A yeast RNA-binding protein shuttles between the nucleus and the cytoplasm

RNA-binding proteins have been suggested to move in association with RNA as it leaves the nucleus. The NPL3 gene of the yeast Saccharomyces cerevisiae encodes a nuclear protein with consensus RNA-binding motifs and similarity to heterogeneous nuclear ribonucleoproteins and members of the S/R protein family. We show that although Np13 is located in the nucleus, it can shuttle between nuclei in yeast heterokaryons. In contrast, other nucleus-targeted proteins do not leave the nucleus under similar conditions. Mutants missing the RNA-binding motifs or the N terminus are still capable of shuttling in and out of the nucleus. Np13 mutants missing the C terminus fail to localize to the nucleus. Overproduction of Np13 in wild-type cells slows cell growth. This toxicity depends on the presence of a series of unique repeats in the N terminus and localization to the nucleus. We suggest that the properties of Np13 are consistent with it being involved in export of RNAs from the nucleus.