Using several resonance energy transfer techniques, we determined that CD4, CXCR4, and CCR5 formed heterotrimers, and that CCR5 coexpression altered the conformation of both CXCR4/CXCR4 homodimers and CD4/CXCR4 heterodimers.     ///    CCR5 reduced HIV-1 envelope-induced CD4/CXCR4-mediated cell-cell fusion. 