Atg12, a post-translational modifier, which can be activated and conjugated to Atg5 by a ubiquitin-like conjugation system, though it has no obvious sequence homology to ubiquitin. It's carboxyl-terminal that is fold into a ubiquitin-like structure is necessary and sufficient for both conjugation and autophagy (1).

1. Hanada T, Ohsumi Y(2005). Structure-function relationship of Atg12, a ubiquitin-like modifier essential for autophagy. Autophagy., 1(2):110-8. PMID:16874032