The Budding Yeast PCH/F‑BAR Proteins
Alan L. Munn and Barbara Winsor
The budding yeast Saccharomyces cerevisiae genome encodes two classical Pombe Cdc15 Homology (PCH) proteins: Hof1p (or Cyk2p) and Bzz1p (or Lsb7p). Like mammalian PCH proteins, both have an N‑terminal F‑BAR domain and C‑terminal Src Homology 3 (SH3) domain(s). The yeast genome also encodes two proteins that possess N‑terminal F‑BAR domains but have a C‑terminal Rho GTPase activating protein (GAP) domain instead of an SH3 domain: Rgd1p and Rgd2p. Hof1p level is regulated during the cell cycle with peak expression at late anaphase/telophase. Hof1p localizes to rings at the site of cytokinesis. Two pathways of cytokinesis in S. cerevisiae differ in their dependency on the contractile actomyosin ring. Loss of Hof1p appears to block the actomyosin‑ring‑independent pathway of cytokinesis. The SH3 domain of Hof1p is dispensable for localization and cytokinesis, but negatively regulates Hof1p localization and both cytokinesis pathways, apparently as part of an important regulatory switch. Bzz1p is found throughout the cell cycle and localizes to dynamic cortical patches that contain actin filaments at sites of polarized surface growth. Loss of Bzz1p has no obvious phenotype, but Bzz1p deficiencies when combined with other mutations and in vitro polymerization assays, show a role for Bzz1p through its SH3 domains in actin filament assembly and endocytosis. Rgd1p and Rgd2p are GAPs for Rho family GTPases that negatively regulate different aspects of polarized cell growth including the actin cytoskeleton, directed secretion and cell wall remodelling. Whether the F‑BAR domains of these proteins bind and bend membranes like those of mammalian PCH proteins has not yet been explored.