The phospholipid growth factors sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA) are ligands for the related G protein-coupled receptors S1P 1 /EDG1 and LPA 1 /EDG2, respectively. We have developed a model of LPA 1 that predicts interactions between three polar residues and LPA. One of these, glutamine 125, which is conserved in the LPA receptor subfamily (LPA 1 /EDG2, LPA 2 /EDG4, and LPA 3 /EDG7), hydrogen bonds with the LPA hydroxyl group. Our previous S1P 1 study identified that the corresponding glutamate residue, conserved in all S1P receptors, ion pairs with the S1P ammonium. These two results predict that this residue might influence ligand recognition and specificity. Characterization of glutamate/glutamine interchange point mutants of S1P 1 and LPA 1 validated this prediction as the presence of glutamate was required for S1P recognition, whereas LPA recognition was possible with either glutamine or glutamate. The most likely explanation for this dual specificity behavior is a shift in the equilibrium between the acid and conjugate base forms of glutamic acid due to other amino acids surrounding that position in LPA 1 , producing a mixture of receptors including those having an anionic glutamate that recognize S1P and others with a neutral glutamic acid that recognize LPA. Thus, computational modeling of these receptors provided valid information necessary for understanding the molecular pharmacology of these receptors.
Lysophosphatidic acid (LPA)1 and sphingosine-1-phosphate (S1P, see Fig. 1A) are members of the phospholipid growth factor family (for reviews, see Refs. 1-3). The responses elicited by phospholipid growth factors are pleiotropic and include the enhancement of cell survival, induction of cell proliferation, regulation of the actin-based cytoskeleton affecting cell shape, adherence, and chemotaxis, and the activation of Cl Ϫ and Ca 2ϩ ion conductances. LPA has been implicated in a number of disease and injury states, due to elevated levels of LPA in fluids surrounding the tissues involved, including corneal injury, lung disease, atherosclerosis, ovarian cancer, and wound healing. The eight receptors in the endothelial differentiation gene (EDG) family encode G protein-coupled receptors activated by the phospholipid growth factors LPA and S1P (4, 5). The EDG family is subdivided into two clusters based on ligand selectivity. S1P 1/3/2/4/5 receptors (formerly EDG1/3/5/6/8) are specifically activated by S1P (4), whereas LPA 1/2/3 receptors (formerly EDG2/4/7) are specifically activated by LPA (5). Members of the S1P receptor subfamily display 40 -50% sequence identity to each other and 30 -35% identity to the members of the LPA receptor subfamily (5, 6). These homologies and a distant relatedness to the cannabinoid receptors (7,8) suggest that the LPA-and S1P-specific subfamilies may have evolved from a common ancestral lipid receptor through the evolutionary development of distinct ligand binding pockets. If so, ligand selectivity should be determined by a limited number of cons...