Induction of Autophagic Flux by Amino Acid Deprivation Is Distinct from Nitrogen Starvation‐Induced Macroautophagy

A number of signaling mechanisms have been implicated in the regulation of autophagic trafficking. Tor kinase activity, cAMP levels, and the GAAC pathway have all been suggested to be involved. We have carefully analyzed the stimuli that underlie induction of autophagic trafficking in Saccharomyces cerevisiae , with a focus on the roles of amino acids and general nitrogen sources. We find evidence for the existence of a novel aspect of the autophagic pathway that is regulated by intracellular amino acids, uncoupled from extracellular nutrient levels, and is absolutely dependent on Gcn2 and Gcn4. This requirement for Gcn2 and Gcn4 distinguishes amino‐acid starvation induced autophagy from classic macroautophagy: Macroautophagic flux in response to nitrogen starvation is only partly diminished in gcn2Δ and gcn4Δ cells. However this maintenance of autophagic flux in gcn mutants during nitrogen starvation reflects the formation of larger numbers of smaller autophagosomes. We find that gcn2Δ and gcn4Δ cells are defective in the induction of Atg8 and Atg4 upon starvation, and we suggest that this defect results, during nitrogen starvation, in the formation of abnormally small autophagosomes.