Fusion Peptide Effects on Epitope Recognition At Membrane Surfaces by the Broadly Neutralizing Anti-HIV-1 2f5 Monoclonal Antibody

Abstract

The HIV-1 glycoprotein fusogenic subunit gp41 is the target for 2F5, a broadly neutralizing monoclonal antibody (MAb2F5) isolated from asymptomatic infected individuals. The 2F5 epitope locates close to the membrane interface within the membrane proximal external region (MPER) that connects the HIV-1 envelope gp41 ectodomain with the transmembrane anchor. Here evidence is presented indicating that the conserved amino-terminal fusion peptide (FP) increases the affinity of this antibody for its membrane-inserted epitope. Structural characterization by circular dichroism together with membrane-disrupting activity measurements suggests the formation of FP-MPER complexes at the surface of lipid bilayer vesicles. MAb2F5 associated more efficiently with lipid vesicles containing FP and MPER peptides as compared to those containing only MPER, or MPER in combination with FPctl, a scrambled version of the FP. Moreover, the N-terminal FP sequence had almost no effect on membrane-inserted epitope binding by MAb4E10, a neutralizing antibody that has been shown to extract its C-terminal MPER epitope from the membrane interface. In combination with recently reported crystallographic data (Julien et al. (2008) J. Mol. Biol., 384, 377-392), these results support a “catch-and-hold” mechanism for the process of MAb2F5-epitope binding at membrane surfaces.