Identification of an N-formyl peptide receptor ligand binding domain by a gain-of-function approach.

Replacement of N-formyl peptide receptor (FPR) domains with those from a homologous receptor, FPR2, resulted in chimeric receptors displaying low binding affinity to fMet-Leu-Phe (fMLF). To characterize fMLF binding domain, we adopted a "gain-of-function" approach by selective replacement of non-conserved residues in the FPR2 portion of the chimeric receptors with those from the FPR. This led to the identification of 3 clusters of residues required for high-affinity fMLF binding. Introduction of 2 positively charged amino acids, Arg84 and Lys85, dramatically improved binding affinity of one chimeric receptor (Kd from 105 nM to 1.6 nM). Similarly, restoration of either Gly89/His90 or Phe102/Thr103 improved the binding affinity of another chimeric receptor from a Kd of 275 nM to a 2.3 Kd and 3.3 nM, respectively. Increased ligand binding affinity was accompanied by a gain in calcium mobilization capability, suggesting functional coupling to G proteins. These results demonstrate the presence of structural determinants in the first extracellular loop and its adjacent transmembrane domains that are essential for high affinity fMLF binding.

cDNA cloning of a novel G protein-coupled receptor with a large extracellular loop structure.

A cDNA designated as AZ3B has been isolated from a differentiated HL-6 0 cell cDNA library with a probe derived from the N-formyl peptide receptor gene. The 1.97-kb cDNA encodes a novel G protein-coupled receptor (GPCR) with 482 amino acids. In addition to the predicted 7 transmembrane domains common to all GPCRs, the protein encoded by AZ3B contains a large extracellular loop of approximately 172 amino acids between the fourth and the fifth transmembrane domains, a feature unique among the hundreds of GPCRs identified to date. High sequence homology exists between the AZ3B protein and a number of chemoattractant receptors in the amino-terminal 170 residues and the carboxyl-terminal 150 residues. Northern and flow cytometric analyses suggested that the AZ3B message and protein are widely expressed in several differentiated hematopoietic cell lines, in the lung, placenta, heart, and endothelial cells. We postulate that the AZ3B protein defines a distinct group of receptors within the GPCR superfamily.

Multiple domains of the N-formyl peptide receptor are required for high-affinity ligand binding. Construction and analysis of chimeric N-formyl peptide receptors.

Binding of the chemotactic tripeptide fMet-Leu-Phe (fMLP) to its receptor on phagocytes activates these cells through a G protein-coupled pathway. To delineate the structural requirement of the N-formyl peptide receptor (FPR) for ligand binding and signaling, we constructed chimeric receptors between FPR and a recently identified granulocyte receptor, FPR2 (Ye, R. D., Cavanagh, S. L., Quehenberger, O., Prossnitz, E. R., and Cochrane, C. G. (1992) Biochem. Biophys. Res. Commun. 184, 582-589). FPR2 shares 69% sequence homology with the FPR; yet it binds fMLP with a low affinity (Kd = 430 nM), as compared with the high affinity (Kd = 1 nM) displayed by the FPR. This property of the FPR2 was utilized for mapping the FPR ligand binding domains. Seven chimeric FPR/FPR2 receptors were generated by sequential replacement of the FPR segments with the corresponding regions from FPR2. Three reciprocal FPR2/FPR chimeric receptors were also constructed by selective substitution of the FPR segments into FPR2. These chimeric receptors were stably expressed in transfected fibroblasts and analyzed for their ligand binding and transmembrane signaling properties. Replacement of the FPR domains, including the first and the third extracellular loops, resulted in 275- and 85-fold decrease in ligand binding affinity, respectively. Introduction of both domains into the FPR2 significantly increased ligand binding affinity (Kd = 18 nM), whereas substitution of the domains containing the first or third extracellular loop alone improved ligand binding to a lesser degree (Kd = 90 and 372 nM, respectively). In contrast, substitution of either the amino or the carboxyl-terminal regions with those of the FPR2 had little effect on ligand binding affinity. An analysis of the sequences of the two receptors revealed several key residues in the first and the third extracellular loops of the FPR and their adjacent transmembrane domains that may be essential for binding of fMLP. We propose that multiple domains of the FPR are required for high-affinity ligand binding, with a major determinant located in the first extracellular loop and its adjacent transmembrane domains.

The rabbit neutrophil N-formyl peptide receptor. cDNA cloning, expression, and structure/function implications.

The rabbit neutrophil N-formyl peptide receptor (FPR) has been well studied for its ligand binding properties. Recent gene cloning experiments have established the existence of a subfamily of G protein-coupled receptors that share extensive sequence homology with the FPR, yet lack the capability of high affinity binding to FMLP. These findings prompted us to identify the structural requirement for formyl peptide ligand binding by delineation of the primary structure of the rabbit FPR. A rabbit neutrophil cDNA library was screened with a cloned human FPR cDNA probe and the insert of one positive isolate (B6) was sequenced. The 1268-bp cDNA insert encodes a peptide of 352 amino acids. Stably transfected L cell fibroblasts expressing the rabbit cDNA displayed specific binding of the ligand fMet-Leu-[3H]Phe with two affinities (Kd = 0.31 and 7.5 nM). Addition of the nonhydrolyzable guanosine triphosphate analogue, GTP gamma S, converted > or = 85% of the high affinity sites to the low affinity sites. FMLP induced mobilization of intracellular calcium in the transfected cells (EC50 = 0.5 nM), a response sensitive to pertussis toxin. FMLP stimulation desensitized the receptor such that subsequent stimulation with the same ligand produced a significantly reduced signal. These results indicate that the cloned rabbit receptor represents a high affinity FPR, and that FPR-mediated early signal transduction events can be fully reconstituted in transfected mammalian cells. The rabbit FPR sequence is 78% identical to that of the human FPR, and 68% identical to FPR2, a homologue of FPR with a low binding affinity (Kd > or = 400 nM) for FMLP. Analysis of the aligned sequences of these three proteins revealed that: 1) the amino termini and the second extracellular loops have the lowest sequence homology; 2) sequence in the intracellular domains that couple to G protein are highly conserved; and 3) the first and the third extracellular loops and their adjacent transmembrane domains of the FPR may contain residues essential for the high affinity binding of FMLP.

Isolation of a cDNA that encodes a novel granulocyte N-formyl peptide receptor.

A cDNA of 1650 base pairs was isolated by screening an HL-60 granulocyte library with an N-formyl peptide receptor (NFPR) cDNA probe under low stringency conditions. The cDNA encodes a protein of 351 amino acids tentatively named FPR2, with a calculated molecular weight of 39 kDa. Sequence analysis revealed that FPR2 is 69% identical in sequence to the human NFPR and shares extensive homology to several other chemoattractant receptors. FPR2 expressed in transfected cells mediated formyl peptide-stimulated calcium mobilization at micromolar concentrations of ligand. FPR2 messenger is detected in granulocytic HL-60 cells, but not in undifferentiated HL-60 cells. These findings suggest that FPR2 is a novel receptor for formyl peptide ligand and a new member of the chemoattractant receptor gene family.