Enables SUMO transferase activity. Acts upstream of or within several processes, including embryonic heart tube development; hematopoietic stem cell homeostasis; and protein sumoylation. Predicted to be active in nucleus. Is expressed in several structures, including immature eye; nervous system; neural crest; pectoral fin bud; and pharyngeal pouch. Orthologous to human UBE2I (ubiquitin conjugating enzyme E2 I).