All chemokines share a common structural scaffold that mediate a remarkable variety of functions from immune surveillance to organogenesis. Chemokines are classified as CXC or CC on the basis of conserved cysteines, and the two subclasses bind distinct sets of GPCR class of receptors and also have markedly different quaternary structures, suggesting that the CXC/CC motif plays a prominent role in both structure and function. For both classes, receptor activation involves interactions between chemokine N-loop and receptor N-domain residues (Site-I), and between chemokine N-terminal and receptor extracellular/ transmembrane residues (Site-II). We engineered a CC variant (labeled as CC-CXCL8) of the chemokine CXCL8 by deleting residue X (CXC 3 CC), and found its structure is essentially similar to WT. In stark contrast, CC-CXCL8 bound poorly to its cognate receptors CXCR1 and CXCR2 (K i > 1 M). Further, CC-CXCL8 failed to mobilize Ca 2؉ in CXCR2-expressing HL-60 cells or recruit neutrophils in a mouse lung model. However, most interestingly, CC-CXCL8 mobilizes Ca 2؉ in neutrophils and in CXCR1-expressing HL-60 cells. Compared with the WT, CC-CXCL8 binds CXCR1 N-domain with only ϳ5-fold lower affinity indicating that the weak binding to intact CXCR1 must be due to its weak binding at Site-II. Nevertheless, this level of binding is sufficient for receptor activation indicating that affinity and activity are separable functions. We propose that the CXC motif functions as a conformational switch that couples Site-I and Site-II interactions for both receptors, and that this coupling is critical for high affinity binding but differentially regulates activation.Chemokines mediate a wide variety of biological functions from recruiting leukocytes to the site of injury and infection to organ development, wound healing, and angiogenesis (1-4). Chemokines are characterized by four conserved cysteines that form disulfide bonds, and are classified into CXC and CC families based on cysteine pattern near the N terminus. CXC ligands bind and activate only CXC receptors and CC ligands bind and activate only CC receptors, indicating functional divergence could be as old as the molecules themselves. Structure-function studies consistently show that binding and receptor activation for both classes involves two interactions: between the ligand N-loop and receptor N-domain residues (Site-I) and between the ligand N-terminal and receptor extracellular/transmembrane residues (Site-II) (5-9).Structures of several CXC and CC chemokines have been solved by NMR and x-ray crystallography, and reveal a common structural fold (known as chemokine fold) at the monomer level (10 -15). The CXC/CC motif connects the functionally important N-loop and N-terminal residues, and also plays a structural role by forming disulfide bonds that tether the N-terminal and N-loop residues to the protein core (Fig. 1). In the CXC chemokine CXCL8 (also known as interleukin-8, IL-8), the residue corresponding to X (Gln) in the CXC motif has been mutated with no effec...