Fig. 7. MAL^TIR nucleates MyD88^TIR assembly formation unidirectionally.

a Time-lapse imaging of MyD88^TIR microcrystal formation. Representative images of microcrystals growing from single GFP-MAL^TIR-MyD88^TIR and MAL^TIR-MyD88^TIR seeds are shown. The seeds were washed to remove MAL and then mixed with MyD88^TIR. Data are representative of five independent experiments. Asterisks denote seeds with unidirectional growth. Scale bars: left panel 5 µm; middle and right panels 10 µm. b Ribbon diagrams of MyD88^TIR (NMR solution structure of monomeric MyD88^TIR (PDB ID 2Z5V) and higher-order assembly structure) and MAL^TIR (NMR solution structure of monomeric MAL^TIR (PDB ID 2NDH) and higher-order assembly cryo-EM structure (PDB 5UZB)), highlighting the rearrangement of the BB loop and αB helix (magenta in MyD88^TIR and green in MAL^TIR) during the monomer-to-oligomer transition. c Two models of interstrand interactions, transitioning between MyD88^TIR monomer (yellow) and MyD88^TIR higher-order assembly (blue): (i) EE surface of MyD88^TIR monomer docks onto BB surface of MyD88^TIR higher-order assembly. This interaction does not require any conformational changes in the BB loop and αB helix to occur prior to binding. (ii) BB surface of MyD88^TIR monomer docks onto EE surface of MyD88^TIR higher-order assembly. This interaction requires significant conformational changes in the BB loop and αB helix to occur prior to binding and is therefore less favoured. d Model of MyD88^TIR unidirectional assembly formation. The conformational changes in BB loop and αB helix required for the recruitment of new TIR domain subunits are induced by interstrand interactions. The higher-order assembly conformations of MAL^TIR and MyD88^TIR, and the monomeric conformation of MyD88^TIR are shown in orange, blue and yellow, respectively.