The Cell Surface Flocculin Flo11 Is Required for Pseudohyphae Formation and Invasion bySaccharomyces cerevisiae

Abstract

Diploid yeast develop pseudohyphae in response to nitrogen starvation, while haploid yeast produce invasive filaments which penetrate the agar in rich medium. We have identified a gene,FLO11, that encodes a cell wall protein which is critically required for both invasion and pseudohyphae formation in response to nitrogen starvation. FLO11 encodes a cell surface flocculin with a structure similar to the class of yeast serine/threonine-rich GPI-anchored cell wall proteins. Cells of theSaccharomyces cerevisiae strain Σ1278b with deletions of FLO11 do not form pseudohyphae as diploids nor invade agar as haploids. In rich media, FLO11 is regulated by mating type; it is expressed in haploid cells but not in diploids. Upon transfer to nitrogen starvation media, however, FLO11transcripts accumulate in diploid cells, but not in haploids. Overexpression of FLO11 in diploid cells, which are otherwise not invasive, enables them to invade agar. Thus, the mating type repression of FLO11 in diploids grown in rich media suffices to explain the inability of these cells to invade. The promoter of FLO11 contains a consensus binding sequence for Ste12p and Tec1p, proteins known to cooperatively activate transcription of Ty1 elements and theTEC1 gene during development of pseudohyphae. Yeast with a deletion of STE12 does not expressFLO11 transcripts, indicating that STE12is required for FLO11 expression. These ste12-deletion strains also do not invade agar. However, the ability to invade can be restored by overexpressing FLO11. Activation ofFLO11 may thus be the primary means by which Ste12p and Tec1p cause invasive growth.