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The negative regulator of Borna disease virus polymerase is a non-structural protein

¹ Department of Virology, University of Freiburg, Hermann-Herder-Strasse 11, D-79104 Freiburg, Germany
² Department of Chemical Biology, GBF, Braunschweig, Germany
³ Institut für Virologie, Universität Marburg, Germany
⁴ Institut für Immunologie, Friedrich Loeffler-Institut, Tübingen, Germany
⁵ Robert Koch-Institut, D-13353 Berlin, Germany

Correspondence
Martin Schwemmle
martin.schwemmle{at}uniklinik-freiburg.de

The X protein of Borna disease virus (BDV) negatively regulates viral polymerase activity. With a BDV mini-replicon system, 30 % inhibition of polymerase activity was observed at an X to phosphoprotein (P) plasmid ratio of 1 : 6 and 100 % inhibition at a ratio of 1 : 1. It was therefore hypothesized that (i) the X : P ratio in infected cells is not significantly higher than 1 : 6 to prevent complete inhibition of polymerase activity and (ii) X is not efficiently incorporated into viral particles, allowing efficient replication early in infection. To test these assumptions, a monoclonal antibody directed against BDV X was generated. Immunofluorescence analysis revealed co-localization of X with the nucleoprotein (N) and P in the nucleus, as well as in the cytoplasm of BDV-infected cells. Quantification of viral protein levels by Western blot analysis, using purified Escherichia coli-derived X, P and N as protein standards, revealed an X : P : N ratio in BDV-infected cells of approximately 1 : 6 : 40. However, only traces of X could be detected in purified BDV stock, suggesting that X is excluded from virus particles. These results indicate that X is a non-structural protein. The lack of X in virus particles may facilitate polymerase activity early in infection; however, the presence of X in persistently infected cells may result in partial inhibition of the polymerase and thus contribute to viral persistence.

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