Ubiquitin-like protein involved in cytoplasm to vacuole transport (Cvt), autophagy vesicles formation, mitophagy, and nucleophagy. Conjugation with apg-4/atg5 through a ubiquitin-like conjugating system involving also apg-5/atg7 as an E1-like activating enzyme and atg10 as an E2-like conjugating enzyme, is essential for its function. The atg12-apg-4/atg5 conjugate functions as an E3-like enzyme which is required for lipidation of apg-6/atg8 and apg-6/atg8 association to the vesicle membranes (By similarity). Forms a conjugate with apg-4/atg5. Belongs to the ATG12 family.