Unconventional myosins.

Myosins constitute a large superfamily of F-actin-based motor proteins found in many organisms from yeast to humans. A phylogenetic comparison of their head sequences has allowed them to be grouped into 15 different classes. Unconventional myosins can be monomeric or dimeric, but are thought not to form filaments, unlike conventional myosin. The double-headed class-V myosins are good candidates for transporting vesicles, organelles and (mRNA) particles along actin filaments. Class-I myosins are involved in membrane dynamics and actin organization at the cell cortex, thus affecting cell migration, endocytosis, pinocytosis and phagocytosis. A class-III myosin from Drosophila is required for phototransduction and maintenance of the rhabdomere. Class-IX myosins negatively regulate the small G-protein Rho, a signalling molecule that regulates the organization of the actin cytoskeleton. Protein kinases that are regulated by members of the Rho small G-protein family regulate the motor activities of different myosins.