The yeast cell fusion protein FUS1 is O-glycosylated and spans the plasma membrane.

Previous work has shown that efficient cell fusion during conjugation in Saccharomyces cerevisiae requires a pheromone-induced surface protein encoded by FUS1. We show that the FUS1 protein migrates on SDS/polyacrylamide gels with an apparent molecular mass of 80 kDa, although the mass is predicted to be 58 kDa from the gene coding capacity. This discrepancy results from the presence of O-linked mannose oligosaccharides attached to the clustered serines and threonines at the amino terminus of the protein. The addition of mannose is completely abolished in the early secretory mutant sec53, attenuated in the late-endoplasmic reticulum-blocked sec18, and unaffected in sec7, which is blocked late in the Golgi phase of secretion. Membrane fractionation and protease protection experiments indicate that FUS1 spans the plasma membrane, with its glycosylated amino terminus projecting into the periplasmic space.