Functional expression of the human formyl peptide receptor in Xenopus oocytes requires a complementary human factor

Human phagocytic cells express receptors for bacterial N-formyl peptides (formyl peptide receptor or FPR) which mediate chemotaxis, degranulation, and the respiratory burst. Although cDNA encoding a human phagocyte formyl peptide-binding protein has been reported recently (Boulay, F., Tardif, M., Brouchon, L., and Vignais, P. (1990) Biochem. Biophys. Res. Commun. 168, 1103-1109), functional coupling to signal transduction processes was not demonstrated. We describe corresponding full-length cDNA clones and prove that they encode the calcium-mobilizing human formyl peptide receptor by demonstrating functional reconstitution in the Xenopus oocyte. We further demonstrate that in contrast to all other cloned guanine nucleotide-binding regulatory protein (G-protein) coupled receptors expressed in this system, microinjection of FPR transcripts is not sufficient to confer ligand responsiveness to the oocyte: co-injection of phagocyte RNA encoding a complementary human factor that is not the alpha subunit of the heterotrimeric G-proteins Gi1, Gi2 or Gi3 is also required. Whereas a 1.4-kilobase FPR transcript is expressed exclusively in differentiated phagocytic cells, the complementary factor activity localizes to a 3.5-kilobase RNA fraction and is expressed in both differentiated and undifferentiated myeloid cells as well as in liver. The deduced human FPR protein possesses seven hydrophobic putative membrane spanning segments, three sites for N-linked glycosylation, and a short 18-amino acid predicted third cytoplasmic loop. Surprisingly, the human FPR possesses only 28% amino acid identity with the rabbit FPR reported recently by Thomas and co-workers (Thomas, K. M., Pyun, H. Y., and Navarro, J. (1990) J. Biol. Chem. 265, 20061-20065). Moreover, the rabbit FPR does not require a complementary factor for calcium mobilization in the oocyte. Structural alignment reveals at most 20% amino acid identity of the human FPR with other G-protein coupled receptors, indicating a common ancestral gene. Functional reconstitution of the recombinant FPR will now permit precise delineation of its functional and regulatory domains. Moreover, discovery of a complementary factor for oocyte expression of the human FPR establishes a novel approach to the qualification by ligand screening of cDNA encoding other suspected G-protein coupled receptors.