Structure-based Analysis of GPCR Function: Conformational Adaptation of both Agonist and Receptor upon Leukotriene B4 Binding to Recombinant BLT1

We produced the human leukotriene B4 (LTB4) receptor BLT1, a G-protein-coupled receptor, in Escherichia coli with yields that are sufficient for the first structural characterization of this receptor in solution. Overexpression was achieved through codon optimization and the search for optimal refolding conditions of BLT1 recovered from inclusion bodies. The detergent-solubilized receptor displays a 3D-fold compatible with a seven transmembrane (TM) domain with ca 50% α-helix and an essential disulfide bridge (circular dichroism evidence); it binds LTB4 with Ka=7.8(±0.2)×108 M−1 and a stoichiometric ratio of 0.98(±0.02). Antagonistic effects were investigated using a synthetic molecule that shares common structural features with LTB4. We report evidence that both partners, LTB4 and BLT1, undergo a rearrangement of their respective conformations upon complex formation: (i) a departure from planarity of the LTB4 conjugated triene moiety; (ii) a change in the environment of Trp234 (TM-VI helix) and in the exposure of the cytoplasmic region of this transmembrane helix.