Motor proteins are a class ofmolecular motors that can move along thecytoskeleton of cells. They do this by converting chemical energy into mechanical work by thehydrolysis ofATP.[1]
Motor proteins are the driving force behind mostactive transport ofproteins andvesicles in thecytoplasm.Kinesins andcytoplasmic dyneins play essential roles in intracellular transport such asaxonal transport and in the formation of thespindle apparatus and the separation of thechromosomes duringmitosis andmeiosis.Axonemal dynein, found incilia andflagella, is crucial tocell motility, for example inspermatozoa, and fluid transport, for example in trachea. The muscle protein myosin "motors" the contraction of muscle fibers in animals.
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 forCharcot-Marie-Tooth disease and somekidney diseases. Dynein deficiencies can lead tochronicinfections of therespiratory tract ascilia fail to function without dynein. Numerous myosin deficiencies are related to disease states and genetic syndromes. Becausemyosin 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 toUsher syndrome and non-syndromicdeafness.[2]
Motor proteins utilizing thecytoskeleton for movement fall into two categories based on theirsubstrate:microfilaments ormicrotubules.Actin-based motor proteins (myosin) move alongmicrofilaments through interaction withactin, andmicrotubule motors (dynein andkinesin) move alongmicrotubules through interaction withtubulin.
There are two basic types ofmicrotubule motors: plus-end motors and minus-end motors, depending on the direction in which they "walk" along themicrotubule cables within the cell.
Myosins are asuperfamily ofactin 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 generatingmuscle 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 ofcell 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.[3][4] Myosin XI is involved incytoplasmic streaming, wherein movement alongmicrofilament networks in the cell allowsorganelles andcytoplasm to stream in a particular direction.[5] Eighteen different classes of myosins are known.[6]
Genomic representation of myosin motors:[7]
Kinesins are asuperfamily of related motor proteins that use amicrotubule track inanterograde movement. They are vital to spindle formation in mitotic and meioticchromosome separation during cell division and are also responsible for shuttlingmitochondria,Golgi bodies, andvesicles withineukaryotic cells. 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.[8] 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.[9]
Genomic representation of kinesin motors:[7]
Dyneins are microtubule motors capable of aretrograde 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.[10] The dynein family has two major branches.Axonemal dyneins facilitate the beating ofcilia andflagella 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 twocytoplasmic forms are known.[11]
Genomic representation of dynein motors:[7]
In contrast toanimals,fungi andnon-vascular plants, the cells offlowering plants lack dynein motors. However, they contain a larger number of different kinesins. Many of these plant-specific kinesin groups are specialized for functions duringplant cellmitosis.[12] Plant cells differ from animal cells in that they have acell wall. During mitosis, the new cell wall is built by the formation of acell plate starting in the center of the cell. This process is facilitated by aphragmoplast, a microtubule array unique to plant cell mitosis. The building of cell plate and ultimately the new cell wall requires kinesin-like motor proteins.[13]
Another motor protein essential for plant cell division iskinesin-like calmodulin-binding protein (KCBP), which is unique to plants and part kinesin and part myosin.[14]
Besides the motor proteins above, there are many more types of proteins capable of generatingforces andtorque in the cell. Many of these molecular motors are ubiquitous in bothprokaryotic andeukaryotic cells, although some, such as those involved withcytoskeletal elements orchromatin, are unique to eukaryotes. The motor proteinprestin,[15] 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 possessespiezoelectric properties.