ATG12 is transferred from ATG10, becoming conjugated to the target protein ATG5 at Lys-149 through an isopeptide bond (Mizushima et al. 1998). There is no E3-like enzyme involved in Atg12-Atg5 conjugation. Nuclear magnetic resonance experiments, mutational analyses, and crosslinking experiments showed that ATG10 directly recognizes ATG5, binding it to place the side chain of ATG5 Lys-145 into an optimal orientation for its conjugation reaction with ATG12, thereby enabling ATG10 to mediate the formation of the ATG12-ATG5 conjugate without a specific E3 enzyme (Yamaguchi et al. 2012).