Microcrystal electron diffraction structure of Toll-like receptor 2 TIR-domain-nucleated MyD88 TIR-domain higher-order assembly

Eukaryotic TIR (Toll/interleukin-1 receptor protein) domains signal via TIR-TIR interactions, either by self-association or by interaction with other TIR domains. In mammals, TIR domains are found in Toll-like receptors (TLRs) and cytoplasmic adaptor proteins involved in pro-inflammatory signaling. Previous work revealed that the MAL TIR domain (MAL^TIR) nucleates the assembly of MyD88^TIR into crystalline arrays in vitro. A microcrystal electron diffraction (MicroED) structure of the MyD88^TIR assembly has previously been solved, revealing a two-stranded higher-order assembly of TIR domains. In this work, it is demonstrated that the TIR domain of TLR2, which is reported to signal as a heterodimer with either TLR1 or TLR6, induces the formation of crystalline higher-order assemblies of MyD88^TIR in vitro, whereas TLR1^TIR and TLR6^TIR do not. Using an improved data-collection protocol, the MicroED structure of TLR2^TIR-induced MyD88^TIR microcrystals was determined at a higher resolution (2.85 Å) and with higher completeness (89%) compared with the previous structure of the MAL^TIR-induced MyD88^TIR assembly. Both assemblies exhibit conformational differences in several areas that are important for signaling (for example the BB loop and CD loop) compared with their monomeric structures. These data suggest that TLR2^TIR and MAL^TIR interact with MyD88 in an analogous manner during signaling, nucleating MyD88^TIR assemblies unidirectionally.