Noncanonical recognition and UBL loading of distinct E2s by autophagy-essential Atg7.

Autophagy requires ubiquitin-like Atg8 and Atg12 conjugation systems, where Atg7 has a critical role as the sole E1 enzyme. Although Atg7 recognizes two distinct E2s, Atg3 and Atg10, it is not understood how Atg7 correctly loads these E2s with their cognate ubiquitin-like proteins, Atg8 and Atg12. Here, we report the crystal structures of the N-terminal domain of Atg7 bound to Atg10 or Atg3 of thermotolerant yeast and plant homologs. The observed Atg7-Atg10 and Atg7-Atg3 interactions, which resemble each other but are quite distinct from the canonical E1-E2 interaction, makes Atg7 suitable for transferring Atg12 to Atg10 and Atg8 to Atg3 by a trans mechanism. Notably, in vitro experiments showed that Atg7 loads Atg3 and Atg10 with Atg8 and Atg12 in a nonspecific manner, which suggests that cognate conjugate formation in vivo is not an intrinsic quality of Atg7.