Screening for glycosylphosphatidylinositol (GPI)-dependent cell wall proteins in Saccharomyces cerevisiae

Open reading frames in the genome of Saccharomyces cerevisiae were screened for potential glycosylphosphatidylinositol (GPI)-attached proteins. The identi®cation of putative GPI-attached proteins was based on three criteria: the presence of a GPI-attach-ment signal sequence, a signal sequence for secretion and a serine- or threonine-rich sequence. In all, 53 ORFs met these three criteria and 38 were further analyzed as follows. The sequence encoding the 40 C- terminal amino acids of each was fused with the struc- tural gene for a reporter protein consisting of a secretion signal, a-galactosidase and a hemagglutinin (HA)epitope, and examined for the ability to become incorpo- rated into the cell wall. On this basis, 14 of fusion proteins were classi®ed as GPI-dependent cell wall proteins because cells expressing these fusion proteins: (i)had high levels of a-galactosidase activity on their surface; (ii)released signi®cant amounts of the fusion proteins from the membrane on treatment with phosphatidylinositol-speci®c phospholipase C (PI-PLC); and (iii)released fusion proteins from the cell wall following treatment with laminarinase. Of the 14 identi®ed putative GPI-dependent cell wall proteins, 12 had novel ORFs adjacent to their GPI-attachment signal sequence. Amino acid sequence alignment of the C-terminal sequences of the 12 ORFs, together with those of known cell wall proteins, reveals some sequence similarities among them.