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 vivo2,3. 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.
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