Accepts the ubiquitin-like protein SUMO/SMT3 from the E1 complex and catalyzes its covalent attachment to other proteins. Mediates the selective degradation of short-lived and abnormal proteins. Protein modification; protein sumoylation. Interacts with CHIP. Highest expression in young stems and old leaves. Lowest levels in floral buds, anthers and young leaves. Up-regulated during senescence, but not during the G0 to S phase transition. Not induced by heat shock or wounding. Belongs to the ubiquitin-conjugating enzyme family.