Selective interactions of polyanions with basic surfaces on human immunodeficiency virus type 1 gp120

It is well established that the gp120 V3 loop of T-cell-line-adapted human immunodeficiency virus type 1 (HIV-1) binds both cell-associated and solubl e polyanions, Virus infectivity is increased by interactions between HIV-1 and heparan sulfate proteoglycans on some cell types, and soluble polyanion s such as heparin and dextran sulfate neutralize HIV-1 in vitro. However, t he analysis of gp120-polyanion interactions has been limited to T-cell-line -adapted, CXCR4-using virus and virus-derived gp120, and the polyanion bind ing ability of gp120 regions other than the V3 loop has not been addressed. Here we demonstrate by monoclonal-antibody inhibition, labeled heparin bin ding, and surface plasmon resonance studies that a second site, most probab ly corresponding to the newly defined, highly conserved coreceptor binding region on gp120, forms part of the polyanion binding surface. Consistent wi th the binding of polyanions to the coreceptor binding surface, dextran sul fate interfered with the gp120-CXCR4 association while having no detectable effect on the gp120-CD4 interaction. The interaction between polyanions an d X4 or R5X4 gp120 was readily detectable, whereas weak or undetectable bin ding was observed with R5 gp120. Analysis of mutated forms of X4 gp120 demo nstrated that the V3 loop is the major determinant for polyanion binding wh ereas other regions, including the V1/V2 loop structure and the NH2 and COO H termini, exert a more subtle influence. A molecular model of the electros tatic potential of the conserved coreceptor binding region confirmed that i t is basic but that the overall charge on this surface is dominated by the V3 loop. These results demonstrate a selective interaction of gp120 with po lyanions and suggest that the conserved coreceptor binding surface may pres ent a novel and conserved target for therapeutic intervention.