Upon infection the influenza virus starts multiplying in the cells of an infected host. The polymerase is crucial in this process because it copies the viral genome and directs the production of its proteins. Interfering with polymerase function would prevent the virus replicating, thereby reducing the spread of the virus and the severity of the infection.
"For many years scientists have tried to understand the flu polymerase and to look for weak points that could be targeted by drugs," says Darren Hart, whose team participated in the research at EMBL Grenoble. "But no one could get enough protein to analyse its structure. We developed a way to use robots to screen tens of thousands of experimental conditions and discovered a piece of the influenza polymerase that we could work with. It is a small part of the entire protein, but it provides interesting insights into how the protein works and how mutations may affect hos t range."
Together with scientists at the IBS they visualized the atomic structure of the protein and discovered a previously overlooked signal that labels it for transport to the human nucleus where the genetic material of the virus is replicated. Cell microscopy studies at EMBL Heidelberg revealed that the human nuclear transport protein, importin alpha, recognises this signal and shuttles the polymerase into the nucleus. To find out how the polymerase and importin interact, Stephen Cusack, head of EMBL Grenoble, and collaborators at the UVHCI, used the high intensity X-ray source of the European Synchrotron Radiation Facility to generate a high-resolution image of the two proteins interacting with each other. The image revealed that mutations known to play a role in the transmission of avian influenza virus to mammals were located within, or close to, this site of interaction. This suggests that mutations may affect the efficiency of nuclear transport and through this the ability of the virus to replicate in different species.
"Interfering with polymerase function could provide new ways to treat or prevent flu," says Cusack, "but this will require a detailed picture of the rest of the polymerase. This is what we are aiming for in our new FLUPOL project. In a joint effort with other European laboratories, and with financial support by the European Commission, we will explore both structure and function of this key drug target and try to characterise other mutations implicated in bird-to-human transmission."