SUMOylation of substrate proteins takes place in a manner analogous to that of the ubiquitin conjugation system, utilising E1, E2 and E3 enzymes in an ATP dependent process. The SUMO E1 activating enzyme is a heterodimeric complex consisting of Aos1 and Uba2. Aos1 is similar to the N-terminal half of the UbE1 ubiquitin activating enzyme whilst Uba2 has similarity to the C-terminal half, and contains the active site cysteine residue required for formation of thioester bonds. Uba2 alone is insufficient to catalyse SUMOylation. A short sequence containing the consensus
