HIV-1 Membrane Fusion Mechanism: Structural Studies of the Interactions between Biologically-Active Peptides from gp41

Two synthetic peptides corresponding to sequences in HIV-1LAI gp41, T21 (aa 558−595) and T20 (aa 643−678), are strong inhibitors of HIV-1 viral fusion, having EC50 values of 1 μg/mL and 1 ng/mL, respectively. Previous work suggested that T21 forms a coiled-coil structure in PBS solution, while T20 is primarily nonhelical, and that the inhibitory action of these peptides occurs after the interaction between the viral gp120 protein and the cellular CD4 receptor [Wild, C. T., Shugars, D. C., Greenwell, T. K., McDanal, C. B., Matthews, T. J. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 9770 and references therein]. The current study uses sedimentation equilibrium (SE), circular dichroism (CD), and viral-fusion assays to quantitatively investigate peptide structure and peptide−peptide interactions. SE analyses of T21 (1−100 μM) indicate that the peptide self-associates via a monomer/dimer/tetramer equilibrium; in addition, T20 is monomeric in the range of 1−10 μM and exhibits a complicated monomer/tetramer equilib...