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.