Requirement of yeast fimbrin for actin organization and morphogenesis in vivo

THE SAC6 gene was found by suppression of a yeast actin mutation¹. Its protein product, Sac6p (previously referred to as ABP67), was independently isolated by actin-filament affinity chromatogra-phy and colocalizes with actin in vivo². Thus Sac6p binds to actin in vitro, and functionally associates with actin structures involved in the development and maintenance of cell polarity in vivo^2,3. We report here that Sac6p is an actin-filament bundling protein 43% identical in amino-acid sequence to the vertebrate bundling protein fimbrin⁴. This yeast fimbrin homologue contains two putative actin-binding regions⁵homologous to domains of dystrophin, β-spectrin, filamin, actin-gelation protein and α-actinin. Mutants lacking Sac6p do not form normal actin structures and are defective in morphogenesis. These findings demonstrate an in vivo role for the well-documented biochemical interaction between fimbrin and actin.