Virus caught in the act of infecting a cell
The detailed changes in the structure of a virus as it infects
an E. colibacterium have been observed
for the first time, report researchers from The University of Texas at Austin
and The University of Texas Health Science Center at Houston (UT Health) Medical
School this week inScience Express. To infect a cell, a virus must be
able to first find a suitable cell and then eject its genetic material into its
host. This robot-like process has been observed in a virus called T7 and
visualized by Ian Molineux, professor of biology at The University of Texas at
Austin, and his colleagues.
The
researchers show that when searching for its prey, the virus briefly extends --
like feelers -- one or two of six ultra-thin fibers it normally keeps folded at
the base of its head.
Once a
suitable host has been located, the virus behaves a bit like a planetary rover,
extending these fibers to walk randomly across the surface of the cell and find
an optimal site for infection.
At the
preferred infection site, the virus goes through a major change in structure in
which it ejects some of its proteins through the bacterium's cell membrane,
creating a path for the virus's genetic material to enter the host.
After
the viral DNA has been ejected, the protein path collapses and the infected
cell membrane reseals.
"Although
many of these details are specific to T7," said Molineux, "the
overall process completely changes our understanding of how a virus infects a
cell."
For
example, the researchers now know that most of the fibers are usually bound to
the virus head rather than extended, as was previously thought. That those
fibers are in a dynamic equilibrium between bound and extended states is also
new.
Molineux
said that the idea that phages "walk" over the cell surface was
previously proposed, but their paper provides the first experimental evidence
that this is the case.
This is
also the first time that scientists have made actual images showing how the
virus's tail extends into the host -- the very action that allows it to infect
a cell with its DNA.
"I
first hypothesized that T7 made an extended tail more than 10 years ago,"
said Molineux, "but this is the first irrefutable experimental evidence
for the idea and provides the first images of what it looks like."
The
researchers used a combination of genetics and cryo-electron tomography to
image the infection process. Cryo-electron tomography is a process similar to a
CT scan, but it is scaled to study objects with a diameter a thousandth the
thickness of a human hair.
Source: University
of Texas at Austin
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