Ubiquitin-like protein involved in cytoplasm to vacuole transport (Cvt), autophagy vesicles formation, mitophagy, and nucleophagy (By similarity). 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 (By similarity). The ATG12-ATG5 conjugate acts as an E3-like enzyme which is required for lipidation of ATG8 and ATG8 association to the vesicle membranes (PubMed:27309377). ATG12-ATG5 rearranges the ATG3 catalytic center and enhances its E2 activity (By similarity). plays a role in sexual development and perithecia formation (PubMed:27309377). Forms a conjugate with ATG5 (By similarity). Forms a thioester bond with the 'Cys-116' of ATG10 (By similarity). Interacts with the ATG7 C-terminal 40 amino acids domain (PubMed:27309377). The ATG12-ATG5 conjugate forms a complex with several units of ATG16 (By similarity). The ATG12-ATG5 conjugate associates also with ATG3 (PubMed:27309377). Localizes to the isolation membrane (IM), a membrane sac which is generated from the pre-autophagosomal structure (PAS) (By similarity). Ultimately, the IM expands to become a mature autophagosome (By similarity). Localizes also to a dot at the junction between the IM and the vacuolar membrane, termed the vacuole-IM contact site (VICS) (By similarity). Displays slower vegetative growth under nutrient starvation conditions and leads to the inability to form fruiting bodies (PubMed:27309377). Belongs to the ATG12 family.