Background: Import of proteins into the mitochondria is regulated by the translocase of the outer mitochondrial membrane (TOM) complex, which facilitates transport through the outer mitochondrial membrane, and a complementary translocase of the inner membrane (TIM) complex, responsible for protein transport to the mitochondrial matrix. The TOM complex consists of the receptors Tom20, Tom22, and Tom70, and the channel-forming protein Tom40 (1). Tom20 is localized in the outer mitochondrial membrane and initially recognizes precursors with a presequence to facilitate protein import across the outer mitochondrial membrane (2).Changes in mitochondrial dynamics regulated by environmental cues affect mitochondrial size and shape and have been shown to dramatically impact mitochondrial metabolism, apoptosis, and autophagy (3). These processes are largely controlled by mitochondrial dynamin-related GTPases, including mitofusin-1, mitofusin-2, OPA1, and DRP1. DRP1 regulates mitochondrial fission, while the mitofusins and OPA1 control fusion at the outer and inner mitochondrial membrane, respectively. These proteins are tightly regulated. OPA1 activity is regulated through alternative splicing and post-translational modifications, including complex proteolytic processing by multiple proteases (4-9). In addition, OPA1 expression can be induced under conditions of metabolic demand through a pathway involving Parkin induced NF-κB activation (10). DRP1 is regulated in part through multiple phosphorylation sites (11). Phosphorylation of DRP1 at Ser616 by MAPK or during mitosis by CDKs stimulates mitochondrial fission (12-14). Mitochondrial fission factor (MFF) is a tail-anchored protein that resides within the outer mitochondrial membrane and is part of the mitochondrial fission complex. MFF participates in mitochondrial fission by serving as one of multiple receptors for the GTPase dynamin-related protein 1 (Drp1) (15-18). AMPK directly phosphorylates MFF at two sites to allow for enhanced recruitment of Drp1 to the mitochondria (19).
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).