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. Interacts with UBA3, DCUN1D1 and RBX1. Belongs to the ubiquitin-conjugating enzyme family. UBC12 subfamily. Note: This description may include information from UniProtKB.