosins may interact, via their tails, with a large number of different cargoes, while the goal in each case - to move along actin filaments - remains the same and therefore requires the same machinery in the motor. For example, the [[human genome]] contains over 40 different myosin [[genes]].
These differences in shape also determine the speed at which myosins can move along actin filaments. The hydrolysis of ATP and the subsequent release of the [[phosphate]] group causes the "[[power stroke]]," in which the "lever arm" or "neck" region of the heavy chain is dragged forward. Since the power stroke always moves the lever arm by the same angle, the length of the lever arm determines how fast the cargo will move. 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. Myosin V, for example, has a much longer neck region than myosin II, and therefore moves 30-40 nanometers with each stroke as opposed to only 5-10.
===Myosin Classes===
====Myosin I====
Myosin I's function is unknown, but it is believed to be responsible for vesicle transport or the contraction vacuole of cells.<ref>{{cite web | author= Sutherland Macive | title=Myosin I | url=http://www.bms.ed.ac.uk/research/others/smaciver/Myosin%20I.htm | date=6/4/03 | accessdate=2007-05-23}}</ref>
====Myosin II====
Myosin II is perhaps the best-studied example of these properties.
* 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-terminus|N-terminal]] head domain, while the [[C-terminus|C-terminal]] tails take on a [[coiled-coil]] morphology, holding the two heavy chains together (imagine two snakes wrapped around each other, such as in a [[caduceus]]). Thus, myosin II has two heads.
* It also contains 4 ''light chains'' (2 per head), which bind the heavy chains in the "neck" region between the head and tail.
In muscle cells, it is myosin II that is responsible for producing the contractile force. Here, the long [[coiled-coil]] tails of the individual myosin molecules join together, 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.
==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]], [[MYH9]], [[MYH10]], [[MYH11]], [[MYH13]], [[MYH14]], [[MYH15]], [[MYH16]]
* Class III: [[MYO3A]], [[MYO3B]]
* Class V: [[MYO5A]], [[MYO5B]], [[MYO5C]]
* Class VI: [[MYO6]]
* Class VII: [[MYO7A]], [[MYO7B]]
* Class IX: [[MYO9A]], [[MYO9B]]
* Class X: [[MYO10]]
* Class XV: [[MYO15A]]
* 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]], [[MYL6B]], [[MYL7]], [[MYL9]], [[MYLIP]], [[MYLK]], [[MYLK2]], [[MYLL1]]
==Footnotes==
<references/>
==References==
*{{cite journal |author=Berg JS, Powell BC, Cheney RE |title=A millennial myosin census |journal=Mol. Biol. Cell |volume=12 |issue=4 |pages=780–94 |year=2001 |month=April |pmid=11294886 |pmc=32266 |doi= |url=http://www.molbiolcell.org/cgi/pmidlookup?view=long&pmid=11294886}}
*{{cite journal |author=Cheney RE, Mooseker MS |title=Unconventional myosins |journal=Curr. Opin. Cell Biol. |volume=4 |issue=1 |pages=27–35 |year=1992 |month=February |pmid=1558751 |doi= |url=}}
*{{cite journal |author=Cheney RE, Riley MA, Mooseker MS |title=Phylogenetic analysis of the myosin superfamily |journal=Cell Motil. Cytoskeleton |volume=24 |issue=4 |pages=215–23 |year=1993 |pmid=8477454 |doi=10.1002/cm.970240402 |url=}}
*{{cite journal |author=Hodge T, Cope MJ |title=A myosin family tree |journal=J. Cell. Sci. |volume=113 Pt 19 |issue= |pages=3353–4 |year=2000 |month=October |pmid=10984423 |doi= |url=http://jcs.biologists.org/cgi/pmidlookup?view=long&pmid=10984423}}
*{{cite journal |author=Gavin RH |title=Myosins in protists |journal=Int. Rev. Cytol. |volume=206 |issue= |pages=97–134 |year=2001 |pmid=11407764 |doi= |url=}}
*{{cite journal |author=Goodson HV |title=Molecular evolution of the myosin superfamily: application of phylogenetic techniques to cell biological questions |journal=Soc. Gen. Physiol. Ser. |volume=49 |issue= |pages=141–57 |year=1994 |pmid=7939893 |doi= |url=}}
*{{cite journal |author=Mooseker MS, Cheney RE |title=Unconventional myosins |journal=Annu. Rev. Cell Dev. Biol. |volume=11 |issue= |pages=633–75 |year=1995 |pmid=8689571 |doi=10.1146/annurev.cb.11.110195.003221 |url=}}
*{{cite journal |author=Oliver TN, Berg JS, Cheney RE |title=Tails of unconventional myosins |journal=Cell. Mol. Life Sci. |volume=56 |issue=3-4 |pages=243–57 |year=1999 |month=October |pmid=11212352 |doi= |url=http://link.springer.de/link/service/journals/00018/bibs/90563-4/90560243.htm}}
*{{cite journal |author=Pollard TD, Korn ED |title=Acanthamoeba myosin. I. Isolation from Acanthamoeba castellanii of an enzyme similar to muscle myosin |journal=J. Biol. Chem. |volume=248 |issue=13 |pages=4682–90 |year=1973 |month=July |pmid=4268863 |doi= |url=http://www.jbc.org/cgi/pmidlookup?view=long&pmid=4268863}}
*{{cite journal |author=Sellers JR |title=Myosins: a diverse superfamily |journal=Biochim. Biophys. Acta |volume=1496 |issue=1 |pages=3–22 |year=2000 |month=March |pmid=10722873 |doi= |url=http://linkinghub.elsevier.com/retrieve/pii/S0167-4889(00)00005-7}}
*{{cite journal |author=Soldati T, Geissler H, Schwarz EC |title=How many is enough? Exploring the myosin repertoire in the model eukaryote Dictyostelium discoideum |journal=Cell Biochem. Biophys. |volume=30 |issue=3 |pages=389–411 |year=1999 |pmid=10403058 |doi=10.1007/BF02738121 |url=}}
*Molecular Biology of the Cell. Alberts, Johnson, Lewis, Raff, Roberts, and Walter. 4th Edition. 949-952.
==External links==
==Additional images==
<gallery>
Image:Querbrückenzyklus 1.png|Phase 1
Image:Querbrückenzyklus 2.png|Phase 2
Image:Querbrückenzyklus 3.png|Phase 3
Image:Querbrückenzyklus 4.png|Phase 4
Image:Myosin-painting.JPG|Myosin-painting
Image:Myosin powerstroke.JPG|Myosin powerstroke
</gallery>
==See also==
*[[Motor protein]]
*[[Actin]]
==External links==
*[http://www.banyantree.org/jsale/actinmyosin/index.html Myosin Video] A video of a moving myosin motor protein.
* {{MeshName|Myosins}}
*[http://www.mrc-lmb.cam.ac.uk/myosin/myosin.html The Myosin Homepage]
* {{EC number|3.6.4.1}}
{{Cytoskeletal Proteins}}
{{ATPases}}
{{Muscle tissue}}
[[Category:Proteins]]
[[Category:Motor proteins]]
[[Category:Cytoskeleton]]
[[Category:Skeletal muscle]]
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