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

Several human immunodeficiency virus type 2 (HIV-2) strains have been shown to infect some CD4-negative cell lines (P. R. Clapham, A. McKnig ht, and R. A. Weiss, J. Virol. 66:3531-3537, 1992). Using molecular cl ones of HIV-2 with a CD4-independent tropism, we have identified criti cal 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 I 528M) and flanking the base of the V4 loop (L378F and K403R) are cruci al for the CD4-independent phenotype. Of several mutations conferring a positive charge in V1, V2, and V3, only the change in V3 (Q310K) hel ped 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 lowe r the activation threshold for the fusion process. After binding to ce ll surface-anchored CD4, a CD4-independent recombinant envelope protei n showed an increased binding of anti-envelope protein antibodies, sug gesting either an enhanced binding to cell surfaces or more extensive conformational changes in CD4-independent compared to CD4-dependent en velope proteins. The reduced activation threshold of CD4-independent e nvelope proteins may thus enable them to utilize a membrane molecule f or entry which is not as efficient as CD4 in triggering the conformati onal changes required for the membrane fusion process, CD4-independent HIV-2 variants may be conceptually similar to influenza virus variant s capable of fusing at a higher than normal pH.