Abstract
The removal of misfolded, ubiquitinated proteins is an essential part of the protein quality control. The ubiquitin‐proteasome system (UPS) and autophagy are two interconnected pathways that mediate the degradation of such proteins. During autophagy, ubiquitinated proteins are clustered in a p62‐dependent manner and are subsequently engulfed by autophagosomes. However, the nature of the protein substrates targeted for autophagy is unclear. Here, we developed a reconstituted system using purified components and show that p62 and ubiquitinated proteins spontaneously coalesce into larger clusters. Efficient cluster formation requires substrates modified with at least two ubiquitin chains longer than three moieties and is based on p62 filaments cross‐linked by the substrates. The reaction is inhibited by free ubiquitin, K48‐, and K63‐linked ubiquitin chains, as well as by the autophagosomal marker LC3B, suggesting a tight cross talk with general proteostasis and autophagosome formation. Our study provides mechanistic insights on how substrates are channeled into autophagy.
Highlights
Protein aggregation is a major threat to cellular homeostasis
The clearance of misfolded proteins is crucial for the maintenance of intracellular homeostasis
Misfolded, ubiquitinated proteins are normally degraded via the ubiquitin-proteasome system (UPS), but when this is overloaded or malfunctioning, these proteins accumulate and are subsequently sequestered into larger structures that become targets for aggrephagy (Kageyama et al, 2014; Demishtein et al, 2017; Dikic, 2017; Galluzzi et al, 2017)
Summary
Protein aggregation is a major threat to cellular homeostasis. The accumulation of misfolded aggregated proteins is associated with many pathological conditions, including several neurodegenerative diseases (Menzies et al, 2015). When the UPS is impaired or its capacity is exceeded, misfolded poly-ubiquitinated proteins can accumulate forming larger structures, which subsequently become substrates (cargoes) for selective autophagy ( referred to as aggrephagy; Dikic, 2017; Galluzzi et al, 2017). P62 is required for the nucleation of several types of aggregates containing misfolded proteins (Zatloukal et al, 2002; Nan et al, 2004; Bjorkoy et al, 2005; Komatsu et al, 2007; Pankiv et al, 2007; Stumptner et al, 2007; Kageyama et al, 2014; Lahiri et al, 2016) During this process, p62 acts together with other proteins, including ALFY, WDR81, Huntingtin, and the cargo receptor NBR1 (Kirkin et al, 2009; Clausen et al, 2010; Rui et al, 2015; Liu et al, 2017). We further discover a mechanism potentially allowing the coordination of cargo nucleation and autophagosome formation
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