Yeast myosin II: a new subclass of unconventional conventional myosins?

Myosin II is the founder member of a large and structurally diverse clan of actin-based motor proteins. The native myosin II molecule is a hexamer consisting of two heavy chains, two essential light chains (ELC), and two regulatory light chains (RLC). For convenience, the myosin IIs are often subdivided into four subclasses: vertebrate skeletal and cardiac muscle myosin II form one subclass, vertebrate smooth muscle and nonmuscle myosin II a second, invertebrate muscle a third, and protozoan myosin II a fourth [Sellers and Goodson, 1995]. Different mechanisms of regulation may exist between myosins within a single subclass yet all myosin IIs share a common three-domain structure; the N-terminus of the heavy chain forms two globular heads that contain the ATP- and actin-binding sites and the alpha-helical neck region that is stabilised by the binding of the two classes of light chains, whilst the C-terminus forms an extended coiled-coil tail that can consist of anywhere between 700 and 1,200 amino acids. In nonmuscle cells, myosin II has at least two well-defined functions, cell locomotion and cytokinesis. Yeast cells do not locomote, and their mechanism of cytokinesis involves the deposition of a cross-wall or septum. However, in the fission yeast, Schizosaccharomyces pombe, deposition of the septum is anticipated by the appearance of a contractile actomyosin ring [Marks and Hyams, 1985; May et al., 1997; Kitayama et al., 1997] and actin is also present at the bud neck during cytokinesis in the budding yeast, Saccharomyces cerevisiae [Kilmartin and Adams, 1984]. Here we report a phylogenetic analysis of the N-terminal head domains of the myosin IIs from both yeasts, a structural analysis of the tail domains of these proteins and we speculate as to the nature of the light chains that regulate their function. On the basis of these findings, we propose that the yeast myosin IIs constitute a divergent fifth class of "unconventional" conventional myosins.