Proteasomes are complexes that mediate degradation of intracellular proteins as part of the ubiquitin-proteasome system. In yeast, proteasome localisation is altered by nutrient stress, where carbon starvation triggers the translocation of nuclear proteasomes into cytosolic condensates known as proteasome storage granules (PSGs). PSGs are believed to act as protective structures to prevent the autophagic degradation of proteasomes. However, the signalling mechanisms linking nutrient stress to PSG formation are poorly understood. In this study, Kenrick Waite and Jeroen Roelofs (Waite and Roelofs, 2022) identify mitochondrial respiration as a key parameter for controlling PSG formation. The authors find that PSG formation is reduced when yeast cells are actively respiring prior to carbon starvation. Mechanistically, the MAPK Mpk1 is important in this process, given that Mpk1-deficient yeast do not form PSGs. Additionally, chemical inhibition of mitochondrial function induces PSG formation even in the presence of a carbon source, a process requiring the yeast homologue of AMP kinase, Snf1. Interestingly, Snf1 is dispensable for PSG formation in carbon-starved cells, suggesting differential mechanisms are at play, dependent on the inducing stimuli. Together, these findings suggest mitochondrial activity regulates proteasome localisation and offers insights into how glycolytic versus respiratory states influence PSG formation.
