The SUMO pathway. All SUMO isoforms are synthetized as a precursor, containing a C-terminal extension, which is cleaved by specific hydrolases. Mature SUMO is then activated by formation of a thioester bond between its C-terminal glycine and the catalytic cysteine of the Uba2 subunit from the E1 activating enzyme (Aos1/Uba2). This step requires ATP hydrolysis. SUMO is then tranfered to the catalytic cysteine of the E2 activating enzyme Ubc9. The last step of the conjugating cascade consists in the transfer of SUMO from Ubc9 to the ε-NH2 group of a lysine side chain (isopeptide bond formation). Efficient modification usually requires E3 ligases. SUMOylation is a reversible and highly dynamic modification due to the presence of specific cysteine proteases of the Ulp/Senp family that cleave the isopeptide bond.