Ubiquitin-like protein involved in cytoplasm to vacuole transport (Cvt), autophagy vesicles formation, mitophagy, and nucleophagy. 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 and ATG8 association to the vesicle membranes. ATG12-ATG5 rearranges the ATG3 catalytic center and enhances its E2 activity. Forms a conjugate with ATG5. Forms a thioester bond with the 'Cys-133' of ATG10. Interacts with the ATG7 C-terminal 40 amino acids domain. The ATG12-ATG5 conjugate forms a complex with several units of ATG16. The ATG12-ATG5 conjugate associates also with ATG3. Interacts with COG2. Localizes to the isolation membrane (IM), a membrane sac which is generated from the pre-autophagosomal structure (PAS). Ultimately, the IM expands to become a mature autophagosome. Localizes also to a dot at the junction between the IM and the vacuolar membrane, termed the vacuole-IM contact site (VICS). Small amount of ATG5-ATG12 conjugate is enough to perform normal autophagy. Belongs to the ATG12 family.