Electron microscopes make high-resolution images of their tiny targets by hitting them with electrons rather than photons of light. The images revealed numerous well-defined RNP complexes. To Moeller and his colleagues' surprise, many of these appeared to have new, partial RNPs growing out of them. "They were branchingthis was very exciting," he said.
"Essentially these were snapshots of flu RNPs being replicated, which had never been seen before," said Kirchdoerfer. These and other data, built up from images of tens of thousands of individual RNPs, allowed the team to put together the most complete model yet for flu-RNP structure and functions. The model includes details of how the viral polymerase binds to its RNA, how it accomplishes the tricky task of viral gene transcription, and how a separate copy of the viral polymerase assists in carrying out RNP replication. "We're now able to take a lot of what we knew before about flu virus RNP and map it onto specific parts of the RNP structure," said Kirchdoerfer.
The new flu RNP model highlights some viral weak points. One is a shape-change that a polymerase subunitwhich grabs viral RNA and feeds it to the polymerase's active site on a second subunithas to undergo during viral gene transcription. Another is key interaction between the polymerase and viral nucleoproteins. Flu RNPs are long and flexible, curving and bending in electron microscope images; and thus the structural model remains only modestly fine-grained. "You wouldn't be able to design drugs based on this model alone," said Kirchdoerfer, "but we now have a much better idea of how flu RNPs work, and that does suggest some possibilities for better flu drugs."
|Contact: Jann Coury|
Scripps Research Institute