Caini Liu, Shadi Swaidani, Wen Qian, Zizhen Kang, Paige Sun, Yue Han, Chenhui Wang, Muhammet Fatih Gulen, Weiguo Yin, Chunjiang Zhang, Paul L Fox, Mark Aronica, Thomas A Hamilton, Saurav Misra, Junpeng Deng, Xiaoxia Li

Interleukin-17 (IL-17) and IL-25 signaling induce the expression of genes encoding inflammatory factors and are implicated in the pathology of various inflammatory diseases. Nuclear factor κB (NF-κB) activator 1 (Act1) is an adaptor protein and E3 ubiquitin ligase that is critical for signaling by either IL-17 or IL-25, and it is recruited to their receptors (IL-17R and IL-25R) through heterotypic interactions between the SEFIR [SEF (similar expression to fibroblast growth factor genes) and IL-17R] domain of Act1 and that of the receptor. SEFIR domains have structural similarity with the Toll-IL-1 receptor (TIR) domains of Toll-like receptors and IL-1R. Whereas the BB' loop of TIR is required for TIR-TIR interactions, we found that deletion of the BB' loop from Act1 or IL-17RA (a common subunit of both IL-17R and IL-25R) did not affect Act1-IL-17RA interactions; rather, deletion of the CC' loop from Act1 or IL-17RA abolished the interaction between both proteins. Surface plasmon resonance measurements showed that a peptide corresponding to the CC' loop of Act1 bound directly to IL-17RA. A cell-permeable decoy peptide based on the CC' loop sequence inhibited IL-17- or IL-25-mediated signaling in vitro, as well as IL-17- and IL-25-induced pulmonary inflammation in mice. Together, these findings provide the molecular basis for the specificity of SEFIR-SEFIR versus TIR-TIR domain interactions and consequent signaling. Moreover, we suggest that the CC' loop motif of SEFIR domains is a promising target for therapeutic strategies against inflammatory diseases associated with IL-17 or IL-25 signaling.