New images show how flu jumps species boundary
European researchers have produced a three-dimensional image of a part of the flu virus which is involved in the transmission of the diseases between birds and humans, opening up new options for ways to treat or prevent flu. The work, which was partly funded by the EU's Fifth Framework Programme (FP5), is published online by the journal Nature Structural and Molecular Biology. As outbreaks of the virulent H5N1 strain of avian influenza continue to pop up around the world, concerns remain high that a mutation will arise which permits the virus to be transferred directly from human to human. According to World Health Organisation figures, since the outbreak started in 2003 there have been 274 confirmed cases of H5N1 in people and 167 deaths. Most of these people lived or worked in close proximity with infected poultry. If the virus jumps the species barrier, the results could be catastrophic; in 1918, 50 million people world-wide died during a flu pandemic caused by a mutation in an avian strain of the disease. Now, for the first time, researchers have produced a three dimensional image of part of the polymerase protein, leading to a greater understanding of how small changes in the structure of polymerase could contribute to giving the virus a helping hand across the species boundary from birds into humans. When a person or bird is first infected with flu, the virus starts to replicate itself inside the host's cells. Polymerase plays a key role in this process, as it is responsible for copying the viral genome and directing the production of its proteins. Although it has been known for some time that small changes to parts of the polymerase are involved in the transmission from avian to human hosts, the nature of the protein makes it very hard to study. 'For many years scientists have tried to understand the flu polymerase and to look for weak points that could be targeted by drugs,' commented Darren Hart of the European Molecular Biology Laboratory (EMBL), one of the authors of the paper. 'But no one could get enough protein to analyse its structure.' To obtain a sample of the protein, the researchers developed a new screening technique called 'expression of soluble proteins by random incremental truncation' (ESPRIT). '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,' explained Dr Hart. 'If is a small part of the entire protein, but it provide interesting insights into how the protein works and how mutations may affect host range.' Investigations into the atomic structure of the protein revealed a previously overlooked signal that is recognised by the human nuclear transport protein, importin alpha. On recognising the signal, importin alpha transports the polymerase into the nucleus of the cell, where the genetic material of the virus is replicated. Using the high intensity x-ray source of the European Synchrotron radiation Facility, the researchers were able to generate a high-resolution image of how the polymerase and importin alpha interact with each other. The images showed that the mutations known to be involved in the transmission of bird flu to humans were located in or near to the site of interaction. According to the researchers these mutations may influence the ability of the virus to replicate in different species by affecting the efficiency of nuclear transport. 'Interfering with polymerase function could provide new ways to treat or prevent flu, but this will require a detailed picture of the rest of the polymerase' explained Stephen Cusack, Head of the EMBL's Grenoble laboratory. Obtaining more information on the polymerase protein is the topic of the EU-funded FLUPOL project, which is coordinated by the EMBL and will last for three years. It is one of the 17 projects selected for funding last year following a special call for proposals under the Sixth Framework Programme (FP6). '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,' commented Dr Cusack.