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Structural similarity between the hydrophobic fluorescent probe and lipid A as a ligand of MD‐2

Toll‐like receptors (TLRs) belong to the family of pattern recognition receptors, as they recognize molecules sharing a broad structural pattern rather than a single defined structure. Bacterial LPS is recognized by MD‐2, which is associated with the extracellular domain of TLR4. Understanding the molecular recognition pattern of MD‐2 could lead to efficient inhibitors of the excessive LPS signaling needed for early treatment of sepsis. The effect of the acyl chain variability of lipid A on its biological activity indicates that in addition to electrostatic interactions, the recognition must also involve hydrophobic interactions. We show that the fluorescent hydrophobic probe bis‐ANS binds to MD‐2 with a dissociation constant in the 10 nanomolar range, both to glycosylated and to nonglycosylated MD‐2, and requires its native conformation. The binding site of bis‐ANS overlaps with the binding site of LPS and is in the proximity of the single tryptophan residue. Furthermore, photoincorporation of bis‐ANS by UV light inhibits the ability of MD‐2 to confer the LPS responsiveness to the TLR4‐transfected HEK293 cell line. Our results show that the structural pattern recognized by MD‐2 is defined by the hydrophobic patch and a pair of separated negative charges.—Mancek‐Keber, M., Jerala, R. Structural similarity between the hydrophobic fluorescent probe and lipid A as a ligand of MD‐2. FASEB J. 20, 1836–1842 (2006)