HIV-1 CORECEPTOR ACTIVITY OF CCR5 AND ITS INHIBITION BY CHEMOKINES - INDEPENDENCE FROM G-PROTEIN SIGNALING AND IMPORTANCE OF CORECEPTOR DOWN-MODULATION

HIV-1 infection requires the presence of specific chemokine receptors on CD4+ target cells to enable the fusion reactions involved in virus entry. CCR5 is a major fusion coreceptor for macrophage-tropic HIV-1 i solates. HIV-1 entry and fusion are mediated by the viral envelope gly coprotein (Env) and are inhibited by CCR5 ligands, but the mechanisms are unknown. Here, we test the role of G protein signaling and CCR5 su rface downmodulation by two separate approaches; direct inactivation o f CCR5 signaling by mutagenesis and inactivation of G(i)-type G protei ns with pertussis toxin. A CCR5 mutant lacking the last 45 amino acids of the cytoplasmic C-terminus (CCR5(306)) was created that was expres sed on transfected cells at levels comparable to cells expressing CCR5 and displayed normal chemokine binding affinity, CCR5 ligands induced calcium flux and receptor downmodulation in cells expressing CCR5, bu t not in cells expressing CCR5(306). Nevertheless, CCR5 or CCR5(306), when coexpressed with CD4, supported comparable HIV-l Env-mediated cel l fusion. Consistent with this, treatment of CCR5-expressing cells wit h pertussis toxin completely blocked ligand-induced transient calcium flux, but did not affect Env-mediated cell fusion or HIV-1 infection. Also, pertussis toxin did not block chemokine inhibition of Env-mediat ed cell fusion or HIV-1 infection. However, chemokines inhibited Env-m ediated cell fusion less efficiently for CCR5(306) than for CCR5. We c onclude that the C-terminal domain of CCR5 is critical for G protein s ignaling and receptor downmodulation from the surface, but that neithe r function is required for CCR5 fusion coreceptor activity. The contra sting phenotypes of CCR5 and CCR5(306) suggest that coreceptor downmod ulation and direct blockage of Env interaction sites both contribute t o chemokine inhibition of HIV-1 infection. (C) 1997 Academic Press.