| Since human Atg4B was found to cleave the carboxyl terminus of MAP1LC3B in vitro , we hypothesized that this exposes its carboxyl terminal Gly ( 120 ) . ^^^ An in vitro assay showed that Gly ( 120 ) is essential for carboxyl terminal cleavage by human Atg4B as well as for formation of the intermediates Atg 7 MAP1LC3B ( ubiquitin activating enzyme substrate ) and Atg 3 MAP1LC3B ( ubiquitin carrier protein substrate ) . ^^^ |