Proteasome Subunit Alpha 1 Overexpression Preferentially Drives Canonical Proteasome Biogenesis And Enhances Stress Tolerance In Yeast

The 26S proteasome conducts the majority of regulated protein catabolism in eukaryotes. At the heart of the proteasome is the barrel-shaped 20S core particle (CP), which contains two beta-rings sandwiched between two alpha-rings. Whereas canonical CPs contain alpha-rings with seven subunits arranged alpha 1-alpha 7, a non-canonical CP in which a second copy of the alpha 4 subunit replaces the alpha 3 subunit occurs in both yeast and humans. The mechanisms that control canonical versus non-canonical CP biogenesis remain poorly understood. Here, we have repurposed a split-protein reporter to identify genes that can enhance canonical proteasome assembly in mutant yeast producing non-canonical alpha 4-alpha 4 CPs. We identified the proteasome subunit alpha 1 as an enhancer of alpha 3 incorporation, and find that elevating alpha 1 protein levels preferentially drives canonical CP assembly under conditions that normally favor alpha 4-alpha 4 CP formation. Further, we demonstrate that alpha 1 is stoichiometrically limiting for alpha-ring assembly, and that enhancing alpha 1 levels is sufficient to increase proteasome abundance and enhance stress tolerance in yeast. Together, our data indicate that the abundance of alpha 1 exerts multiple impacts on proteasome assembly and composition, and we propose that the limited alpha 1 levels observed in yeast may prime cells for alternative proteasome assembly following environmental stimuli.