The solubility state of inactive 26S proteasomes determines their cellular fate

Ubiquitin-mediated proteasomal degradation of many proteins plays a key role in the protein quality control (PQC) machinery. However, the proteasome itself can become dysfunctional as a result of transcriptional and translational failures, genomic mutations, or diverse stress conditions,. We previously demonstrated that among the multilayered PQC mechanisms available to yeast, proteasome-mediated degradation of its own impaired subunits is the favored pathway, and that upon proteasome inhibition, the small heat shock protein, Hsp42, mediates an alternative pathway, characterized by accumulation of these subunits in the IPOD, a cytoprotective cytoplasmic aggregates that reside juxtavacuolar. Interestingly, it was recently shown that Hsp42 also mediates the ubiquitination of inactive proteasomes, which is then targeted to autophagy.

In this study we examined the solubility state proteasomes subjected to autophagy as a result of their inactivation, or under nitrogen starvation. The results clearly show that in both cases only soluble proteasomes could serve as a substrate to autophagy, and that the deposition in the IPOD serves as an alternative pathway, when the system becomes overloaded. We also identified the E3 ligase that plays a key role in the ubiquitination of inactive proteasomes and suggest a mechanistic insight to this process.

Altogether, our study shows that the regulation of inactive proteasome homeostasis is regulated through the interplay between refolding, aggregation, or autophagy.