Toll-like receptor 4, commonly abbreviated TLR4, is a membrane receptor involved in innate immunity. Together with the accessory protein MD-2, it recognizes lipopolysaccharide and helps couple extracellular recognition to intracellular signal transduction. The relevant recognition unit is a receptor complex, rather than TLR4 acting independently of every partner.[ref]
The structural basis of lipopolysaccharide recognition by the TLR4–MD-2 complex by Beom Seok Park and others. Abstract, Nature 458 (2009), pp. 1191–1195; DOI 10.1038/nature07830.
A motivating experiment determined the complex's structure by X-ray crystallography. Lipopolysaccharide occupied a largely hydrophobic pocket in MD-2 and participated in the interface joining two TLR4–MD-2 units. This supplied a structural explanation for how binding can accompany assembly of a signaling-competent arrangement, instead of merely filling an isolated binding site.[ref]
The structural basis of lipopolysaccharide recognition by the TLR4–MD-2 complex by Beom Seok Park and others. Figures 1–3: overall complex, ligand pocket and dimerization interface.
The example connects chemical structure with receptor organization: different portions of one ligand can interact with an accessory protein and with the assembled receptor interface. A crystal structure constrains that recognition model; it does not by itself measure the magnitude or duration of every downstream immune response in a living organism.
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