We report the characterization of a gene encoding a novel flocculin related to 
the STA genes of yeast, which encode secreted glucoamylase. The STA genes 
comprise sequences that are homologous to the sporulation-specific glucoamylase 
SGA and to two other sequences, S2 and S1. We find 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 chromosome IX whose predicted product is similar in 
overall structure to the class of yeast serine/threonine-rich GPI-anchored cell 
wall proteins. An amino-terminal domain containing a signal sequence and a 
carboxy-terminal domain with homology to GPI (glycosyl-phosphatidyl-inositol) 
anchor-containing proteins are separated by a central domain containing a highly 
repeated threonine- and serine-rich sequence. Yeast cells that express FLO11 
aggregate in the calcium-dependent process of flocculation. Flocculation is 
abolished when FLO11 is disrupted. The product of STA1 also is shown to have 
flocculating activity. When a green fluorescent protein fusion of FLO11 was 
expressed from the FLO11 promoter 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 flocculation, its structural 
similarity to other members of the FLO gene family, and the cell surface 
location of its product. FLO11 gene sequences are present in all yeast strains 
tested, including all standard laboratory strains, unlike the STA genes which 
are present only in the variant strain Saccharomyces cerevisiae var. 
diastaticus. 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 al/alpha2.