Substitutions in a homologous region of extracellular loop 2 of CXCR4 and CCR5 alter coreceptor activities for HIV-1 membrane fusion and virus entry

CXCR4 and CCR5 are the principal coreceptors for human immunodeficiency vir us type-1 (HIV-1) infection. Previously, mutagenesis of CXCR4 identified si ngle amino acid changes that either impaired CXCR4's coreceptor activity fo r CXCR4-dependent (X4) isolate envelope glycoproteins (Env) or expanded its activity, allowing it to serve as a functional coreceptor for CCR5-depende nt (R5) isolates. The most potent of these point mutations was an alanine s ubstitution for the aspartic acid residue at position 187 in extracellular loop 2 (ecl-2), and here we show that this mutation also permits a variety of primary R5 isolate Envs, including those of other subtypes (clades), to employ it as a coreceptor. We also examined the corresponding region of CCR 5 and demonstrate that the substitution of the serine residue in the homolo gous ecl-2 position with aspartic acid impairs CCR5 coreceptor activity for isolates across several clades. These results highlight a homologous and c ritical element in ecl-2, of both the CXCR4 and CCR5 molecules, for their r espective coreceptor activities. Charge elimination expands CXCR4 corecepto r activity, while a similar charge introduction can destroy the coreceptor function of CCR5. These findings provide further evidence that there are co nserved elements in both CXCR4 and CCR5 involved in coreceptor function.