Bni1p and Bnr1p: downstream targets of the Rho family small G-proteins which interact with profilin and regulate actin cytoskeleton in Saccharomyces cerevisiae

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Abstract
The RHO1 gene encodes a homologue of mammalian RhoA small G‐protein in the yeast Saccharomyces cerevisiae. Rho1p is required for bud formation and is localized at a bud tip or a cytokinesis site. We have recently shown that Bni1p is a potential target of Rho1p. Bni1p shares the FH1 and FH2 domains with proteins involved in cytokinesis or establishment of cell polarity. In S.cerevisiae, there is an open reading frame (YIL159W) which encodes another protein having the FH1 and FH2 domains and we have named this gene BNR1 (BNI1Related). Bnr1p interacts with another Rho family member, Rho4p, but not with Rho1p. Disruption of BNI1 or BNR1 does not show any deleterious effect on cell growth, but the bni1 bnr1 mutant shows a severe temperature‐sensitive growth phenotype. Cells of the bni1 bnr1 mutant arrested at the restrictive temperature are deficient in bud emergence, exhibit a random distribution of cortical actin patches and often become multinucleate. These phenotypes are similar to those of the mutant of PFY1, which encodes profilin, an actin‐binding protein. Moreover, yeast two‐hybrid and biochemical studies demonstrate that Bni1p and Bnr1p interact directly with profilin at the FH1 domains. These results indicate that Bni1p and Bnr1p are potential targets of the Rho family members, interact with profilin and regulate the reorganization of actin cytoskeleton.