Hirimburegama et al. investigated how nutrient availability triggers trehalase activation in Saccharomyces cerevisiae, specifically examining whether cAMP functions as a second messenger. When nitrogen-starved yeast cells were re-supplied with nitrogen sources, phosphate, or sulfate, trehalase was rapidly activated within minutes. Surprisingly, nitrogen and sulfate addition did not increase cAMP levels, while phosphate showed only a small transient increase. Using immunoaffinity chromatography, researchers demonstrated that trehalase activation results from post-translational modification rather than new protein synthesis. Temperature-sensitive mutants defective in cAMP synthesis (cdc25-5 and cdc35-10) still showed nutrient-induced trehalase activation at restrictive temperatures with very low cAMP levels, strongly suggesting cAMP is not the second messenger. However, nutrient-induced trehalase activation required glucose at concentrations (~20 mM) identical to those activating the RAS-adenylate cyclase pathway, suggesting a shared glucose-sensing mechanism. The activation occurred uniformly across all cell sizes regardless of their ability to progress through the cell cycle. The authors propose a model where nutrients activate free catalytic subunits of cAMP-dependent protein kinase independently of cAMP levels, depending instead on basal cAMP availability.

Key findings

• Nitrogen, phosphate, and sulfate addition to starved yeast cells rapidly activated trehalase without significant increases in cAMP levels
• Trehalase activation in temperature-sensitive cAMP synthesis mutants at restrictive temperatures (very low cAMP) indicates cAMP is not the second messenger for nutrient signaling
• Nutrient-induced trehalase activation requires post-translational modification, likely phosphorylation, rather than new protein synthesis
• Glucose concentration requirement for nitrogen-induced trehalase activation (~20 mM) matches that for RAS-adenylate cyclase pathway activation, suggesting shared glucose sensing
• Trehalase activation by nutrients occurs independently of critical cell size requirements for cell cycle progression