THE 2ND EXTRACELLULAR LOOP OF CCR5 IS THE MAJOR DETERMINANT OF LIGANDSPECIFICITY

The chemokine receptor CCR5 binds macrophage inflammatory protein (MIP )-1 alpha, MIP-1 beta, and regulated on activation, normal T-cell expr essed and secreted (RANTES), and constitutes the major co-receptor all owing infection of CD4(+) T lymphocytes, macrophages, and microglial c ells by macrophage-tropic strains of human and simian immunodeficiency virus, CCR5 is most closely related to CCR2b, another chemokine recep tor that responds to monocyte chemoattractant protein (MCP)-1, MCP-2, MCP-3, and MCP-4, We have investigated by mutagenesis the regions of C CR5 and CCR2b involved in the specificity of binding and functional re sponse to their respective ligands, We demonstrate that the key region of CCR5 involved in its specific interaction with MIP-1 alpha, MIP-1 beta, and RANTES, and its subsequent activation, lies within the secon d extracellular loop (and possibly the adjacent transmembrane segments ), Conversely, the NH2-terminal domain of CCR2b is responsible for the high affinity binding of MCP-1, but is not sufficient to confer activ ation of the intracellular cascades. Extracellular loops of the recept or, among which the second loop plays a prominent role, are necessary to achieve efficient signaling of the receptor, These data complement our previous mapping of CCR5 domains functionally involved in the fusi on process with the human immunodeficiency virus envelope, and will he lp in the development of agents able to interfere with the early steps of viral infection.