Motor proteins (not to be confused with the movement proteins of some plant viruses ) are one of five functional groups of the cytoskeletal proteins in addition to the filamentary scaffold proteins , bridging proteins , limiting proteins and regulating proteins. They are pronounced allosteric proteins that primarily serve to generate movement while consuming ATP , but sometimes also take on regulatory tasks. Mainly with these movements biological loads ( vesicles , cell organelles, etc.) are transported or cytoskeletal elements are shifted against each other.
Motor proteins always consist of a so-called head (or motor) domain and a tail domain.
This is how the protein binds to the cytoskeletal element. The motor domain also contains the ATP binding site and the structures that move the molecule when it changes its conformation. In the individual families of motor proteins, the motor domains are very uniform and highly conserved.
The binding points for the load to be transported are located here. The structure of the tail domain also determines whether several proteins can assemble into larger complexes. The structure of the tail domain is very variable.
Motor protein classes
- Kinesin : binds to microtubules . Mostly forms dimers. The direction of movement of kinesin is mostly from the minus to the plus end of the microtubule. It is primarily used to transport cell organelles and vesicles. For example, vesicles with neurotransmitters are transported by means of kinesin in the nerve cells from the cell nucleus via the axon to the synapse. So in most cases it is a matter of transport from the cell nucleus to the cell membrane. Kinesin also has important regulatory functions in cell division.
- Dynein : Dyneins mostly form dimers and bind to microtubules. They mostly move from the plus to the minus end of a microtubule (from the cell membrane to the cell nucleus). There are special types of dynein found in the axonema of the cilia and flagella .
- Myosin and tropomyosin : Myosins bind to actin filaments and can also transport vesicles or similar here as dimers. Their direction of movement is mostly from the minus to the plus end of an actin filament (exception: myosin VI). Myosins also serve to move cytoskeletal elements against each other. You therefore take over z. B. Functions in cell adhesion, endo- and exocytosis, cell locomotion through crawling, general deformation of cells (e.g. muscle contraction) etc. Myosin is a motor protein that occurs only in eukaryotes , but always here. Tropomyosin is similar to myosin.
- Prestin: A very fast moving motor protein found in the outer hair cells of the inner ear.
Immunolocation shows that prestin is found in the lateral plasma membranes of the outer hair cells. This is the area in which electromotility occurs. Prestin (molar mass: 80 kDa ) belongs to the family of anion transporters, SLC26. The proteins in this family are structurally well preserved and can mediate the electroneutral exchange of chlorides and carbonates across the plasma membrane of mammalian cells. Two anions prove to be important for the motility of the outer hair cells. In contrast to the enzymatically operated motor proteins, the direct conversion of electrical voltages causes a mechanical shift in prestin. In this way, motility is achieved which is orders of magnitude faster than cellular motor proteins. In order to be able to develop this effect, the protein is controlled directly by the sensory elements of the outer hair cells via a short route.
Structural analyzes have shown that myosin and kinesin have a very similar core in the motor domain in which the ATP binding site is located and the protein's conformational change begins. It is therefore assumed that myosin and kinesin have a common evolutionary origin, they form a protein family .
Although kinesin and dynein use microtubules as the rail system, no common origin has been identified.
There is also a functional relationship to the G proteins which undergo a conformational change through consumption of GTP , but use this conformational change primarily for signal transmission.
- ^ J. Menetrey et al .: The structure of the myosin VI motor reveals the mechanism of directionality reversal. Nature 435/7043/ 2005 . Pp. 779-785, PMID 15944696 .
- Cymobase - A database for cytoskeletal and motor protein sequence information