Polymeric Chains of SUMO-2 and SUMO-3 Are Conjugated to Protein Substrates by SAE1/SAE2 and Ubc9

Conjugation of the small ubiquitin-like modifier SUMO-1/SMT3C/Sentrin-1 to proteins in vitro is dependent on a heterodimeric El (SAE1/SAE2) and an E2 (Ubc9). Although SUMO-2/SMT3A/Sentrin-3 and SUMO-3/SMT3B/Sentrin-2 share 50% sequence identity with SUMO-1, they are functionally distinct. Inspection of the SUMO-2 and SUMO-3 sequences indicates that they both contain the sequence psi KXE, which represents the consensus SUMO modification site. As a consequence SAE1/SAE2 and Ubc9 catalyze the formation of polymeric chains of SUMO-2 and SUMO-3 on protein substrates in vitro, and SUMO-2 chains are detected in vivo. The ability to form polymeric chains is not shared by SUMO-1 and although all SUMO species use the same conjugation machinery, modification by SUMO-1 and SUMO-2/-3 may have distinct functional consequences.