Tetherin antagonism by the Ebola virus glycoprotein: Molecular mechanism and contribution to viral spread

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The interferon (IFN) system is an important component of innate immunity and plays a key role in the defence against viral infections. Invading viruses are detected by cellular sensors which trigger the production and secretion of IFN. Binding of IFN to uninfected cells triggers the expression of IFN-induced genes, some of which exert a direct antiviral activity. Tetherin is an IFN-induced protein, which can inhibit release of several enveloped viruses from infected cells. However, some viruses encode tetherin antagonists, which allow viral spread in tetherin expressing cells. The HIV-1 protein Vpu reduces tetherin expression at the cell surface, the site of viral budding. The glycoprotein of Ebola virus (EBOV-GP1,2) also inhibits tetherin, but the underlying mechanism is unclear and will be defined within the proposed studies.Our preliminary data suggest that EBOV-GP1,2, in particular GP2, could relocalize the tetherin N-terminus from the cytoplasm to the extracellular space, and this hypothesis will be investigated. For this, we will clarify if the tetherin N-terminus becomes accessible to antibody binding upon coexpression of EBOV-GP1,2, which would demonstrate that the N-terminus has indeed been relocated into the extracellular space. Control experiments conducted in parallel will ensure that antibody binding is not due to interference of EBOV-GP1,2 with membrane integrity. An alternative hypothesis regarding tetherin antagonism is that EBOV-GP1,2 might prevent tetherin localization in compartments of the cytoplasmic membrane, which are used by EBOV for budding. This scenario will be investigated with the help of superresolution microscopy (STORM) and immunogold labeling and electronmicroscopy. Moreover, mutagenic analysis will be employed to identify amino acid substitutions, which selectively block tetherin antagonism but do not compromise EBOV-GP1,2 expression and EBOV-GP1,2-driven virus cell fusion. Finally, reverse genetics will be employed to mutate GP1,2 in the context of authentic EBOV, in order to address if tetherin antagonism is required for viral spread and pathogenesis. These studies will reveal important insights into viral defense strategies against antiviral host cell proteins and might define novel targets for antiviral intervention.