The Karyopherin Sal3 is Required for Nuclear Import of the Core RNA Interference Pathway Protein Rdp1

RNA-dependent RNA polymerase activity is required for RNA interference (RNAi) in many lower eukaryotes including the fission yeast Schizosacchromyces pombe. Together with Ago1 and Dcr1, the RNA-dependent RNA polymerase Rdp1 is critical for RNA-dependent transcriptional- and post-transcriptional gene silencing. Although the bulk of Rdp1 is localized to the nucleus, Ago1 and Dcr1 are primarily cytoplasmic. This may reflect the fact that Rdp1 is required early in the RNAi pathway to generate double strand RNA from transcripts that originate from centromeric loci. The relatively large size of Rdp1 (139.4 kD) precludes passive diffusion of the enzyme into the nucleus suggesting that karyopherin-dependent transport is involved in nuclear targeting of this enzyme. In this study, we report that the karyopherin/importin beta 3 homolog Sal3 is required for nuclear import of Rdp1 in S. pombe. Loss of nuclear Rdp1 was associated with substantially reduced transcriptional gene silencing, and surprisingly, post-transcriptional gene silencing which occurs in the cytoplasm of other eukaryotes, was also significantly affected. Together, these results identify Sal3 as a modulator of RNAi-dependent transcriptional gene silencing as well as a potential link between nuclear import and post-transcriptional gene silencing.