Background: Autophagy is a catabolic process for the autophagosome-lysosomal degradation of bulk cytoplasmic contents (1,2). Selective autophagy targets the degradation of distinct sets of substrates and organelles and can occur through the utilization of a number of autophagy cargo receptors (3-5). Autophagy cargo receptors contain an LC3-interacting region (LIR) required for interaction with Atg8/LC3 family members targeted to the autophagosome. SQSTM1/p62-like receptors (SLRs) are a family of autophagy cargo receptors that contain domains for binding to ubiquitin. This family includes prototypical member SQSTM1/p62, NBR1, NDP52, Optineurin, and TAX1BP1. Targets of SLRs include ubiquitylated protein aggregates (aggrephagy), organelles such as mitochondria (mitoophagy) and peroxisomes (pexophagy), and intracellular bacteria (xenophagy).Upon binding of cargo to these receptors, the complex is delivered to the autophagosome where both the cargo and receptor are degraded through the autophagic process. While some redundancy may exist among SLR family members, they can have unique activities. Many SLRs can have additional roles as scaffolding proteins for various signaling pathways. For example, SQSTM1/p62 interacts with KEAP1, a cytoplasmic inhibitor of NRF2, a key transcription factor involved in cellular responses to oxidative stress (6). Thus, accumulation of SQSTM1/p62 can lead to an increase in NRF2 activity.
Background: Autophagy is a catabolic process for the autophagosome-lysosomal degradation of bulk cytoplasmic contents (1, 2). Selective autophagy targets the degradation of distinct sets of substrates and organelles (3-5). One of the best studied examples of selective autophagy involves the clearance of damaged mitochondria through a process called mitophagy. Several pathways have been described for various contexts of mitophagy, including the FUNDC1 pathway, the BNIP3 and BNIP3L/Nix pathway, and the PINK1/Parkin pathway. FUNDC1 is a mitochondrial protein that is phosphorylated by the autophagy kinase ULK1 and regulates hypoxia induced mitophagy (6, 7). BNIP3L/Nix and BNIP3 are members of the Bcl-2 family of apoptosis regulators that are expressed on mitochondria, induced by hypoxia, and have have been shown to play a role in mitophagy (8). BNIP3L/Nix is also important in the autophagic maturation of erythroid cells (9). FUNDC1, BNIP3 and BNIP3L/Nix bind to LC3 family members, targeting the mitochondria to the autophagosome.Non-hypoxic induction of mitophagy can be regulated by the PINK1/Parkin pathway, which plays causative roles in neurodegenerative disease, most notably Parkinson’s disease (10, 11). PINK1 is a mitochondrial serine/threonine kinase that is stabilized on the outer mitochondrial membrane of damaged mitochondria. Substrates of PINK1 include the E3 ubiquitin ligase Parkin and ubiquitin itself (12-14). Phosphorylation of Parkin as well as binding to phosphorylated ubiquitin leads to accumulation of ubiquitinated chains on multiple mitochondrial proteins. Ubiquitinated proteins are recognized by selective cargo receptors including SQSTM1/p62, Optineurin, and NDP52 (15-16). Autophagy cargo receptors contain an LC3-interacting region (LIR) required for binding to Atg8/LC3 family members and targeting to the autophagosome (3).