Funktionelle Bedeutung der Protein-Sequestrierung in zytosolische Nipahvirus-Einschlusskörperchen

Nipahvirus (NiV) is a highly pathogenic virus that causes respiratory and neurological diseases. To get a better understanding of the productive NiV replication, which is still poorly characterized at the cellular level, this thesis focuses on the functional significance of so-called viral inclusion bodies (IBs). These IBs are formed by all non-segmented negative strand RNA viruses in the cytosol of infected cells and are generally described as “liquid organelles” that likely represent the sites of viral RNA replication. However, the role of NiV-IBs is largely unknown and this work shows for the first time that NiV-IBs not only possess the classical characteristics of liquid organelles but also have important, not yet described functions. For example, NiV-IBs contain components that are found in cellular aggresomes (γ-Tubulin, HDAC6, BAG3) and are able to sequester overabundant viral and non-viral proteins from the cytosol. Thus, NiV-IBs exhibit aggresome-like properties and functions. However, both compartments differ substantially in their structure and protein composition. Most importantly, protein sequestration to NiV-IBs does not require microtubule-dependent aggresomal transport pathways. Sequestration of viral and non-viral proteins into NiV-IBs presumably serves to reduce proteotoxic stress resulting from protein overexpression during infection, thereby improving cellular fitness and ensuring efficient viral replication. Another function of NiV-IBs, which is associated with the sequestration of cytosolic proteins, is the antagonization of the interferon (IFN) response. This thesis shows for the first time, that NiV-IBs recruit the signal transducers STAT1 and STAT2 via a specific interaction with the viral phosphoprotein, which accumulates in IBs. This sequestration results in a depletion of free STAT proteins in the cytosol, preventing STAT phosphorylation at IFN receptors upon stimulation with IFN. As a result, nuclear translocation of phosphorylated STAT and subsequent induction of antiviral gene expression is blocked. Therefore, in NiV-IB containing infected cells, autocrine IFN signal transduction and expression of antiviral proteins are blocked, supporting successful viral replication. In summary, this work demonstrates that NiV-IBs represent viral compartments that, in addition to their assumed role in viral RNA replication, indirectly support viral propagation by sequestering cytosolic proteins, thereby limiting proteotoxic stress in the cells on one hand and antagonizing the IFN-mediated immune response on the other.