The function of SAE1 (SUMO1 activating enzyme subunit 1, Ensembl gene identifier ENSG00000142230) is as follows. 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. {ECO:0000269|PubMed:10187858, ECO:0000269|PubMed:10217437, ECO:0000269|PubMed:11451954, ECO:0000269|PubMed:11481243, ECO:0000269|PubMed:15660128, ECO:0000269|PubMed:20164921, ECO:0000269|PubMed:9920803}.