HnRNP proteins and the nuclear export of mRNA

Pre-mRNAs associate in the nucleus with specific RNA-binding proteins to form heterogeneous nuclear ribonucleoprotein (hnRNP) complexes. The hnRNP proteins participate directly or indirectly in the processing of pre-mRNAs into mature mRNAs. Recent studies have shown that some hnRNP proteins shuttle continuously between the nucleus and the cytoplasm. The export of shuttling hnRNP proteins from the nucleus is mediated by specific nuclear export sequences (NESs) within the proteins. In addition, shuttling hnRNP proteins appear to remain bound to exported mRNAs in transit through nuclear pores. As discussed in this review, the picture that is emerging is that nuclear export of mRNAs is mediated by the export of NES-containing hnRNP proteins to which they are bound.