Close association of the first and fourth extracellular domains of the Duffy antigen/receptor for chemokines by a disulfide bond is required for ligand binding.

It has been demonstrated that the promiscuous chemokine binding profile of the Duffy antigen/receptor for chemokines (DARC) is given by its extracellular NH2-terminal region. However, the relationship among the Fy6, Fya/b, and Fy3 epitopes, localized in the first and fourth extracellular domains of DARC, respectively, and the chemokine binding sites remained a matter of controversy. Here, we performed cross-displacement and cross-inhibition experiments indicating that all anti-Fy6, anti-Fya, and anti-Fy3 monoclonal antibodies and interleukin 8 are antagonists for binding to red cells. Biopanning of phage peptide libraries with an anti-Fy6 monoclonal antibody led to the identification of the motif Phe22-Glu23, the mutation of which altered the binding of both anti-Fy6 and chemokines (interleukin 8, MGSA, RANTES (regulated on activation normal T cell expressed)) to DARC transfectants. These results characterized the core of the Fy6 epitope and provided definitive proof of the tight relationship between Fy6 and the chemokine receptor site. Analysis of red cells treated by sulfhydryl group-modifying reagents suggested that the chemokine receptor function of DARC required the integrity of disulfide bond(s) but not that of free sulfhydryl group(s). Accordingly, mutation of cysteines 51 and 276 abolished chemokine binding to DARC transfectants. Altogether, our results suggested that the chemokine binding pocket of DARC included sequences located in the first and fourth extracellular domains which are brought into close vicinity by a disulfide bridge.