Roles of a fimbrin and an alpha-actinin-like protein in fission yeast cellpolarization and cytokinesis

Eukaryotic cells contain many actin-interacting proteins, including the alp ha -actinins and the fimbrins, both of which have actin cross-linking activ ity in vitro. We report here the identification and characterization of bot h an a-actinin-like protein (Ain1p) and a fimbrin (Fim1p) in the fission ye ast Schizosaccharomyces pombe. Ain1p localizes to the actomyosin-containing medial ring in an F-actin-dependent manner, and the Ain1p ring contracts d uring cytokinesis. ain1 deletion cells have no obvious defects under normal growth conditions but display severe cytokinesis defects, associated with defects in medial-ring and septum formation, under certain stress condition s. Overexpression of Ain1p also causes cytokinesis defects, and the ain1 de letion shows synthetic effects with other mutations known to affect medial- ring positioning and/or organization. Fim1p localizes both to the cortical actin patches and to the medial ring in an F-actin-dependent manner, and se veral lines of evidence suggest that Fim1p is involved in polarization of t he actin cytoskeleton. Although a fim1 deletion strain has no detectable de fect in cytokinesis, overexpression of Fim1p causes a lethal cytokinesis de fect associated with a failure to form the medial ring and concentrate acti n patches at the cell middle. Moreover, an ain1 fim1 double mutant has a sy nthetical-lethal defect in medial-ring assembly and cell division. Thus, Ai n1p and Fim1p appear to have an overlapping and essential function in fissi on yeast cytokinesis. In addition, protein-localization and mutant-phenotyp e data suggest that Fim1p, but not Ain1p, plays important roles in mating a nd in spore formation.