MYO1 encodes the only class II myosin heavy chain in budding yeast (2). As part of the actomyosin ring, Myo1p plays a critical role in cytokinesis (3). Early in the cell cycle, Myo1p localizes to a ring at the presumptive bud site and remains at the mother-bud neck until cytokinesis is completed (5, 1). Formation, but not maintenance, of the contractile ring requires the intact septin collar at the bud neck (The septins are encoded by CDC3, CDC10, CDC11, and CDC12) (5, 1, 6). Late in anaphase, F-actin (Act1p) and the IQGAP-related protein, Iqg1p, also accumulate in the neck ring (5). Incorporation of F-actin into the neck ring depends on Myo1p. At the end of anaphase the actinomyosin ring exhibits an F-actin-dependent decrease in size, presumably due to contraction (1).

There is controversy in the literature over whether MYO1 is an essential gene (5, 1). A myo1 mutant is inviable in W303, displaying a terminal phenotype of chains of large round cells, indicative of a cytokinesis defect (5). However, in other strains, myo1 mutants are viable and able to undergo a form of cell division (1). Genetic analysis suggests that at least one of these strains harbors an extragenic suppressor that may function to upregulate distinct pathways to partition the cytoplasm. For example, the suppressor may promote the insertion of membrane and/or septum formation at the division site (3). In fact, both of these processes are directly coupled to cytokinesis (7, 4, 8).

The catalytic domain of Myo1p is located in the N-terminus while the C-terminus contains a coiled-coiled domain interrupted by a hinge region (9). Remarkably, the non-catalytic tail domain can be recruited to the site of division and is sufficient for constriction and cytokinesis (10). Between the head and tail are two IQ domains. Class II myosins are regulated by two light chains, an essential light chain (ELC) and a regulatory light chain (RLC) that interact with the myosin through the IQ motifs (9). Mlc1p is the ELC for Myo1p and interacts with the IQ1 motif. However, binding of Mlc1p to Myo1p does not appear to play a major role in regulating Myo1p, but instead regulates actin ring formation and targeted secretion through interactions with Myo1p, Iqg1p, and Myo2p. Mlc2p is the RLC for Myo1p and interacts with the IQ2 motif. Mlc2p most likely plays a role in the disassembly of the Myo1p ring in vivo (4).

Class II myosins are found in many other organisms, and include both muscle myosins and nonmuscle myosins (11). Nonmuscle class II myosins have been implicated in cytokinesis in several organisms including Drosophila, C. elegans, and S. pombe (12). MYO1 is similar to human MYH11, a smooth muscle myosin heavy chain (OMIM). In some people, mutations in MYH11 give rise to a leukemia or familial aortic aneurysm.

Last updated: 2006-09-20