Ubiquitin-like modifier involved in autophagosomes formation. With ATG4, mediates the delivery of the autophagosomes to the vacuole via the microtubule cytoskeleton. Required for selective autophagic degradation of the nucleus (nucleophagy) as well as for mitophagy which contributes to regulate mitochondrial quantity and quality by eliminating the mitochondria to a basal level to fulfill cellular energy requirements and preventing excess ROS production. Participates also in membrane fusion events that take place in the early secretory pathway. Also involved in endoplasmic reticulum-specific autophagic process and is essential for the survival of cells subjected to severe ER stress. The ATG8-PE conjugate mediates tethering between adjacent membranes and stimulates membrane hemifusion, leading to expansion of the autophagosomal membrane during autophagy. The C-terminal 20 residues are removed to expose Gly-116 at the C-terminus. The C-terminal Gly is then amidated with phosphatidylethanolamine by an activating system similar to that for ubiquitin. Belongs to the ATG8 family.