Entry of R5X4 and X4 human immunodeficiency virus type 1 strains is mediated by negatively charged and tyrosine residues in the amino-terminal domain and the second extracellular loop of CXCR4.

CXCR4 mediates the fusion and entry of X4 and R5X4 strains of human immunodeficiency virus type 1 (HIV-1). The residues involved in CXCR4 coreceptor function have not all yet been identified, but tyrosine and negatively charged residues in the amino-terminal domain of CCR5 were shown to be indispensable for gp120 binding and entry of R5 and R5X4 strains. We therefore evaluated the role of such residues in CXCR4 coreceptor function by replacing tyrosines (Y), aspartic acids (D), and glutamic acids (E) with alanines (A) and testing the ability of these mutants to mediate the entry of X4 and R5X4 HIV-1 isolates. Our results show that viral entry depends on YDE-rich clusters in both the amino-terminus and the second extracellular loop of CXCR4. Different viral isolates vary in their dependence on residues in one or the other domain. The determinants of CXCR4 coreceptor function are, therefore, more diffuse and isolate-dependent than those of CCR5.