The amino-terminal extracellular domain of the MCP-1 receptor, but not the RANTES/MIP-1alpha receptor, confers chemokine selectivity. Evidence for a two-step mechanism for MCP-1 receptor activation.
The chemoattractant cytokines, MCP-1 (monocyte chemoattractant protein) and MIP-1alpha (macrophage inflammatory protein), are recognized by highly homologous but distinct receptors. To identify receptor domains involved in determining ligand specificity, we created a series of chimeric MCP-1 and RANTES (regulated on activation, normal T cell expressed and secreted)/MIP-1alpha receptors that progressively interchanged the amino terminus and each of the three extracellular loops. Radiolabeled MCP-1 bound with high affinity to the wild-type MCP-1 receptor, but not to the RANTES/MIP-1alpha receptor (C-C CKR-1). Chimeras that retained the amino-terminal extension of the MCP-1 receptor bound MCP-1 with high affinity. In contrast, chimeric MCP-1 receptors, in which the wild-type amino terminus was replaced with the corresponding portion of the RANTES/MIP-1alpha receptor, bound MCP-1 with low affinity. These data indicate that the amino terminus of the MCP-1 receptor is necessary for high affinity binding of the ligand. Very different results were obtained using the RANTES/MIP-1alpha receptor. Radiolabeled MIP-1alpha bound with high affinity to chimeras that expressed the extracellular loops of the RANTES/MIP-1alpha receptor. In contrast to the MCP-1 receptor, substitution of the wild-type amino-terminal extension had little or no effect on MIP-1alpha binding. For the MCP-1, but not the RANTES/MIP-1alpha receptor, the presence of the wild-type amino terminus also significantly lowered the ligand concentration required for maximal signaling. We conclude that the amino-terminal extension of the MCP-1 receptor, but not the RANTES/MIP-1alpha receptor, is critically involved in ligand binding and signal transduction. These data reveal significant functional differences between the two C-C chemokine receptors and suggest a two-step mechanism for activation of the MCP-1 receptor.
Monteclaro
F S
FS
Gladstone Institute of Cardiovascular Disease, University of California San Francisco, California 94141-9100, USA.
Charo
I F
IF
D002460:Cell Line; D018932:Chemokine CCL2 / Q000378:metabolism*; D018925:Chemokines / Q000378:metabolism*; D006801:Humans; D011485:Protein Binding; D054390:Receptors, CCR2; D019713:Receptors, CCR5; D019707:Receptors, Chemokine; D018121:Receptors, Cytokine / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction
C515334:CCR2 protein, human; D018932:Chemokine CCL2; D018925:Chemokines; D054390:Receptors, CCR2; D019713:Receptors, CCR5; D019707:Receptors, Chemokine; D018121:Receptors, Cytokine; D011993:Recombinant Fusion Proteins; C080633:macrophage inflammatory protein 1alpha receptor