myosins

Myosins are a large family of ATP-dependent motor proteins that interact with actin filaments to generate force and movement within cells. They convert chemical energy from ATP hydrolysis into mechanical work through conformational changes in their motor domains, typically moving toward the plus end of actin. An exception is myosin VI, which moves toward the minus end.

Most myosins share a head motor domain that binds actin and hydrolyzes ATP, a neck region that binds light chains such as calmodulin, and a tail domain that determines dimerization and cargo binding. The motor is regulated by light-chain binding, phosphorylation, calcium signals in muscle cells, and cargo interactions. In muscle, calcium-triggered regulation via the troponin-tropomyosin complex controls access to actin binding sites.

Classification and roles vary widely. Conventional myosin II forms thick filaments and drives muscle contraction as a dimer with two heads, while nonmuscle myosins and other tissue-specific forms function in intracellular transport and force generation for processes such as cytokinesis, cell migration, and organelle positioning. Myosin I proteins are typically single-headed motors associated with membranes and cortical actin; myosin V and myosin VI are processive motors that transport vesicles and organelles; myosin VII participates in organelle transport and sensory cell function. Other classes contribute to specialized cellular tasks across tissues and organisms, reflecting broad functional diversity.

Myosins are found throughout eukaryotes, with humans harboring numerous myosin genes and roughly 40 identified classes. Mutations in specific myosins are linked to diseases, including Usher syndrome associated with myosin VIIA. Research on myosins informs understanding of muscle physiology, cell motility, and intracellular trafficking.