Accepts the ubiquitin-like proteins sumo1, sumo2 and sumo3 from the uble1a-uble1b E1 complex and catalyzes their covalent attachment to other proteins with the help of an E3 ligase such as ranbp2 or cbx4. Essential for nuclear architecture and chromosome segregation. Protein modification; protein sumoylation. Forms a tight complex with rangap1 and ranbp2. Interacts with sox9 and sox10. Belongs to the ubiquitin-conjugating enzyme family.