Macrophages detect pathogen infection via the activation of their plasma membrane-bound Toll-like receptor proteins (TLRs). The heterotypic interaction between the Toll/interleukin-1 receptor (TIR) domains of TLRs and adaptor proteins, likeMyeloid differentiation primary response gene 88 (MyD88), is the first intracellular step in the signaling pathway of the mammalian innate immune response. The hetero-oligomerization of the TIRs of the receptor and adaptor brings about the activation of the transcription factor NF-B, which regulates the synthesis of pro-inflammatory cytokines. Here, we report the first crystal structure of a bacterial TIR domain solved at 2.5 Å resolution. The three-dimensional fold of Paracoccus denitrificans TIR is identical to that observed for the TIR of human TLRs and MyD88 proteins. The structure shows a unique dimerization interface involving the DD-loop and EE-loop residues, whereas leaving the BB-loop highly exposed. Peptide amide hydrogendeuterium exchange mass spectrometry also reveals that the same region is used for dimerization in solution and in the context of the full-length protein. These results, together with a functional interaction between P. denitrificans TIR and MyD88 visualized in a co-immunoprecipitation assay, further substantiate the model that bacterial TIR proteins adopt structural mimicry of the host active receptor TIR domains to interfere with the signaling of TLRs and their adaptors to decrease the inflammatory response.
Toll/interleukin-1 receptor (TIR)2 domain is a key mediator in the Toll-like receptor (TLR) signaling. TLRs are involved in early detection of pathogen-associated molecular patterns to enable quick responses to infection by triggering innate immune reactions through activation of the gene program regulated by the transcription factor nuclear factor-B (NF-B) and the recruitment of macrophages to the infection sites (1). The signaling of TLRs requires the homo-or heterodimerization of their extracellular leucine-rich repeats region mediated by the microbial pathogen-associated molecular patterns, leading to the dimerization of the receptor cytoplasmic TIR domains (2). Only in this active conformation are the receptor TIR domains capable of a functionally productive interaction with TIR domains of adaptor molecules, such as Myeloid differentiation primary response gene 88 (MyD88), MyD88 adaptor-like (Mal, also known as Toll/IL-1 receptor domain-containing adaptor protein (TIRAP)), TIR domain-containing adapter-inducing interferon- (TRIF), or TRIF-related adaptor molecule (TRAM), to initiate the signaling cascade (3).Structures of TIR domains from human TLR1(4), TLR2(5), TLR10(6), IL-1RAPL(7), and MyD88 (Protein Data Bank (PDB) IDs: 2JS7; 2Z5V) have been determined, and they all showed a flavodoxin-like fold consisting of three layers ␣//␣ with fivestranded parallel -sheet ordered 2,1,3,4,5 surrounded by ␣-helices on each side. However, neither the homotypic interactions of TIR domains nor the heterotypic ones between the TIRs of receptors and a...