Inhibition of KR13-induced virus breakdown by the gp41-binding fusion inhibitor T20. (a) BaL pseudovirus. Virus was treated with 1 μM KR13 in the absence and presence of serial dilutions of T20 starting from 1 μM. Relative p24 release was measured using p24 ELISA by comparing the residual virion (18.2 - 19% Optiprep) and soluble protein (6-8% Optiprep) fractions. Relative gp120 release was measured using western blot analysis by comparing content of gp120 in the analogous residual virion and soluble protein fractions. Western blot values were obtained using Image J analysis of the protein bands. The IC50 value T20 inhibition of KR13-induced p24 release was 15.9 ± 4.9 nM using Origin Pro .8 (Origin Lab). No significant effect on the gp120 release in the presence of T20. Error bars represent the standard deviation of the mean, n = 3. (b) R3A and R3A V38A pseudoviruses; the latter, mutant virus has been found previously to be resistant to T20 inhibition. Release of p24 was quantified as for BaL pseudovirus in part (a). The IC50 of T20 inhibition of peptide induced p24 release from the virus R3A was calculated to be 21.9 ± 5.9 nM. The mutant virus V38A R3A did not exhibit inhibition of p24 release. Sigmoidal fits were obtained using Origin Pro .8 (Origin Lab). Error bars represent the standard deviation of the mean, n = 3.