Glucose-induced degradation of yeast fructose-1,6-bisphosphatase requires additional triggering events besides protein phosphorylation.

Glucose addition to yeast cells stimulates a cAMP overshoot with concomitant activation of cAMP-dependent protein kinase, which in turn rapidly phosphorylates fructose-1,6-bisphosphatase. The phosphorylated enzyme subsequently undergoes a slow proteolytic breakdown. Also, it has been proposed that phosphorylation represents the mechanism that initiates proteolysis. Here we present experiments carried out on a yeast mutant defective in adenylate cyclase [(1982) Proc. Natl. Acad. Sci. USA 79, 2355-2359] in which extracellular cAMP triggers full enzyme phosphorylation but a scanty proteolysis, whereas glucose plus cAMP provoke both phosphorylation and complete proteolytic breakdown. Thus, besides a glucose-induced cAMP peak, which results in enzyme phosphorylation, other effects evoked by the sugar are indispensable for its proteolytic degradation.