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. Interacts with UBA3 and RBX1. Interacts (N-terminally acetylated form) with (via DCUN1 domain) DCUN1D1, DCUN1D2, DCUN1D3, DCUN1D4 and DCUN1D5 (PubMed:23201271, PubMed:28581483). 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.