Diploid yeast cells repeatedly polarize and bud from their poles, probably because of highly stable marks of unknown composition. Here, Rax2, a membrane protein, was shown to behave as such a mark. The Rax2 protein itself was inherited immutably at the cell cortex for multiple generations, and Rax2 was shown to have a half-life exceeding several generations. The persistent inheritance of cortical protein markers would provide a means to couple a cell's history to the future development of a precise morphogenetic form.

In the budding yeast, Saccharomyces cerevisiae, protein kinases Ste20p (p21 Cdc42p͞Rac -activated kinase), Ste11p [mitogen-activated protein kinase (MAPK) kinase kinase], Ste7p (MAPK kinase), Fus3p, and Kss1p (MAPKs) are utilized for haploid mating, invasive growth, and diploid filamentous growth. Members of the highly conserved Ste20p͞ p65 PAK protein kinase family regulate MAPK signal transduction pathways from yeast to man. We describe here a potent negative regulator of Ste20p in the yeast filamentous growth-signaling pathway. We identified a mutant, hsl7, that exhibits filamentous growth on rich medium. Hsl7p belongs to a highly conserved protein family in eukaryotes. Hsl7p associates with the noncatalytic region within the amino-terminal half of Ste20p as well as Cdc42p. Deletions of HSL7 in haploid and diploid strains led to cell elongation and enhancement of both haploid invasive growth and diploid pseudohyphal growth. However, deletions of STE20 in haploid and diploid greatly diminished these hsl7-associated phenotypes. In addition, overexpression of HSL7 inhibited pseudohyphal growth. Thus, Hsl7p may inhibit the activity of Ste20p in the S. cerevisiae filamentous growth-signaling pathway. Our genetic analyses suggest the possibility that Cdc42p and Hsl7p compete for binding to Ste20p for pseudohyphal development when starved for nitrogen.

Bud-site selection in yeast offers an attractive system for studying cell polarity and asymmetric division. Haploids divide in an axial pattern, whereas diploids divide in a bipolar pattern. AXL1 is expressed in haploids but not diploids, and ectopic expression of AXL1 in diploids converts their bipolar budding pattern to an axial pattern. How Axl1 acts as a switch between the bipolar and axial patterns is not understood. Here we report that Axl1 localizes to the mother-bud neck and division site remnants of haploids. Axl1 is absent from diploids. Axl1 colocalizes with Bud3, Bud4, and Bud10, components of the axial landmark structure. This localization suggests that Axl1 couples the axial landmark with downstream polarity establishment factors. Consistent with such a role, Axl1 associated biochemically with Bud4 and Bud5. Genetic evidence suggests that Axl1 works with Bud3 and Bud4 to promote the activity of the Bud10 membrane protein. Given Axl1's suggested role in morphogenesis and cell fusion during mating, we also examined its localization during this process. Axl1 redistributes independently of the axial landmark to a tight cell surface dot at the tip of each mating projection. These dots are rapidly lost as prezygotes form.