The heterodimer acts as an E1 ligase for SUMO1, SUMO2, SUMO3, and probably SUMO4. 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). Shuttles between the cytoplasm and the nucleus, sumoylation is required either for nuclear translocation or nuclear retention. Broadly expressed, with highest levels in testis. Sumoylated with SUMO1 and SUMO2/3 and by UBC9. Sumoylation at Lys-236 inhibits enzymatic activity. Sumoylation at the C-terminal lysine cluster plays an essential role in nuclear trafficking (By similarity). Belongs to the ubiquitin-activating E1 family.