Bipartite Nuclear Localization Signal Controls Nuclear Import and DNA-Binding Activity of IFN Regulatory Factor 3

Accurate cellular localization plays a crucial role in the effective function of most signaling proteins, and nuclear trafficking is central to the function of transcription factors. IFN regulatory factor (IRF)3 is a master transcription factor responsible for the induction of type I IFN, which plays a crucial role in host antiviral innate immune responses. However, the mechanisms for control and regulation of IRF3 nuclear import largely remain to be elucidated. In our study, we identified a bipartite nuclear localization signal (NLS) in IRF3, with two interdependent basic clusters separated by a 7-aa linker. Our study further demonstrated that the bipartite NLS of IRF3 is also critical for IRF3 DNA-binding activity, indicating that the two functions of this region are integrated, which is in contrast to other IRFs. Furthermore, the IFN bioassay and infection studies suggest that IRF3 NLS is essential to the IRF3-mediated IFN responses and antiviral immunity. Overall, our results reveal a previously unrecognized bipartite NLS for IRF3 that contains both DNA-binding activity and nuclear import function, and they shed light on the regulatory mechanisms of IRF3 activation and IRF3-mediated antiviral responses.