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 functions as an E3-like enzyme which is required for lipidation of atg8 and atg8 association to the vesicle membranes (By similarity). Forms a conjugate with atg5. Belongs to the ATG12 family.