SUMO-1 conjugation selectively modulates STAT1-mediated gene responses

Signal transducers and activators of transcription 1 (STAT1) is a critical mediator of interferon (IFN)-induced gene responses. Recently, STAT1 was found to become modified by small ubiquitinlike modifier 1 (SUMO-1) conjugation at Lys703 through the SUMO E3 ligase function of protein inhibitors of activated STAT (PIAS) proteins. However, the physiologic function of sumoylation in STAT1 is still unclear. Here, we show that mutations in the SUMO attachment site in STAT1 result in increased transcriptional activity in a fashion that is selective among IFN-gamma target genes. The sumoylation-defective STAT1 mutant displayed increased induction of guanylate-binding protein 1 (GBP1) and transporters associated with antigen presentation 1 (TAP1) transcription but not interferon regulatory factor 1 (IRF1) transcription. Moreover, the sumoylation-defective mutant STAT1-KR showed a prolonged DNA-binding activity and nuclear localization in response to IFN-gamma stimulation. These results suggest that sumoylation has a defined negative regulatory effect on selective STAT1-mediated transcription responses.