Conserved cysteine protease required for autophagy; functions in both early and late stages of autophagosome formation; primes Atg8p by removing the C-terminal arginine residue of nascent Atg8p in preparation for conjugation with phosphatidylethanolamide (PE), and also cleaves the PE modified, membrane-bound form to reverse conjugation; regulates the membrane-binding state of Atg8p, that is required for autophagosome formation and autophagy, as well as the cytoplasm-to-vacuole targeting pathway