Nuclear pores mediate the transport of proteins and RNA between the nucleus and cytoplasm in a bi-directional fashion. While small molecules under 40Kda can diffuse readily, larger molecules are actively transported through the pore. We have used fission yeast Schizosaccharomyces pombe as a model system to study how eukaryotic cells export their mRNA out of the nucleus. Through genetic approaches we have identified genes whose products influence the export of mRNA from the nucleus into the cytoplasm. We identified mRNA export factor spmex67 through its genetic interaction with rae1 . Both spMex67p and Rae1p genetically and functionally interact with nucleoporins, Npp106p and Nup184p to export mRNA out of the nucleus. Our results suggest a model in which both Rae1p and spMex67p can function in mRNA export by interacting with common mRNA export complexes. Furthermore, we demonstrate that spMex67p acts as an accessory factor in Rae1p-dependent mRNA export by participating at various common steps in mRNA export with Rae1p. The prevailing model of mRNA export is that RNA binds to carrier proteins, hnRNP like proteins that shuttle. In S. pombe we have identified an RNA-binding protein, Crp79p, that contains three RNA recognition motifs called RRM. Our results demonstrate Crp79p functions as a mRNA carrier protein. It binds mRNA, complements the mRNA export defects associated with rae1-167 nup184-1 double mutant, shuttles between the nucleus and cytoplasm. The nuclear import of Crp79p requires the functioning of the Ran-GTPase switch system. For nuclear export this protein contains three export signals. These signals are novel in nature that have not been previously identified.
| Thakurta, Anjan G; Gopal, Ganesh; Yoon, Jin Ho et al. (2004) Conserved nuclear export sequences in Schizosaccharomyces pombe Mex67 and human TAP function in mRNA export by direct nuclear pore interactions. J Biol Chem 279:17434-42 |
