Proteasome Localization is Regulated Through Mitochondrial Respiration and Kinase Signaling.

In most organisms, proteasomes are enriched in cell nuclei under optimal physiological conditions, indicating an important need for protein degradation in this organelle. However, stress conditions can result in proteasome nuclear export, possibly reflecting changes in demand for protein degradation. To understand this adaptive response, it is crucial to know when and how proteasome localization and activity are altered upon changing cellular needs. In yeast, carbon starvation triggers a reversible re-localization of proteasomes to cytosolic granules known as proteasome storage granules (PSGs). These PSGs behave like liquid-liquid phase separated structures and highlight a link between cellular metabolism and proteasome localization. We hypothesized that mitochondrial respiration is important in this process. To test this, we manipulated the glycolytic and respiratory activity in yeast using different carbon sources as well as chemical inhibitors of mitochondrial function and monitored proteasome localization by fluorescent microscopy. Growing cells in carbon sources that necessitate respiration prior to starvation, caused a strong reduction in proteasome storage granule formation upon carbon starvation. This suggests that the mitochondrial activity of cells is a determining factor in proteasome localization. Consistent with this, upon chemical inhibition of mitochondrial function, we observed proteasomes re-localize to cytosolic granules independent of starvation stress. Interestingly, the stress response kinase Snf1, which is not required for proteasome re-localization upon carbon starvation, was required for proteasome re-localization following mitochondrial inhibition. In addition, MAP kinases of the cell wall integrity cascade were required for proteasome granule formation, specifically following respiratory but not glycolytic growth, upon carbon starvation or mitochondrial inhibition. In all, our data point to a model where mitochondria regulate proteasome localization via kinases that sense the cells metabolic state.