Plays an essential role in autophagy: interacts with ATG12-ATG5 to mediate the conjugation of phosphatidylethanolamine (PE) to LC3 (MAP1LC3A, MAP1LC3B or MAP1LC3C), to produce a membrane-bound activated form of LC3 named LC3-II. Thereby, controls the elongation of the nascent autophagosomal membrane. Homooligomer. Part of either the minor and major complexes respectively composed of 4 sets of ATG12-ATG5 and ATG16L1 (400 kDa) or 8 sets of ATG12-ATG5 and ATG16L1 (800 kDa). Recruited to omegasomes membranes by WIPI2. Omegasomes are endoplasmic reticulum connected strutures at the origin of preautophagosomal structures. Localized to preautophagosomal structure (PAS) where it is involved in the membrane targeting of ATG5. Belongs to the WD repeat ATG16 family. Extended N-terminus.