Polymerase activity of BDV. By immunofluorescence analysis, using mammalian and avian cell lines, we showed that the X and P proteins of the nonmammalian bornaviruses tested exhibit a similar distribution to those of BDV in transfected cells (Figure 3) and that the coexpression of X with P induces efficient translocation of the P protein from the nucleus to the cytoplasm. The immunoprecipitation assay also confirmed the interaction between the X and P proteins of non-mammalian bornaviruses. These data strongly suggest that the functional interaction between the X and P proteins of bornaviruses has been conserved during their evolution and that control of the intranuclear level of the P protein may be a fundamental role of the bornavirus X protein. On the other hand, we also found that the nuclear export of the P by the X of RBV may not be optimal in the mammalian and avian cells (Figure 4). This observation suggested either that the RBV X may employ the different mechanism to transport the P to the cytoplasm from the nucleus, or that some reptile-specific hostConserved Interaction of Bornavirus ProteinsFigure 6. Inter-genotypic interaction between the bornavirus X and P proteins. Immunoprecipitation analysis was carried out using cells co-transfected with plasmids expressing Flag-tagged X proteins of BDV (A), ABV4 (B), ABV5 (C) or RBV (D) and HA-tagged P expression plasmids from each genotype indicated. After immunoprecipitation with anti-HA antibody, the precipitates were detected by anti-Flag antibody. A long exposure image of the membrane is shown for the inter-genotypic interaction of RBV X (D). doi:10.1371/journal.pone.0051161.gFigure 7. Compatible function of ABV X and P in a BDV minireplicon assay. BDV minireplicon assays were performed using the expression plasmids indicated, together with 0.125 ng of the minigenome construct and helper plasmids expressing BDV N (0.125 ng) and L (0.125 ng). The graph shows the mean 6 SE of three independent experiments. At least three independent experiments were performed, except for RBV-Btransfection assay (n = 2). The differences were statistically significant (P,0.01, student t test), except for the assay using RBV X and RBV-BP. n.s., not significant. doi:10.1371/journal.pone.0051161.gConserved Interaction of Bornavirus Proteinsfactors may be required for the proper function of the RBV X in the cells. In a previous study, we showed that the 59 UTR of the X/P mRNA of BDV contains elements that control the translation of the X protein [16]. We showed that interaction of the RNA helicase DDX21 with the predicted stem-loop structure in the 59 UTR negatively regulates ribosomal initiation at the AUG codon of the X ORF. It was also shown that the P protein may enhance ribosomal reinitiation at the X ORF by inhibition of the interaction of DDX21 with the stem-loop structure, via interference with its phosphorylation [16]. Considering that the X proteins of non-mammalian bornaviruses seem to have a conserved function as regulatory proteins for maintenance of the optimal level of the P protein in the nucleus, the genotypes with a short 59 UTR in the putative X/P mRNA may use a different mechanism to control the translation of the X protein in infected cells. Intriguingly, we found that, despite the short length of the 59 UTR of the 16985061 putative X/P mRNA in ABV4 and RBV, these seem to form stem-loop structures in a short stretch encompassing the 59 UTR and the X encoding region (data not shown). This fin.