Control of nuclear export of hnRNP A1.

mRNA export is mediated by RNA-binding proteins which shuttle between the nucleus and cytoplasm. Using an in vitro unidirectional export assay, we observe that the shuttling mRNA-binding protein, hnRNP A1, is exported only extremely slowly unless incubations are supplemented with snRNA-specific oligonucleotides which inhibit splicing. In vivo microinjection experiments support this conclusion. Like many examples of nucleocytoplasmic transport, export of hnRNP A1 requires energy and is sensitive to the presence of wheat germ agglutinin. It does not, however, require supplementation with cytoplasmic proteins. Although the exportin, Crm1, is needed for export of several varieties of RNA, both the in vitro assay and in vivo assays show that it is not required for export of hnRNP A1. In vitro and in vivo studies also show that inhibition of transcription allows continued shuttling of hnRNP A1 and in fact accelerates its export. Judging from the stimulatory effects of targeted destruction of snRNAs, this is likely to reflect completion of the covalent maturation of the RNAs with which hnRNP A1 associates. These observations therefore provide a simple explanation of why multiple RNA-binding proteins relocate to the cytoplasm upon inhibition of transcription in vivo.