The Role of Ubiquitin on Yeast Proteasome Dynamics in Quiescence

The ubiquitin-proteasome system regulates protein degradation. Although proteasomes localize in the nucleus of proliferating Saccharomyces cerevisiae, they are sequestered into cytoplasmic proteasome storage granules (PSG) in quiescence. Although important for cell cycle regulation and mediating external stressors, the content and structure of these membraneless PSGs remain unknown. Yeast deletion genetic screens identified several ubiquitin-related genes involved in proteasome sequestration into PSGs. This study aims to determine whether changes in free ubiquitin levels or ubiquitin post-translational modifications affect proteasome dynamics in quiescence. Unlike the wild-type, PSGs were not seen in catalytically inactive Ubp6 mutant strains in quiescence and proteasomes failed to be imported into the nucleus upon the resumption of cell growth. Although no significant differences in proteasome configurations were observed, Western blot analysis of these mutants suggests the presence of post-translationally modified monoubiquitinated proteasomes in quiescence. Ubiquitin modification may target proteasomes towards lysozomal degradation rather than into PSGs for storage.MAS