The molecular model of the active TLR4/MD-2/LPS homodimer, which was suggested by our research team and afterwards confirmed by the determination of its crystal structure, provides us with an excellent base to investigate the molecular mechanism of negative regulation of TLR4 signaling by RP105 and its co-receptor MD-1. Briefly, the lipid A moiety of monomeric LPS binds to the hydrophobic pocket of MD-2, which causes dimerization of the TLR4 membrane receptor mediated by two binding sites on the extracellular domain of TLR4. Based on the homology between the RP105/MD-1 and TLR4/MD-2 pairs and known biochemical data, we proposed a hypothesis that the RP105/MD-1:TLR4/MD-2 heterodimerizes and prevents activation due to the lack of TIR domain in RP105. To test our hypothesis, we prepared a chimeric protein where the TIR domain of TLR4 is attached to the ectodomain of RP105. We found that the chimeric protein acts as an even better inhibitor of TLR4 signaling than RP105 alone, which tells us that lack of the TIR domain in RP105 is not the only factor for its inhibitory effect. On the basis of our experimental results, we proposed a revised model of TLR4/MD-2:RP105/MD-1 interaction.
COBISS.SI-ID: 4829210