SUMO Activating Enzyme E1 (SAE1/UBA2): ProductsSmall Ubiquitin-like Modifier (SUMO) Activating Enzyme Subunit 1 (SAE1) is the highly conserved human ortholog of yeast Ubiquitin-activating enzyme E1-like (UBA2). These SUMO-activating (E1) enzymes are critical for the enzymatic attachment of SUMO molecules to a target protein by a post-translational modification process termed SUMOylation. The ATP-dependent E1 enzyme charges the SUMO by forming a high-energy thiol ester intermediate which is transferred to the UBE2I/Ubc9 SUMO-conjugating (E2) enzyme. The second step is the trans-esterification reaction whereby SUMO is transferred to Cys93 of UbcH9. UBE2I/Ubc9 is the only known E2 that is able to mediate the conjugation of SUMO to lysine residues on a variety of cellular targets, usually in the absence of a Ubiquitin ligase (E3). Although UBE2I/Ubc9 can directly recognize and modify lysine residues contained in a SUMOylation motif, E3-like factors most likely facilitate the SUMOylation of specific substrates.