The hole story. Streptolysin-O punched 30-nanometer holes in this cell membrane.

Soft Poke Through the Membrane

A once brutish technique to deliver proteins into cells has been civilized. The method allows scientists to install working proteins while keeping the cells alive, which until now was difficult. A study describing the work appeared in the 6 March issue of The Proceedings of the National Academy of Sciences.

Proteins are the cell's workhorses, and scientists can learn a lot from sticking different ones inside cells to see what happens. But scientists have only a few ways to get water-loving proteins to cross the oily membrane around cells. They can link the protein to "smart" peptides--produced by some bacteria, viruses, and flies--that are appetizing to eukaryotic cells; when the cells encounter these hybrids, they gobble up the peptide and the protein too. But the bacteria that produce the proteins don't always properly trim and fold them before they're smuggled in. Another technique is to let proteins slip in through holes punched with a bacterial toxin. But that doesn't leave much time to study the cell before it dies from its wounds.

The solution: Go light on the toxin, say Sucharit Bhakdi and colleagues at Johannes Gutenberg University in Mainz, Germany. The team tinkered with streptolysin-O, a protein from the microbe Streptococcus pyogenes, until they had found a dose at which 80% of human leukemia cells allowed in a dye. After rinsing them for an hour in toxin-free medium, about 50% of cells refused entry to a second dye, proving that the holes had resealed and the cells had survived. Next, the researchers showed that the same dose of toxin could load cells with three proteins, all of which worked upon arrival.

The refined technique could aid in deciphering slow phenomena like cell movement and physiological changes provoked by contact between cells, says Stanford cell biologist Chuck Yeaman. "I think it's an important technical step forward," he says. But for many applications, the peptide fusion strategy won't be losing customers, says cell biologist and physician Jonathan Chernoff of the Fox Chase Cancer Center in Philadelphia. When it works, the fusion technique can deliver protein to 100% of cells. In some cases, Chernoff notes, that's the only way to discover subtle cell behaviors.

Related site
Information about Streptococcus pyogenes