Protein chain mail offers amor for viruses.

To Robert L. Duda, the protein shells protecting the DNA of viruses that infect bacteria evoke images of battles from long ago.

"When craftsmen in Japan made katabira for ninjas and the guildsmen of Europe forged chainmail for knights, they were unaware that their armor reflected an ancient biological design used by bacterial viruses. The fabric of interlocking rings of metal that shielded medieval warriors has a protein analog in [the bacterial virus] HK97," says the University of Pittsburgh biologist in the July 10 Cell.

Duda came to this unlikely connection while trying to explain puzzling data concerning the protein shell, or capsid, assembled by HK97. Like many viruses, this bacterial virus, or bacteriophage, creates its shell from hundreds of copies of a single protein (SN: 3/25/95, p. 186). The capsid forms through a multistep process in which five or six of these copies assemble into a ring, and the resulting pentamers and hexamers come together in a shell reminiscent of a soccer ball. The finished HK97 capsid has 72 faces: 60 hexamers and 12 pentamers.

When Duda and other researchers tried to dismantle this capsid into its components, they ended up with unusually large breakdown products. Capsids normally disassemble into individual proteins or into the pentamers or hexamers created during shell assembly. But HK97's capsid broke into pieces so large that they couldn't even be measured by traditional means.

Duda initially thought that an undetected chemical bond was forming extra links between the capsid's many pentamers and hexamers, but experiments did not reveal such bonds. He then realized that the protein rings might not just chemically bond to each other but might also physically interlink like the metal rings of chain mail.

This intertwining apparently occurs right before the capsid proteins form into rings. "Imagine you're joining hands in a ring of six people," says Duda's colleague Roger Hendrix. "But before you join hands, you reach around the arms of the guys in a neighboring ring, and so you get linked rings."

"It's really neat stuff," says Peter E. Prevelige Jr. of the University of Alabama at Birmingham, who studies viral shell assembly. "People don't know how to fold proteins, let alone fold them, make them assemble, and then transform them into something as complex as this."

HK97 isn't alone in its chain mail strategy. "We've now found quite a few other bacteriophages that do this," says Hendrix. He and Duda suggest that the inter-linking stabilizes the capsid.

Viruses that infect animal cells may not employ protein chain mail, notes Hendrix, because they must shed their shells to release their DNA. In contrast, bacteriophages inject their DNA into bacteria with a syringelike mechanism. Although Duda deduced the chain mail strategy from indirect experimental data, investigators plan to directly map HK97's capsid structure at atomic resolution by shining X rays through crystallized versions of it. That should soon determine whether bacteriophages are indeed pioneers in the wearing of chain mail.

In HK97's outer shell, each face intertwines (arrow, for example) with neighboring ones.