Ubiquitin-like protein involved in autophagy vesicles formation. Conjugation with ATG5 through a ubiquitin-like conjugating system involving also ATG7 as an E1-like activating enzyme and ATG10 as an E2-like conjugating enzyme, is essential for its function. The ATG12-ATG5 conjugate acts as an E3-like enzyme which is required for lipidation of ATG8 family proteins and their association to the vesicle membranes. (Microbial infection) May act as a proviral factor. In association with ATG5, negatively regulates the innate antiviral immune response by impairing the type I IFN production pathway upon vesicular stomatitis virus (VSV) infection (PubMed:17709747). Required for the translation of incoming hepatitis C virus (HCV) RNA and, thereby, for the initiation of HCV replication, but not required once infection is established (PubMed:19666601). Forms a conjugate with ATG5 (PubMed:11096062, PubMed:12207896, PubMed:11825910, PubMed:17709747, PubMed:26812546, PubMed:23202584). The ATG12-ATG5 conjugate forms a complex with several units of ATG16L1 (PubMed:23202584). Forms an 800-kDa complex composed of ATG12-ATG5 and ATG16L2 (By similarity). Interacts with DHX58/RIG-1, IFIH1/MDA5 and MAVS/IPS-1 in monomeric form as well as in ATG12-ATG5 conjugate. The interaction with MAVS is further enhanced upon vesicular stomatitis virus (VSV) infection (PubMed:17709747). Interacts with ATG3 and ATG7 (PubMed:11096062, PubMed:12207896, PubMed:11825910, PubMed:23202584). Interacts with ATG10 (By similarity). Interacts with TECPR1 (PubMed:22342342). TECPR1 recruits the ATG12-ATG5 conjugate to the autolysosomal membrane. Ubiquitous. Expression is induced by mitochondrial DNA deletions, chloramphenicol and nicotinamide. Shares weak sequence similarity with ubiquitin family, but contains an 'ubiquitin superfold' and the C-terminal Gly is required for isopeptide linkage. Acetylated by EP300. Small amount of ATG5-ATG12 conjugate is enough to perform normal autophagy. Belongs to the ATG12 family. Extended N-terminus.