YEAST MYOSIN-II - A NEW SUBCLASS OF UNCONVENTIONAL CONVENTIONAL MYOSINS

Myosin II is the founder member of a large and structurally diverse da n of actin-based motor proteins. The native myosin II molecule is a he xamer consisting of two heavy chains, two essential light chains (ELC) , and two regulatory light chains (RLC). For convenience, the myosin I Is are often subdivided into four subclasses: vertebrate skeletal and cardiac muscle myosin II form one subclass, vertebrate smooth muscle a nd nonmuscle myosin IT a second, invertebrate muscle a third, and prot ozoan myosin II a fourth [Sellers and Goodson, 1995]. Different mechan isms of regulation may exist between myosins within a single subclass yet all myosin IIs share a common three-domain structure; the N-termin us of the heavy chain forms two globular heads that contain the ATP-an d actin-binding sites and the ct-helical neck region that is stabilise d by the binding of the two classes of light chains, whilst the C-term inus forms an extended coiled-coil tail that can consist of anywhere b etween 700 and 1,200 amino acids. In nonmuscle cells, myosin II has at least two well-defined functions, cell locomotion and cytokinesis. Ye ast cells do not locomote, and their mechanism of cytokinesis involves the deposition of a cross-wall or septum. However, in the fission yea st, 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 prese nt at the bud neck during cytokinesis in the budding yeast, Saccharomy ces cerevisiae [Kilmartin and Adams, 1984]. Here we report a phylogene tic analysis of the N-terminal head domains of the myosin IIs from bot h yeasts, a structural analysis of the tail domains of these proteins and we speculate as to the nature of the light chains that regulate th eir function. On the basis of these findings, we propose that the yeas t myosin IIs constitute a divergent fifth class of ''unconventional'' conventional myosins. (C) 1998 Wiley-Liss, Inc.