A Role for a Protease in Morphogenic Responses during Yeast Cell Fusion

Abstract

Cell fusion during yeast mating provides a model for signaling-controlled changes at the cell surface. We identified the AXL1 gene in a screen for genes required for cell fusion in both mating types during mating. AXL1 is a pheromone-inducible gene required for axial bud site selection in haploid yeast and for proteolytic maturation of a-factor. Two other bud site selection genes, RSR1, encoding a small GTPase, and BUD3, were also required for efficient cell fusion. Based on double mutant analysis, AXL1 in a MATα strain acted genetically in the same pathway with FUS2, a fusion-dedicated gene. Electron microscopy of axl1, rsr1, and fus2 prezygotes revealed similar defects in nuclear migration, vesicle accumulation, cell wall degradation, and membrane fusion during cell fusion. The axl1 and rsr1 mutants exhibited defects in pheromone-induced morphogenesis. AXL1 protease function was required in MATα strains for fusion during mating. The ability of the Rsr1p GTPase to cycle was required for efficient cell fusion, as it is for bud site selection. During conjugation, vegetative functions may be redeployed under the control of pheromone signaling for mating purposes. Since Rsr1p has been reported to physically associate with Cdc24p and Bem1p components of the pheromone response pathway, we suggest that the bud site selection genes Rsr1p and Axl1p may act to mediate pheromone control of Fus2p-based fusion events during mating.

Specific Protein Targeting during Cell Differentiation: Polarized Localization of Fus1p during Mating Depends on Chs5p in Saccharomyces cerevisiae

“…It has been shown by electron microscopy that there are secretory vesicles implicated in the cell fusion process during mating in yeast (3,6). We propose that Chs5p could be specifically required for the incorporation of Fus1p into these vesicles for its transport to the cell surface.…”

Characterization of the Yeast Amphiphysins Rvs161p and Rvs167p Reveals Roles for the Rvs Heterodimer In Vivo

“…PRM9 encodes a pheromone-regulated membrane protein (Heiman and Walter, 2000), and its mutant phenotype has not yet been characterized. AXL1 encodes a protease required for cell fusion during mating (Elia and Marsh, 1998) and is required for axial budding (Fujita et al, 1994). FIG2 is involved in agglutination during mating .…”

The Exchange Factor Cdc24 Is Required for Cell Fusion during Yeast Mating

“…Furthermore, endocytosis mutants, such as end4, are not defective for cell fusion (6). Vesicle accumulation proximal to the zone of cell fusion has been observed in prezygotes by electron microscopy (5,9,18,22), yet the origin (exocytic or endocytic) of these vesicles is unknown. Another possibility is that Cdc24p regulates the mobility of cell fusion proteins either directly or via Cdc42p.…”

“…As these proteins are all important in a-factor biogenesis and production, it is likely that high levels of mating pheromone are important for cell fusion. It also appears that Axl1p and Ste6p play a role in cell fusion that is independent of their function in a-factor biogenesis and secretion, respectively (18,19). The last class of proteins includes Rvs161p, Fps1p, Spa2p, Pea2p, Bni1p, Chs5p, Kel1p, Bud1p (Rsr1p), and Bud3p, which are also important in processes such as morphogenesis, polarity, actin organization, bud site selection, osmotic balance, or cell wall integrity (6,15,18,22,40,41,45,56).…”

“…It is likely that some of these proteins make up the fusion machinery. The second class includes Ram1p, Axl1p, and Ste6p; mutations in any of the corresponding genes result in a-cell-specific fusion defects (5,18,19). As these proteins are all important in a-factor biogenesis and production, it is likely that high levels of mating pheromone are important for cell fusion.…”