Molecular analysis of the binding mode of Toll/interleukin-1 receptor (TIR) domain proteins during TLR2 signaling

Toll-like receptor (TLR) signaling is initiated by the binding of various adaptor proteins through ligand-induced oligomerization of the Toll/interleukin-1 receptor (TIR) domains of the TLRs. TLR2, which recognizes peptidoglycans, lipoproteins or lipopeptides derived from Gram-positive bacteria, is known to use the TIR domain-containing adaptor proteins myeloid differentiating factor 88 (MyD88) and MyD88 adaptor-like (Mal). Molecular analyses of the binding specificity of MyD88, Mal, and TLR2 are important for understanding the initial defenses mounted against Gram-positive bacterial infections such as Streptococcus pneumoniae. However, the detailed molecular mechanisms involved in the multiple interactions of these TIR domains remain unclear. Our study demonstrates that the TIR domain proteins MyD88, Mal, TLR1, and TLR2 directly bind to each other in vitro. We have also identified two binding interfaces of the MyD88 TIR domain for the TLR2 TIR domain. A residue at these interfaces has recently been found to be mutated in innate immune deficiency patients. These novel insights into the binding mode of TIR proteins will contribute to elucidation of the mechanisms underlying innate immune deficiency diseases, and to future structural studies of hetero-oligomeric TIR–TIR complexes.

► Binding mode of TIR domain proteins during TLR2 signaling.
► Two common binding sites of MyD88 TIR domain for TLR2 and Mal TIR domain.
► Interaction kinetics for TLR2 with MyD88 is similar to TLR2 with Mal.
► A mutation of MyD88, R196C, does not affect the protein folding.
► This mutation disrupts TIR–TIR interactions and causes MyD88-deficiency.