FLO11, A YEAST GENE-RELATED TO THE STA GENES, ENCODES A NOVEL CELL-SURFACE FLOCCULIN

We report the characterization of a gene encoding a novel flocculin re lated to the STA genes of yeast, which encode secreted glucoamylase. T he STA genes comprise sequences that are homologous to the sporulation -specific glucoamylase SGA and to two other sequences, S2 and S1. We f ind that S2 and S1 are part of a single gene which we have named FLO11 . The sequence of FLO11 reveals a 4,104-bp open reading frame on chrom osome IX whose predicted product is similiar in overall structure to t he class of yeast serine/threonine-rich GPI-anchored cell walls protei ns. An amino-terminal domain containing a signal sequence and a carbox y-terminal domain with homology to GPI (glycosyl-phosphatidyl-inositol ) anchor-containing proteins are seperated by a central domain contain ing a highly repeated threonine- and serine-rich sequence. Yeast cells that express FLO11 aggregate in the calcium-dependent process of floc culation. Flocculation is abolished when FLO11 is disrupted. The produ ct of STA1 also is shown to have flocculating activity. When a green f luorescent protein fusion of FLO11 was expressed from the FLO11 promot er on a single-copy plasmid, fluorescence was observed in vivo at the periphery of cells. We propose that FLO11 encodes a flocculin because of its demonstrated role in the flocculation, its structural similarit y to other members of the FLO gene family, and the cell surface locati on of its product. FLO11 gene sequences are present in all yeast strai ns tested, including all standard laboratory strains, unlike the STA g enes which are present only in the variant strain Saccharomyces cerevi siae var. distaticus. FLO11 differs from all other yeast flocculins in that it is located near a centromere rather than a telomere, and its expression is regulated by mating type. Repression of FLO11-dependent flocculation in diploids is conferred by the mating-type repressor a1/ alpha 2.