A. M. Dranginis

The Flo11/Muc1 flocculin has diverse phenotypic effects. Saccharomyces cerevisiae cells of strain background Σ1278b require Flo11p to form pseudohyphae, invade agar, adhere to plastic, and develop biofilms, but they do not flocculate. We show that S. cerevisiae var. diastaticus strains, on the other hand, exhibit Flo11-dependent flocculation and biofilm formation but do not invade agar or form pseudohyphae. In order to study the nature of the Flo11p proteins produced by these two types of strains, we examined secreted Flo11p, encoded by a plasmid-borne gene, in which the glycosylphosphatidylinositol anchor sequences had been replaced by a histidine tag. A protein of approximately 196 kDa was secreted from both strains, which upon purification and concentration, aggregated into a form with a very high molecular mass. When secreted Flo11p was covalently attached to microscopic beads, it conferred the ability to specifically bind to S. cerevisiae var. diastaticus cells, which flocculate, but not to Σ1278b cells, which do not flocculate. This was true for the 196-kDa form as well as the high-molecular-weight form of Flo11p, regardless of the strain source. The coated beads bound to S. cerevisiae var. diastaticus cells expressing FLO11 and failed to bind to cells with a deletion of FLO11 , demonstrating a homotypic adhesive mechanism. Flo11p was shown to be a mannoprotein. Bead-to-cell adhesion was inhibited by mannose, which also inhibits Flo11-dependent flocculation in vivo, further suggesting that this in vitro system is a useful model for the study of fungal adhesion.

Diploid strains of S. cerevisiae are known to develop pseudohyphae in response to starvation for nitrogen. We report that both haploid and diploid yeast grow in a filamentous form when embedded in solid media. This is not a response to starvation, since yeast grown on rich media and overlaid with rich agar grow within the agar as pseudohyphae. While we find that the only element of diploidy required for formation of pseudohyphae in response to nitrogen starvation is the a1/α2 repressor, pseudohyphal development by embedded cells does not require a1/α2. Deletion of BUD 5 prevented the formation of pseudohyphae by embedded cells, suggesting that these structures are the result of ordered filament formation rather than agar penetration. Deletion of STE 12 prevented the formation of pseudohyphae by all cell types, showing that the same signal transduction pathway is used by embedded cells as by those responding to nitrogen starvation. Different cell types of yeast thus form filaments in response to several kinds of environmental stimuli.