IRF-3 releases its inhibitions

Interferon regulatory factor (IRF) 3 plays a critical role in triggering the activation of interferon antiviral genes. The structure of IRF-3 in association with the CBP/p300 coactivator by in this issue of Structure illuminates the mechanism of IRF activation and the structural flexibilities inherent in CBP/p300.     

Direct involvement of CREB-binding protein/p300 in sequence-specific DNA binding of virus-activated interferon regulatory factor-3 holocomplex

Infections of bacteria and viruses induce host defense reactions known as innate responses including the activation of interferon regulatory factor-3 (IRF-3), critical for the activation of type I interferon system. Upon immediate early signals triggered by the infection, IRF-3 is phosphorylated and a homodimer results. The homodimer complexes with the coactivator CREB-binding protein (CBP)/p300 in the nucleus; thus, holocomplex of IRF-3 competent in DNA binding is generated. We showed CBP/p300 to be indispensable for the DNA binding activity of the holocomplex and to aid the binding through direct interaction with the DNA. We demonstrated that p300 binds with the IRF-3 homodimer via a Q-rich domain and that an intact histone acetyltransferase (HAT) domain is indispensable for the DNA binding of the holocomplex along with a CH3 domain, which connects the HAT and Q-rich domains. These results highlight a novel function of CBP/p300: direct involvement in sequence-specific DNA binding. Furthermore, the critical function of these domains in virus-induced gene activation was demonstrated in vivo by using p300 mutants.