Scientists get inside look at viruses

By Corey Binns

updated 1:28 p.m. ET June 5, 2006

Exactly 25 years ago, in the body of the world's first diagnosed AIDS case, the full capabilities and mysterious workings of a virus unfolded.

Three years later, in 1984, Luc Montagnier of the Pasteur Institute of Paris and Robert Gallo, then of the National Cancer Institute, announced their discovery of HIV, the virus that infects the human immune system and causes AIDS.

Even though the smallest viruses are only about one-millionth of an inch long, they live up to their Latin namesake — poison. They are capable of infecting and hijacking a human body, creating health hazards as minor as the common flu and as disastrous as the AIDS epidemic.

On the inside
Viruses are neatly organized, petite packages of genetic material, shaped like rods, filaments, harpoons, or spheres.

Proteins surround the package, which is called a capsid. Some viruses have an added layer of lipids that coat the capsid. Little extensions on the virus are called antigens, which help the virus hunt down the target host cell.

The diminutive nature of viruses, with the exception of the relatively gargantuan mimivirus, has made determining their looks difficult. The invention of the electron microscope in the 1940s first made viruses visible.

Scientists at Florida State University recently produced a new 3-D image of HIV and the protein spikes on its surface that match up with host cells. The pictures may help researchers better understand how the virus fuses with a host T-cell and inspire new ways to design vaccines.

"Until now, despite intensive study by many laboratories, the design details of the spikes and their distribution pattern on the surface of the virus membrane have been poorly understood, which has limited our understanding of how the virus infection actually occurs, and frustrated efforts to create vaccines," said principal investigator and immunologist Kenneth Roux.

The findings were published this month in the online edition of the journal Nature.

Everyone's a target
Viruses prey upon all living organisms, turning them into virus copy machines.

Unlike a bacterium or a cell of an animal, a virus lacks the ability to replicate on its own. A virus does contain some genetic information critical for making copies of itself, but it can't get the job done without the help of a cell's duplicating equipment, borrowing enzymes and other molecules to concoct more virus.

"It's not a living organism," said immunologist Fabio Romerio of the Institute of Human Virology, founded and directed by Gallo. "It's simply a well-organized molecular parasite."

Stuck in a microscopic purgatory somewhere between life and unlife, viruses can remain dormant for long periods of time.

On the long end of the time spectrum is a variant of the herpes virus, HHV6, that infects more than 97 percent of the population without causing serious health problems. Most of us are infected at a young age, which feels like a mild case of the flu. Afterwards, the virus can harmlessly stick with us for our lifetime.

Lying in wait
HIV can also be dormant inside the body for months or even years. It can stealthily replicate at low levels, constantly producing a few new viral agents without killing the host cell.

By attaching to a host cell and forcing the cell to follow its genetic orders, a virus can turn a host into a viral army-making machine.

Some viruses are very specific about which cells they target, while others are less selective. Like matching puzzle pieces, the virus searches for cells to stick to. Proteins on the surface of the virus recognize its target by the proteins or sugars on the surface of the host cell.

For example, a protein called GP120 sits on the surface of HIV and allows the virus to attach to the target cell. GP120 binds to two proteins on the cell. After the proteins connect, the virus delivers its genetic material into the cell.

HIV only has a limited number of proteins, so it relies on the host cell's proteins to synthesize new genetic material, including more GP120, and regulate its replication.

Viruses come prepared with various amounts and types of genetic material. Some viruses carry double-stranded DNA, while others, like HIV, have only a single strand of RNA. The kind of genetic material a virus carries determines how the replication process works inside the host cell.

An infected host cell becomes a virus factory.

In the case of HIV, each infiltrated cell produces and spits out hundreds of new viral particles. The whole hostile takeover, from the time when HIV attaches itself to a host cell and releases new HIV progeny, takes one to two days.

If HIV production is vigorous, it kills the cell immediately. Otherwise, the cell will survive just a few more days.

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