Autophagic Clearance of PolyQ Proteins Mediated by Ubiquitin-Atg8 Adaptors of the Conserved CUET Protein Family

Abstract

Selective ubiquitin-dependent autophagy plays a pivotal role in the elimination of protein aggregates, assemblies, or organelles and counteracts the cytotoxicity of proteins linked to neurodegenerative diseases. Following substrate ubiquitylation, the cargo is delivered to autophagosomes involving adaptors like human p62 that bind ubiquitin and the autophagosomal ubiquitin-like protein Atg8/LC3; however, whether similar pathways exist in lower eukaryotes remained unclear. Here, we identify by a screen in yeast a new class of ubiquitin-Atg8 adaptors termed CUET proteins, comprising the ubiquitin-binding CUE-domain protein Cue5 from yeast and its human homolog Tollip. Cue5 collaborates with Rsp5 ubiquitin ligase, and the corresponding yeast mutants accumulate aggregation-prone proteins and are vulnerable to polyQ protein expression. Similarly, Tollip depletion causes cytotoxicity toward polyQ proteins, whereas Tollip overexpression clears human cells from Huntington's disease-linked polyQ proteins by autophagy. We thus propose that CUET proteins play a critical and ancient role in autophagic clearance of cytotoxic protein aggregates.

Autophagy at the crossroads of catabolism and anabolism

“…For example, the autophagy cargo receptors p62 (also known as SQSTM1), NBR1 (next to BRCA1 gene 1 protein) and histone deacetylase 6 (HDAC6) all promote the autophagic clearance of protein aggregates in a process known as aggrephagy, which is dependent on both the UBD and LIR 2,10 . In yeast, although orthologues of UBD-containing adaptor proteins such as p62 are missing, a recent mass spectrometric study identified Cue5 as a potential autophagy cargo receptor 23 . Cue5 possesses a coupling of ubiquitinconjugation to ER degradation (CUE) domain, which is structurally related to mammalian UBDs and is capable of binding to ubiquitin as well as interacting with Atg8.…”

“…Hence, Cue5 is a ubiquitin-Atg8 adaptor protein that functions analogously to p62 in mammals to mediate the selective autophagic degradation of ubiquitylated targets. Both yeast Cue5 and its mammalian orthologue, Toll interacting protein (TOLLIP), target aggregationprone proteins that cannot be cleared by the UPS, such as huntingtin, for autophagic degradation 23 .…”

How the proteasome is degraded

“…Instead, Cue5 has been identified as selective autophagy receptor for ubiquitinated proteasomes in yeast. Cue5 and its human homolog Tollip have been implicated only recently in the autophagic clearance of polyQ proteins (11). Moreover, proteaphagy in yeast requires the action of the chaperone Hsp42.…”

Atg20- and Atg24-family proteins promote organelle autophagy in fission yeast

“…Autophagy receptors link selective cargos to the core autophagy machinery, especially to the Atg8 protein. At least eight autophagy receptors have been identified in fungal species: S. cerevisiae Atg19 and Atg34 for the Cvt pathway (Leber et al, 2001;Scott et al, 2001;, S. cerevisiae Atg30 and P. pastoris Atg36 for pexophagy (Farré et al, 2008;Motley et al, 2012), S. cerevisiae Atg32 for mitophagy (Kanki et al, 2009a;Okamoto et al, 2009), S. cerevisiae Atg39 for nucleophagy (Mochida et al, 2015), S. cerevisiae Atg40 for ER-phagy (Mochida et al, 2015) and S. cerevisiae Cue5 for aggrephagy (Lu et al, 2014). Among these receptors, only Cue5 has an obvious S. pombe homolog (SPBC16E9.02c), which is a yet uncharacterized protein.…”