Identification of the enzyme required for activation of the small ubiquitin-like protein SUMO-1

The ubiquitin-like protein SUMO-1 is conjugated to a variety of proteins including Ran GTPase-activating protein 1 (RanGAP1), I kappa B alpha, and PML, SUMO-1-modified proteins display altered subcellular targeting and/or stability. We have purified the SUMO-1-activating enzyme from human cells and shown that it contains two subunits of 38 and 72 kDa, Isolation of cDNAs for each subunit indicates that they are homologous to ubiquitin-activating enzymes and to the Saccharomyces cerevisiae enzymes responsible for conjugation of Smt3p and Rub-1p, In vitro, recombinant SAE1/SAE2 (SUMO-1-activating enzyme) was capable of catalyzing the ATP-dependent formation of a thioester linkage between SUMO-1 and SAE2. The addition of the SUMO-1-conjugating enzyme Ubch9 resulted in efficient transfer of the thioester-linked SUMO-1 from SAE2 to Ubch9, In the presence of SAE1/SAE2, Ubch9, and ATP, SUMO-1 was efficiently conjugated to the protein substrate I kappa B alpha. As SAE1/SAE2, Ubch9, SUMO-1, and I kappa B alpha are all homogeneous, recombinant proteins, it appears that SUMO-1 conjugation of I kappa B alpha in vitro does not require the equivalent of an E3 ubiquitin protein ligase activity.