Abstract

The cAMP-protein kinase A (PKA) pathway in the yeast Saccharomyces cerevisiae plays a major role in the control of metabolism, proliferation and stress resistance. Derepressed cells show a rapid increase in the cAMP level (within 1 min) after addition of glucose or after intracellular acidification. A specific mutation in adenylate cyclase, the enzyme that catalyzes the synthesis in cAMP, largely prevents both cAMP responses. The responsible mutation was originally called lcr1 (for l ack of c AMP r esponses); lcr1 was later identified as allelic with CYR1/CDC35. The mutation was introduced into the CYR1 gene of a W303-1A wild type strain, which resulted in a large decrease in cAMP signalling. Furthermore, there was a strong reduction in GTP/Mg 2+-stimulated but not in Mn 2+-stimulated adenylate cyclase activity in isolated plasma membranes, which is consistent with the absence of signalling through adenylate cyclase in vivo. Glucose-induced activation of trehalase was reduced and mobilization of trehalose and glycogen and loss of stress resistance were delayed in the lcr1 mutant. Because of the absence of cAMP signalling during exponential growth on glucose, it was concluded that glucose-induced cAMP signalling is restricted to the transition from gluconeogenic/respiratory to fermentative growth. Activation of the PKA pathway is mediated by a G protein (either Ras1/Ras2 or Gpa2). Constitutive activation of the pathway by Ras2 val19 or Gpa2 val132 has a negative effect on glycogen and trehalose accumulation and heat shock survival. The lcr1 mutation partially suppresses this effect indicating that the target sites of the two G-proteins on adenylate cyclase might have at least a part in common.

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