Abstract
The monoclonal antibody 13H11 shares part of its epitope in the HIV-1 gp41 membrane-proximal external region (MPER) with the rare, broadly neutralizing human antibody 2F5. Although 13H11 partially cross-blocked 2F5 binding, 13H11 is non-neutralizing and does not block 2F5 neutralization. We show that unlike 2F5, 13H11 binds to a well-defined helical MPER structure that is consistent with the structure of gp41 in a post-fusion six-helix bundle conformation.