{{Short description|Class of molecular proteins}} [[Image:Kinesin_walking.gif|thumb|300px| [[Kinesin]] "walking" on a [[microtubule]] using [[protein dynamics]] on [[Nanoscopic scale|nanoscale]]s. [[Protein domain dynamics]] can now be seen by [[neutron spin echo]] spectroscopy. ]] '''Motor proteins''' are a class of [[molecular motors]] that can move along the [[cytoskeleton]] of cells. They do this by converting chemical energy into mechanical work by the [[hydrolysis]] of [[Adenosine triphosphate|ATP]].<ref>{{Citation |last=Alberts |first=Bruce |title=Molecular Motors |date=2002 |work=Molecular Biology of the Cell. 4th edition |url=https://www.ncbi.nlm.nih.gov/books/NBK26888/ |access-date=2025-04-03 |publisher=Garland Science |language=en |last2=Johnson |first2=Alexander |last3=Lewis |first3=Julian |last4=Raff |first4=Martin |last5=Roberts |first5=Keith |last6=Walter |first6=Peter}}</ref>
== Cellular functions == [[File:1003 Thick and Thin Filaments.jpg|thumb|The action of [[myosin]] along the [[actin]] filaments causes the shortening and lengthening of the [[sarcomere]]; responsible for muscle contraction and relaxation, respectively.]] Motor proteins are the driving force behind most [[active transport]] of [[protein]]s and [[Vesicle (biology)|vesicles]] in the [[cytoplasm]]. [[Kinesin]]s and [[dyneins#Structure|cytoplasmic 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]]. [[Axonemal dynein]], found in [[cilia]] and [[flagella]], is crucial to [[cell motility]] in [[spermatozoa]], and fluid transport in trachea.{{cn|date=June 2025}} The muscle protein myosin "motors" the contraction of muscle fibers in animals.
== 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 the 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. Numerous myosin deficiencies are related to disease states and genetic syndromes. Because [[myosin]] II is essential for muscle contraction, defects in muscular myosin predictably cause myopathies. Myosin is necessary in the process of hearing because of its role in the growth of stereocilia so defects in myosin protein structure can lead to [[Usher syndrome]] and non-syndromic [[deafness]].<ref name="Hirokawa">{{cite journal | vauthors = Hirokawa N, Takemura R | title = Biochemical and molecular characterization of diseases linked to motor proteins | journal = Trends in Biochemical Sciences | volume = 28 | issue = 10 | pages = 558–65 | date = October 2003 | pmid = 14559185 | doi = 10.1016/j.tibs.2003.08.006 }}</ref>
== Cytoskeletal motor proteins ==
Motor proteins utilizing the [[cytoskeleton]] for movement fall into two categories based on their [[substrate (biochemistry)|substrate]]: [[microfilaments]] or [[microtubules]]. [[Actin]]-based motor proteins ([[myosin]]) move along [[microfilament]]s through interaction with [[actin]], and [[microtubule]] motors ([[dynein]] and [[kinesin]]) move along [[microtubule]]s through interaction with [[tubulin]].{{cn|date=June 2025}}
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 a [[Protein superfamily|superfamily]] of [[actin]] motor proteins that convert chemical energy in the form of ATP to mechanical energy, thus generating force and movement. The first identified myosin, myosin II, is responsible for generating [[muscle contraction]]. Myosin II is an elongated protein that is formed from two heavy chains with motor heads and two light chains. Each myosin head contains actin and ATP binding site. The myosin heads bind and hydrolyze ATP, which provides the energy to walk toward the plus end of an actin filament. Myosin II are also vital in the process of [[cell division]]. For example, non-muscle myosin II bipolar thick filaments provide the force of contraction needed to divide the cell into two daughter cells during cytokinesis. In addition to myosin II, many other myosin types are responsible for variety of movement of non-muscle cells. For example, myosin is involved in intracellular organization and the protrusion of actin-rich structures at the cell surface. [[Myosin V]] is involved in vesicle and organelle transport.<ref>{{Cite journal|title = Molecular Motors|url = https://www.ncbi.nlm.nih.gov/books/NBK26888/|date = 2002-01-01|first1 = Bruce|last1 = Alberts|first2 = Alexander|last2 = Johnson|first3 = Julian|last3 = Lewis|first4 = Martin|last4 = Raff|first5 = Keith|last5 = Roberts|first6 = Peter|last6 = Walter | name-list-style = vanc |website=NCBI - National Institutes of Health}}</ref><ref name="Warshaw">{{cite journal |last1=Warshaw |first1=DM |title=Tilting and twirling as myosin V steps along actin filaments as detected by fluorescence polarization. |journal=The Journal of General Physiology |date=February 2012 |volume=139 |issue=2 |pages=97–100 |doi=10.1085/jgp.201210769 |pmid=22291143|pmc=3269787 }}</ref> [[Myosin XI]] is involved in [[cytoplasmic streaming]], wherein movement along [[microfilament]] networks in the cell allows [[organelle]]s and [[cytoplasm]] to stream in a particular direction.<ref>{{cite journal | vauthors = Hartman MA, Spudich JA | title = The myosin superfamily at a glance | journal = Journal of Cell Science | volume = 125 | issue = Pt 7 | pages = 1627–32 | date = April 2012 | pmid = 22566666 | pmc = 3346823 | doi = 10.1242/jcs.094300 }}</ref> Eighteen different classes of myosins are known.<ref name="Thompson">{{cite journal | vauthors = Thompson RF, Langford GM | title = Myosin superfamily evolutionary history | journal = The Anatomical Record | volume = 268 | issue = 3 | pages = 276–89 | date = November 2002 | pmid = 12382324 | doi = 10.1002/ar.10160 | s2cid = 635349 }}</ref>
Genomic representation of myosin motors:<ref name="Vale">{{cite journal | vauthors = Vale RD | title = The molecular motor toolbox for intracellular transport | journal = Cell | volume = 112 | issue = 4 | pages = 467–80 | date = February 2003 | pmid = 12600311 | doi = 10.1016/S0092-8674(03)00111-9 | doi-access = free }}</ref> * [[Fungus|Fungi]] ([[yeast]]): 5 * [[Plant]]s ([[Arabidopsis thaliana|Arabidopsis]]): 17 * [[Insect]]s ([[Drosophila]]): 13 * [[Mammal]]s ([[human]]): 40 * [[Chromadorea]] ([[Caenorhabditis elegans|nematode C. elegans]]): 15
=== Microtubule motors ===
==== Kinesin ==== [[Kinesin]]s are a [[Kinesin superfamily|superfamily]] of related motor proteins that use a [[microtubule]] track in '''anterograde''' movement. They are vital to spindle formation in mitotic and meiotic [[chromosome]] separation during cell division and are also responsible for shuttling [[mitochondria]], [[Golgi bodies]], and [[Vesicle (biology)|vesicles]] within [[eukaryotic cell]]s. Kinesins have two heavy chains and two light chains per active motor. The two globular head motor domains in heavy chains can convert the chemical energy of ATP hydrolysis into mechanical work to move along microtubules.<ref>{{cite journal | vauthors = Verhey KJ, Kaul N, Soppina V | title = Kinesin assembly and movement in cells | journal = Annual Review of Biophysics | volume = 40 | pages = 267–88 | date = 2011-01-01 | pmid = 21332353 | doi = 10.1146/annurev-biophys-042910-155310 }}</ref> The direction in which cargo is transported can be towards the plus-end or the minus-end, depending on the type of kinesin. In general, kinesins with N-terminal motor domains move their cargo towards the plus ends of microtubules located at the cell periphery, while kinesins with C-terminal motor domains move cargo towards the minus ends of microtubules located at the nucleus. Fourteen distinct kinesin families are known, with some additional kinesin-like proteins that cannot be classified into these families.<ref name="Miki">{{cite journal | vauthors = Miki H, Okada Y, Hirokawa N | title = Analysis of the kinesin superfamily: insights into structure and function | journal = Trends in Cell Biology | volume = 15 | issue = 9 | pages = 467–76 | date = September 2005 | 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 thaliana]]): 61 * [[Insect]]s ([[Drosophila melanogaster]]): 25 * [[Mammal]]s ([[human]]): 45
==== Dynein ==== [[Dynein]]s are microtubule motors capable of a '''retrograde''' sliding movement. Dynein complexes are much larger and more complex than kinesin and myosin motors. Dyneins are composed of two or three heavy chains and a large and variable number of associated light chains. Dyneins drive intracellular transport toward the minus end of microtubules which lies in the microtubule organizing center near the nucleus.<ref>{{cite journal | vauthors = Roberts AJ, Kon T, Knight PJ, Sutoh K, Burgess SA | title = Functions and mechanics of dynein motor proteins | journal = Nature Reviews. Molecular Cell Biology | volume = 14 | issue = 11 | pages = 713–26 | date = November 2013 | pmid = 24064538 | pmc = 3972880 | doi = 10.1038/nrm3667 }}</ref> The dynein family has two major branches. [[Axonemal dynein]]s facilitate the beating of [[cilia]] and [[flagella]] by rapid and efficient sliding movements of microtubules. Another branch is cytoplasmic dyneins which facilitate the transport of intracellular cargos. Compared to 15 types of axonemal dynein, only two [[cytoplasm]]ic forms are known.<ref name="Mallik">{{cite journal | vauthors = Mallik R, Gross SP | title = Molecular motors: strategies to get along | journal = Current Biology | volume = 14 | issue = 22 | pages = R971-82 | date = November 2004 | pmid = 15556858 | doi = 10.1016/j.cub.2004.10.046 | s2cid = 14240073 | doi-access = free | bibcode = 2004CBio...14.R971M }}</ref>
Genomic representation of dynein motors:<ref name="Vale"/> * [[Fungus|Fungi]] ([[yeast]]): 1 * [[Plant]]s ([[Arabidopsis thaliana]]): 0 * [[Insect]]s ([[Drosophila melanogaster]]): 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 | vauthors = Vanstraelen M, Inzé D, Geelen D | title = Mitosis-specific kinesins in Arabidopsis | journal = Trends in Plant Science | volume = 11 | issue = 4 | pages = 167–75 | date = April 2006 | pmid = 16530461 | doi = 10.1016/j.tplants.2006.02.004 | bibcode = 2006TPS....11..167V | url = https://biblio.ugent.be/publication/364298 | hdl = 1854/LU-364298 | hdl-access = free }}</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 | vauthors = Smith LG | title = Plant cytokinesis: motoring to the finish | journal = Current Biology | volume = 12 | issue = 6 | pages = R206-8 | date = March 2002 | pmid = 11909547 | doi = 10.1016/S0960-9822(02)00751-0 | doi-access = free | bibcode = 2002CBio...12.R206S }}</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 | vauthors = Abdel-Ghany SE, Day IS, Simmons MP, Kugrens P, Reddy AS | title = Origin and evolution of Kinesin-like calmodulin-binding protein | journal = Plant Physiology | volume = 138 | issue = 3 | pages = 1711–22 | date = July 2005 | pmid = 15951483 | pmc = 1176440 | doi = 10.1104/pp.105.060913 }}</ref>
== Other molecular motors == {{main article|Molecular motors}} Besides the motor proteins above, there are many more types of proteins capable of generating [[force]]s and [[torque]] in the cell. 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. The motor protein [[prestin]],<ref>{{cite journal | vauthors = Dallos P, Fakler B | title = Prestin, a new type of motor protein | journal = Nature Reviews. Molecular Cell Biology | volume = 3 | issue = 2 | pages = 104–11 | date = February 2002 | pmid = 11836512 | doi = 10.1038/nrm730 | s2cid = 7333228 }}</ref> expressed in mammalian cochlear outer hair cells, produces mechanical amplification in the cochlea. It is a direct voltage-to-force converter, which operates at the microsecond rate and possesses [[piezoelectric]] properties.
== See also == * [[ATP synthase]] * [[Cytoskeleton]] * [[Protein dynamics]]
== References == {{reflist}}
== External links == *{{usurped|1=[https://archive.today/20160615202251/http://www.mechanobio.info/modules/go-0003774#1_go-0003774 MBInfo - What are Motor Proteins?]}} *[https://www.ibiology.org/cell-biology/motor-proteins/ Ron Vale's Seminar: "Molecular Motor Proteins"] *[http://www.motorprotein.de/research.html Biology of Motor Proteins ][[Institute]] for [[Biophysical Chemistry]], [[Göttingen]] * Jonathan Howard (2001), Mechanics of motor proteins and the cytoskeleton. {{ISBN|9780878933334}}
[[Category:Cell movement]] [[Category:Motor proteins| ]] [[Category:Molecular machines]]