PROTEIN-O-GLYCOSYLATION IN YEAST - PROTEIN-SPECIFIC MANNOSYLTRANSFERASES

S.cerevisiae contains at least six genes (PMT1-6) for dolicholphosphat e-D-mannose: protein-O-D-mannosyltransferases. The in vivo mannosylati on of seven O-mannosylated yeast proteins has been analyzed in a numbe r of pmt mutants, The results clearly indicate that the various protei n O-mannosyltransferases have different specificities for protein subs trates. Five of the proteins tested (chitinase, a-agglutinin, Kre9p, B ar1p, Pir2p/hsp150) are mainly underglycosylated in pmt1 and pmt2 muta nts, whereby qualitative differences exist among the various proteins, Two of the O-mannosylated proteins (Ggp1p and Kex2p) are not at all a ffected in pmt1 and pmt2 mutants but are clearly underglycosylated whe n PMT4 is mutated, Although the PMT4 gene product is shown to be respo nsible for O-mannosylating a Ser-rich region of Ggp1p in vivo, a penta -seryl-peptide is not an in vitro substrate for this transferase, A PM T3 mutation does affect O-mannosylation of chitinase only in the genet ic background of a pmt1pmt2 double mutation, indicating that PMT2 and PMT2 can compensate for a deleted PMT3 gene.