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. Forms a conjugate with ATG5 (PubMed:11266458, PubMed:12482611, PubMed:12890687, PubMed:12665549, PubMed:18768753, PubMed:19417210). The ATG12-ATG5 conjugate forms a complex with several units of ATG16L1 (PubMed:12665549). Forms an 800-kDa complex composed of ATG12-ATG5 and ATG16L2 (PubMed:22082872). 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. Interacts with ATG3 and ATG7 (By similarity). Interacts with ATG10 (PubMed:12482611). Interacts with TECPR1 (By similarity). TECPR1 recruits the ATG12-ATG5 conjugate to the autolysosomal membrane. Ubiquitous. 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. Belongs to the ATG12 family.