An emerging theme in our understanding of gene expression is the integration and coordination of the many nuclear events of mRNA biogenesis. mRNA export depends upon both accurate completion of pre-mRNA processing and proper packaging of mRNAs into ribonucleoprotein complexes (mRNPs). The THO complex (Tho2p, Hrp1p, Mft1p, Thp2p) is believed to play a major role in the formation of export-competent mRNPs in Saccharomyces cerevisiae
by recruiting key proteins to the mRNA before export (37
; for a review, see reference 29
). An example of this coordination is the recruitment of Sub2p and Yra1p to the THO complex, forming the TREX complex, which is required for efficient elongation and for subsequent recruitment of the mRNA export receptor Mex67p to the elongating mRNA. The overall pathway for gene expression and almost all of the proteins required for packaging and export of mRNA are very highly conserved among eukaryotes, suggesting that mRNA export occurs by the same mechanisms in all eukaryotic cells.
Multiple mechanisms are present to ensure that mRNPs with incompletely or incorrectly processed mRNAs are retained in the nucleus (for reviews, see references 1
, and 33
). The nuclear exosome, a complex of 3′-to-5′ exoribonucleases, acts to retain defective mRNPs at or near their sites of transcription (14
; for a review, see reference 39
). Surveillance also involves the Mlp proteins (Mlp1p and Mlp2p), which are associated with the nuclear basket of the nuclear pore complex (NPC) and are thought to participate in quality control over mRNA export by interacting with mRNP proteins, including Nab2p (8
Heat shock and other stresses cause a radical shift in the pattern of gene expression. At the level of transcription, many genes, including those encoding heat shock proteins, are induced and transcribed at a high rate, while the expression of many others ceases (9
). At the level of mRNA processing, splicing is blocked following heat shock (5
). Because heat shock mRNAs generally lack introns, they are unaffected by inactivation of splicing. After heat shock, polyadenylated mRNAs accumulate in nuclei of both budding and fission yeast, whereas heat shock mRNAs are exported efficiently (24
). In the cytoplasm, the translation of many mRNAs is interrupted following heat shock, and this facilitates efficient synthesis of large amounts of heat shock proteins.
The mechanistic basis for differential mRNA export following heat shock in yeast is not known. Export after stress requires the same nucleoporins as export under normal growth conditions (3
). The mRNA export receptor Mex67p (34
) and the export factor Dbp5p (15
) are also required for heat shock mRNA export.
It is not known how distinct the pathways for general mRNA export in growing cells and export of heat shock mRNAs following heat shock are. Krebber et al. showed that Npl3p, an hnRNP protein that associates with mRNAs during transcription, dissociates from mRNAs following heat shock (23
). We reported previously that heat shock mRNA export was not affected when cells carrying the temperature-sensitive (TS) npl3
allele were shifted to 42°C (28
). Because Npl3p is normally required for mRNA export, its dissociation from mRNA following heat shock could be part of the mechanism underlying selective mRNA export. Whether another protein performs the functions of Npl3p for heat shock mRNAs is not known. The studies described here were conducted to examine the effects of mutations affecting other mRNA-binding proteins and additional mRNA export factors on heat shock mRNA export. We also analyzed recruitment of mRNA-binding proteins to SSA4
mRNA (encoding Hsp70).
Heat shock mRNA export was not affected by mutation of several other mRNA-binding proteins, including Yra1p. Interestingly, double mutants carrying the npl3-1 allele and several yra1 mutant alleles were still capable of heat shock mRNA export. Yra1p was loaded onto heat shock mRNAs in wild-type (WT) cells but not in npl3-1 cells, suggesting that Npl3p participates in the recruitment of Yra1p to mRNA. Export of SSA4 mRNA following heat shock does not require the THO complex. Because heat shock mRNA export occurs efficiently in strains that contain mutations affecting proteins normally important for efficient mRNA export, export after heat shock may have a reduced requirement for accurate formation of mRNPs.