SED1P IS A MAJOR CELL-WALL PROTEIN OF SACCHAROMYCES-CEREVISIAE IN THESTATIONARY-PHASE AND IS INVOLVED IN LYTIC ENZYME RESISTANCE

A 260-kDa structural cell wall protein was purified from sodium dodecy l sulfate-treated cell walls of Saccharomyces cerevisiae by incubation with Rarobacter faecitabidus protease I, which is a yeast-lytic enzym e, Amino acid sequence analysis revealed that this protein is the prod uct of the SED1 gene, SED1 was formerly identified as a multicopy supp ressor of erd2, which encodes a protein involved in retrieval of lumin al endoplasmic reticulum proteins from the secretory pathway. Sed1p is very rich in threonine and serine and, like other structural cell wal l proteins, contains a putative signal sequence for the addition of a glycosylphosphatidylinositol anchor. However, the fact that Sed1p, unl ike other cell wall proteins, has six cysteines and seven putative N-g lycosylation sites suggests that Sed1p belongs to a new family of cell wall proteins, Epitope-tagged Sed1p was detected in a beta-1,3-glucan ase extract of cell walls by immunoblot analysis, suggesting that Sed1 p is a glucanase-extractable cell wall protein, The expression of Sed1 p mRNA increased in the stationary phase and was accompanied by an inc rease in the Sed1p content of cell walls. Disruption of SED1 had no ef fect on exponentially growing cells but made stationary-phase cells se nsitive to Zymolyase. These results indicate that Sed1p is a major str uctural cell wall protein in stationary-phase cells and is required fo r lytic enzyme resistance.