Involved in cytoplasm to vacuole transport (Cvt) and autophagic vesicle formation (PubMed:26442587). Autophagy is essential for maintenance of amino acid levels and protein synthesis under nitrogen starvation (By similarity). Required for selective autophagic degradation of the nucleus (nucleophagy) (By similarity). Also required for mitophagy, which eliminates defective or superfluous mitochondria in order to fulfill cellular energy requirements and prevent excess ROS production (By similarity). 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 (PubMed:22682742). The ATG12-ATG5 conjugate acts as an E3-like enzyme which is required for lipidation of ATG8 and ATG8 association to the vesicle membranes (By similarity). ATG12-ATG5 rearranges the ATG3 catalytic center and enhances its E2 activity (By similarity). Conjugated with ATG12 (PubMed:22682742). Interacts with ATG10 (PubMed:22682742). The ATG5-ATG12 conjugate forms a complex with several units of ATG16 (By similarity). The ATG12-ATG5 conjugate associates also with ATG3 (By similarity). Localizes to the isolation membrane (IM), a membrane sac which is generated from the pre-autophagosomal structure (PAS) (By similarity). Ultimately, the IM expands to become a mature autophagosome (By similarity). Localizes also to a dot at the junction between the IM and the vacuolar membrane, termed the vacuole-IM contact site (VICS) (By similarity). Correct localization to the PAS requires ATG21 (By similarity). Conjugated to ATG12; which is essential for autophagy (PubMed:22682742). Conjugation with ATG12 involves ATG7 as an E1-like activating enzyme and ATG10 as an E2-like conjugating enzyme (By similarity). Impairs the formation of preautophagosomal structures (PubMed:26442587). Kluyveromyces marxianus proteins are shorter in length and have a more ordered secondary structure than their S.cerevisiae counterparts, which might contribute to the superior thermotolerance and solubility (PubMed:26442587). K.marxianus could be therefore useful as a new model organism for further elucidation of the molecular details of autophagy (PubMed:26442587). Belongs to the ATG5 family.