An FH domain-containing Bnr1p is a multifunctional protein interacting with a variety of cytoskeletal proteins in Saccharomyces cerevisiae

Proteins containing formin homology domains, FH1 and FH2, are involved in c ytokinesis or establishment of cell polarity in a variety of organisms. Bni 1p and Bnr1p are FPF proteins and potential targets of the Rho family small GTP-binding proteins in S. cerevisiae. We have shown that Bnr1p is localiz ed at the bud neck to interact with Hof1p, involved in cytokinesis. We repo rt here that the overexpression of BNR1 causes a cytokinesis deficiency whi ch is similar to the phenotypes of the septin mutants, including cdc3, cdc1 0, cdc11, and cdc12. The region required for the septin mutant phenotypes w as mapped to Bnr1p (35-500), which coincided,vith the region required for t he bud-neck localization. To further isolate a gene interacting with BNI1 o r BNR1, a multicopy suppressor of the bni1 bnr1 mutant was isolated. This g ene encoded Smy1p, a kinesin-related protein. Bnr1p, but not Bni1p, directl y interacted with the C-terminal region of Smy1p. The Smy1p-interacting reg ion of Bnr1p was mapped to a region containing the FH2 domain. Bnr1p also d irectly interacted with Bud6p, a novel actin-binding protein. Bnr1p is thus a multifunctional protein which interacts with the septin system, a microt ubule-dependent motor protein, and the actin system, to regulate cytoskelet al functions in S. cerevisiae.