Involved in autophagic vesicle formation. Conjugation with atg12, through a ubiquitin-like conjugating system involving 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 its association to the vesicle membranes (By similarity). Conjugated to atg12; which is essential for autophagy. Belongs to the ATG5 family.