Role of the first extracellular loop in the functional activation of CCR2 - The first extracellular loop contains distinct domains necessary for bothagonist binding and transmembrane signaling

The physiological cellular responses to monocyte che moattractant protein-1 (MCP-1), a potent chemotactic and activating factor for mononuclear leukoc ytes, are mediated by specific binding to CCR2, The aim of this investigati on is to identify receptor microdomains that are involved in high affinity agonist binding and receptor activation, The results from our functional st udies in which we utilized neutralizing antisera against CCR2 are consisten t with a multidomain binding model, previously proposed by others. The firs t extracellular loop was of particular interest, because in addition to a l igand-binding domain it contained also information for receptor activation, crucial for transmembrane signaling. Replacement of the first extracellula r loop of CCR2 with the corresponding region of CCR1 decreased the MCP-1 bi nding affinity about 10-fold and prevented transmembrane signaling. A more detailed analysis by site directed mutagenesis revealed that this receptor segment contains two distinct microdomains. The amino acid residues Asn(104 ) and Glu(105) are. essential for high affinity agonist binding but are not involved in receptor activation. In contrast, the charged amino acid resid ue His(100) does not contribute to ligand binding but is vital for receptor activation and initiation of transmembrane signaling; We hypothesize that the interaction of agonist with this residue initiates the conformational s witch that allows the formation of the functional CCR2-G protein complex.