The C34 Peptide Fusion Inhibitor Binds to the Six-Helix Bundle Core Domain of HIV‑1 gp41 by Displacement of the C‑Terminal Helical Repeat Region
John M. Louis, James L. Baber, G. Marius Clore
The conformational transition of the core domain of HIV-1 gp41 from a prehairpin intermediate to a six-helix bundle is responsible for virus–cell fusion. Several inhibitors which target the N-heptad repeat helical coiled-coil trimer that is fully accessible in the prehairpin intermediate have been designed. One such inhibitor is the peptide C34 derived from the C-heptad repeat of gp41 that forms the exterior of the six-helix bundle. Here, using a variety of biophysical techniques, including dye tagging, size-exclusion chromatography combined with multiangle light scattering, double electron–electron resonance EPR spectroscopy, and circular dichroism, we investigate the binding of C34 to two six-helix bundle mimetics comprising N- and C-heptad repeats either without (coreSP) or with (coreS) a short spacer connecting the two. In the case of coreSP, C34 directly exchanges with the C-heptad repeat. For coreS, up to two molecules of C34 bind the six-helix bundle via displacement of the C-heptad repeat. These results suggest that fusion inhibitors such as C34 can target a continuum of transitioning conformational states from the prehairpin intermediate to the six-helix bundle prior to the occurrence of irreversible fusion of viral and target cell membranes.
Circular dichroism, Protein folding, Ligand binding, Biochemistry