January 12, 2001
New Protein Thwarts HIV Attachment
Researchers have synthesized a protein that jams the "grappling
hook" that HIV uses to attach to target cells. The synthetic protein
prevents a spring-loaded component of the grappling hook from snapping
shut and drawing the virus to its target—one of the key steps in
HIV infection. If HIV cannot fuse with the membrane on its target cell,
infection cannot occur.
The researchers believe that the new protein may be useful in
treating patients with drug-resistant HIV or those patients who
experience side effects when taking antiviral medications. Howard
Hughes Medical Institute (HHMI) investigator Peter S. Kim and his
colleagues at the Whitehead Institute for Biomedical Research at MIT
reported on the new anti-HIV protein in a research article published
online on January 12, 2001, by the journal Science.
“I think we are only a few steps away from testing in monkeys to determine whether 5-Helix or a derivative can reduce the viral load in the bloodstream.”
Peter Kim
Development of the HIV-inhibiting protein, called 5-Helix, is based
on earlier studies that showed that HIV uses a spring-loaded mechanism
to attach to T cells, its primary targets in the immune system. In
attaching to T cells, HIV first uses a protein called gp120 to
recognize the CD4 receptor on the surface of a T cell. Once gp120
recognizes CD4, an HIV attachment protein called gp41 launches a
harpoon-like component into the T cell’s membrane. Next,
gp41’s spring mechanism snaps shut—forming what is known as
the "trimer of hairpins," (named for its triple helical, u-shaped
protein structure)—and draws the virus to the T cell like a
grappling hook. The virus can then fuse its membrane with the T
cell’s membrane.
In previous studies, a research team led by Kim and a second that
included HHMI investigators Stephen C Harrison and Don Wiley at Harvard
independently solved the structure of gp41. Kim's group focused on deep
pockets in the molecule that looked to be excellent targets for
inhibitory molecules. Last year, Kim's team identified D-peptide
inhibitors of HIV infection that bound solely to the pocket, laying the
groundwork for the potential development of "orally bioavailable
inhibitors of HIV entry," said Kim. In addition, Harrison and HHMI
investigator Stuart L. Schreiber created a library of compounds that
fit into the pocket, when attached to a longer peptide, to block the
action of gp41.
Other research on gp41 inhibitors has also shown that synthetic
peptides could be created to bind to the grappling hook end of gp41
nearest the target cell—called the N-terminal end. In fact, such
peptides have proven to be potent inhibitors of HIV, and one is
currently in clinical trials.
Kim and his colleagues wanted to test the idea that protein
inhibitors that bind to the C-terminal end of gp41—the end
nearest the virus—could also be potent inhibitors of HIV
infection by stopping the closing of the trimer-of-hairpins. To test
this hypothesis, they synthesized 5-Helix, a small protein designed to
bind to the C-terminal region, and looked at whether it could jam gp41
in cell culture.
"We really didn't know whether binding to the C-terminal region
would actually stop the virus," said Kim. "So, we were pleasantly
surprised to find that not only did it inhibit the virus, but it did so
quite effectively. What’s more, we were very pleased to find that
5-Helix was capable of inhibiting a wide range of HIV isolates."
Analyzing the structure of the C-terminal region of gp41 in many
versions of HIV, Kim and his colleagues found that, although there is
variability in the region, the "face" of the C-terminal helix that
actually interacts with 5-Helix is highly conserved among the different
viral strains.
According to Kim, the 5-Helix molecule is stable, and is therefore
likely to be resistant to degradation by the body’s enzymes. This
is one reason why Kim believes that 5-Helix may be a good candidate for
becoming an injectable anti-HIV therapy. Also, he said, the molecule
can be altered—for instance, making it larger to reduce its
elimination in the kidneys—without diminishing its ability to jam
gp41. And carbohydrate molecules could also be added to the molecule to
help shield it from the immune system, he added.
Kim foresees 5-Helix or its molecular cousins proving to be
effective as "salvage therapies" to treat HIV patients who are
suffering side effects from other drugs, or in whom HIV has mutated to
become drug resistant. The durability of 5-Helix and its effectiveness
bode well for rapid progress toward a therapy, he said.
"I think we are only a few steps away from testing in
monkeys—for which good models of HIV infection exist—to
determine whether 5-Helix or a derivative can reduce the viral load in
the bloodstream," Kim said.
Other clinically important viruses also use similar
trimer-of-hairpins proteins in infection, and a similar approach to
inhibiting attachment might also work against them. "Work from our lab
and from Don Wiley's lab has shown that the influenza and Ebola viruses
use a similar mechanism, and we have recently shown that the human
respiratory syncytial virus also uses the trimer-of-hairpins motif. All
of these viruses are significant threats to human health, and we are
hopeful that our approach to HIV inhibition can be broadly effective
against them as well," said Kim.
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