April 27, 2000
Getting to the Core of Reovirus
HHMI researchers have solved the structure of an important component
of the reovirus, a double-stranded RNA virus that bears similarity to
pathogens such as rotavirus, a potentially deadly cause of diarrhea in
infants.
Using X-ray crystallography, a team led by Stephen C. Harrison,
an HHMI investigator at Children's Hospital in Boston and Harvard
University, determined the three-dimensional architecture of the core
structure of reovirus.
The team, which also included lead author Karin M. Reinisch, a
postdoctoral fellow in Harrison's laboratory, and Max L. Nibert at the
Institute for Molecular Virology at the University of Wisconsin,
Madison, reported their findings in the April 27, 2000, issue of the
journal Nature.
"This work will provide much insight on two levels," says Aaron
Shatkin, director of the Center for Advanced Biotechnology and Medicine
at Rutgers University and a professor at the University of Medicine and
Dentistry of New Jersey-Robert Wood Johnson Medical School. He notes
that understanding the reoviral structure will inform studies of
cellular genetic messaging in higher life forms, including humans. Many
double-stranded RNA viruses are found in nature, so the structure could
serve as a model for these related viruses, says Shatkin.
The reoviral core is an internal component of the virus that remains
intact after the virus penetrates the host cell, as in other plant and
animal double-stranded RNA viruses. The core synthesizes, modifies, and
exports viral messenger RNA (mRNA), which eventually makes its way to
the host-cell ribosome—where viral proteins are constructed from
the mRNA, thus completing the viral takeover of the host cell.
Once they solved the core structure, Harrison's group began to
examine its role in viral replication. The researchers found that the
core encases the viral genome, which is made up of ten distinct
segments of double-stranded RNA. They also showed that the core
organizes RNA so that it can easily be transcribed into many
copies.
Harrison's team found how the core ensures that mRNA is modified
with a methyl guanosine cap in a sequence of reactions discovered many
years ago by Shatkin.
"Reovirus makes a cage to trap the end of the messenger RNA as it
emerges from the core," explains Harrison. "It makes a hollow that
holds the RNA long enough with five possible sites that it can hit, any
one of which can do the job in order to make sure the RNA gets
capped."
These "hollows" resemble turrets projecting from the core and are
the capping machinery. The synthesized mRNAs are capped the instant
they emerge from the active site of the polymerase. The guanosine cap
is essential for RNA stability and for the ribosome to recognize the
viral RNA.
In Harrison's opinion, the structural study of important viral
pathogens has revealed a number of fundamental principles in
macromolecular and protein assembly.
"I've always been interested in problems of viral structure from
both ends," says Harrison. In the case of the reovirus core, he asks,
how do you organize a structure like that to be able smoothly— and
without entanglement— to make multiple copies of ten different
messenger RNAs, cap them, and get them out?
The reovirus core has also interested Harrison's lab because it may
bear similarities to other double-stranded RNA viruses that are
important pathogens.
In upcoming studies, Harrison's group will be looking at the
relationship between the structure and function of the outer shell
proteins that get reoviruses and rotaviruses into cells.
Broadly speaking, viruses use two styles of entry. Enveloped
viruses, like influenza and HIV, have bilayer membranes, and get in by
fusing their envelope with the cell's membrane. Solving how
membrane-less, non-enveloped viruses like reovirus and poliovirus, gain
entry has been harder to do, Harrison says.
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