Differential regulation of G-protein-mediated signaling by chemokine receptors.

Monocyte chemoattractant protein-1 (MCP-1) is a member of a family of chemotactic cytokines that induce directed migration of leukocytes via activation of seven-transmembrane domain receptors. To identify G-proteins that couple to the two forms of the MCP-1 receptor, as well as to related chemokine receptors, we have performed cotransfection experiments in mammalian cells. In COS-7 cells, the type A and type B MCP-1 receptors coupled to Galphai, Galphaq, and Galpha16, whereas the macrophage inflammatory protein-1alpha/RANTES (regulated on activation, normal T cell-expressed and secreted) receptor (C-CR1) coupled to Galphai and Galphaq but failed to couple to Galpha16. In HEK-293 cells, however, the MCP-1 receptors and C-CR1 coupled to Galphaq but failed to couple to Galpha16. In contrast, the interleukin-8 and C5a receptors did not couple to Galphaq in either COS-7 or HEK-293 cells but did couple to Galpha16. Exchange of intracellular loops between the MCP-1 and interleukin-8 receptors to create chimeric receptors revealed that the third loop of the MCP-1 receptor accounted for virtually all of the coupling to Galphaq. We conclude that the MCP-1 and related chemokine receptors couple to multiple G-proteins, that coupling is cell type-specific, and that the third intracellular loop of the C-C type receptors mediates Galphaq coupling.