The breast cancer tumor suppressor BRCA2-interacting protein, DSS1, and its homologs are critical for DNA recombination in eukaryotic cells. We found that Dss1p, along with Mlo3p and Uap56p, Schizosaccharomyces pombe homologs of two messenger RNA (mRNA) export factors of the NXF-NXT pathway, is required for mRNA export in S. pombe. Previously, we showed that the nuclear pore-associated Rae1p is an essential mRNA export factor in S. pombe. Here, we show that Dss1p and Uap56p function by linking mRNA adapter Mlo3p to Rae1p for targeting mRNA-protein complex (mRNP) to the proteins of the nuclear pore complex (NPC). Dss1p preferentially recruits to genes in vivo and interacts with -FG (phenylalanine glycine) nucleoporins in vivo and in vitro. Thus, Dss1p may function at multiple steps of mRNA export, from mRNP biogenesis to their targeting and translocation through the NPC.
Rae1p and Mex67p/Tap are conserved mRNA export factors. We have used synthetic lethal genetic screens in Schizosaccharomyces pombe to identify mutations in genes that are functionally linked to rae1 and mex67 in mRNA export. From these screens, we have isolated mutations in a putative S. pombe homologue of the Candida albicans elf1 gene. The elf1 of S. pombe is not an essential gene. When elf1 mutations are combined with rae1-167 mutation, growth and mRNA export is inhibited in the double mutants. This inhibition can be suppressed by the multicopy expression of mex67 suggesting that Mex67p can substitute for the loss of Elf1p function. Elf1p is a non-membrane member of the ATPbinding cassette (ABC) class of ATPase and the GFPElf1p fusion localizes to the cytoplasm. Elf1p, expressed and purified from Escherichia coli, binds and hydrolyzes ATP. A mutant Elf1p that carries a glycine to aspartic acid (G731D) mutation within the Walker A domain of the second ATP site retains the ATP binding but loses its ATPase activity in vitro. This mutant protein no longer functions in mRNA export. Taken together, our results show that Elf1p functions as a mRNA export factor along with Rae1p and Mex67p in S. pombe.In the eukaryotic cell, the nuclear envelope (NE) 1 separates the nucleus from the cytoplasmic compartment. Embedded within the NE are the nuclear pore complexes (NPC) through which nucleo-cytosolic exchanges take place. While small molecules of molecular mass 40 kDa and under can diffuse passively through the NPC, receptor proteins interact with the NPC to mediate the transport of macromolecules (1, 2). The mature mRNAs are exported out of the nucleus as ribonucleoprotein (RNP) complexes. Concurrent with transcription and splicing, the coordinated assembly of export-competent RNP complexes takes place in the nucleus by association of proteins with the maturing transcripts (3-5). In the cytoplasm, the RNP complexes are disassembled to release the mRNA and soluble export factors; the latter return to the nucleus for participating in the next round of mRNA export.The major components of the mRNA export machinery appear to be evolutionarily conserved. Genetic and biochemical approaches have led to the identification of several conserved export factors, Tap/Mex67p, Rae1p/Gle2p, Gle1, and Dbp5/ RHA from yeasts to metazoans (6 -13). The molecular mechanism of Mex67p/Tap function in mRNA export is understood in some detail, but how Rae1p/Gle2p function is largely unknown (5, 14). Nonetheless, there is a close reciprocal relationship between Rae1p and Mex67p in the two yeast systems that may indicate their functional homology and mechanistic similarity. Mex67/Tap are non- karyopherin receptors. They associate with nucleoporins, bind mRNA, and interact with the RNAbinding protein, Aly/Yra1p, to mediate mRNA export (3, 5). Rae1p, on the other hand, is an NPC-associated, conserved WD-domain protein that is also required for cell-cycle progression at the G 2 /M boundary (8). While the hRae1p has been shown to bind mRNA and to shutt...
Mex67, the homolog of human TAP, is not an essential mRNA export factor in Schizosaccharomyces pombe. Here we show that S. pombe encodes a homolog of the TAP cofactor that we have also named p15, whose function in mRNA export is not essential. We have identified and characterized two distinct nuclear export activities, nuclear export signal ( Evolutionarily conserved nuclear export factors (NXFs)
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