Molecular characterization of the chemokine receptor CXCR3: evidence for the involvement of distinct extracellular domains in a multi-step model of ligand binding and receptor activation.

CXCR3 is a chemokine receptor predominantly expressed on T lymphocytes, and binds the chemokines CXCL9 (Mig), CXCL10 (IP-10) and CXCL11 (I-TAC). Here, we have investigated the role of the extracellular domains of CXCR3 in ligand selectivity and receptor activation by assessing the ligand binding and chemotactic responses of chimeric CXCR3/CXCR1 constructs. Our data reveal that the secondextracellular loop of CXCR3 is essential for receptor activation in response to all CXCR3 ligands. In contrast, the N terminus and first extracellular loop of CXCR3 play some role in CXCL10- and CXCL11-mediated activation but are dispensable for CXCL9-induced signaling. The third extracellular loop of CXCR3 is important only for CXCL9- and CXCL10-induced chemotaxis. Binding studies suggest that the CXCR3 ligands bind to distinct sites composed of multiple domains of CXCR3 and that high-affinity binding and receptor activation are disparate functions. Collectively, our data support a multi-site model for CXCR3 interactions with its agonists, in which several extracellular domains of CXCR3 contribute to ligand binding and the induction of receptor activation. The development of antagonists targeting the second extracellular loop of CXCR3 should impede receptor activation and aid the treatment of several human inflammatory disorders.