September 16, 2005
Learning How SARS Spikes Its Quarry
Researchers have determined the first detailed molecular images of a
piece of the spike-shaped protein that the SARS virus uses to grab host
cells and initiate the first stages of infection. The structure, which
shows how the spike protein grasps its receptor, may help scientists
learn new details about how the virus infects cells. The information
could also be helpful in identifying potential weak points that can be
exploited by novel antiviral drugs or vaccines.
The SARS (severe acute respiratory syndrome) coronavirus was
responsible for a worldwide outbreak in 2002-2003 that affected more
than 8,000 people and killed 774 before being brought under control.
Public health experts worry about another outbreak of the virus, which
originates in animals such as civet cats.
“One of the critical issues in a SARS epidemic would be to predict whether a given variant of the virus will jump species or move laterally from one human to the other.”
Stephen C. Harrison
The research team, led by Howard Hughes Medical Institute
investigator Stephen C. Harrison at Children's Hospital and Harvard
Medical School, and colleague Michael Farzan, also at Harvard Medical
School, reported its findings in the September 16, 2005, issue of the
journal Science. Lead author Fang Li in Harrison's laboratory
and Wenhui Li in Farzan's laboratory, also collaborated on the
study.
According to Harrison, prior to these studies, researchers knew that
one of the key steps in SARS infection occurs when the virus's spike
protein attaches to a receptor on the surface of target cells.
Attachment of the spike protein permits the virus to fuse with a host
cell and inject its RNA to infect the cell.
A detailed understanding of how the spike protein complexes with its
receptor, ACE2 (angiotensin-converting enzyme 2), could have important
clinical implications. “The interest in understanding this
complex has to do with the fact that this virus jumps from animals to
humans, laterally among humans, and in some cases from animals to
humans but without subsequent human-to-human transmission,” said
Harrison. “And we know that those modes of transmission depend on
specific mutations in the spike protein that affect spike-receptor
interaction.
“One of the critical issues in a SARS epidemic would be to
predict whether a given variant of the virus will jump species or move
laterally from one human to the other. Understanding the structure of
this complex will help us understand what these mutations in the spike
protein mean in terms of infectivity,” Harrison said.
According to Harrison, Farzan and his colleagues laid the scientific
groundwork for determining the structure of the spike-ACE2 complex. In
2003, Farzan's team discovered that the ACE2 protein is the receptor
for the SARS virus. They also identified a specific fragment of the
spike protein that is involved in viral attachment.
As a result of those studies, researchers in Harrison's and Farzan's
laboratories could concentrate their efforts on creating crystals of
the relevant fragments of the spike protein in complex with the ACE2
receptor. After they had crystallized the protein complex, the crystals
were then subjected to structural analysis using x-ray crystallography.
In this widely used technique, x-rays are directed through crystals of
a protein. The resulting diffraction pattern is analyzed to deduce the
atomic structure of the protein or protein complex under study.
The x-ray structure revealed that the spike protein fragment showed
a slightly concave surface that fits a complementary surface on the
receptor, said Harrison. There was nothing surprising about the
interaction itself, he noted. However, the studies revealed important
new information about two specific amino acids on the spike protein.
These were the amino acids that Farzan and his colleagues had
previously determined to be the most critical for determining how the
SARS virus adapted from infecting only civets to infecting humans.
“Both of these critical amino acids turned out to be right in
the middle of the interface between the spike protein and the
receptor,” said Harrison. Thus, the structure reveals details
about how even small mutations in the spike protein gene that alter the
identity of amino acids at those sites can affect the virus's ability
to infect humans. Such mutations enable viral transmission by altering
the shape of the spike protein, which affects how well it binds to the
ACE2 receptor, explained Harrison. In particular, he said, the new
structure shows how mutation at one of the two sites can enable the
animal SARS virus to infect humans, but by itself this mutation does
not appear to allow subsequent human-to-human transmission.
“The observation is that a dramatic epidemiological difference
can result from what looks like an almost trivial mutation,” said
Harrison. “These findings give us the beginnings of information
needed — if a new virus were isolated — to make predictive guesses
about infectivity, so that we can better give advance
warning.”
He also noted that laboratory studies indicate that the fragment of
the spike protein they used could provide the basis of a vaccine
against SARS, since it appears to be recognized by the immune system of
the host.
In future studies, Harrison and his colleagues plan to explore the
steps that occur after the spike protein attaches to the receptor. The
researchers know that the spike protein undergoes a conformational
change that enables the virus to fuse with the host cell.
“When there's a conformational change, it gives you an
opportunity to explore the possibility of antiviral
therapeutics,” said Harrison. “When you have two
conformational structures, you can think about how to prevent infection
by inhibiting the transition from one state to another.”
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Jennifer Michalowski
