Exploiting the power of heterologous expression in plants to discover new virus structure.

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For a virus to be able to spread from one organism to another, it is absolutely essential that a protective protein (and sometimes membrane-containing) capsid is assembled to protect its genetic material (genome) from the harsh external environment. Typically, the protein capsid is formed from one (or a few) type of coat protein that assembles to form a highly symmetric container into which the genome is packaged. These capsids are characteristic of the virus: each virus has a particular size, shape and configuration that uniquely identifies it. A large number of 3D structures have been determined for virus capsids, and these structures have helped revolutionise research into viruses. Structural information enables a myriad of experiments, including the design of mutant versions of the viruses to help understand their basic biology, informing the design of new molecules that have antiviral properties and are thus potential anti-viral medicines, and helping to validate the design and efficacy of new vaccines. However, despite these enormous strides, large holes exist in our structural understanding of the viruses in nature. A great many different types of viruses, including viruses that are devastating pathogens of crops around the world, and a thus are a major source of food insecurity in the developing world, currently have no structures. In part this is because many viruses, especially those that are extremely toxic to plants, are exquisitely difficult to propagate in the amounts required for structural studies. We are now in a position to remedy this problem. Using cryo-electron microscopy, a technique in structural biology that is now capable of generating structures for viruses at atomic resolution using relatively small amounts of virus (at the University of Leeds), and new capabilities to express virus proteins in plants (at the John Innes Centre in Norwich), we have shown that virus-like particles that are identical to the authentic virus can be produced, and their 3D structures can be relatively rapidly determined. We will now use these techniques to fill in some of the gaps in our structural knowledge of viruses present in Nature. We will start with the Luteoviridae, a family of viruses that infect plants, and are commercially important pathogens of cereals and potatoes. We have already determined a preliminary structure for one: potato leaf roll virus, showing that our approach is highly likely to yield rapid results. We will improve our existing structure and solve the structure of other important family members, before beginning to work on more challenging viruses (with more complicated capsids). These will include a large number of different families of plant viruses, which again include important pathogens that devastate food and commercial crops across the developing world (e.g. rice tungro spherical virus, that is implicated in rice crop losses of >$1.5 billion p.a.). They will also include human pathogens.Clearly a greater understanding of the structure that viruses assemble to protect their genomes, and of processes essential for virus spread would be of huge significance to our ability to combat the diseases these viruses cause. Such understanding might help to develop virus particles that can act vaccines, or as vehicles for the delivery of molcules to cells for a variety of medical applications. As a routine part of our work, we will generate a novel protein-based binding reagent that can specifically recognise the virus in question. These molecules, called 'Adhirons' are functionally analogous to antibodies, and will be an invaluable resource for researchers interested in the virus in question, potentially allowing for example the rapid diagnosis of infection in a simple, in-field testing device, or the purification of small amounts of authentic virus from infected tissues for future research. The knowledge gained from these studies would therefore also aid applications in biotechnology.