Differential adapter recruitment by TLR2 co-receptors

TLR2 heterodimers with TLR1 or TLR6 recognize distinct pathogen-associated molecules such as tri- and di-acylated lipopeptides. The activated TLR2 heterodimers recruit Toll-IL-1R domain- (TIR-) containing adapter proteins, TIRAP and MyD88, through the receptor TIR domains. Molecular recognition mechanisms responsible for agonist-driven, TIR domain-mediated receptor-adapter interactions as well as the structure of resultant signaling complexes remain unknown. We previously reported that the cell-permeable peptide derived from helix D of TLR2 TIR (2R9) specifically binds TIRAP in vitro and in cells and thereby inhibits TIRAP-dependent TLR signaling. This study demonstrates that cell-permeable peptides from D helix of TLR1 or TLR6, peptides 1R9 and 6R9 respectively, inhibit signaling mediated by cognate TLR2 co-receptors. Interestingly, 1R9 and 6R9 bind different TLR2 adapters, as they selectively bind MyD88 and TIRAP TIR, respectively. Both peptides block the agonist-induced co-immunoprecipitation (co-IP) of TLR2 with TIRAP or MyD88, but not TLR2 co-IP with co-receptors. Our data suggest that D helices of TLR1 and TLR6 TIR domains are adapter recruitment sites in both co-receptors; yet the sites recruit different adapters. The D helix in TLR1 is the MyD88 docking site, whereas in TLR6 this site recruits TIRAP.