Structural basis for the multiple interactions of the MyD88 TIR domain in TLR4 signaling.
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ABSTRACT:
SUBMITTER: Ohnishi H
PROVIDER: S-EPMC2693180 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:Toll-like receptor (TLR) signaling is a key innate immunity response to pathogens. Recruitment of signaling adapters such as MAL (TIRAP) and MyD88 to the TLRs requires Toll/interleukin-1 receptor (TIR)-domain interactions, which remain structurally elusive. Here we show that MAL TIR domains spontaneously and reversibly form filaments in vitro. They also form cofilaments with TLR4 TIR domains and induce formation of MyD88 assemblies. A 7-Å-resolution cryo-EM structure reveals a stable MAL protofilament consisting of two parallel strands of TIR-domain subunits in a BB-loop-mediated head-to-tail arrangement. Interface residues that are important for the interaction are conserved among different TIR domains. Although large filaments of TLR4, MAL or MyD88 are unlikely to form during cellular signaling, structure-guided mutagenesis, combined with in vivo interaction assays, demonstrated that the MAL interactions defined within the filament represent a template for a conserved mode of TIR-domain interaction involved in both TLR and interleukin-1 receptor signaling.
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Project description:MyD88 adaptor-like protein (Mal) is a crucial adaptor that acts as a bridge to recruit the MyD88 molecule to activated TLR4 receptors in response to invading pathogens. The specific assembly of the Toll/interleukin-1 receptor (TIR) domains of TLR4, Mal and MyD88 is responsible for proper signal transduction in the TLR4 signaling pathway. However, the molecular mechanism for the specificity of these TIR domains remains unclear. Here, we present the crystal structure of the TIR domain of the human Mal molecule (Mal-TIR) at a resolution of 2.4 Å. Unexpectedly, Mal-TIR exhibits an extraordinarily long AB loop, but no ?B helix or BB loop, distinguishing it from other TIR domains. More importantly, the Mal-TIR AB loop is capable of mediating direct binding to the TIR domains of TLR4 and MyD88 simultaneously. We also found that Mal-TIR can form a back-to-back dimer that may resemble the dimeric assembly of the entire Mal molecule. Our data demonstrate the bridge role of the Mal-TIR domain and provide important information about TIR domain specificity.
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Project description:Among the diseases caused by Toll-like receptor 4 (TLR4) abnormal activation by bacterial endotoxin, sepsis is the most dangerous one. The reprogramming of macrophages plays a crucial role in orchestrating the pathogenesis of sepsis. However, the precise mechanism underlying TLR4 activation in macrophages remained incompletely understood. Our studies revealed that upon lipopolysaccharide (LPS) stimulation, CREB-binding protein (CBP) was recruited to the TLR4 signalosome complex and resulted in pronounced acetylation in the TIR domains of TLR4, Myeloid differentiation factor 88 (MyD88) and MyD88 adapter-like (MAL), which significantly enhanced the activation of the NF-κB signaling pathway and polarization of M1 macrophages. In sepsis patients, significantly elevated TLR4-TIR acetylation was detected in CD16+ monocytes combined with elevated expression of M1 macrophage markers and production of pro-inflammatory cytokines. In contrast, histone deacetylase 1 (HDAC1) served as a key deacetylase in the deacetylation of the TIR domain complex. The inhibition of HDAC1 accelerated sepsis-associated syndromes, while the inhibition of CBP alleviated this process. Overall, our findings highlighted the crucial role of TIR domain complex acetylation in the regulation of inflammatory immune response and suggested that the reversible acetylation of the complex emerged as a promising therapeutic target for M1 macrophages during the progression of sepsis.
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