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Prodigious protein production sans cells.

Prodigious protein production sans cells

In the biotech equivalent of leading a horse to water and trying to make it drink, gene engineers often find they can insert a new gene into living cells but still cannot get those cells to produce much useful protein. In some cases, other cell products break down the protein, or the protein might be toxic to the cells that produce it.

Now five Soviet scientists report creating a highly efficient system for making proteins without using living cells, thereby bypassing many of these sorts of problems. Their system, detailed in the NOV. 25 SCIENCE, should "open new fields" for scientific and biotechnological development, they say.

"Cell-free" systems throw together genes, amino acids, the cell's protein-making machinery and a source of chemical energy into a solution to make proteins without cell membranes to contain the reaction. The drawback common to such systems has been that they produce very little protein, say Alexander S. Spirin and his colleagues at the Institute of Protein Research in Moscow. Typically such systems produce only two to three proteins per gene copy, they say.

But the Soviet scientists have developed a "flow-through" system that creates hundreds of times as much protein as previously possible, they report. In the new system, the cell machinery and messenger RNA (mRNA) copies of the desired gene are held in a reaction chamber while a fresh solution containing the energy-storing molecules ATP and GTP continuously flows through the chamber. The flow of solution not only replenishes the supply of amino acids, ATP and GTP in the chamber, but also carries away the proteins already produced. Filters keep the larger protein-making machinery and mRNA from leaving the chamber.

Spirin and his colleagues say they kept this reaction going for more than 40 hours, producing up to 300 copies of protein per gene copy. The efficiency of the new system surprised even the researchers, who say they had expected to see some of the mRNA chewed up by enzymes that are difficult to remove from the reaction.

They also expected the efficiency of the reaction to decrease over time as some of the smaller pieces of the protein-making machinery slipped through the filters. This didn't happen either, perhaps because such molecules were always bound up with other, larger molecules, they say. The huge increase in reaction efficiency may make the system useful for creating difficult-to-manufacture proteins, several scientists note.

"You might want to make toxic proteins that can't be produced in cells," says Shing Chang, director of research at Cetus Corp. in Emeryville, Calif. Such proteins, which would quickly kill any cell that made them, could be attached to monoclonal antibodies that could then seek out and kill cancer cells, Chang says.

One drawback to the system might be the high cost of constantly replacing the expensive ATP, Chang says. "ATP is like currency," he says. "The utility of the system depends on how much you have to spend."

There are also questions of whether the process will work on the very large scale required for industrial use and whether it can produce structurally correct proteins, Chang says.

"Certain modifications of proteins occur in [the cell]," Chang says. "I would want to know if this system could make those modifications too."

The new cell-free expression system is not the best system for the production of all proteins, but it may be best for certain kinds of proteins, the Soviet scientists say. It can be used to produce not only toxic proteins but also proteins that are unstable and need to be isolated quickly, they say. It may also give researchers simple proteins to study how proteins are modified and folded by cellular machinery, and "significantly extend" the possibilities of protein engineering, they add.
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Author:Vaughan, Christopher
Publication:Science News
Date:Nov 26, 1988
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