Selective degradation of intracellular targets, such as misfolded proteins and damaged organelles, is an important homeostatic function that autophagy has acquired in addition to its more general role in restoring the nutrient balance during stress and starvation. Although the exact mechanism underlying selection of autophagic substrates is not known, ubiquitination is a candidate signal for autophagic degradation of misfolded and aggregated proteins. p62/SQSTM1 was the first protein shown to bind both target-associated ubiquitin (Ub) and LC3 conjugated to the phagophore membrane, thereby effectively acting as an autophagic receptor for ubiquitinated targets. Importantly, p62 not only mediates selective degradation but also promotes aggregation of ubiquitinated proteins that can be harmful in some cell types. Is p62 the only autophagic receptor for selective autophagy? Looking for proteins that interact with ATG8 family proteins, we identified NBR1 (neighbor of BRCA1 gene 1) as an additional LC3- and Ub-binding protein. NBR1 is degraded by autophagy depending on its LC3-interacting region (LIR) but does not strictly require p62 for this process. Like p62, NBR1 accumulates and aggregates when autophagy is inhibited and is a part of pathological inclusions. We propose that NBR1 together with p62 promotes autophagic degradation of ubiquitinated targets and simultaneously regulates their aggregation when autophagy becomes limited.
V Kirkin, T Lamark, YS Sou, G Bjørkøy, JL Nunn, JA Bruun, E Shvets, DG McEwan, TH Clausen, P Wild, I Bilusic, JP Theurillat, A Øvervatn, T Ishii, Z Elazar, M Komatsu, I Dikic, T Johansen. A role for NBR1 in autophagosomal degradation of ubiquitinated substrates. Mol Cell 2009; 33: 505- 16.
PMID: 19268617 DOI: 10.1016/j.molcel.200