Yale Scientists Visualize Details Of How Hepatitis C Unwinds RNA

The work published in the journal Nature focuses on an enzyme, helicase NS3, that unwinds the RNA virus for replication inside cells. NS3 is one member of an extensive family of helicases and is used as a model for studying unwinding activities of motor proteins.

Their findings are particularly important because NS3 is a major drug target against HCV and understanding the helicase function will aid in the development of HCV inhibitors.

Pyle's results contradict the idea that helicases move smoothly with the continuous action of a snow plow. Instead, NS3 moves with a discontinuous stepping motion that alternates rapid translocation with pausing. "We observe that the helicase proceeds through discreet spatial and kinetic microstates," Pyle said. "We actually track the speed and processivity of the helicase as it passes through each base pair of its substrate."

"While this report is the first of its kind, and has produced highly significant results, it is only the beginning of a new understanding in HCV enzymology," said Pyle. "In the future, our approaches will be used to understand the composition of the HCV replication complex and the interplay between its constituent proteins. Comparative studies will be done on other viruses and in other systems where helicase function is critical."

These novel features were revealed using a new type of combinatorial enzymology that allows the behavior of helicase enzymes to be directly compared on a sequence panel. This is the first time that the behavior of a nucleic acid remodeling protein has been monitored at this high resolution, as it acts upon or passes each subunit of its target, according to Pyle. Since the work was conducted on RNA, it helps to bring RNA helicases to the forefront of motor research.

"By visualizing one of the key steps in how hepatitis C makes copies of itself, Dr. Pyle unexpectedly discovered that the molecular motor that unzips the virus' genetic material looks a lot like the motor that drives muscles," said Dr. Richard Ikeda, a chemist at the National Institute of General Medical Sciences, which funded the work. "This is a perfect example of how a basic investigation revealed surprise insights into the similarities among widely different organisms.'

The work was co-authored by postdoctoral fellow, Victor Serebrov, and funded by grants from the National Institutes of Health and the Howard Hughes Medical Institute.

Citation: Nature 430: 476-480 (July 22, 2004)