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

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.