Diploid Saccharomyces cerevisiae strains starved for nitrogen undergo a developmental transition from a colonial form of growth to a filamentous pseudohyphal form. This dimorphism requires a polar budding pattern and elements of the MAP kinase signal transduction pathway essential for mating pheromone response in haploids. We report here that haploid strains exhibit an invasive growth behavior with many similarities to pseudohyphal development, including filament formation and agar penetration. Haploid filament formation depends on a switch from an axial to a bipolar mode of bud site selection. Filament formation is distinct from agar penetration in both haploids and diploids. We find that the same components of the MAP kinase cascade necessary for diploid pseudohyphal development (STE20, STEI1, STE7, and STE12) are also required for both filament formation and agar penetration in haploids. Thus, haploid yeast cells can enter either of two developmental pathways: mating or invasive growth, both of which depend on elements of a single MAP kinase cascade. Our results provide a novel developmental model to study the dynamics of signal transduction, with implications for higher eukaryotes.

RAS2val19, a dominant activated form of Saccharomyces cerevisiae Ras2, stimulates both filamentous growth and expression of a transcriptional reporter FG(TyA)::lacZ but does not induce the mating pathway reporter FUS1::lacZ. This induction depends upon elements of the conserved mitogen-activated protein kinase (MAPK) pathway that is required for both filamentous growth and mating, two distinct morphogenetic events. Full induction requires Ste20 (homolog of mammalian p65PAK protein kinases), Ste11 [an MEK kinase (MEKK) or MAPK kinase (MEK) kinase], Ste7 (MEK or MAPK kinase), and the transcription factor Ste12. Moreover, the Rho family protein Cdc42, a conserved morphogenetic G protein, is also a potent regulator of filamentous growth and FG(TyA)::lacZ expression in S. cerevisiae. Stimulation of both filamentous growth and FG(TyA)::lacZ by Cdc42 depends upon Ste20. In addition, dominant negative CDC42Ala118 blocks RAS2val19 activation, placing Cdc42 downstream of Ras2. Our results suggest that filamentous growth in budding yeast is regulated by an evolutionarily conserved signaling pathway that controls cell morphology.

14-3-3 proteins are highly conserved ubiquitous proteins whose explicit functions have remained elusive. Here, we show that the S. cerevisiae 14-3-3 homologs BMH1 and BMH2 are not essential for viability or mating MAPK cascade signaling, but they are essential for pseudohyphal-development MAPK cascade signaling and other processes. Activated alleles of RAS2 and CDC42 induce pseudohyphal development and FG(TyA)-lacZ signaling in Bmh+ strains but not in ste20 (p65PAK) or bmh1 bmh2 mutant strains. Moreover, Bmh1p and Bmh2p associate with Ste20p in vivo. Three alleles of BMH1 encode proteins defective for FG(TyA)-lacZ signaling and association with Ste20p, yet these alleles complement other 14-3-3 functions. Therefore, the 14-3-3 proteins are specifically required for RAS/MAPK cascade signaling during pseudohyphal development in S. cerevisiae.

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