NBR1 mediates pexophagy

Large cytoplasmic materials are selectively sequestered and degraded in lysosomes in a process called selective autophagy. Substrate selection is mediated by cargo receptors such as NBR1, p62, NDP52 and optineurin, although their precise roles are not well understood. On page 939, Peter Kim and colleagues test the hypothesis that autophagy receptors confer substrate selectivity in autophagy. Specifically, they examine the role of autophagy receptors NBR1 and p62 in the selective autophagy of peroxisomes (pexophagy) in mammalian cells. Overexpressing NBR1 induced peroxisome localisation to lysosomes, indicating that NBR1 can promote the activation of pexophagy. p62, however, is not required when NBR1 is in excess, but instead increases the efficiency of NBR1-driven pexophagy. Mutagenesis studies of NBR1 were then used to investigate the mechanism underlying NBR1-mediated pexophagy. The authors identify four domains that are necessary to mediate pexophagy: the amphipathic α-helical J domain, the ubiquitin-associated (UBA) domain, the LC3-interacting region and the coiled-coil domain. Interestingly, some of the substrate specificity was shown to come from NBR1 itself by coincident binding of the phospholipid-binding J and ubiquitin-binding UBA domains to peroxisomes. Taken together, these findings indicate that NBR1 is the specific autophagy receptor for pexophagy in mammalian cells.