UBA3, also known as NEDD8-activating enzyme E1 catalytic subunit, is the catalytic subunit of the dimeric UBA3-NAE1 E1 enzyme. E1 activates NEDD8 by first adenylating its C-terminal glycine residue with ATP, thereafter linking this residue to the side chain of the catalytic cysteine, yielding a NEDD8-UBA3 thioester and free AMP. E1 finally transfers NEDD8 to the catalytic cysteine of UBE2M. Recombinant human UBA3 protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography.