The hydrophobic pocket contributes to the structural stability of the N-terminal coiled coil of HIV gp41 but is not required for six-helix bundle formation

In models of HIV fusion, the glycoprotein gp41 is thought to form a six-helix bundle during viral fusion with the target cell. This bundle is comprised of three helical regions (from the heptad repeat 2, or HR2, region of gp41) bound to an inner, trimeric, coiled-coil core (from the HR1 region). Although much has been learned about the structure and thermodynamics of this complex, the energetics of the isolated HR1 self-associated oligomer remain largely unknown. By systematically studying self-association through a series of truncations based on a 51-mer HR1 peptide (T865), we have identified amino acid segments which contribute significantly to the stability of the oligomeric HR1 complex. Biophysical characterization of C-terminal truncations of T865 identifies a 10-15-amino acid region that is essential for HR1 oligomerization. This region coincides with a hydrophobic pocket that provides important contacts for the interaction of HR2 helices. Complete removal of this pocket abolishes HR1 oligomerization. Despite the dramatic reduction in stability, the monomeric HR1 peptides are still able to form stable six-helix bundles in the presence of HR2 peptides. Truncations on the N-terminal side of T865 have little effect on oligomerization but significantly reduce the stability of the HR1-HR2 six-helix bundle. Unlike the HR2 binding site, which extends along a hydrophobic groove on the HR1 oligomer, the residues that are critical for HR1 oligomenzation are concentrated in a 10-15-amino acid region. These results demonstrate that there are localizations of binding energy, or "hot spots", in the self-association of peptides derived from the HR1 region of gp41.