Alignment of the protein sequences of the human FPRs. The putative transmembrane domains (I to VII) are shaded. Sequence of the FPR-98 isoform is shown. Comparison of the three receptors has identified highly conserved regions, including most of TM-I and TM-II, and the short intracellular loop connecting TM-I and TM-II. The second intracellular loops from these receptors, known for G protein interaction, are nearly identical. TM-VII, including the NPXXY motif and a stretch of ∼25 amino acids extending toward the C-terminal tail, are also conserved among these receptors. Major differences are found in the extracellular domains between FPR1 and the other two receptors, especially in the amino termini (∼50% different), the second extracellular loops (56% different), and the third extracellular loops (∼50% different). The two putative N-glycosylation sites in the N-terminal domains are conserved among all three receptors. Most of the serines and threonines in the C-terminal tail, along with charged residues that constitute consensus GRK phosphorylation sites, are also conserved among these receptors.

Alignment of the predicted receptor sequence of the mouse Fpr genes. The putative transmembrane domains (TM-I–TM-VII) are shaded. Dashes indicate gaps in sequence created for alignment purposes. It is noteworthy that there is an eight-residue insertion in the N-terminal region of Fpr-1, just before TM-I. A three-residue insertion is found in the second extracellular loop in Fpr-1. The positively charged residues Arg84 and Lys85, found in human FPR1 and known for the interaction with fMLF, are missing from Fpr-1 and other mouse Fpr-related sequences. In its place are the noncharged residues Ser92 and Met93. The predicted Fpr-rs5 sequence is truncated at amino acid 246, resulting in a putative protein with only five TMs. Fpr-rs4 encodes a protein of 323 residues with a short C-terminal tail. In Fpr-rs1, there is a four-residue deletion in TM-IV, whereas the cloned mouse LXA4 receptor gene encodes a protein with the sequence of ARNV in its place. Polymorphisms exist in the Fpr-rs1 gene that result in amino acid substitutions at positions 3 (Thr/Ser), 8 (Pro/His), 13 (Asp/Glu), 16 (Ile/Val), 222 (Thr/Tyr), 236 (Phe/Ser), 296 (Ile/Met), and 318 (Gln/Pro) (Takano et al., 1997; Gao et al., 1998; Wang et al., 2002). The highest sequence identity (81%) is found between Fpr-rs1 and Fpr-rs2, and between Fpr-rs3 and Fpr-rs4.

Sequence homology between the FPR family members and their tissue distribution. The predicted protein sequences of the three human (h) FPR genes, the eight mouse (m) Fpr genes, and the rabbit (r) FPR1 gene were compared (Boulay et al., 1990a,b; Ye et al., 1993; Gao et al., 1998; Wang et al., 2002). Based on sequence homology, the hFPR1, mFpr1, and rFPR1 are in the same cluster. The mFpr-rs1, mFpr-rs2 (also termed mFpr2), and mFpr-rs8 are in another cluster closely related to hFPR2/ALX and hFPR3. The mFpr-rs3, mFpr-rs4, mFpr-rs6, and mFpr-rs7 (and, to a lesser extent, mFpr-rs5) are closely related based on their protein sequences (see Table 4 for sequence identity between the gene products). Note that some of these genes are not expressed in neutrophils and monocytes. The tissue expression profiles for mFpr-rs4, mFpr-rs5, and mFpr-rs8 have not been determined. Mo, monocytes; PMN, polymorphnuclear leukocytes; iDC, immature dendritic cells; astro, astrocytes; T, T lymphocytes.

Chemical structures of selected ligands for the formyl peptide receptors. Despite their abilities to bind to FPR1 and/or FPR2/ALX, these ligands have quite different structures. Of the ligands shown, t-Boc-FLFLF and CsH are antagonists and others are agonists. Note that the N-formyl group that defines agonistic activities in peptides such as fMLF is replaced with a bulky t-butyloxycarbonyl group that defines antagonistic activities in peptides such as t-Boc-FLFLF. LXA4, Quin-C1, and compound 43 are highly selective agonists for FPR2/ALX, whereas AG-14 and fMLF are selective for FPR1. t-Boc-FLFLF is selective for FPR1 at low concentrations but the selectivity is lost at high micromolar concentrations (e.g., 100 μM).