A Bromodomain-Containing Host Protein Mediates the Nuclear Importation of a Satellite RNA of Cucumber Mosaic Virus

Replication of the satellite RNA (satRNA) of Cucumber Mosaic Virus is dependent on replicase proteins of helper virus (HV). However, we recently demonstrated that like with Potato spindle tuber viroid (PSTVd), a satRNA associated with Cucumber Mosaic Virus strain Q (Q-satRNA) has the propensity to localize in the nucleus and generate multimers that subsequently serve as templates for HV-dependent replication. But the mechanism regulating the nuclear importation of Q-satRNA is unknown. Here we show that the nuclear importation of Q-satRNA is mediated by a bromodomain-containing host protein (BRP1), which is also apparently involved in the nuclear localization of PSTVd. A comparative analysis of nuclear and cytoplasmic fractions from Nicotiana benthamiana plants coinfected with Q-satRNA and its HV confirmed the association of Q-satRNA but not HV with the nuclear compartment. A combination of the MS2-capsid protein-based RNA tagging assay and confocal microscopy demonstrated that the nuclear localization of Q-satRNA was completely blocked in transgenic lines of Nicotiana benthamiana (ph5.2nb) that are defective in BRP1 expression. This defect, however, was restored when the ph5.2nb lines of N. benthamiana were trans-complemented by ectopically expressed BRP1. The binding specificity of BRP1 with Q-satRNA was confirmed in vivo and in vitro by coimmunoprecipitation and electrophoretic mobility shift assays, respectively. Finally, infectivity assays involving coexpression of Q-satRNA and its HV in wild-type and ph5.2nb lines of N. benthamiana accentuated a biological role for BRP1 in the Q-satRNA infection cycle. The significance of these results in relation to a possible evolutionary relationship to viroids is discussed.