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Abstract: Monocyte chemotactic protein-1 (MCP-1) binds its G-protein-coupled seven transmembrane (TM) receptor, CCR2B, and causes infiltration of monocytes/macrophages into areas of injury, infection or inflammation. To identify functionally important amino acid residues in CCR2B, we made specific mutations of nine residues selected on the basis of conservation in chemokine receptors and located TM1 (Tyr49), TM2 (Leu95), TM3 (Thr117 and Tyr120), and TM7 (Ala286, Thr290, Glu291, and His297) and in the extracellular loop 3 (Glu278). MCP-1 binding was drastically affected only by mutations in TM7. Reversing the charge at Glu291 (E291K) and at His297 (H297D) prevented MCP binding although substitution with Ala at either site had little effect, suggesting that Glu291 and His297 probably stabilize TM7 by their ionic interaction. E291A elicited normal Ca2+ influx. H297A, Y49F in TM1 and L95A in TM2 that showed normal MCP-1 binding did not elicit Ca2+ influx and elicited no adenylate cyclase inhibition at any MCP-1 concentration. MCP-1 treatment of HEK293 cells caused lamellipodia formation only when they expressed CCR2B. The mutants that showed no Ca2+ influx and adenylate cyclase inhibition by MCP-1 treatment showed lamellipodia formation and chemotaxis. Our results show that induction of lamellipodia formation, but not Ca2+ influx and adenylate cyclase inhibition, is necessary for chemotaxis. [Copyright &y& Elsevier]