SUMO-conjugating enzyme that accepts the SUMO proteins from the E1 SUMO-activating heterodimer SAE1/SAE2 and catalyzes its covalent attachment to other proteins with the E3 SUMO ligases SIZ1 and MMS21. Associates with SIZ1 for sumoylation of the transcription factor GTE3. Protein modification; protein sumoylation. Interacts with SIZ1 (via PHD domain) and MMS21 (PubMed:18502747, PubMed:19682286). Interacts with TCP14 and TCP15 (PubMed:29250092). Interacts with KIN10 (PubMed:20855607, PubMed:26662259). Embryonic lethal. Belongs to the ubiquitin-conjugating enzyme family.