Schwardt M, Mayer D, Frank R, Schneider U, Eickmann M, Planz O, Wolff T, Schwemmle M (2005): The negative regulator of Borna Disease Virus polymerase is a non-structural protein
J. Gen. Virol. 86 (11): 3163-3169.

The X protein of Borna Disease Virus (BDV) negatively regulates the viral polymerase activity. With a BDV mini replicon system, 30 % inhibition of polymerase activity is observed at an X to phosphoprotein (P) plasmid ratio of 1 to 6 and 100% inhibition at a ratio of 1:1. We therefore hypothesized that i) the X to P protein ratio in infected cells is not significantly higher than 1 to 6 to prevent complete inhibition of polymerase activity, and ii) that X is not efficiently incorporated into viral particles to allow efficient replication early in infection. To test these assumptions, we generated a monoclonal antibody directed against BDV-X. Immunofluorescence analysis revealed co-localiziation 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 E. coli-derived X, P and N as protein standards, revealed an X to P to N ratio in BDV-infected cells of approximately 1:6:40. However, only traces of X could be detected in purified BDV virus 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.