E1 and E2 enzymes play a central role in ubiquitin and ubiquitin-like protein transfer cascades. This is an E2 binding domain that is found on NEDD8 activating E1 enzyme. The domain resembles ubiquitin, and recruits the catalytic core of the E2 enzyme Ubc12 in a similar manner to that in which ubiquitin interacts with ubiquitin binding domains [1].

1: Structural basis for recruitment of Ubc12 by an E2 binding domain in NEDD8's E1. Huang DT, Paydar A, Zhuang M, Waddell MB, Holton JM, Schulman BA; Mol Cell. 2005;17:341-350. PMID:15694336