FH3, A DOMAIN FOUND IN FORMINS, TARGETS THE FISSION YEAST FORMIN FUS1TO THE PROJECTION TIP DURING CONJUGATION

Formins are involved in diverse aspects of morphogenesis, and share tw o regions of homology: FH1 and FH2. We describe a new formin homology region, FH3. FH3 is an amino-terminal domain that differs from the Rho binding site identified in Bni1p and p140mDia. The Schizosaccharomyce s pombe formin Fus1 is required for conjugation, and is localized to t he projection tip in cells of mating pairs. We replaced genomic fus1() with green fluorescent protein (GFP) tagged versions that lacked eit her the FH1, FH2, or FH3 domain. Deletion of any FH domain essentially abolished mating. FH3, but neither FH1 nor FH2, was required for Fus1 localization. An FH3 domain-GFP fusion protein localized to the proje ction tips of mating pairs. Thus, the FH3 domain alone can direct prot ein localization. The FH3 domains of both Fus1 and the S. pombe cytoki nesis formin Cdc12 were able to localize GFP to the spindle pole body in half of the late G2 cells in a vegetatively growing population. Exp ression of both FH3-GFP fusions also affected cytokinesis. Overexpress ion of the spindle pole body component Sad1 altered the distribution o f both Sad1 and the FH3-GFP domain. Together these data suggest that p roteins at multiple sites can interact with FH3 domains.