= Toll-like receptor 4
{c}
\b[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.<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.]{ref}
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.<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.]{ref}
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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