A protein that shuttles between the nucleus and the cytoplasm is an important mediator of RNA export

The connection between RNA and protein export from the nucleus was examined in the budding yeast Saccharomyces cerevisiae. NPL3 encodes an RNA- binding protein that shuttles in and out of the nucleus. Export of poly(A)⁺ RNA has been shown previously to be blocked in npl3-1 mutants. To understand the role of Npl3p in RNA export, we have developed a novel assay that effectively uncouples nuclear protein export from reimport. With this assay, we show that Npl3p satisfies several of the predicted requirements for a protein carrier for mRNA export. Temperature-sensitive mutations in the RNA recognition motifs of Npl3p result in nuclear accumulation of poly(A)⁺ RNA. One such mutation prevents nuclear export of Npl3p. Moreover, Npl3p export depends on ongoing RNA polymerase II transcription. Export ceases in either the presence of the RNA synthesis inhibitor thiolutin or in a temperature- sensitive RNA polymerase (rpb1) mutant. Together, these findings support a model in which Npl3p exits the nucleus in association with poly(A)⁺ RNA, deposits the RNA in the cytoplasm, and is rapidly reimported for another cycle of export.