Motor protein

Overview
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 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 proteins and vesicles in the cytoplasm. Kinesins and dyneins play essential roles in intracellular transport such as axonal transport and in the formation of the spindle apparatus and the separation of the chromosomes 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 diseases. Dynein deficiencies can lead to chronic infections of the respiratory tract as cilia fail to function without dynein. Defects in muscular myosin predictably cause myopathies, whereas defects in unconventional myosin are the cause for Usher syndrome and deafness.

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 chromosomes during anaphase of mitosis (kinesin) or 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 conformation depending on ATP hydrolysis. The tail end of the molecule normally binds adaptor proteins that allow for stable interactions with the cargo to be moved along the substrate. These motor proteins typically form a complex of longer "heavy chains" with motor head domains and shorter "light chains" 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 microfilaments through interaction with actin. Microtubule motors such as dynein and kinesin move along microtubules 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.

Myosin
Myosins are actin motors and form myosin complexes consisting of two heavy chains with motor heads and two light chains. Derived from the 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 organelles and cytoplasm to stream in a particular direction. Eighteen different classes of myosins are known.

Genomic representation of myosin motors:
 * Fungi (yeast): 5
 * Plants (Arabidopsis): 17
 * Insects (Drosophila): 13
 * Mammals (human): 40

Kinesin
Kinesins are a group of related motor proteins that use a microtubule track along which to "walk." They are vital to movement of chromosomes during mitosis and are also responsible for shuttling mitochondria, Golgi bodies, and vesicles within eukaryotic cells. Kinesins typically contain two heavy chains with motor heads which move along microtubules via a pseudo-processive asymmetric walking motion, 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.

Genomic representation of kinesin motors:
 * Fungi (yeast): 6
 * Plants (Arabidopsis): 61
 * Insects (Drosophila): 25
 * Mammals (human): 45

Dynein
Dyneins 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 cytoplasmic forms are known.

Genomic representation of dynein motors:
 * Fungi (yeast): 1
 * Plants (Arabidopsis): 0
 * Insects (Drosophila): 13
 * Mammals (human): 14-15

Plant-specific motors
In contrast to animals, fungi and non-vascular plants, the cells of flowering plants 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. 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.

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.

Other molecular motors
Besides the motor proteins above, there are many more types of proteins capable of generating forces and torque in the cell. Among the processes regulated by force-generating proteins are:
 * transcription of RNA
 * DNA replication
 * chromatin remodeling
 * chromosome condensation by SMC proteins
 * Helicases

Many of the molecular motors that regulate these processes are ubiquitous in both prokaryotic and eukaryotic cells, although some, like those involved with cytoskeletal elements or chromatin, are unique to eukaryotes.