The TIR-domain containing proteins including Interleukin-1 receptors (IL-1Rs), Toll-like receptors (TLRs) and cytoplasmic adaptor proteins such as MAL and MyD88 play a major role in immune signaling and are vital to innate host defense, inflammation, injury and stress [1]. IL-1R8, also known as single immunoglobulin interleukin-1 receptor-related protein (SIGIRR) is an inhibitory receptor from IL-1R family which regulates signaling of both IL-1Rs and TLRs. The mechanism of inhibition is not yet known, but the only available genetic evidence suggests that the conserved intracellular TIR domain alone of IL-1R8 is necessary to inhibit LPS-induced TLR4 signaling [2]. The recent cryo-EM structure of the MAL protofilament has revealed the molecular mechanism of TIR-TIR interactions in the MAL and MyD88 dependent TLR4 signaling [3]. Based on this, we hypothesize that a similar TIR:TIR interaction between the TIR domain of IL-1R8 and the TIR domains of either TLR4/MAL/MyD88 would be involved in the inhibition mechanism.
The TIR domain of human IL-1R8 was cloned, expressed and purified using E. coli host system. Turbidity assays, negative-stain electron microscopy (EM) and single-molecule fluorescence spectroscopy (SMFS) analysis indicated a potential interaction between IL-1R8TIR and MALTIR. MALTIR forms filamentous assemblies when incubated with IL-1R8TIR (Fig. 1). We are currently focusing on solving the 3D structure of MALTIR:IL-1R8TIR filaments using cryo-EM to obtain molecular insights into the interaction interfaces and binding sites of IL-1R8TIR and MALTIR. This study will eventually lead to an understanding of how TLR4 signaling is regulated by IL-1R8 and can potentially pave way in development of new therapeutic agents in future.