Study sheds light on how viruses enter cells

West Lafayette, Ind. -Purdue University research is providing clues on how viruses infect cells.

The bacteriophage T4 virus is one of the largest of the bacterial viruses. It consists of a head, tail, baseplate and a dozen tail fibers. The baseplate serves as a "nerve center" of the virus. When the tail fibers attach to E. coli, the baseplate transmits a message to the tail, which contracts like a muscle, bringing the internal pin-like tube in contact with the outer membrane of the E. coli cell. As the tube punctures the outer and inner membranes, the virus' DNA is injected into the host cell.

The study took a detailed look at a syringe-like structure designed to inject viral DNA into a host cell, and the results reveal "a unique and complex entry scheme for viruses," the university reports.

Ultimately, the study provides clues on how similar viruses infect cells and may offer ways for developing a new class of antibiotics and other drugs to prevent illnesses caused by viral pathogens.

Published in Nature (Jan. 31), the study has solved the three-dimensional structure of the bacteriophage T4 virus, and describes how the virus binds to the surface of the host, punctures the cell wall with a syringe-like tube and injects its own genetic blueprint into the cell. This genetic information then sets the cell's machinery to work creating replicas of the virus.

"Though the T4 virus has been studied extensively in the past, this study provides the first detailed information on the virus structure and how it works," says Michael Rossmann, Hanley Distinguished Professor of Biological Sciences at Purdue University who directed the study.

Bacteriophage T4 is a virus that infects only bacteria, in this case E. coli, a bacteria used extensively in molecular biology research. The study of bacterial viruses such as T4 is useful in understanding many basic functions in biology, Rossmann says.

"This particular study tells us a great deal about how a virus infects a cell," he says. "These processes tend to be quite general, so mechanisms used by one virus often are similar to mechanisms used by other viruses, including those that infect humans."

Bacteriophages may play a future role in controlling disease-causing bacteria, says Kamal Shukla, the National Science Foundation project officer for this research.

"Knowing the exact mechanism of T4 bacteriophage infectivity is a significant breakthrough," Shukla says. "This information could eventually help in creating designer viruses that could be the next class of antibiotics."

Analysis of the cell-puncturing device also reveals a structure that may hold potential for applications in nanotechnology, such as microscopic probes, Rossmann says.

"This is a very stable structure that looks like a small stylus," he says. "It might be useful as a probe in an atomic force microscope, which employs a probe of molecular dimension."

The T4 virus consists of an elongated head, which carries the virus' genetic material, and a tail made up of a hexagonal baseplate and six leg-type structures, called long-tail and short-tail fibers.

In the study, the Purdue group analyzed atom-by-atom, the structure of the virus' baseplate. The baseplate is the key component of the virus, Rossmann says, serving as a "nerve center" and sending signals to and from the virus' head and tail fibers.

While transmitting its messages, the baseplate also prepares the virus machinery to eject its DNA into the host cell.

"A whole series of events are required to recognize, attach and confirm the attachment, and then contract so that the viral DNA can be ejected into the host," Rossmann says. "It's a very complicated system for infecting a cell."

How it works

The virus uses its long-tail fibers to recognize its host and to send a signal back to the baseplate, the university explains. Once the signal is received, the short-tail fibers help anchor the baseplate into the cell surface receptors. As the virus sinks down onto the surface, the baseplate undergoes a change - shifting from a hexagon to a star-shaped structure. At this time, the whole tail structure shrinks and widens, bringing the internal pin-like tube in contact with the outer membrane of the E. coli cell, Purdue explains.

As the tail tube punctures the outer and inner membranes of the E. coli cell, the virus' DNA is injected through the tail tube into the host cell.

The DNA then instructs the bacterium to produce new viruses. So many are produced, in fact, that the E. coli eventually bursts, setting masses of new virus free to infect other cells.

The new detailed images provided by Rossmann's group also show a structure slightly different than what scientists had envisioned.

"We found that the baseplate is shaped like a cup or small dome," Rossmann says. "Previously it was believed that the baseplate was a rather flat structure."

The study also is the first to show how the syringe-like tube is situated in the center of the baseplate, positioned in line with the DNA contained in the virus head.

The studies were done at Purdue using X-ray crystallography, a technique often used to study structures such as proteins and viruses, in atomic detail.

But the process works only if the substances can be made to form crystals. Crystals are used because the diffraction pattern from one single molecule could be insignificant, but the many individual, identical molecules in a crystal amplify the pattern.

Diffraction patterns are created when an X-ray beam hits a crystal, causing the electrons surrounding each atom to bend the beam. Computers can then be used to interpret this pattern and reconstruct the positions of the atoms.

"Because the structure is so complex, we could not crystallize the entire virus structure at once," Rossmann says. "Instead, we crystallized the various components and gradually pieced together a picture of the structure."