DIMINISHED ACTIVITY OF THE FIRST N-GLYCOSYLATION ENZYME, DOLICHOL-P-DEPENDENT N-ACETYLGLUCOSAMINE-1-P TRANSFERASE (GPT), GIVES RISE TO MUTANT PHENOTYPES IN YEAST

The enzyme which initiates the dolichol pathway of protein N-glycosyla tion, dolichol-P-dependent N-acetylglucosamine-1-P transferase (GPT), is encoded by the ALG7 gene. Essential for viability, ALG7 has been ev olutionarily conserved and shown to be involved in a variety of functi ons. ALG7 is an early growth-response gene in yeast, and downregulatio n of ALG7 expression results in diminished N-glycosylation and secreti on of Xenopus oocyte proteins. We have now investigated the consequenc es of diminished GPT activity in yeast using mutant ALG7 genes with de letions in the 3' untranslated region (3' UTR). We show that a 2.5- to 4-fold reduction in GPT activity gave rise to distinct phenotypes, wh ose severity was inversely related to the level of GPT activity. These phenotypes included hypersensitivity to tunicamycin, enlarged cell si ze, extensive aggregation, lack of a typical stationary (G(0)) arrest, and defective spore germination. We conclude that yeast cells are sen sitive to GPT dosage, and that attenuation of GPT activity interferes with various functions in the yeast life cycle.