Mother Cell–Specific HO Expression in Budding Yeast Depends on the Unconventional Myosin Myo4p and Other Cytoplasmic Proteins

Abstract

Certain cell types give rise to progeny that adopt different patterns of gene expression in the absence of any differences in their environment. Cells of budding yeast give birth to mother and daughter cells that differ in that only mother cells express the HO endonuclease gene and thereby switch mating types. We describe the identification of five genes, called SHE1-SHE5, that encode cytoplasmic proteins required for mother-specific HO expression. She1p, which is identical to the minimyosin Myo4p, and She3p are not, however, mother-specific proteins. On the contrary, they accumulate in growing buds. She proteins might be required for the transport of factors that promote HO repression from the mother cell into its bud. In an accompanying paper, we show that SHE genes are needed for the accumulation in daughter nuclei of Ash1p, a repressor of HO.

Stable and Dynamic Axes of Polarity Use Distinct Formin Isoforms in Budding Yeast

“…Both formins promote actin cable assembly Sagot et al, 2002a), but the loss of Bnr1p causes only a mild delay in cell separation (Vallen et al, 2000), whereas the absence of Bni1p causes a widened bud neck (Jansen et al, 1996;Zahner et al, 1996;Mosch and Fink, 1997;Sheu et al, 2000), an inability to engage in tip-directed growth (Evangelista et al, 1997;Mosch and Fink, 1997;Sheu et al, 2000;Ozaki-Kuroda et al, 2001), a defect in bipolar bud site selection (Zahner et al, 1996), a partial defect in cytokinetic ring contraction (Vallen et al, 2000), a defect in early spindle alignment Lee et al, 1999), and a variety of synthetic lethal genetic interactions (Kohno et al, 1996;Longtine et al, 1996;Fujiwara et al, 1998;Fujiwara et al, 1999;Lee et al, 1999;Tong et al, 2001Tong et al, , 2004. We suggest that many, if not all, of these can be attributed to the actin assembly activity of the two formins in conjunction with their specific localizations.…”

“…Presumably, the septins, which are polarized in both wild-type and bni1⌬ unbudded cells ( Figure S2A and B), provide the cue by which Bnr1p is weakly polarized in unbudded cells, because yeast lacking both Bni1p function and septin function are unable to initiate a normal bud ( Figure 6A). The guidance of bud emergence by this disorganized mesh of Bnr1p-dependent cables may provide an explanation for the widened neck of bni1⌬ cells (Jansen et al, 1996;Zahner et al, 1996;Mosch and Fink, 1997;Sheu et al, 2000), a feature that also may indirectly explain the cytokinetic ring contraction defects (Vallen et al, 2000), and minor defects seen in the septin ring of bni1⌬ yeast ( Figure S2A, middle bni1⌬/bni1⌬ cell).…”

“…This provides an explanation for the inability of bni1⌬ yeast to undergo tip-directed growth (Evangelista et al, 1997;Mosch and Fink, 1997;Sheu et al, 2000;OzakiKuroda et al, 2001) or to bud in a bipolar manner (Zahner et al, 1996), which in turn requires tip-directed growth (Sheu et al, 2000). Other myosin-V-dependent cargoes also lose their tip-association in bni1⌬ cells, including the microtubule anchoring protein Kar9p (Miller et al, 1999), cytoplasmic microtubules (Segal et al, 2000), and the ASH1 mRNA (Takizawa et al, 1997;Beach et al, 1999), explaining the inability of bni1⌬ yeast to orient the mitotic spindle early in the cell cycle Lee et al, 1999) or to properly restrict Ash1p-mediated cell differentiation to daughter cells (Jansen et al, 1996).…”

“…As has been seen for similar Bni1p-derived constructs, this construct stimulated the polymerization of actin from monomers, whereas GST did not. Thus, the conserved COOH-terminal regions of the two yeast formins stimulate actin assembly.Based on studies of overexpressed Bni1p and Bnr1p, the NH 2 termini of these formins help confer distinct localizations on the two (Jansen et al, 1996;Evangelista et al, 1997;Fujiwara et al, 1998;Kamei et al, 1998;Ozaki-Kuroda et al, 2001). We wished to examine whether these localizations impart differences in the distribution of the actin filaments assembled by the two isoforms.…”

Isolation and characterization of a novel actin filament-binding protein from Saccharomyces cerevisiae

“…Glucan synthase is involved in cell wall synthesis (Shematek and Cabib, 1980). Bni1p is known to be implicated in cytokinesis and establishment of cell polarity (Jansen et al, 1996;Zahner et al, 1996). We have moreover found that Bni1p directly interacts with pro®lin, an actin monomer-binding protein, which accelerates the polymerization of actin (Imamura et al, 1997).…”