SUMO (small Ub-related modifier) is a Ub-like protein. Three types of SUMO are most commonly studied, SUMO 1, SUMO 2, and SUMO 3. SUMO 2 and SUMO 3 are almost identical isoforms and thus share many functions. Like Ub, SUMO can be conjugated to its target proteins as a polymeric chain. However, SUMO 1 forms chains inefficiently as compared to SUMO 2 and SUMO 3. SUMO is conjugated to target proteins by the E1 (SAE1/SAE2), E2 (Ube2I or Ubc9), E3 (RanBP2/Nup358, amongst others). Protein sumoylation is involved in many cellular processes including gene transcription. The SUMO2(Q90P) mutant can still form polySUMO2 chains, but these chains are often resistant to desumolyation.
