85NISR RESEARCH GRANT
2002 Young Investigator Research Grant

Selective Nuclear Export of mRNAs under Stressed Conditions in Saccharomyces cerevisiae
Shingo IZAWA
Division of Applied Life Science, Graduate School of Agriculture, Kyoto University
It is well known that cells show adaptive stress responses
both at the transcriptional level and the translational level
under stressed conditions. Recently, it has been reported
that adaptive responses are also induced at the level ofmRNA export from the nucleus to the cytoplasm. Severalstresses such as heat shock and ethanol stress cause selective mRNA export in Saccharomyces cerevisiae. Bulk poly(A) mRNAs accumulate in the nucleus, whereas mRNAsof heat shock proteins are exported under stressed conditions. However, the mechanisms of mRNA export still remain to be clarified, and we have little information about
the selective export of mRNAs under stressed conditions.
In this study, we investigated the response to stresses including high salt stress in mRNA export in yeast cells. We
also investigated the changes in mRNA export factors
under stressed conditions. Through this research, we got
several novel and interesting findings about stress responses
in mRNA export.
Export of mRNAs under high salt stressed conditions inyeast
We investigated the export of bulk poly (A) mRNAs
under stressed conditions by in situ hybridization using
oligo dT probes. In S. cerevisiae, high salt stress (0.4 MNaCl) rapidly blocked the export of bulk poly (A) mRNAsand induced the accumulation of poly (A) mRNAs in thenucleus. On the other hand, the ENA1 mRNA, a high salt
stress responsive gene, was synthesized and exported to thecytoplasm under high salt stressed conditions. Intriguingly,
the export of bulk poly (A) mRNAs was resumed approximately 5 min after the treatment with high salt stress,
whereas heat shock and ethanol stress blocked the export ofbulk poly (A) mRNAs over 1 hour. Furthermore, the nuclear accumulation of mRNAs under 1.0 M NaCl was not
induced by the pretreatment with mild salt stress (0.1 M
NaCl for 1 hour). These results indicate that the adaptation
to high salt stress in mRNA export is presumably induced
easily and rapidly in S. cerevisiae. We also investigated themRNA export in Zygosaccharomyces rouxii. Bulk poly(A) mRNAs were exported even under 1.0 M NaCl and noaccumulation of mRNAs in the nucleus was induced in Z.
rouxii, indicating that Z. rouxii has higher ability to adapt tohigh salt stress also in mRNA export. We are now trying to
identify other mRNAs exported under high salt stressed
conditions as well as ENA1 in S. cerevisiae. The cloning ofgenes encoding mRNA export factors of Z. rouxii is alsounderway.
Stress responsive changes in the localization of mRNA
export factors
We investigated the effects of stresses on mRNA export
factors. Several export factors and nucleoporins showed thechanges in their localizations under stressed conditions.
One of the export factors, Rat8p, showed the stress response to ethanol but not to other stresses. In yeast cells
treated with ethanol stress, Rat8p showed reversible andrapid change in its localization, indicating that the blockingof bulk poly (A) mRNAs export under ethanol stressed
conditions is partly caused by the stress-responsive change
in the localization of Rat8p. Intriguingly, Rat8p did notchange its localization under heat shocked conditions, suggesting a possibility that mechanisms of the blocking ofmRNA export are different between ethanol stressed conditions and other stressed conditions. We also found that
Rat8p is dispensable for the export of stress inducible
mRNAs under ethanol stressed conditions. This is the firstreport of the stress-responsive changes in the localization ofmRNA export factors. These results may aid understandingof the contribution of mRNA export factors to selectivemRNA export under stressed conditions.
Reference
Shingo Izawa, Reiko Takemura, and Yoshiharu Inoue. Stress Response inYeast mRNA Export Factors: Reversible Change in the Localization ofRat8p Is Caused by Ethanol Stress but Not by Heat Shock. (submitted)