"There's a whole universe of other kinds of motor proteins out there," says Anna Marie Pyle, an HHMI investigator at Yale University's school of medicine. Pyle's lab studies RNA helicases, which traverse RNA strands rather than protein cables. Pyle's lab recently measured the movements of the NS3 helicase, which hepatitis C virus uses to smooth out its RNA genome as part of its replication cycle. Instead of observing individual helicase molecules under the microscope, Pyle and postdocs Victor Serebrov and Jane Kawaoka devised innovative enzyme-mixing experiments to demonstrate that the helicase operates just like those pliers you use to separate speaker wires.
"You attach that little tool onto one of the wires and pull it through the hole, and then the other strand gets stripped off. And just like your hand has to let go and then come closer to the pliers as you pull the wires through, that's how these proteins appear to behave," Pyle says.
Pyle's analysis showed that the helicase plows through exactly 18 base pairs with every rip, and then pauses to regain leverage. The researchers credit the unprecedented accuracy of their measurements to the fact that they were able to synchronize the helicase molecules with extreme precision, allowing them to time the motions of many motors simultaneously.
"We think single-molecule experiments are great, and we are doing them, too. But bulk enzyme experiments are often discounted by people who say, 'Oh well, you can't hear the notes if everybody's singing together,'" Pyle jokes, defending her different approach. "But that's not true if you have a good choir. You can hear them perfectly well, and you can often hear them louder."
Photo: Jennifer Altman
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Reprinted from the HHMI Bulletin,
Winter 2005, pages 14-18.
©2005 Howard Hughes Medical Institute