EBV LMP1 Induces the Sumoylation of Interferon Regulatory Factor 7

Interferon regulatory factor (IRF) 7, a transcription factor that is a key regulator of Type I interferons (IFN α/β) in innate and adaptive immune responses in viral infections, is constitutively expressed in lymphoid cells. Increased expression can be induced by various stimuli including the EBV oncoprotein LMP1, a constitutively active membrane protein that mimics the signaling induced by the TNF receptor family and is required for EBV transformation. Previous work in our laboratory demonstrated that LMP1 does activate IRF7. Activation is accomplished through the post-translational modification of IRF7 by both phosphorylation and K63-linked ubiquitination and its nuclear translocation. Therefore, we investigated other post-translational modifications of IRF7 that might be initiated by LMP1. One such modification is by small ubiquitin-like protein modifier (SUMO)-1, which regulates protein function through various mechanisms. Sumoylation of IRF1 results in its transcriptional repression, and we initially hypothesized that sumoylation of IRF7, induced by LMP1 during EBV latency during EBV latency, regulates its transcriptional activity and inhibits the induction of immune responses. We now show that IRF7 interacts with Ubc9, the only reported SUMO-conjugating enzyme, and that IRF7 is sumoylated, endogenously in EBV transformed lymphoblastoid cells and as well as when over-expressed exogenously, resulting in its increased nuclear accumulation. In addition, sumoylated IRF7 is only detected when LMP1 is co-expressed, suggesting that LMP1 induces IRF7 sumoylation. Our investigations into the function of sumoylated IRF7 revealed that sumoylation resulted in the activation of the Type I interferons, and further studies are in progress to examine the mechanism by which this occurs. Recently it has been shown that murine IRF7 is sumoylated following vesicular stomatitis virus infection. Our new data are the first to show that a specific viral protein induces the sumoylation of IRF7 and identify a potential target for regulation of anti-viral immune responses.