The impact of protein glycosylation on Flo11-dependent adherence in Saccharomyces cerevisiae

Fungal cell adhesion molecules are critical for the attachment of cells to each other and to surfaces and in pathogens contribute to virulence. Fungal adhesins are typically heavily glycosylated. The impact of protein glycosylation on the function and regulation of adhesion glycoproteins is not clear. We examined the role of protein glycosylation on the adherence properties of the major adhesion molecule Muc1/Flo11 in the budding yeast Saccharomyces cerevisiae. Using a conditional mutant required for an early step in protein glycosylation, pmi40-101, we show that the glycosylation of Flo11 is required for invasive growth and biofilm/mat formation. Underglycosylated Flo11 was not defective in cell-surface localization or binding to wild-type cells in trans. However, wild-type Flo11 was defective for binding to the surface of cells undergoing a glycosylation stress. Shed Flo11 and other shed glycoproteins (Msb2 and Hkr1) were extremely stable with half-lives on the order of days. The glycosylation of Flo11 contributed to its stability. Moreover, the overall balance between Flo11 production, shedding, and turnover favored accumulation of the shed protein over time. Our findings may be applicable to fungal adhesion molecules in other species including pathogens.