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. The ATG12-ATG5 conjugate also regulates negatively the innate antiviral immune response by blocking the type I IFN production pathway through direct association with RARRES3 and MAVS. Also plays a role in translation or delivery of incoming viral RNA to the translation apparatus (By similarity). Forms a conjugate with ATG5. The ATG12-ATG5 conjugate forms a complex with several units of ATG16L1. Forms an 800-kDa complex composed of ATG12-ATG5 and ATG16L2 (By similarity). Interacts with ATG3, ATG7 and ATG10. ATG12-ATG5 also interacts with MAVS, MGA, RARRES3 and TECPR1 (By similarity). TECPR1 recruits the ATG12-ATG5 conjugate to the autolysosomal membrane. 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.