Identification of residues of CXCR4 critical for human immunodeficiency virus coreceptor and chemokine receptor activities

CXCR4 is a G-coupled receptor for the stromal cell-derived factor (SDF-1) c hemokine, and a CD4-associated human immunodeficiency virus type 1 (HIV-1) coreceptor. These functions were studied in a panel of CXCR4 mutants bearin g deletions in the NH2-terminal extracellular domain (NT) or substitutions in the NT, the extracellular loops (ECL), or the transmembrane domains (TMs ). The coreceptor activity of CXCR4 was markedly impaired by mutations of t wo Tyr residues in NT (Y7A/ Y12A) or at a single Asp residue in ECL2 (D193A ), ECL3 (D262A), or TMII (D97N). These acidic residues could engage electro statical interactions with basic residues of the HIV-1 envelope protein gp1 20, known to contribute to the selectivity for CXCR4. The ability of CXCR4 mutants to bind SDF-1 and mediate cell signal was consistent with the two-s ite model of chemokine-receptor interaction. Site I involved in SDF-1 bindi ng but not signaling was located in NT with particular importance of Glu(14 ) and/or Glu(15) and Tyr(21). Residues required for both SDF-1 binding and signaling, and thus probably part of site II, were identified in ECL2 (Asp( 187)), TMII (Asp(97)), and TMVII (Glu(288)). The first residues (2-9) of NT also seem required for SDF-1 binding and signaling. A deletion in the thir d intracellular loop abolished signaling, probably by disrupting the coupli ng with G proteins. The identification of CXCR4 residues involved in the in teraction with both SDF-1 and HIV-1 may account for the signaling activity of gp120 and has implications for the development of antiviral compounds.