Motor protein 3017886 215476985 2008-05-28T11:14:43Z DOI bot 6652755 Citation maintenance. You can [[WP:DOI|use this bot]] yourself! Please [[User:DOI_bot/bugs|report any bugs]]. '''Motor proteins''' are a class of [[molecular motors]] that are able to move along the surface of a suitable substrate. They are powered by the hydrolysis of [[Adenosine triphosphate|ATP]] and convert chemical energy into mechanical work. == Cellular functions == The most prominent example of a motor protein is the [[muscle]] protein [[myosin]] which "motors" the contraction of muscle fibers in animals. Motor proteins are the driving force behind most [[active transport]] of [[protein]]s and [[Vesicle (biology)|vesicles]] in the [[cytoplasm]]. Kinesins and dyneins play essential roles in intracellular transport such as [[axoplasmic transport|axonal transport]] and in the formation of the [[spindle apparatus]] and the separation of the [[chromosome]]s during [[mitosis]] and [[meiosis]]. Dynein is found in [[flagella]] and is crucial to [[cell motility]], for example in [[spermatozoa]]. == Diseases associated with motor protein defects == The importance of motor proteins in cells becomes evident when they fail to fulfill their function. For example, kinesin deficiencies have been identified as cause for [[Charcot-Marie-Tooth disease]] and some [[kidney disease]]s. Dynein deficiencies can lead to [[Chronic (medical)|chronic]] [[infection]]s of the [[respiratory tract]] as [[cilia]] fail to function without dynein. Defects in muscular myosin predictably cause [[Myopathy|myopathies]], whereas defects in unconventional myosin are the cause for [[Usher syndrome]] and [[deafness]].<ref name="Hirokawa">{{cite journal|author= Hirokawa N, Tekamura R | title = Biochemical and molecular characterization of diseases linked to motor proteins | journal = Trends in Biochemical Sciences | date = 2003 | volume = 28 | pages = 558–565 | pmid = 14559185 | doi = 10.1016/j.tibs.2003.08.006}}</ref> == Structure == Most eukaryotic motor proteins consist of two distinct domains: A motor head domain with [[ATPase]] function and a tail domain that can either form fibers (muscle myosin) or attach to a cargo such as for example [[chromosome]]s during [[anaphase]] of [[mitosis]] (kinesin) or [[Vesicle (biology)|vesicles]] during [[endocytosis]] (dynein). The head domain of the proteins carries out the movement by binding to a specific site on the substrate and changing [[Protein structure|conformation]] depending on [[ATP hydrolysis]]. The tail end of the molecule normally binds [[adaptor protein]]s that allow for stable interactions with the cargo to be moved along the substrate.<ref name="Kamal">{{cite journal|author= Kamal A, Goldstein LSB | title = Principles of cargo attachment to cytoplasmic motor proteins | journal = Current Opinion in Cell Biology | date = 2002 | volume = 14 | pages = 63–68 | pmid = 11792546 | doi = 10.1016/S0955-0674(01)00295-2}}</ref> These motor proteins typically form a complex of longer "[[heavy chain]]s" with motor head domains and shorter "[[light chain]]s" for stabilization. == Cytoskeletal motor proteins == Motor proteins utilizing the [[cytoskeleton]] for movement fall into two categories based on their substrates: Actin motors such as myosin move along [[microfilament]]s through interaction with [[actin]]. Microtubule motors such as dynein and kinesin move along [[microtubule]]s through interaction with [[tubulin]]. There are two basic types of microtubule motors: plus-end motors and minus-end motors, depending on the direction in which they "walk" along the microtubule cables within the cell. === Actin motors === ==== Myosin ==== [[Myosin]]s are actin motors and form myosin complexes consisting of two heavy chains with motor heads and two light chains. Derived from the [[Greek language|Greek]] word for muscle, myosin is the protein responsible for generating [[muscle contraction]]. By non-processively walking along [[actin]] filaments, many molecules of myosin generate enough force to contract muscle tissue. Myosins are also vital in the process of [[cell division]]. They are also involved in [[cytoplasmic streaming]], wherein movement along [[microfilament]] networks in the cell allows [[organelle]]s and [[cytoplasm]] to stream in a particular direction. Eighteen different classes of myosins are known.<ref name="Thompson">{{cite journal|author= Thompson RF, Langford GM | title = Myosin superfamily evolutionary history | journal = The Anatomical Record | date = 2002 | volume = 268 | pages = 276–289 | pmid = 12382324 | doi = 10.1002/ar.10160}}</ref> Genomic representation of myosin motors: <ref name="Vale">{{cite journal|author= Vale RD | title = The molecular motor toolbox for intracellular transport | journal = Cell | date = 2003 | volume = 112 | pages = 467–480 | pmid = 12600311 | doi = 10.1016/S0092-8674(03)00111-9}}</ref> * [[Fungus|Fungi]] ([[yeast]]): 5 * [[Plant]]s ([[Arabidopsis]]): 17 * [[Insect]]s ([[Drosophila]]): 13 * [[Mammal]]s ([[human]]): 40 === Microtubule motors === ==== Kinesin ==== [[Kinesin]]s are a group of related motor proteins that use a [[microtubule]] track along which to "walk." They are vital to movement of [[chromosome]]s during [[mitosis]] and are also responsible for shuttling [[mitochondria]], [[Golgi bodies]], and [[Vesicle (biology)|vesicles]] within [[eukaryotic cell]]s. Kinesins typically contain two heavy chains with motor heads which move along microtubules via a pseudo-processive asymmetric walking motion (gansta' limp), that can be towards the plus-end or the minus-end, depending on the type of kinesin. Fourteen distinct kinesin families are known, with some additional kinesin-like proteins that cannot be classified into these families.<ref name="Miki">{{cite journal|author= Miki H, Okada Y, Hirokawa N | title = Analysis of the kinesin superfamily: insights into structure and function | journal = Trends in Cell Biology | date = 2005 | volume = 15 | pages = 467–476 | pmid = 16084724 | doi = 10.1016/j.tcb.2005.07.006}}</ref> Genomic representation of kinesin motors: <ref name="Vale"/> * [[Fungus|Fungi]] ([[yeast]]): 6 * [[Plant]]s ([[Arabidopsis]]): 61 * [[Insect]]s ([[Drosophila]]): 25 * [[Mammal]]s ([[human]]): 45 ==== Dynein ==== [[Dynein]]s are microtubule motors capable of a sliding movement. Dynein complexes are much larger and more complex than kinesin and myosin motors. Dynein facilitates the movement of [[cilia]] and [[flagella]]. Compared to 15 types of dynein for this function, only two [[cytoplasm]]ic forms are known.<ref name="Mallik">{{cite journal|author= Mallik R, Gross SP | title = Molecular motors: strategies to get along | journal = Current Biology | date = 2004 | volume = 14 | pages = R971–R982 | pmid = 15556858 | doi = 10.1016/j.cub.2004.10.046}}</ref> Genomic representation of dynein motors: <ref name="Vale"/> * [[Fungus|Fungi]] ([[yeast]]): 1 * [[Plant]]s ([[Arabidopsis]]): 0 * [[Insect]]s ([[Drosophila]]): 13 * [[Mammal]]s ([[human]]): 14-15 === Plant-specific motors === In contrast to [[animal]]s, [[fungi]] and [[non-vascular plant]]s, the cells of [[flowering plant]]s lack dynein motors. However, they contain a larger number of different kinesins. Many of these plant-specific kinesin groups are specialized for functions during [[plant cell]] [[mitosis]].<ref name="Vanstraelen">{{cite journal|author= Vanstraelen M, Inze D, Geelen D | title = Mitosis-specific kinesins in ''Arabidopsis'' | journal = Trends in Plant Science | date = 2006 | volume = 11 | pages = 167–175 | pmid = 16530461 | doi = 10.1016/j.tplants.2006.02.004}}</ref> Plant cells differ from animal cells in that they have a [[cell wall]]. During mitosis, the new cell wall is built by the formation of a [[cell plate]] starting in the center of the cell. This process is facilitated by a [[phragmoplast]], a microtubule array unique to plant cell mitosis. The building of cell plate and ultimately the new cell wall requires kinesin-like motor proteins.<ref name="Smith">{{cite journal|author= Smith LG | title = Plant cytokinesis: motoring to the finish | journal = Current Biology | date = 2002 | volume = 12 | pages = R202–R209 | pmid = 11909547}}</ref> Another motor protein essential for plant cell division is [[kinesin-like calmodulin-binding protein]] (KCBP), which is unique to plants and part kinesin and part myosin.<ref name="Abdel-Ghany">{{cite journal|author= Abdel-Gany I, Day IS, Simmons PK, Reddy ASN | title = Origin and evolution of kinesin-like calmodulin-binding protein | journal = Plant Physiology | date = 2005 | volume = 138 | pages = 1711–1722 | pmid = 15951483 | doi = 10.1104/pp.105.060913}}</ref> == Other molecular motors == Besides the motor proteins above, there are many more types of proteins capable of generating [[force]]s and [[torque]] in the cell. Among the functions performed by force-generating proteins are: :* [[Transcription (genetics)|transcription]] of RNA ([[RNA polymerase]]) :* [[DNA replication]] ([[DNA polymerase]]) :* nucleic acid double strand separation prior to transcription or replication ([[helicase]]) :* chromatin remodeling ([[Chromatin Structure Remodeling (RSC) Complex|RSC complex]]) :* [[chromosome condensation]] ([[SMC protein]])<ref>{{cite journal |author=Peterson C |title=The SMC family: novel motor proteins for chromosome condensation? |journal=Cell |volume=79 |issue=3 |pages=389–92 |year=1994 |pmid=7954805 |doi=10.1016/0092-8674(94)90247-X}}</ref> :* ATP synthesis ([[ATP synthase]]) Many of these [[molecular motors]] are ubiquitous in both [[Prokaryote|prokaryotic]] and [[Eukaryote|eukaryotic]] cells, although some, such as those involved with [[Cytoskeleton|cytoskeletal]] elements or [[chromatin]], are unique to eukaryotes. == See also == * [[Molecular modelling]] * [[Molecular motors]], for a general discussion of natural and synthetic motor molecules * [[Cytoskeleton]] == References == <references/> [[Category:Cell movement]] [[Category:Motor proteins]] [[Category:Molecular machine]] [[pl:Białka motoryczne]] [[zh:馬達蛋白]]