The export of mRNA from the nucleus to the cytoplasm is a multistep process that is mediated by mRNA-protein complexes (mRNP). The formation of mRNP begins by recruitment of some mRNA export factors to the elongating transcripts followed by targeting of the mRNP to the nuclear pore complex (NPC). The mRNP-complex is then translocation through the NPC channel. Finally the mRNP-complex is dissembled in the cytoplasm so that the RNA can be translated and mRNA export factors are returned to the nucleus for another round of mRNA export. We found in contrast to other organisms, that in fission yeast Schizosaccharomyces pombe (S. pombe) the putative RNA-helicase Uap56p functions non-enzymatically in mRNA export. It has been shown to be released from the mRNP-complex in the nucleus, however, in S. pombe we found it to function as a carrier of mRNA from the nucleus to the cytoplasm. We also found that its export to the cytoplasm through the nuclear pore complex is mediated by directly interacting with the mRNA export factor Rae1p. We have also found that DNA-damage checkpoint protein Rad24p a member of the 14-3-3 family of proteins is involved in the nuclear export of mRNA in S. pombe. Rad24p interacted directly with mRNA export factor Rae1p and with BRCA-2 interacting protein Dss1p. Previously we had reported that Dss1p was also required for the nuclear export of mRNA. We have recently found that Rae1p also functions in monitoring DNA damage in G2 /M phase of the cell cycle. For the first time we found that Dss1p was required for the recruitment of DNA-damage checkpoint proteins Rad24p-Rae1p to the DSB site in vivo. In DNA-damage Rad24p is though to mediate nuclear export of Cdc25p a mitotic activator of Cdc2p. Interestingly, for the first time, we demonstrate that Cdc25p is also recruited to DSB break site upon DNA-damage and this recruitment is mediated by Rad24p. Rad24p mediated sequestration of Cdc25p onto DSB adds a new dimension to G2/M checkpoint control by first preventing Cdc25ps interaction with Cdc2p during DNA damage, and subsequently providing a nuclear source of Cdc25p for activating Cdc2p immediately upon completion of DNA repair. Our demonstration of direct physical interaction between Rae1p and Rad24p in vitro and Rad24p-dependent recruitment of Rae1p in vivo provides a plausible molecular basis for the role of Rae1p in cell cycle progression.
|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|