The structural basis of lipopolysaccharide recognition by the TLR4–MD-2 complex

Abstract

The lipopolysaccharide (LPS) of Gram negative bacteria is a well-known inducer of the innate immune response. 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 approximately 5 A towards the solvent area. 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 family, which is essential for defence against diverse microbial infection.

Intrinsic and Extrinsic Regulation of Innate Immune Receptors

“…TLR4 recognizes lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria, with the assistance of co-receptors such as CD14 and MD2 16,17. LPS binds first to LPS binding protein (LBP) and membrane-bound GPI (glycosylphosphatidylinositol)-anchored CD14, and is then transferred to the TLR4 and MD2 (myeloid differentiation proten-2) complexes 18,19. In the MD2 complex, LPS binds to a large hydrophobic pocket, through non-covalent interactions such as hydrogen bonding and hydrophobic and hydrophilic interactions, which results in the dimerization of the two TLR4/MD2 complexes.…”

Comparative Biology of the Pentraxin Protein Family: Evolutionarily Conserved Component of Innate Immune System

“…Other species of Vibrio have been found to secrete serine (Park et al, 2009) and metalloproteases (Chang et al, 2005;Kwon et al, 2007) with fibrinolytic activities to proteins of the human clotting system. To our knowledge, this present study is the first to report a similar situation in arthropods in which an arthropod pathogen is allowed to interact with an arthropod blood clot.…”

Extracts of Larix Leptolepis effectively augments the generation of tumor antigen-specific cytotoxic T lymphocytes via activation of dendritic cells in TLR-2 and TLR-4-dependent manner

“…TLR2/4 interacts with other molecules (e.g., CD36, TLR1, TLR6 and CD14 etc.) and each complex recognizes different ligands, respectively [43][44][45]. These post-transcriptional regulations might be responsible for recognition of components of ELL by TLR2/4.…”