"But, lo and behold, it appears to make its own tail," said Michael Rossmann. "It doesn't carry its tail around with it, but when it is about to infect the host it makes a tail." (Credit: Jack Amick/Flickr)

Virus grows a temporary tube to inject DNA

During an infection a certain type of virus forms a tube-like structure to deliver DNA to its host. The tube dissolves when the job is done.

The researchers discovered the mechanism in the phiX174 virus, which attacks E. coli bacteria. The virus, called a bacteriophage because it infects bacteria, is in a class of viruses that do not contain an obvious tail section for the transfer of its DNA into host cells.

“But, lo and behold, it appears to make its own tail,” said Michael Rossmann, a professor of biological sciences at Purdue University. “It doesn’t carry its tail around with it, but when it is about to infect the host it makes a tail.”

Researchers were surprised to discover the short-lived tail.

“This structure was completely unexpected,” says Bentley A. Fane, a professor in the BIO5 Institute at the University of Arizona. “No one had seen it before because it quickly emerges and then disappears afterward, so it’s very ephemeral.”

Although this behavior had not been seen before, another phage called T7 has a short tail that becomes longer when it is time to infect the host, says Purdue postdoctoral research associate Lei Sun, lead author of a research paper published in the journal Nature.

Unable to infect

Researchers at the BIO5 institute mutated the virus so that it could not form the tube. The mutated viruses were unable to infect host cells, Fane says.

The virus’s outer shell, or capsid, is made of four proteins, labeled H, J, F, and G. The structures of all but the H protein had been determined previously. The new findings show that the H protein assembles into a tube-shaped structure. The E. coli cells have a double membrane, and the researchers discovered that the two ends of the virus’s H-protein tube attach to the host cell’s inner and outer membranes.

Images created with a technique called cryoelectron tomography show this attachment. The H-protein tube was shown to consist of 10 “alpha-helical” molecules coiled around each other. Findings also showed that the inside of the tube contains a lining of amino acids that could be ideal for the transfer of DNA into the host.

“This may be a general property found in viral-DNA conduits and could be critical for efficient genome translocation into the host,” Rossmann says.

Like many other viruses, the shape of the phiX174 capsid has icosahedral symmetry, a roughly spherical shape containing 20 triangular faces.

Additional researchers from Purdue, the University of Arizona, and the University of Texas at Austin contributed to the research, which was funded by the National Science Foundation, US Department of Energy, and the US Department of Agriculture.

Source: Purdue University

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