Rational design of an HIV immunogen
Ali Emileh
The envelope glycoprotein gp120 of HIV is key for viral entry. Upon binding the broadly
neutralizing recombinant antibody b12, gp120 undergoes a relatively small degree of
conformational fixation. The crystallographic structure of a complex between b12 and a gp120
core has previously been resolved after introduction of multiple mutations into gp120, among
which I109C/Q428C disulfide “stitched” the inner domain α1 helix to B20/B21 of the bridging
sheet. However, in this structure, both alpha1 and β20/β21 are misfolded compared to the
canonical CD4-bound conformation of gp120. Here we propose that the I109C/Q428C mutation
is responsible for the unfolded α1 helix and that, consequently, the native gp120/b12 complex
may permit a completely folded α1. Using Targeted Molecular Dynamics (TMD), we folded
alpha1 in the b12-bound conformation of gp120, and the results showed that with misfolded
β20/ β21, folded α1 is stable regardless of the disulfide stitch. Folding of alpha1 in the presence
of the stitch brings misfolded β20/ β21 closer to the core of the molecule compared to the
unstitched case. When b12 is overlaid on the α1-folded structures generated from these
calculations, a large number of overlaps between gp120 and b12 develop. Mimicking b12
binding to a disulfide-stitched mutant with a folded alpha1 by pushing β20/β21 away from the
core of the molecule partially unfolds α1, supporting the hypothesis that binding of b12 to the
disulfide-stitched mutant, rather than to wild-type gp120, leads to unfolding of the α1 helix.