U. TEXAS-AUSTIN (US) — Researchers have observed for the first time the detailed changes in the structure of a virus as it infects an E. coli bacterium.
For infection, a virus must first be able to find a suitable cell and then eject its genetic material into its host. The 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.
The new study, published in the journal Science Express, shows 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 the 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,” Molineux says, “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.
The idea that phages “walk” over the cell surface was previously proposed, but the new paper provides the first experimental evidence that this is the case. It 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,” Molineux says, “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