Researchers open door to new HIV therapy
By Robert Sanders , Media Relations | January 28, 2014
|The AIDS virus enters immune cells by binding to CD4 receptors embedded in the membrane (parallel lines) of the cell. But once a virus enters the cell, it makes a protein, Nef, that binds to the protein complex underlying CD4, tagging it for the waste bin. Potential anti-HIV drugs would disable one of the proteins (colored blobs) to which Nef binds, interfering with HIV's strategy for spreading through the body. Image by James Hurley, UC Berkeley.
January 28, 2014 - BERKELEY - People infected with the Human Immunodeficiency
Virus (HIV) can stave off the symptoms of AIDS thanks to drug cocktails that mainly target three enzymes
produced by the virus, but resistant strains pop up periodically that threaten to thwart these drug combos.
Researchers at the University of California, Berkeley, and the National Institutes of Health have instead
focused on a fourth protein, Nef, that hijacks host proteins and is essential to HIV's lethality. The
researchers have captured a high-resolution snapshot of Nef bound with a main host protein, and
discovered a portion of the host protein that will make a promising target for the
next-generation of anti-HIV drugs. By blocking the part of a key host protein to
which Nef binds, it may be possible to slow or stop HIV.
"We have imaged the molecular details for the first time," said structural biologist James H. Hurley,
UC Berkeley professor of molecular and cell biology. "Having these details in hand puts us in striking distance
of designing drugs to block the binding site and, in doing so, block HIV infectivity."
Hurley, cell biologist Juan Bonifacino of the Eunice Kennedy Shriver National Institute of Child Health and Human
Development (NICHD) of the National Institutes of Health and their colleagues report their findings in a paper
published today (Jan. 28) by the open-access, online journal eLife.
The report comes a month after President Barack Obama pledged to redirect $100 million in the
NIH budget to accelerate development of a cure for AIDS, though therapies to halt the symptoms of AIDS will
remain necessary for the immediate future, Bonifacino said.
"For many patients, current drug therapies have transformed HIV infection into a chronic condition that doesn't
lead to AIDS, but anything we can develop to further interfere with replication and propagation of the virus
would help keep it in check until we find a way to completely eliminate the virus from the body," he said.
HIV hijacks cellular machinery
HIV contains a small number of genes that produce only 15 different proteins, all of which hijack some aspect of immune cells' internal
machinery to produce more copies of the virus.
Current anti-AIDS drugs block three HIV enzymes - proteins that transcribe and insert the virus' genetic material, and snip some of the
encoded proteins - but researchers are now looking for drugs that target the other proteins as backups to current therapies. In
particular, they are searching for ways to block sites on host proteins to which the virus proteins bind in order to stop
HIV. This has to be done without interfering with normal cell function, however.
Nearly 20 years ago, scientists showed that without the protein Nef - short for negative factor - HIV is far less infective. Some patients
can live for decades without problems if infected with a Nef-defective virus.
Since then, more details have emerged about Nef's role. HIV enters immune system cells via a receptor called CD4, but once "HIV gets into
cells through the CD4 door, it slams the door shut behind itself to prevent unproductive re-infection," Hurley said.
Scientists aren't sure why HIV slams the door on other viruses, he said, though it may be a way to prevent too many viruses entering the
same cell, making the cellular environment less favorable for productive viral replication.
Whatever the reason, the virus prevents further HIV infection by ridding the cell surface of all other CD4 receptors. Nef achieves
this by tagging the CD4 receptor so that the cell thinks it is trash and carries it to the cell's incinerator, the lysosome, where
it is destroyed. Six years ago, Bonifacino and colleagues found that Nef does this by directly binding to a host protein, AP2,
that latches onto a protein called clathrin. This causes the cell membrane to bulge inward and pinch off to form a small
membrane bubble that carries attached CD4 receptors to the lysosome for destruction.
"The new high-resolution image reveals a cavity at the site where Nef binds to AP2, that could be a good site for drug targeting,"
"This cavity on AP2 is not known to be used by any other host protein, so if we interfere with the cavity we are not going to
interfere with any host cell function, only the function of Nef," he said. "This will inform better searches for inhibitors."
Hurley cautioned, however, that the research "needs more validation to prove that the cavity is a target. But we are excited because it
is a potential target, and these things don't come along every day."
Taking a snapshot
UC Berkeley scientist Xuefeng "Snow" Ren obtained the high-resolution snapshot of Nef bound to AP2 by crystallizing the proteins and
zapping the complex with X-rays. She calculated the 3-D structure from the resulting X-ray diffraction pattern. She is now at work
crystallizing the Nef-AP2 complex bound to the CD4 receptor to look for other possible drug targets on AP2 or CD4 that could
prevent Nef from trashing CD4.
Hurley and Bonifacino are primarily interested in the function of adaptors like AP2 - there are at least five known adaptors in human
cells - that control intracellular membrane traffic.
"This work was an extension of our work on clathrin adaptors, an opportunity to do something relevant to fighting HIV that was based
on the purely basic research we are doing on the sorting of proteins to lysosomes," Hurley said.
Sang Yoon Park of NICHD also is an author of the paper. The work was supported by Grant P50GM082250 from the National Institute of
General Medical Sciences of the NIH, the intramural program of the NICHD and the NIH Intramural AIDS Targeted Antiviral Program.
"Reproduced with permission - University of California, Berkeley "
University of California, Berkeley
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