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Bio-Imaging of Zoonotic and Emerging Bunyaviruses

Periodic Reporting for period 3 - BIZEB (Bio-Imaging of Zoonotic and Emerging Bunyaviruses)

Reporting period: 2018-04-01 to 2019-04-30

We aim to understand host cell entry of enveloped viruses at molecular level. A crucial step in this process is when the viral membrane fuses with the cell membrane. Similarly to cell–cell fusion, this step is mediated by fusion proteins (classes I–III). Several medically important viruses, notably dengue and many bunyaviruses, harbour a class II fusion protein. Class II fusion protein structures have been solved in pre- and post-fusion conformation and in some cases different factors promoting fusion have been determined. However, questions about the most important steps of this key process remain unanswered.

We are focusing on the entry mechanism of bunyaviruses by using cutting-edge, high spatial and temporal resolution bio-imaging techniques. These viruses have been chosen as a model system as they form an emerging viral threat to humans and animals, no approved vaccines or antivirals exist for human use and they are less studied than other class II fusion protein systems. Cryo-electron microscopy and tomography are being used to solve high-resolution structures of viruses, in addition to virus–receptor and virus–membrane complexes. Advanced fluorescence microscopy techniques are being used to probe the dynamics of virus entry and fusion in vivo and in vitro. Deciphering key steps in virus entry is expected to contribute to rational vaccine and drug design.

The project greatly benefits from our unique biosafety level 3 laboratory offering advanced bio-imaging techniques. Furthermore it will also pave way for similar projects on other infectious viruses. Finally the novel computational image processing methods developed in the project are becoming broadly applicable for the analysis of flexible biological structures, which often pose the most challenging yet interesting questions in structural biology.
Novel computational methods have been developed to analyse different structural components of virus particles from electron microscopy images. We are now applying the developed methods in order to determine the structures of different bunyaviruses at high resolution. First results show how the viral glycoproteins are organised on the viral surface. This is important as these proteins mediate entry of the virus into a host cell and thus better understanding of the structure can inform development of antiviral strategies. These methods have also general applications in the field of structural biology of large macromolecular assemblies.
By the end of the project we expect to be able to resolve, for the first time, the interaction between a viral fusion protein and a target membrane. This would be one of the many elusive intermediates in the viral fusion process. Mechanistic understanding of such intermediates holds potential for designing and understanding inhibitors that block the viral entry at this step.
Cryogenic electron microscopy model of Rift Valley fever virus