title: The CD4-independent tropism of human immunodeficiency virus type 2 involves several regions of the envelope protein and correlates with a reduced activation threshold for envelope-mediated fusion.
abstract: Several human immunodeficiency virus type 2 (HIV-2) strains have been shown to infect some CD4-negative cell lines (P. R. Clapham, A. McKnight, and R. A. Weiss, J. Virol. 66:3531-3537, 1992). Using molecular clones of HIV-2 with a CD4-independent tropism, we have identified critical amino acid residues in the envelope protein which are required for CD4-independent infection. Mutations located immediately upstream of a proposed coiled coil domain in the transmembrane protein (A526T or I528M) and flanking the base of the V4 loop (L378F and K403R) are crucial for the CD4-independent phenotype. Of several mutations conferring a positive charge in V1, V2, and V3, only the change in V3 (Q310K) helped to enhance the CD4-independent phenotype but could not mediate it on its own. These mutations reduce the amount of soluble CD4 required to trigger CD4-independent cell-cell fusion, suggesting that they lower the activation threshold for the fusion process. After binding to cell surface-anchored CD4, a CD4-independent recombinant envelope protein showed an increased binding of anti-envelope protein antibodies, suggesting either an enhanced binding to cell surfaces or more extensive conformational changes in CD4-independent compared to CD4-dependent envelope proteins. The reduced activation threshold of CD4-independent envelope proteins may thus enable them to utilize a membrane molecule for entry which is not as efficient as CD4 in triggering the conformational changes required for the membrane fusion process. CD4-independent HIV-2 variants may be conceptually similar to influenza virus variants capable of fusing at a higher than normal pH (R. S. Daniels, J. C. Downie, J. A. Hay, M. Knossow, J. J. Skehel, M. L. Wang, and D. C. Wiley, Cell 40:431-439, 1985).
authors.affiliation: Virology Laboratory, The Institute of Cancer Research, London, United Kingdom.
authors.forename: J D
authors.initials: JD
authors.lastname: Reeves
authors.forename: T F
authors.initials: TF
authors.lastname: Schulz
mesh_terms: D015704:CD4 Antigens; D015496:CD4-Positive T-Lymphocytes / Q000821:virology*; D015967:Gene Expression Regulation, Viral; D005814:Genes, Viral; D015658:HIV Infections / Q000821:virology*; D015498:HIV-2 / Q000502:physiology*; D006801:Humans; D009154:Mutation; D014759:Viral Envelope Proteins / Q000235:genetics*; D014779:Virus Replication / Q000235:genetics*
chemical_list: D015704:CD4 Antigens; D014759:Viral Envelope Proteins