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 G alpha(i), G alpha(q), and G alpha(16), whereas the macrophage inflammatory protein-1 alpha/RANTES (regulated on activation, normal T cell-expressed and secreted) receptor (C-CR1) coupled to G alpha(i) and G alpha(q) but failed to couple to G alpha(16). In HEK-293 cells, however, the MCP-1 receptors and C-CR1 coupled to G alpha(q) but failed to couple to G alpha(16). In contrast, the interleukin-8 and C5a receptors did not couple to G alpha(q) in either COS-7 or HEK-293 cells but did couple to G alpha(16). 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 G alpha(q). 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 G alpha(q) coupling.