Abstract text (incl. figure legends and references)

Toll-like receptors (TLRs) activate the innate immune response during pathogen infection. Upon ligand-induced receptor dimerization, intracellular adapter proteins, like TIRAP (TIR-domain containing adaptor protein), initiate the assembly of large signaling complexes. In resting TLR-positive cells, TIRAP multimerizes into filaments at the plasma membrane and is subsequently converted into dimers during TLR signaling. However, the exact process of TIRAP filament formation and interaction with TLR dimers remains unknown.

Here, we utilize negative-stain and cryo-EM to investigate the mechanism of TIRAP multimerization. We demonstrate that the full-length human TIRAP forms filaments in a temperature-dependent and reversible manner in the absence of TLRs. Our 3.2 Å cryo-EM structure, together with the negative-stain analysis, indicates a sequential filament formation, starting from stable dimers that polymerize into thin filamentous intermediates and tubes with a diameter of about 30 nm. Together, our data support the presence of TIRAP filaments in resting TLR-positive cells. Furthermore, filamentous TIRAP may provide a pool of required adapter proteins to stimulate TLR-based signal transduction.