The amino-terminal extracellular domain of the MCP-1 receptor, but not the RANTES/MIP-1 alpha 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-1 alpha (macrophage inflammatory protein), are recognized by highly homologous but distinct receptors, To identify receptor domains involved ill determining ligand specificity, we created a series of chimeric MCP-1 and RANTES (regulated on activation, normal T cell expressed and secreted)/MIP-1 alpha 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/MLP-1 alpha receptor (C-C CKR-1), Chimeras that retained the amino-terminal extension of the MCP-I receptor bound MCP-1 with high affinity, In contrast, chimeric MCP-I receptors, in which the wild-type amino terminus was replaced with the corresponding portion of the RANTES/MIP-1 alpha: receptor, bound MIP-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-1 alpha: receptor, Radiolabeled MIP-1 alpha bound with high affinity to chimeras that expressed the extracellular loops of the RANTES/MIP-1 alpha receptor, In contrast to the MCP-1 receptor; substitution of the wild-type amino-terminal extension had little or no effect on MIP-1 alpha binding, For the MCP-1, but not the RANTES/MIP-1 alpha 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-1 alpha 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.