Structural and functional characterization of the human formyl peptide receptor ligand-binding region

The formyl peptide (N-formyl-1-methionyl-1-leucyl-1-phenylalanine [FMLP]) receptor is involved in the activation of neutrophils and their subsequent response to chemotactic N-formulated peptides. Recently, we found that the first extracellular loop closest to the N-terminal end of the FMLP receptor exhibited the strongest ligand binding compared with that shown by other extracellular regions. By constructing amino acid substitutional variants of this domain, we have determined that residues Arg-84 and Lys-85 on this loop play major roles in ligand-binding activity. Furthermore, random rearrangement of the residues of this receptor region demonstrated that the position of these charged amino acids did not affect their involvement in ligand binding, although their presence was essential for this binding to occur. We propose that the portion of the first N-terminal extracellular loop of the FMLP receptor containing residues Arg-84 and Lys-85 contributes significantly to the active site in ligand-receptor binding. We further propose that this binding is not dependent on defined structure but rather that these charged moieties may function as important 'contacts' in receptor- ligand interactions.