THE AMINO-TERMINAL DOMAIN OF CCR2 IS BOTH NECESSARY AND SUFFICIENT FOR HIGH-AFFINITY BINDING OF MONOCYTE CHEMOATTRACTANT PROTEIN-1 - RECEPTOR ACTIVATION BY A PSEUDO-TETHERED LIGAND

High affinity binding of monocyte chemoattractant protein 1 (MCP-1) re quires the presence of the amino-terminal domain of CCR2, the MCP-1 re ceptor. Here we report that the 35 amino-terminal residues of CCR2, ex pressed as a membrane-bound fusion protein, bound MCP-1 with an affini ty similar to that of the intact, wildtype receptor, Furthermore, the amino-terminal fusion protein enhanced, in trans, agonist-dependent ac tivation of a CCR2 variant that was engineered to lacks the high affin ity binding sites for MCP-1. Mutation of highly conserved cysteines in the amino-terminal domain and third extracellular loop of CCR2, but n ot in the fusion protein, resulted in a dramatic loss of MCP-1 binding , suggesting the existence of a critical intramolecular disulfide bond that positions the amino-terminal protein for ligand interaction. The se data indicate that the amino-terminal region of CCR2 is both necess ary and sufficient for the high affinity binding of MCP-1 and provide the first direct evidence for activation of a chemokine receptor by a pseudo-tethered ligand. In this model, high affinity binding by the re latively short amino-terminal domain of CCR2 serves to tether MCP-1 an d enhance low affinity interactions with distal regions of the recepto r.