Autophagic Turnover of Inactive 26S Proteasomes in Yeast Is Directed by the Ubiquitin Receptor Cue5 and the Hsp42 Chaperone

Abstract

The autophagic clearance of 26S proteasomes (proteaphagy) is an important homeostatic mechanism within the ubiquitin system that modulates proteolytic capacity and eliminates damaged particles. Here, we define two proteaphagy routes in yeast that respond to either nitrogen starvation or particle inactivation. Whereas the core autophagic machineries required for Atg8 lipidation and vesiculation are essential for both routes, the upstream Atg1 kinase participates only in starvation-induced proteaphagy. Following inactivation, 26S proteasomes become extensively modified with ubiquitin. Although prior studies with Arabidopsis implicated RPN10 in tethering ubiquitylated proteasomes to ATG8 lining the autophagic membranes, yeast proteaphagy employs the evolutionarily distinct receptor Cue5, which simultaneously binds ubiquitin and Atg8. Proteaphagy of inactivated proteasomes also requires the oligomeric Hsp42 chaperone, suggesting that ubiquitylated proteasomes are directed by Hsp42 to insoluble protein deposit (IPOD)-type structures before encapsulation. Together, Cue5 and Hsp42 provide a quality control checkpoint in yeast directed at recycling dysfunctional 26S proteasomes.

Limited and digestive proteolysis: crosstalk between evolutionary conserved pathways

“…K48 vs K63 substrate ubiquitination (Park & Cuervo, 2013) It is tempting to speculate that proteophagy may encompass Ub chains linked by K63, as in yeast Ub-proteasomes first aggregate and then embark on proteophagy (Marshall et al, 2016). The proteasome aggregation is indispensable for yeast proteophagy and requires the chaperone HSP42 (Table 1).…”

“…Interestingly, RPN10 and Ub-proteasomes are sufficient but not necessary for proteophagy induced by nitrogen depletion in plants (Marshall et al, 2015), highlighting the presence of additional proteophagic pathways. Ubiquitin-proteasome system (UPS) -dependent degradation N-end rule (Varshavsky, 2011) N-end rule (Gibbs et al, 2016) N-end rule (Varshavsky, 2011) Interplay between two routes of digestive proteolysis: autophagy vs UPS Receptors of proteophagy Cue5 (Marshall et al, 2016) RPN10 (Marshall et al, 2015) p62 (Cohen-Kaplan et al, 2016a)…”

How the proteasome is degraded

“…First insights into these processes were gained in 2015 when Marshall et al observed that proteasomes can be degraded by autophagy in Arabidopsis (5), and in 2016 when two independent reports described the autophagic elimination of proteasomes in Saccharomyces cerevisiae (6,7). Autophagy is the second major degradation system in eukaryotic cells specialized on long-lived, large, heterogeneous material (8).…”

Role of sHsps in organizing cytosolic protein aggregation and disaggregation

“…In contrast to these generic features valid for bulk misfolded proteins sequestered in CytoQs, a specific role of Hsp42 and CytoQs was recently reported for dysfunctional and inactive proteasomes, which cannot assemble properly because of mutations or inhibitions (Marshall et al 2016;Peters et al 2015). While misassembled proteasomes are preferentially degraded by the ubiquitin-proteasome system involving 26S wild type proteasomes, they become substrate of Hsp42 if stabilized upon proteasome inhibition and are deposited at CytoQs.…”

“…Hsp42 gains its functional specificity as cellular aggregase via its unusual long N-terminal extension (NTE). NTE deletion abrogates CytoQ formation (Marshall et al 2016;Specht et al 2011), whereas a hybrid Hsp26 protein harboring the NTE of Hsp42 is largely proficient in CytoQ formation (Specht et al 2011). Notably, the long Hsp42 NTE includes a prion-like domain (Alberti et al 2009).…”