The lipopolysaccharide (LPS) of Gram negative bacteria is a well-known inducer of the innate immune response1. Toll-like receptor (TLR) 4 and myeloid differentiation factor 2 (MD-2) form a heterodimer that recognizes a common ‘pattern’ in structurally diverse LPS molecules. To understand the ligand specificity and receptor activation mechanism of the TLR4–MD-2–LPS complex we determined its crystal structure. LPS binding induced the formation of an m-shaped receptor multimer composed of two copies of the TLR4–MD-2–LPS complex arranged symmetrically. LPS interacts with a large hydrophobic pocket in MD-2 and directly bridges the two components of the multimer. Five of the six lipid chains of LPS are buried deep inside the pocket and the remaining chain is exposed to the surface of MD-2, forming a hydrophobic interaction with the conserved phenylalanines of TLR4. The F126 loop of MD-2 undergoes localized structural change and supports this core hydrophobic interface by making hydrophilic interactions with TLR4. Comparison with the structures of tetra-acylated antagonists bound to MD-2 indicates that two other lipid chains in LPS displace the phosphorylated glucosamine backbone by ∼5 Å towards the solvent area2,3. This structural shift allows phosphate groups of LPS to contribute to receptor multimerization by forming ionic interactions with a cluster of positively charged residues in TLR4 and MD-2. The TLR4–MD-2–LPS structure illustrates the remarkable versatility of the ligand recognition mechanisms employed by the TLR family4,5, which is essential for defence against diverse microbial infection.
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We thank the staff of beamline 4A at the Pohang Accelerator Laboratory and beamline ID23-2 at ESRF for help with data collection. We thank J. Gross for critical reading of the manuscript. J.-O.L and co-workers are funded by the Creative Research Initiative (Center for Membrane Receptor Research) from the Ministry of Education, Science and Technology of Korea.
Author Contributions B.S.P., D.H.S. and H.M.K. performed the experiments. B.S.P. and J.-O.L. designed the experiments and analysed the data. B.S.P., H.L. and J.-O.L. wrote the paper. J.-O.L. managed the project and had overall responsibility for data interpretation and writing the manuscript. All authors discussed and commented on the manuscript.
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Park, B., Song, D., Kim, H. et al. The structural basis of lipopolysaccharide recognition by the TLR4–MD-2 complex. Nature 458, 1191–1195 (2009). https://doi.org/10.1038/nature07830
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