Protein-O-glycosylation in yeast: protein-specific mannosyltransferases. Glycobiology 7:481–486

'To wbom correspondence should be addressed S.cerevisiae contains at least six genes (PMT1-6) for dolichol-phosphate-D-mannose: protein-O-D-mannosyltransferases. The in vivo mannosylation of seven O-mannosylated yeast proteins has been analyzed in a number of pmt mutants. The results clearly indicate that the various protein O-mannosyltransferases have different specificities for pro-tein substrates. Five of the proteins tested (chitinase, a-agglutinin, Kre9p, Barlp, Pir2p/hspl50) are mainly under-glycosylated in pmtl and pmt2 mutants, whereby qualitative differences exist among the various proteins. Two of the O-mannosylated proteins (Ggplp and Kex2p) are not at all affected in pmtl and pmt2 mutants but are clearly underglycosylated when PMT4 is mutated. Al-though the PMT4 gene product is shown to be responsible for O-mannosylating a Ser-rich region of Ggplp in vivo, a penta-seryl-peptide is not an in vitro substrate for this transferase. A PMT3 mutation does affect 0-manno-sylation of chitinase only in the genetic background of a pmtlpmt2 double mutation, indicating that PMT1 and PMT2 can compensate for a deleted PMT3 gene. Key words: dolichol-phosphate/PATr gene family/protein gly-cosylation/5. cerevisia