FIGURE 1.

Overview of receptor- and transceptor-mediated nutrient activation of PKA in the yeast Saccharomyces cerevisiae. (A) Glucose binding to the G-protein-coupled receptor Gpr1, together with intracellular activation of the Ras proteins triggers activation of adenylate cyclase (Cyr1), to synthesize cAMP. The rapid increase in cAMP releases the Tpk catalytic subunits from the Bcy1 regulatory subunits of PKA, resulting in activation or downregulation of the many downstream targets of PKA: trehalase activity, trehalose and glycogen levels, stress tolerance, heat shock protein gene expression, cell wall strength, ribosomal protein gene expression and many other targets. (B) Addition of other essential nutrients, in the presence of a fermentable carbon source, to appropriately starved yeast cells triggers activation of the catalytic subunits of PKA, without any increase in the cAMP level. Nutrient sensing happens with a specific nutrient transceptor, different for each nutrient and previously known as substrate starvation-induced high-affinity transporters. How the signal is transmitted from the nutrient transceptor to PKA is under investigation.