Proteasome Disassembly and Downregulation is Correlated with Viability During Stationary Phase

During prolonged starvation, yeast cells enter a stationary phase (SP) during which the synthesis of many proteins is dramatically decreased. We show that a parallel decrease in proteasome-dependent proteolysis also occurs. The reduction in proteolysis is correlated with disassembly of 26S proteasome holoenzymes into their 20S core particle (CP) and 19S regulatory particle (RP) components. Proteasomes are reassembled, and proteolysis resumes prior to cell cycle reentry. Free 20S CPs are found in an autoinhibited state in which the N-terminal tails from neighboring alpha subunits are anchored by an intricate lattice of interactions blocking the channel that leads into the 20S CPs. By deleting channel gating residues of CP alpha subunits, we generated an "open channel" proteasome that exhibits faster rates of protein degradation both in vivo and in vitro, indicating that gating contributes to regulation of proteasome activity. This open channel mutant is delayed in outgrowth from SP and cannot survive following prolonged starvation. In summary, we have found that the ubiquitin-proteasome pathway can be subjected to global downregulation, that the proteasome is a target of this regulation, and that proteasome downregulation is linked to survival of SP cells. Maintaining high viability during SP is essential for evolutionary fitness, which may explain the extreme conservation of channel gating residues in eukaryotic proteasomes.