A YEAST RNA-BINDING PROTEIN SHUTTLES BETWEEN THE NUCLEUS AND THE CYTOPLASM

RNA-binding proteins have been suggested to move in association with R NA 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 member s 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 und er similar conditions. Mutants missing the RNA-binding motifs or the N terminus are still capable of shuttling in and out of the nucleus. Np 13 mutants missing the C terminus fail to localize to the nucleus. Ove rproduction of Np13 in wild-type cells slows cell growth. This toxicit y depends on the presence of a series of unique repeats in the N termi nus and localization to the nucleus. We suggest that the properties of Np13 are consistent,vith it being involved in export of RNAs from the nucleus.