|Structure of Viral Harpoon Protein Reveals How Viruses Enter
A team of Northwestern University researchers has solved the structure of a
molecule that controls the ability of viruses of the paramyxovirus family, including
the viruses that cause measles, mumps, and many human respiratory diseases, to
fuse with and infect human cells. Determining the structure of this molecule
and its role in the viral fusion mechanism may aid the development of drugs and
vaccines that target these types of viruses, say the scientists, whose work was
funded by the National Institute of General Medical Sciences (NIGMS) and the
National Institute of Allergy and Infectious Diseases (NIAID), both parts of
the National Institutes of Health (NIH).
As described in the latest issue of the journal Nature, this large protein,
called F, studs the surfaces of certain RNA viruses that are encased in a membrane
envelope. As soon as such a virus comes in contact with a cell it can infect,
the F protein changes shape and extends like a harpoon into the outer membrane
of that cell. Then the protein undergoes a conformational (shape) change and
collapses upon itself, pulling the virus against the host cell, and fusing the
viral membrane with the target cell's membrane. The fusion unleashes the viral
RNA into the cell, which then hijacks the cell's machinery to make and spread
"Because of F protein's central role in viral infection, solving the structure
of this critical protein is truly a great advance in biomedical science," says
Elias A. Zerhouni, M.D., NIH director.
Even though the basic concept of viral fusion has been understood for some time,
the complete conformations of the structures of the before- and after-fusion
forms of the F protein had eluded scientists until the recent work of the Northwestern
team, which was led by Theodore Jardetzky, Ph.D., and Robert Lamb, Ph.D., Sc.D.
What has allowed Drs. Jardetzky, Lamb, and their colleagues to understand these
new mechanistic details is the fact that they determined the structure of the
pre-fusion form of the protein-before the protein has harpooned a cell.
"Such structural details offer valuable insights into how viruses infect cells
and underscore the contributions of basic science to improving human health," says
Jeremy M. Berg, Ph.D., NIGMS director.
"The findings may point the way to new medical interventions, such as drugs
or vaccines, for infections caused by enveloped RNA viruses," adds Anthony S.
Fauci, M.D., NIAID director.
About the Protein and How its Structure was Solved
The F protein that the research team solved is from a parainfluenza virus. Not
to be confused with the similar yet distinct Orthomyxoviridae viruses that cause
influenza, parainfluenza viruses belong to a family of viruses known as paramyxoviruses.
Besides human parainfluenza virus, this family includes human and animal pathogens
such as mumps virus, measles virus, human respiratory syncytial virus (a common
cause of pneumonia in children), and the animal pathogens canine distemper virus
and rinderpest virus. In addition to these paramyxoviruses, there are several
other enveloped RNA viruses that use a similar fusion mechanism to enter human
cells, including those that cause influenza, AIDS, and SARS.
Solving the structure of this protein proved difficult, the scientists say,
because F is an unusual protein that exists in two different forms, including
the metastable shape that it adopts before it harpoons a cell and collapses into
its stable post-fusion conformation. Solving the metastable structure was difficult
because the anchoring of F to the virus surface (or membrane) is important for
holding F in this active state. The protein's structure could not be solved unless
it was in the membrane, but solving a protein structure like this required that
it be separated from the membrane.
To accomplish this, the scientists utilized a bit of molecular trickery. They
replaced the part of the protein that is embedded in the viral membrane with
an engineered piece of protein that acts as a substitute. Thus, the F protein
was stabilized in its pre-fusion form and could be crystallized. Then, using
the Advanced Photon Source at the U.S. Department of Energy's Argonne National
Laboratory, the research team employed high intensity X-rays to obtain data from
the crystals, which they then interpreted in order to reconstruct the structure
of the F fusion protein-the culmination of several years' worth of research.
Drs. Jardetzky, Lamb, and their colleagues also compared this structure to the
structure of the protein in the transformed post-fusion form. This allowed them
to observe how the protein undergoes a radical shape change upon harpooning a
target cell. This turned out to be one of the most dramatic rearrangements of
a protein ever observed. "[The fusion protein] breaks a lot of rules about protein
folding," says Dr. Jardetzky, explaining that it does not adopt a single, stable
conformation as one normally expects from a protein but rather exists in two
very different conformations depending on whether it has harpooned a cell or
not. "It's giving us new ideas about how flexible protein structures can be."
And by seeing these F structures in atomic detail, scientists will now be able
to target for intervention similar proteins on the measles virus, mumps virus,
human respiratory disease viruses, and others in ways that were not possible
before. "What you learn about one paramyxovirus fusion protein applies to all
the others," adds Dr. Lamb.
a component of the National Institutes of Health, supports basic biomedical
research that is the foundation for advances in disease diagnosis, treatment,
NIAID is a component of the National Institutes of Health, an agency of
the U.S. Department of Health and Human Services. NIAID supports basic and
applied research to prevent, diagnose and treat infectious diseases such as
HIV/AIDS and other sexually transmitted infections, influenza, tuberculosis,
malaria and illness from potential agents of bioterrorism. NIAID also supports
research on transplantation and immune-related illnesses, including autoimmune
disorders, asthma and allergies.
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