Accepts the ubiquitin-like protein NEDD8 from the UBA3-NAE1 E1 complex and catalyzes its covalent attachment to other proteins. The specific interaction with the E3 ubiquitin ligase rbx1, but not rbx2, suggests that the rbx1-ube2m complex neddylates specific target proteins, such as cul1, cul2, cul3 and cul4. Involved in cell proliferation. [E1 NEDD8-activating enzyme]-S-[NEDD8 protein]-yl-L-cysteine + [E2 NEDD8-conjugating enzyme]-L-cysteine = [E1 NEDD8-activating enzyme]-L-cysteine + [E2 NEDD8-conjugating enzyme]-S-[NEDD8-protein]-yl-L-cysteine. Protein modification; protein neddylation. Both the N-terminal docking peptide and the catalytic core domain must bind the uba3-nae1 complex simultaneously for optimal transfer of nedd8. The acetylation of Met-1 increases affinity for DCUN1D1 by about 2 orders of magnitude and is crucial for NEDD8 transfer to cullins. Belongs to the ubiquitin-conjugating enzyme family. UBC12 subfamily.