Autophagic Degradation of the 26S Proteasome Is Mediated by the Dual ATG8/Ubiquitin Receptor RPN10 in Arabidopsis

Abstract

SUMMARY Autophagic turnover of intracellular constituents is critical for cellular housekeeping, nutrient recycling, and various aspects of growth and development in eukaryotes. Here we show that autophagy impacts the other major degradative route involving the ubiquitin-proteasome system by eliminating 26S proteasomes, a process we termed proteaphagy. Using Arabidopsis proteasomes tagged with GFP, we observed their deposition into vacuoles via a route requiring components of the autophagy machinery. This transport can be initiated separately by nitrogen starvation and chemical or genetic inhibition of the proteasome, implying distinct induction mechanisms. Proteasome inhibition stimulates comprehensive ubiquitylation of the complex, with the ensuing proteaphagy requiring the proteasome subunit RPN10, which can simultaneously bind both ATG8 and ubiquitin. Collectively, we propose that Arabidopsis RPN10 acts as a selective autophagy receptor that targets inactive 26S proteasomes by concurrent interactions with ubiquitylated proteasome subunits/targets and lipidated ATG8 lining the enveloping autophagic membranes.

Comparative analyses of ubiquitin-like ATG8 and cysteine protease ATG4 autophagy genes in the plant lineage and cross-kingdom processing of ATG8 by ATG4

“…10,50 Emerging evidence also suggests that ATG8 plays an important role in selective autophagy. [50][51][52] The ATG4-mediated processing of ATG8 followed by lipidation is an important step in autophagosome initiation and maturation. ATG4 also plays a role in the recycling of ATG8 once the autophagosome fuses with the vacuole or lysosome.…”

How the proteasome is degraded

“…Similar to other selective autophagy pathways, the sequestration of ubiquitinated proteasomes into the autophagosome critically depends on autophagy receptors that can simultaneously bind to Ub attached to the cargo and Atg8/LC3, a Ub-like modifier exposed on autophagosomal membranes. Marshall et al (5) have shown that, in Arabidopsis, Rpn10a fulfills this function. Of note, besides being an integral proteasomal RP subunit, RPN10a can exist also in free (i.e., extraproteasomal) form, and it possesses a newly identified LC3-interacting region (LIR) enabling Rpn10a to take the role of a selective autophagy receptor for ubiquitinated proteasomes.…”

Proteasome inhibition attenuates rasfonin-induced autophagy concurring with the upregulation of caspase-dependent apoptosis

“…As an important unit in the Ub–proteasome pathway, Ub covalently linked many cellular proteins by the ubiquitination process, which targets proteins for degradation by the 26S proteasome (Ciechanover 1998). MG132, a widely used proteasome inhibitor, was reported to stimulate proteaphagy, a process that describes autophagy affects the UPS by eliminating 26S proteasomes (Marshall, Li, et al 2015). Consistently, we observed that MG132 induced autophagy in ACHN cells, as CQ further increased the ratio of LC3-II to actin, and blocked the p62 degradation (Figure 3B).…”

“…While the UPS is responsible for short-lived proteins, autophagy clears many long-lived proteins and organelles (Lee et al 2013). A recent study shows that the 26S proteasome is degraded by autophagy in Arabidopsis (Marshall, Li, et al 2015). LC3 (a mammalian homolog of yeast Atg8) is the most widely monitored autophagy-related protein.…”