The heterodimer acts as an E1 ligase for sumo1, sumo2, and sumo3. It mediates ATP-dependent activation of sumo proteins followed by formation of a thioester bond between a sumo protein and a conserved active site cysteine residue on uba2/sae2 (By similarity). Protein modification; protein sumoylation. Heterodimer of sae1 and uba2/sae2. The heterodimer corresponds to the two domains that are encoded on a single polypeptide chain in ubiquitin-activating enzyme E1. Interacts with ube2i (By similarity). Belongs to the ubiquitin-activating E1 family.