Ubiquitin-like protein involved in autophagy vesicles formation. 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 family proteins and their association to the vesicle membranes. As part of the ATG8 conjugation system with ATG5 and ATG16L1, required for recruitment of LRRK2 to stressed lysosomes and induction of LRRK2 kinase activity in response to lysosomal stress (By similarity). {ECO:0000250|UniProtKB:Q9CQY1, ECO:0000269|PubMed:12207896, ECO:0000269|PubMed:17999726, ECO:0000269|PubMed:19074260, ECO:0000269|PubMed:19164948, ECO:0000269|PubMed:23202584}. (Microbial infection) May act as a proviral factor. In association with ATG5, negatively regulates the innate antiviral immune response by impairing the type I IFN production pathway upon vesicular stomatitis virus (VSV) infection (PubMed:17709747). Required for the translation of incoming hepatitis C virus (HCV) RNA and, thereby, for the initiation of HCV replication, but not required once infection is established (PubMed:19666601). {ECO:0000269|PubMed:17709747, ECO:0000269|PubMed:19666601}. This is the function of Ensembl gene identifier ENSG00000145782 (ATG12, autophagy related 12).