The dimeric enzyme acts as an E1 ligase for sumo. It mediates ATP-dependent activation of sumo and formation of a thioester with a conserved cysteine residue on sae2 (By similarity). Protein modification; protein sumoylation. Heterodimer of sae1 and sae2. The complex binds sumo via sae2 (By similarity). Belongs to the ubiquitin-activating E1 family.