Mapping regions of gp120 and Tat required for their interaction. (A) SPR analysis of Tat-gp120 interaction (n = 3). Recombinant Tat was coated on a sensor chip surface, and binding of rgp120 was evaluated under a range of concentrations (1.25 to 50 nM). (B) Binding of phage display–selected peptide sequences on rTat-coated microwells (▪). An insertless phage (FdTet) and BSA (▦) were used as negative controls (n = 6; F = 147.5; * = P ≤ .05 within rTat- and BSA-coated microwells). (C) Binding of GST-fused Tat₈₆ on rgp120-coated microwells in the presence of the gp120-like synthetic peptides (♦, CT303; ▪, CT304; □, CT319). GST-Tat₈₆ binding in the presence of the control peptide GACVRLSACGA is referred to as 100%. For the CT304 peptide, a logarithmic regression curve has been drawn and values for IC₅₀ deduced from the x-axis interception. (D) Binding of GST-fused Tat variants on rgp120-coated microwells. GST-Tat₈₆ binding is referred to as 100% (n = 3; F = 76.35; *P ≤ .05 versus binding to Tat₈₆). (E) Effect of incubation with GST-Tat variants on Tat-negative/gp120-LV entry into C8166 cells (n = 3; F = 233.24; *P ≤ .05 versus effect of GST-Tat₈₆). Values are shown as percent variation of transduced cells compared with a control transduction of Tat-untreated C8166 cells. (F) Surface-bound Tat enhances virus attachment. Cells were treated with rTat and successively incubated with Tat-negative/gp120-LV. Values are shown as percent of cells with detectable surface staining for Sp2/0 anti-gp120 mAb (n = 3; *P ≤ .03 versus Tat-negative/gp120-LV binding to Tat-untreated C8166 cells). In B-F, each value indicates mean ± SD of the experiments performed at least in duplicate.