of HIV-1 Envelope Protein
gp120 with the Cellular Receptor Protein CD4 and the CCR5 Co-Receptor
Mimic 412d Antibody
James Beckett '11 and
Maggie Taylor '11
(HIV) has recently become one of the most studied viruses by medical
researchers. These viruses can establish a lifelong infection of a
host’s T-cells, leading
to acquired immunodeficiency syndrome (AIDS), and eventually causing
death of the individual.
1 HIV (HIV-1)
contains the envelope glycoprotein gp120 which recognizes the host cell
receptors CD4 and the co-receptors CCR5 or
CXCR4. Once these proteins have interacted, HIV enters the cell by
fusing the viral and cellular membranes. The envelope protein is
required for the positioning and timing of this fusion (Kwong, et al, 1998).
envelope protein gp120
beta-strands, 5 alpha-helices, and 10 defined loop segments.
conserved regions of gp120 provide most of the
interactions with the T-cell co-receptor proteins: the bridging sheet
V3 loop. The bridging sheet consists of beta sheets 2, 3, 20, and 21
The V3 loop contains a
a flexible stem,
and a beta-hairpin tip, providing a second set of interactions (Huang, et al, 2007).
Invasion of HIV-1 into the host
cell first requires the binding of the HIV-1 envelope protein gp120 to the
cellular receptor protein CD4
, orienting the
spike, stabilizing the bridging sheet, and exposing the V3 loop of gp120.
allows for viral interactions with the
CCR5 (Huang, et
therapies disrupting the CCR5 protein are promising
avenues of research for developing HIV resistance.
In 2006, a man with AIDS was treated
leukemia with a bone marrow transplant of stem cells from a person who
naturally had a mutant, non-functioning version of the CCR5
This mutant CCR5 does not facilitate HIV-1
entry into cells. The recipient of the
transplant has shown no signs of HIV since then, and has been
“functionally” cured by the American Foundation for
AIDS Research. Many reservations exist about the treatment,
but it has been proclaimed a proof of concept.
Vivo Binding Interactions
CCR5 co-receptor is a seven-transmembrane protein involved
HIV-1 gp120 glycoprotein
binds to CD4,
preferentially bind to residues contained in the N terminus (amino
acid residues 2-15) and
the second extracellular loop of
N-terminus of CCR5 approaches from the
same face of gp120 as does CD4
but binds to an orthogonal surface at the
intersection of the bridging sheet and the
(Huang, et al, 2005).
interaction of the co-receptor with HIV-1 requires an unusual
post-translational modification, o-sulfonation of
tyrosine, creating sulfotyrosines (Tys) (Farzan, et al, 1999). On
CCR5, tyrosines at residues
3, 10, 14, and 15 may
but Tys's at residues 10 and 14 are sufficient to facilitate
interactions with gp120.
Huang, et al
were not able to
crystallize the CCR5 protein with the CD4-complexed HIV-1 gp120 .
antigen- binding fragment (FAB) of
the antibody 412d was crystallized with gp120 and
The light and heavy chains
412d contain binding domains almost identical
of CCR5. When bound, the sulfotyrosines of 412d very closely
mimic the sulfotyrosines in the N terminus. Tys100c
provides most of the interactions with residues in the V3 loop, acting
as a replacement for Tys 14 from CCR5, while Tys100 mimics
two sulfotyrosines fulfill most of
the major interactions between gp120 and its co-receptor protein CCR5,
making 412d and excellent mimic for researchers to study (Huang, et al, 2007).
412d Interactions with gp120
The interactions between CCR5
are mimicked by residues contained in the heavy chain of
Hydrophobic residues of the second complementarity-determining region
of the heavy chain
(CDR H2) interact with the hydrophobic bridging sheet of gp120
the second interaction, the acidic CDR H3
of the 412d heavy chain
to basic residues on the gp120
(Tys100 and Tys100c)
contribute the most essential binding interactions. Tys100
mostly exposed, with its aromatic ring making pi-cation interactions
with Arg327 of gp120 and
its sulfate group making
peripheral electrostatic interactions. Tys100c
uses its sulfate
group for most of its interactions with gp120.
Hydrogen bonding between the sulfate group of Tys100c
and Thr303, Asn300, and Asn302 of gp120 are the
The sulfate can also
make a salt bridge to Arg298
(Huang, et al, 2007).
CD4 Interactions with gp120
interatomic contacts are made between 22 CD4 residues and 26 gp120 amino-acid
residues. This represents 219 van der Waals
forces and 12 hydrogen bonds. The most important interactions occurs
with Asp368, Glu370,
and Trp427 of gp120, which are
conserved among all primate immunodeficiency viruses. Phe43 alone accounts for 23
% of the total CD4-gp120 interaction. Phe43 is
buried in a hydrophobic cavity and interacts with Trp112,
Val255, Thr257, Asp368,
Trp427, Ile 371,
. Arg59 interacts with Asp368
. The carboxylate
of Asp368 makes two very strong
hydrogen-bonds with the Arg59 residue
while the strength of the Phe 43 residue comes from its ability
between carboxylate groups and contact other residues hydrophobically.
These interactions provide the strength to make a connection between
the glycoprotein and the initial receptor protein (Kwong, et al, 1998).
Upon binding, CD4
will initiate an energetically-favorable
conformational change in the structure
of gp120. The binding of Phe43 to the hydrophobic cavity of gp120 helps to
form a more hydrophobic core of the protein. This
conformation stabilizes beta-strands 2, 3, 20, and 21
into the bridging-sheet
conformation. Additionally, this binding
stabilizes the V3 loop seen in the crystal, positioning it to bind with
the co-receptor CCR5.
This sequential binding is necessary for HIV-1 entry into the host
cell (Kwong, et al, 1998).
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