A structured retroviral RNA element that mediates nucleocytoplasmic export of intron-containing RNA.

A common feature of gene expression in all retroviruses is that unspliced, intron-containing RNA is exported to the cytoplasm despite the fact that cellular RNAs which contain introns are usually restricted to the nucleus. In complex retroviruses, the export of intron-containing RNA is mediated by specific viral regulatory proteins (e.g., human immunodeficiency virus type 1 [HIV-1] Rev) that bind to elements in the viral RNA. However, simpler retroviruses do not encode such regulatory proteins. Here we show that the genome of the simpler retrovirus Mason-Pfizer monkey virus (MPMV) contains an element that serves as an autonomous nuclear export signal for intron-containing RNA. This element is essential for MPMV replication; however, its function can be complemented by HIV-1 Rev and the Rev-responsive element. The element can also facilitate the export of cellular intron-containing RNA. These results suggest that the MPMV element mimics cellular RNA transport signals and mediates RNA export through interaction with endogenous cellular factors.     

Nuclear retention of unspliced mRNAs in yeast is mediated by perinuclear Mlp1

The molecular mechanism underlying the retention of intron-containing mRNAs in the nucleus is not understood. Here, we show that retention of intron-containing mRNAs in yeast is mediated by perinuclearly located Mlp1. Deletion of MLP1 impairs retention while having no effect on mRNA splicing. The Mlp1-dependent leakage of intron-containing RNAs is increased in presence of ts-prp18 delta, a splicing mutant. When overall pre-mRNA levels are increased by deletion of RRP6, a nuclear exosome component, MLP1 deletion augments leakage of only the intron-containing portion of mRNAs. Our data suggest, moreover, that Mlp1-dependent retention is mediated via the 5' splice site. Intriguingly, we found Mlp-proteins to be present only on sections of the NE adjacent to chromatin. We propose that at this confined site the perinuclear Mlp1 implements a quality control step prior to export, physically retaining faulty pre-mRNAs.     

The ins and outs of HIV Rev.

The Rev protein of the human immunodeficiency virus mediates the nuclear export of the intron-containing viral messages. This export is a consequence of the continuous shuttling of HIV Rev between the nucleus and cytoplasm. This shuttling is mediated by a nuclear localization signal and a nuclear export signal contained within Rev. Recently, several factors which are required for the movement of Rev through the nuclear pore have been identified. This review will focus on these factors and their role the nucleocytoplasmic shuttling of HIV Rev.     

An intron in the YRA1 gene is required to control Yra1 protein expression and mRNA export in yeast

Yra1p is an essential and conserved mRNA export factor in yeast. Strikingly, removal of the intron from YRA1 causes a dominant-negative growth phenotype and a concomitant inhibition of mRNA export. However, both defects are neutralized by replacement of the intron of YRA1 by a different intron. Significantly, Yra1p is overproduced in yeast when expressed from its intronless gene, but Yra1p levels are the same as the wild type when expressed from an intron-containing YRA1 gene. Thus, an intron in YRA1 controls Yra1p expression and mRNA export.     

The shuttling SR protein 9G8 plays a role in translation of unspliced mRNA containing a constitutive transport element

The splicing regulatory SR protein, 9G8, has recently been proposed to function in mRNA export in conjunction with the export protein, Tap/NXF1. Tap interacts directly with the Mason-Pfizer monkey virus constitutive transport element (CTE), an element that enables export of unspliced, intron-containing mRNA. Based on our previous finding that Tap can promote polysome association and translation of CTE-RNA, we investigated the effect of 9G8 on cytoplasmic RNA fate. 9G8 was shown to enhance expression of unspliced RNA containing either the Mason-Pfizer monkey virus-CTE or the recently discovered Tap-CTE. 9G8 also enhanced polyribosome association of unspliced RNA containing a CTE. Hyperphosphorylated 9G8 was present in monosomes and small polyribosomes, whereas soluble fractions contained only hypophosphorylated protein. Our results are consistent with a model in which hypophosphorylated SR proteins remain stably associated with messenger ribonucleoprotein (mRNP) complexes during export and are released during translation initiation concomitant with increased phosphorylation. These results provide further evidence for crucial links between RNA splicing, export and translation.     

Expression of the essential mRNA export factor Yra1p is autoregulated by a splicing-dependent mechanism

Recent evidence supports the idea that pre-mRNA splicing and mRNA export are mechanistically coupled. In metazoans, this process appears to be mediated by a multicomponent complex, which associates with the spliced RNA upstream of the exon-exon junction. One of these components (Aly/REF) has a homolog in the budding yeast Saccharomyces cerevisiae known as Yra1p. The YRA1 gene is essential for growth and required for mRNA export. Notably, YRA1 is one of the only approximately 5% intron-containing genes in yeast. Moreover, the YRA1 intron has several unusual features and is conserved in other budding yeast species. Previously, overexpression of intronless YRA1 was shown to be toxic. We show here that overexpression of the intron-containing gene results in increased levels of unspliced pre-mRNA but normal levels of Yra1 protein; conversely, expression of the cDNA results in increased levels of protein and accumulation of nuclear poly(A)+ RNA. Two additional lines of evidence suggest that expression of Yra1p is autoregulated: First, expression of excess Yra1p from a plasmid reduces the level of tagged, chromosomal Yra1p, and, second, this effect requires wild-type protein. Replacement of the YRA1 intron with that of other S. cerevisiae genes cannot rescue the dominant-negative growth defect of intronless YRA1. We conclude that the level of Yra1p is negatively autoregulated by a mechanism that involves splicing of its unusual intron. Tight control of the levels of Yra1p might be necessary to couple the rates of pre-mRNA splicing and mRNA export.     

Utp8p is an essential intranuclear component of the nuclear tRNA export machinery of Saccharomyces cerevisiae

A yeast tRNA three-hybrid interaction approach and an in vivo nuclear tRNA export assay based on amber suppression was used to identify proteins that participate in the nuclear tRNA export process in Saccharomyces cerevisiae. One of the proteins identified by this strategy is Utp8p, an essential 80-kDa nucleolar protein that has been implicated in 18 S ribosomal RNA biogenesis. Our characterization indicated that the major function of Utp8p is in nuclear tRNA export. Like the S. cerevisiae Los1p and the mammalian exportin-t, which are proteins known to facilitate nuclear tRNA export, overexpression of Utp8p restored export of tRNAamTyr mutants defective in nuclear export. Furthermore, depletion of Utp8p blocked nuclear export of mature tRNAs derived from both intronless and intron-containing pre-tRNAs but did not affect tRNA and rRNA maturation, nuclear export of mRNA and ribosomes, or nuclear tRNA aminoacylation. Overexpression of Utp8p also alleviated nuclear retention of non-aminoacylated tRNATyr in a tyrosyl-tRNA synthetase mutant strain. Utp8p binds tRNA directly and saturably, indicating that it has a tRNA-binding site. Utp8p does not appear to function as a tRNA export receptor, because it does not shuttle between the nucleus and the cytoplasm. Taken together, the results suggest that Utp8p is an essential intranuclear component of the nuclear tRNA export machinery, which may channel tRNA to the various tRNA export pathways operating in S. cerevisiae.