Defective threonine-linked glycosylation of human insulin-like growth factor in mutants of the yeast Saccharomyces cerevisiae.

Mutants of the yeast Saccharomyces cerevisiae were identified, in which O-glycosylation at threonine 29 of a heterologous protein, human insulin-like growth factor (hIGF-1), is defective. In mutant M195, O-glycosylation of hIGF-1, but not of yeast proteins chitinase and a-agglutinin, was reduced; in mutant M577 yeast proteins were affected besides hIGF-1. The mutations of M195 and M577 did not affect viability and could not be complemented by the PMT1 or PMT2 genes. The mutant phenotype of strain M195 was reconstituted in an in vitro system, in which a hIGF-1-derived peptide encompassing residues 24-34 was not used as acceptor for mannosylation, while unrelated peptides were glycosylated at wild-type levels. hIGF-1 glycosylation was drastically reduced in pmt1 disruptants and to a lesser extent in pmt2 disruptants, suggesting interaction between the PMT gene products and components mutated in M195 and M577 cells. The results suggest that mutations may only affect O-glycosylation of a specific subset of secreted proteins in yeast.