a. 10E8 recognizes a highly conserved gp41 helix to neutralize HIV-1. Fab 10E8 is shown in ribbon representation (shades of violet for heavy chain and of gray for light chain) in complex with a gp41 peptide (red) that encompasses the MPER (Asn656-Arg683). b, Interface between 10E8 and gp41 with select 10E8-side chains (green, heavy chain; cyan, light chain) and gp41-side chains (gray) in stick representation. In analogy to a hand, the hinge can be viewed as being gripped by a thumb (represented by the CDR H2), the C-terminal helix as being suspended along a corresponding extended forefinger (represented by the CDR H3), and residues that commence the C-terminal helix as being caught in the cleft between the thumb and forefinger (represented by the juncture of the CDR loops). c-d, Buried contact surfaces and epitope conservation. An examination of the buried contact surface on gp41 (gray; c) reveals that epitope residues (labeled, d) that are directly contacted by 10E8 are highly conserved across 2870 examined strains (; conservation percentages provided in parentheses). e-h, Alanine mutagenesis of paratope and epitope. Residues at the tip of the 10E8 CDR H3 loop and within the hydrophobic cleft are crucial for recognition of gp41 and for virus neutralization (), as mapped onto the buried 10E8-contact surface (e,g). These results mirror the effects of alanine scan mutations of the 10E8 epitope (), as mapped onto the buried gp41-contact surface (f,h). A comparison of the effects of the alanine mutagenesis of the paratope and epitope reveal that residues of the epitope that are most crucial for 10E8 recognition and neutralization, are also the most highly conserved (d).