Sphingosine 1-phosphate analogue recognition and selectivity at S1P4 within the endothelial differentiation gene family of receptors

Synergistic computational and experimental studies provided previously unforeseen details concerning the structural basis of S1P (sphingosine 1-phosphate) recognition by the S1P(4) G-protein-coupled receptor. Similarly to reports on the S1P(1) receptor, cationic and anionic residues in the third transmembrane domain (R3.28 and E3.29 at positions 124 and 125) form ion pairs with the phosphate and ammonium of S1P, and alanine mutations at these positions abolished specific S1P binding, S1P-induced receptor activation and cell migration. Unlike findings on the S1P(1) receptor, no cationic residue in the seventh transmembrane domain interacts with the phosphate. Additionally, two previously undiscovered interactions with the S1P polar headgroup have been identified. Trp(186) at position 4.64 in the fourth transmembrane domain interacts by a cation-pi interaction with the ammonium group of S1P. Lys(204) at position 5.38 forms an ion pair with the S1P. The S1P(4) and S1P(1) receptors show differences in binding-pocket shape and electrostatic distributions that correlate with the published structure-activity relationships. In particular, the binding pocket of mS1P(4) (mouse S1P(4)) has recognition sites for the anionic phosphate and cationic ammonium groups that are equidistant from the end of the non-polar tail. In contrast, the binding pocket of hS1P(1) (human S1P(4)) places the ammonium recognition site 2 angstrom (1 angstrom = 0.1 nm) closer to the end of the non-polar tail than the phosphate recognition site.