A nuclear import pathway exploited by viroid RNAs

Abstract

The prevailing view regarding intracellular RNA trafficking in eukaryotic cells describes that RNAs transcribed in the nucleus either stay in the nucleus or cross the nuclear envelope entering the cytoplasm for function. Interestingly, emerging evidence illustrates numerous functional RNAs trafficking in the reverse direction from the cytoplasm to the nucleus. However, the mechanism underlying the RNA nuclear import has not been well elucidated. Viroids are single-stranded circular noncoding RNAs that infect plants. Using nuclear-replicating viroids as a model, we showed that cellular Importin alpha-4 is likely involved in viroid RNA nuclear import, empirically supporting the involvement of Importin-based cellular pathway in RNA nuclear import. We also confirmed the involvement of a cellular protein (Virp1) that binds both Importin alpha-4 and viroids. Furthermore, a conserved C-loop in nuclear-replicating viroids is critical for Virp1 binding. Disrupting C-loop impairs Virp1 binding, viroid nuclear accumulation and infectivity. Further, C-loop exists in a subviral satellite noncoding RNA that relies on Virp1 for nuclear import. These results have significant implications for understanding the infection process of subviral agents. In addition, our data outline a cellular pathway responsible for the nuclear import of RNAs and uncover a 3-dimensional RNA motif-based regulation over RNA nuclear import.