Caught in the act. Three viruses (yellow, green, and pink lines) infect one cell (outlined in yellow) and head straight to the nucleus (outlined in red).

Viral Infection: Live and Uncut

After several brief kisses for its unwitting victim, a dazzling virus pushes inside the recumbent cell. There, other glowing viruses already glide along protein rails to the nucleus, some entering through nuclear pores. That's the view researchers report from the first live, real-time scenes of a virus infecting a cell. Their technique could open up gene therapy and antiviral research to highly detailed, blow-by-blow analysis.

The team, led by physical chemist Christoph Bräuchle of Ludwig Maximilian University in Munich, Germany, customized an existing imaging system, called single-molecule fluorescence spectroscopy, which until now had been used to view chemical reactions. The researchers tagged individual viruses with one or two fluorescent molecules, each of which is about 1/25 the size of the virus. They infected each cell with 10 to 1000 viral particles and took snapshots every 40 milliseconds.

The documentary revealed new information about the small virus, called adeno-associated virus (AAV). Among the team's findings, reported in the 30 November issue of Science: AAV poked through the cell membrane in about 64 milliseconds, much faster than expected, and reached the nucleus in about 15 minutes. That's about an eighth of the time in which conventional cell culture methods, which rely on viral gene expression, can detect infection. Also, the researchers were surprised to see that some particles, after floating toward the nucleus, hopped aboard microtubule-based "tracks" on the nuclear surface and began to move in a straight line. Bräuchle suggests that the tracks are tube-shaped invaginations of the nuclear membrane, recently discovered structures never before known to ferry viruses.

"This technique will be significant for helping us understand how the virus enters the cell," says molecular virologist R. Jude Samulski, director of the University of North Carolina Gene Therapy Center in Chapel Hill. "We're usually taking a picture after the event happened, but this is real time, the live story."

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