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. Activated in retinal ganglion cells (RGCs) following optic nerve transection. Also induced under starvation conditions, through the action of the foxo1 and foxo3 transcription factors. 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.