Biochemical characterization of the TLR4-TIR domain complex

Abstract

The Toll/interleukin-1 receptor (TIR) domain is a highly conserved signaling domain found in the intracellular regions of Toll-like receptors (TLRs), in interleukin-1 receptors, and in several cytoplasmic adaptor proteins, such as Myeloid differentiating factor 88 (MyD88), MyD88 adaptor-like (MAL), TIR-domain-containing adaptor protein inducing IFNβ (TRIF), TRIF-related adaptor molecule (TRAM) and sterile $\alpha$ - and armadillo-motif containing protein (SARM) in mammalian. TIR domains in plants exist as a component of a family of multi-domain proteins known as resistance (R) genes. These R proteins also contain a nucleotide-binding (NB) domain and/or a leucine-rich repeat (LRR) region. And then, bacteria also have TIR domain-containing protein. TIR domains mediate receptor signal transduction through recruitment of adaptor proteins and play critical roles in the innate immune response and inflammation. Of these, MyD88 and Mal are adaptor molecules critically involved in the TLR4 signaling pathway. While Mal has been proposed to serve as a membrane-sorting adaptor, MyD88 mediates signal transduction from activated TLR4 to downstream components. The TIR domain of MyD88 is responsible for sorting and signaling via direct or indirect TIR-TIR interactions between Mal and TLR4. However, the molecular mechanisms of TIR domains involved in multiple interactions through TLR4 signaling remain undefined. In this paper, we make stable TLR4-MyD88-MAL TIR domain complex, and obtain its crystal. These results are expected to enhance the understanding of the structure of the TLR4-MyD88-MAL TIR complex.