Yeast Ubl conjugation systems. There are two Ubl protein conjugations systems, involving Atg8 and Atg12, which are used to generate the conjugation products Atg8–PE and Atg12–Atg5, respectively. Atg8 is initially processed by Atg4, a cysteine protease, and during autophagy, Atg8 is released from Atg8–PE by a second Atg4-dependent cleavage. The Atg12–Atg5 conjugate, along with Atg16, facilitates the conjugation of Atg8 to PE. Both of the Ubl protein conjugation systems share a single activating enzyme, Atg7. The conjugating enzymes are Atg3 for Atg8, and Atg10 for Atg12.