Transfer of phage-related pathogenicity islands of Staphylococcus aureus (SaPI-s) was recently reported to be activated by helper phage dUTPases. This is a novel function for dUTPases otherwise involved in preservation of genomic integrity by sanitizing the dNTP pool. Here we investigated the molecular mechanism of the dUTPase-induced gene expression control using direct techniques. The expression of SaPI transfer initiating proteins is repressed by proteins called Stl. We found that Φ11 helper phage dUTPase eliminates SaPIbov1 Stl binding to its cognate DNA by binding tightly to Stl protein. We also show that dUTPase enzymatic activity is strongly inhibited in the dUTPase:Stl complex and that the dUTPase:dUTP complex is inaccessible to the Stl repressor. Our results disprove the previously proposed G-protein-like mechanism of SaPI transfer activation. We propose that the transfer only occurs if dUTP is cleared from the nucleotide pool, a condition promoting genomic stability of the virulence elements.

Originating from its DNA sequence, a computational model of the Edg1 receptor has been developed that predicts critical interactions with its ligand, sphingosine 1-phosphate. The basic amino acids Arg 120 and Arg 292 ion pair with the phosphate, whereas the acidic Glu 121 residue ion pairs with the ammonium moiety of sphingosine 1-phosphate. The requirement of these interactions for specific ligand recognition has been confirmed through examination of site-directed mutants by radioligand binding, ligand-induced [35 S]GTP␥S binding, and receptor internalization assays. These ion-pairing interactions explain the ligand specificity of the Edg1 receptor and provide insight into ligand specificity differences within the Edg receptor family. This computational map of the ligand binding pocket provides information necessary for understanding the molecular pharmacology of this receptor, thus underlining the potential of the computational method in predicting ligand-receptor interactions.The G protein-coupled receptor (GPCR) 1 superfamily includes more than 2000 genes encoding receptors for bioactive molecules ranging from hormones through neurotransmitters (1). The physiological significance of these ligands makes GPCRs the target of many drugs and the subject of drug development studies. GPCRs are integral membrane proteins with physical properties that vastly increase the difficulty of standard methods of structure analysis. A precise understanding of their ligand-receptor interactions is essential for the rational design of ligands. Therefore, an atomic resolution map of the binding pocket including the locations of the interactions necessary for ligand binding would provide crucial drug development information. The endothelial differentiation gene (Edg) receptors are GPCRs that are activated by lysophospholipids (Fig. 1). Five members of the Edg family (Edg1, Edg3, Edg5, Edg6, and Edg8) show a preference for sphingosine 1-phosphate (SPP) (2-7). The remaining three Edg family members (Edg2, Edg4, and Edg7) are activated by lysophosphatidic acid (LPA) (8, 9). The receptor-mediated effects of SPP include stimulation of cell proliferation, prevention of apoptosis, regulation of cell shape, adhesion, motility, vascular differentiation (10 -12), and cancer cell invasiveness (11, 13). Therefore, receptors for SPP are important targets for the design of receptor-specific ligands both as potential therapeutic agents and to assist in the elucidation of the physiological function of the receptor.Membership of the Edg receptors in the GPCR family confers significant homology with other GPCRs in regard to sequence, topology, and function (14). These shared features make the Edg receptors good candidates for homology modeling. Homology modeling uses a known template protein structure to build an analogous structure for a protein sequence having unknown structure. Homology modeling thus assumes that homologous function and amino acid sequences confer three-dimensional structural similarity. GPCRs have extracellular amino termini, ...

Lysophosphatidic acid (1-acyl-2-lyso-snglycero-3-phosphate, LPA) is a multifunctional lipid mediator found in a variety of organisms that span the phylogenetic tree from humans to plants. Although its physiological function is not clearly understood, LPA is a potent regulator of mammalian cell proliferation; it is one of the major mitogens found in blood serum. In Xenopus laevis oocytes, LPA elicits oscillatory Cl ؊ currents. This current, like other effects of LPA, is consistent with a plasma membrane receptor-mediated activation of G protein-linked signal transduction pathways. Herein we report the identification of a complementary DNA from Xenopus that encodes a functional high-affinity LPA receptor. The predicted structure of this protein of 372 amino acids contains features common to members of the seven transmembrane receptor superfamily with a predicted extracellular amino and intracellular carboxyl terminus. An antisense oligonucleotide derived from the first 5-11 predicted amino acids, selectively inhibited the expression of the endogenous high-affinity LPA receptors in Xenopus oocytes, whereas the same oligonucleotide did not affect the low-affinity LPA receptor. Expression of the full-length cRNA in oocytes led to an increase in maximal Cl ؊ current due to increased expression of the high-affinity LPA receptor, but activation of the low-affinity receptor was, again, unaffected. Oocytes expressing cRNA prepared from this clone showed no response to other lipid mediators including prostaglandins, leukotrienes, sphingosine 1-phosphate, sphingosylphosphorylcholine, and platelet-activating factor, suggesting that the receptor is highly selective for LPA.

We previously reported that fatty alcohol phosphates (FAP) represent a minimal pharmacophore required to interact with lysophosphatidic acid (LPA) receptors. To improve the activity of the first-generation saturated FAP series, a structure−activity relationship (SAR) study was carried out that includes modifications to the headgroup and alkyl side chain of the FAP pharmacophore. A series of unsaturated (C10−C18) FAP, headgroup-modified hydrolytically stable saturated (C10−C18) alkyl phosphonates, and saturated and unsaturated (C10−C18) thiophosphate analogues were synthesized and evaluated for activity in RH7777 cells transfected with individual LPA1 - 3 receptors, in PC-3 cells and in human platelets that endogenously express all three isoforms. In this series we identified several LPA1- and LPA3-selective antagonists with IC50 values in the nanomolar range. Oleoyl-thiophosphate (15g) was shown to be a pan-agonist, whereas tetradecyl-phosphonate (16c) was identified as a pan-antagonist. These compounds were also tested for the ability to activate the transcription factor PPARγ, an intracellular receptor for LPA, in CV1 cells transfected with the PPRE-Acox-Rluc reporter gene. All the FAP tested, along with the previously reported LPA GPCR antagonists dioctanoyl glycerol pyrophosphate (2), Ki16425 (6), and the agonist OMPT (3), were activators of PPARγ. The pan-agonist oleoyl-thiophosphate (15g) and pan-antagonist tetradecyl-phosphonate (16c) mimicked LPA in inhibiting autotaxin, a secreted lysophospholipase D that produces LPA in biological fluids.

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...

Activation of IK(ACh) is the major effect of the vagal neutrotransmitter acetylcholine in the heart. We report that both lysosphingomyelin (D‐erythro‐sphingosyl‐phosphorylcholine; SPC) and sphingosine 1‐phosphate (SPP) activate IK(ACh) in guinea pig atrial myocytes through the same receptor with an EC50 of 1.5 and 1.2 nM, respectively. Pertussis toxin abolished the activation of IK(ACh) by either lipid. The putative receptor showed an exquisite stereoselectivity for the naturally occurring D‐erythro‐(2S,3R)‐SPC stereoisomer, the structure of which was confirmed by mass spectroscopy and NMR. These lipids caused complete homologous and heterologous desensitization with each other but not with ACh, indicating that both act on the same receptor. This receptor displays a distinct structure‐activity relationship: it requires an unsubstituted amino group because N‐acetyl‐SPC, lysophosphatidic acid and lysophosphatidylcholine were inactive. Because SPP and SPC are naturally occurring products of membrane lipid metabolism, it appears that these compounds might be important extracellular mediators acting on a family of bona fide G protein‐coupled receptors. Expression of these receptors in the heart raises the possibility that sphingolipids may be a part of the physiological and/or pathophysiological regulation of the heart. Based on their ligand selectivity we propose a classification of the sphingolipid receptors.

No abstract

Lysophosphatidic acid (LPA), plasmalogen-glycerophosphate (alkenyl-GP) and, cyclic-phosphatidic acid (cyclic-PA) are naturally occurring phospholipid growth factors (PLGFs). PLGFs elicit diverse biological effects via the activation of G protein-coupled receptors in a variety of cell types. In NIH3T3 fibroblasts, LPA and alkenyl-GP both induced proliferation, whereas cyclic-PA was antiproliferative. LPA and alkenyl-GP decreased cAMP in a pertussis toxin-sensitive manner, whereas cyclic-PA caused cAMP to increase. LPA and alkenyl-GP both stimulated the activity of the mitogen-actived protein kinases extracellular signal regulated kinases 1 and 2 and c-Jun NH2-terminal kinase, whereas cyclic-PA did not. All three PLGFs induced the formation of stress fibers in NIH3T3 fibroblasts. To determine whether these lipids activated the same or different receptors, heterologous desensitization patterns were established among the three PLGFs by monitoring changes in intracellular Ca2+ in NIH3T3 fibroblasts. LPA cross-desensitized both the alkenyl-GP and cyclic-PA responses. Alkenyl-GP cross-desensitized the cyclic-PA response, but only partially desensitized the LPA response. Cyclic-PA only partially desensitized both the alkenyl-GP and LPA responses. We propose that pharmacologically distinct subsets of PLGF receptors exist that distinguish between cyclic-PA and alkenyl-GP, but are all activated by LPA. We provide evidence that the PSP24 receptor is selective for LPA and not activated by the other two PLGFs. RT-PCR and Northern blot analysis indicate the co-expression of mRNAs encoding the EDG-2, EDG-4, and PSP24 receptors in a variety of cell lines and tissues. However, the lack of mRNA expression for these three receptors in the LPA-responsive Rat-1 and Sp2-O-Ag14 cells suggests that a number of PLGF receptor subtypes remain unidentified.