2 DISTINCT CCR5 DOMAINS CAN MEDIATE CORECEPTOR USAGE BY HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1

The chemokine receptor CCR5 is the major fusion coreceptor for macroph age-tropic strains of human immunodeficiency virus type 1 (HIV-1). To define the structures of CCR5 that can support envelope (Env)mediated membrane fusion, we analyzed the activity of homologs, chimeras, and m utants of human CCR5 in a sensitive gene reporter cell-cell fusion ass ay. Simian, but not murine, homologs of CCR5 were fully active as HIV- 1 fusion coreceptors. Chimeras between CCR5 and divergent chemokine re ceptors demonstrated the existence of two distinct regions of CCR5 tha t could be utilized for Env-mediated fusion, the amino-terminal domain and the extracellular loops. Dual-tropic Env proteins were particular ly sensitive to alterations in the CCR5 amino-terminal domain, suggest ing that this domain may play a pivotal role in the evolution of corec eptor usage in vivo. We identified individual residues in both functio nal regions, Asp-11, Lys-197, and Asp-276, that contribute to corecept or function. Deletion of a highly conserved cytoplasmic motif rendered CCR5 incapable of signaling but did not abrogate its ability to funct ion as a coreceptor, implying the independence of fusion and G-protein -mediated chemokine receptor signaling. Finally, we developed a novel monoclonal antibody to CCR5 to assist in future studies of CCR5 expres sion.