Saccharomyces cerevisiae mating pheromones specifically inhibit the synthesis of proteins destined to be N‐glycosylated

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Abstract
α Factor specifically inhibits the synthesis of N-glycosylated proteins in Saccharomyces cerevisiae mating type a cells but not in α cells or in a/α diploids. a Factor has the same effect of α cells. The synthesis of O-glycosylated proteins is not inhibited. Although the mating pheromones act like a ‘physiological tunicamycin’, the mechanism of inhibition is different: not the glycosylation of proteins as such but rather the synthesis of those proteins destined to be N-glycosylated is inhibited. Thus none of a number of glycosylating enzymes tested in vitro is reduced in activity in α-factor-treated cells. The synthesis of the glycoprotein carboxypeptidase Y, on the other hand, is strongly inhibited by tunicamycin as well as by α factor; but only in the former case did carbohydrate-free protein accumulate in the cells. α Factor causes maximal inhibition of glycoprotein formation after as little as 30 min, long before all cells in the population are arrested in G1; moreover, release from this inhibition precedes the increase in budding index (resumption of cell division). It is postulated, therefore, that N-glycosylated proteins are required for the G1/S-phase transition in the yeast cell cycle. This is supported by previous reports that first cycle arrest in G1 occurs when (a) tunicamycin is added to growing cultures, and (b) a temperature-sensitive N-glycosylation mutant is shifted to its restrictive temperature.