O-linked oligosaccharides in yeast glycosyl phosphatidylinositol-anchored protein gp115 are clustered in a serine-rich region not essential for its function

The protein gp115 is an exocellular yeast glycoprotein modified by O- and N-glycosylation and attached to the plasma membrane through a glycosylphosphatidylinositol. The more remarkable structural feature in gp115 is the presence of a 36-amino acid serine-rich region. Similar sequences have been found in mammalian glycoproteins, such as the low density lipoprotein receptor, the decay-accelerating factor, and the mucins, where they are targets of multiple sites of O-glycosylation. The modification of these regions greatly influences their conformation and gives rise to "rodlike" structures. In this work, we have deleted or duplicated the Ser-rich region of gp115. The analysis of the size and glycosylation state of both mutant proteins indicates that about 52% of the total contribution of the O-glycosylation to the mass of the protein is concentrated in this region. The phenotype of ggp1 null mutant expressing the mutant proteins was also analyzed to understand if this region is important for gp115 function. The defects of slow growth rate and resistance to zymolyase of the ggp1 cells are completely complemented by both mutant proteins, suggesting that this region could be dispensable for gp115 function. A tentative model of gp115 structure is presented on the basis of the obtained data.