Species and subtype variants of the N-formyl peptide chemotactic receptor reveal multiple important functional domains.

The N-formyl peptide receptor (FPR) is a seven transmembrane-domain receptor that mediates trafficking and activation of phagocytic leukocytes in response to N-formyl oligopeptides such as fMet-Leu-Phe. cDNAs for high affinity FPRs have been cloned from both human (huFPR) and rabbit (rabFPR). To identify functional domains of FPR, we have studied two structurally related "natural mutants" that are 100-10,000-fold less sensitive than huFPR and rabFPR to fMet-Leu-Phe owing to sequence differences that are located predominantly in the proposed extracellular and transmembrane domains. The first is murine FPR (muFPR, 76% identical to huFPR) whose gene we have now cloned and expressed in Xenopus oocytes; the second is the previously reported human FPR-like 1 receptor (FPRL1R, 69% identical to huFPR) which was used to construct huFPR-FPRL1R chimeras. Comparison of the structure and function of huFPR, FPRL1R, muFPR, rabFPR, and huFPR-FPRL1R chimeras suggests that multiple non-contiguous residues must be apposed by coordinate folding of all of the extracellular and transmembrane domains in order to form the high affinity fMLF-binding site.