Myosins are a 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. Myosin XI is 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
* Chromadorea ( nematode C. elegans): 15