Enhancer of SUMO conjugation. Increases SUMO conjugation to proteins by promoting the: binding of E1 and E2 enzymes, thioester linkage between SUMO and ube2i/ubc9 and transfer of SUMO to specific target proteins which include hif1a, pias, nfkbia, nr3c1 and top1. Has no effect on ubiquitination (By similarity). Colocalizes with ubc9/ube2i in nuclear spots. The RWD domain is required for the sumoylation enhancement activity.