A Los1p-independent pathway for nuclear export of intronless tRNAs in Saccharomycescerevisiae

Possibility of cytoplasmic pre-tRNA splicing: the yeast tRNA splicing endonuclease mainly localizes on the mitochondria

Pre-tRNA splicing has been believed to occur in the nucleus. In yeast, the tRNA splicing endonuclease that cleaves the exon-intron junctions of pre-tRNAs consists of Sen54p, Sen2p, Sen34p, and Sen15p and was thought to be an integral membrane protein of the inner nuclear envelope. Here we show that the majority of Sen2p, Sen54p, and the endonuclease activity are not localized in the nucleus, but on the mitochondrial surface. The endonuclease is peripherally associated with the cytosolic surface of the outer mitochondrial membrane. A Sen54p derivative artificially fixed on the mitochondria as an integral membrane protein can functionally replace the authentic Sen54p, whereas mutant proteins defective in mitochondrial localization are not fully active. sen2 mutant cells accumulate unspliced pre-tRNAs in the cytosol under the restrictive conditions, and this export of the pre-tRNAs partly depends on Los1p, yeast exportin-t. It is difficult to explain these results from the view of tRNA splicing in the nucleus. We rather propose a new possibility that tRNA splicing occurs on the mitochondrial surface in yeast.

PAUSED, a putative exportin-t, acts pleiotropically in Arabidopsis development but is dispensable for viability

Exportin-t was first identified in humans as a protein that mediates the export of tRNAs from the nucleus to the cytoplasm. Mutations in Los1p, the Saccharomyces cerevisiae exportin-t homolog, result in nuclear accumulation of tRNAs. Because no exportin-t mutants have been reported in multicellular organisms, the developmental functions of exportin-t have not been determined. Here, we report the isolation and characterization of two Arabidopsis exportin-t mutants, paused-5 and paused-6. The mutant phenotypes indicate that exportin-t acts pleiotropically in plant development. In particular, paused-5 and paused-6 result in delayed leaf formation during vegetative development. The two paused mutations also cause the transformation of reproductive organs into perianth organs in the hua1-1 hua2-1 background, which is partially defective in reproductive organ identity specification. The floral phenotypes of hua1-1 hua2-1 paused mutants resemble those of mutations in the floral homeotic gene AGAMOUS. Moreover, paused-5 enhances the mutant phenotypes of two floral meristem identity genes, LEAFY and APETALA1. The developmental defects caused by paused mutations confirm the important roles of exportin-t in gene expression in multicellular organisms. In addition, a paused null allele, paused-6, is still viable, suggesting the presence of redundant tRNA export pathway(s) in Arabidopsis.

A diverse set of nuclear RNAs transfer between nuclei of yeast heterokaryons

Small nuclear RNAs and small nucleolar RNAs function in the nucleus of eukaryotic cells during pre-mRNA splicing and ribosomal RNA processing, respectively. In metazoan cells, the small nuclear RNAs shuttle between the nucleus and the cytoplasm during ribonucleoprotein particle assembly. Nuclear export of these small RNAs in yeast, however, has not been demonstrated. Therefore, we have attempted to visualize internuclear RNA movements by in situ hybridization in heterokaryon yeast cells. Using the kar1Delta15 mutation to block karyogamy, we mated two strains, each expressing a unique allele of U1 snRNA. In these heterokaryons, we observed a time-dependent transfer of U1 snRNA from one nucleus to the other. This transfer was reduced two-fold by the addition of the Crm1p-inhibitor leptomycin B. Interestingly, however, we observed identical transfer of the U2 and U6 snRNAs and SNR4, SNR8, SNR9 and SNR11 snoRNAs. Remarkably, when the U2, U6 or SNR4 RNAs were observed in the same heterokaryon as the U1 snRNA, both RNAs always transferred simultaneously. These data suggest a global leaking or transport of material between nuclei of yeast heterokaryons. Our results suggest that caution must be taken when testing nuclear envelope shuttling in yeast heterokaryons.

Closely related CC- and A-adding enzymes collaborate to construct and repair the 3'-terminal CCA of tRNA in Synechocystis sp. and Deinococcus radiodurans

The 3'-terminal CCA sequence of tRNA is faithfully constructed and repaired by the CCA-adding enzyme (ATP(CTP):tRNA nucleotidyltransferase) using CTP and ATP as substrates but no nucleic acid template. Until recently, all CCA-adding enzymes from all three kingdoms appeared to be composed of a single kind of polypeptide with dual specificity for adding both CTP and ATP; however, we recently found that in Aquifex aeolicus, which lies near the deepest root of the eubacterial 16 S rRNA-based phylogenetic tree, CCA addition represents a collaboration between closely related CC-adding and A-adding enzymes (Tomita, K. and Weiner, A. M. (2001) Science 294, 1334-1336). Here we show that in Synechocystis sp. and Deinococcus radiodurans, as in A. aeolicus, CCA is added by homologous CC- and A-adding enzymes. We also find that the eubacterial CCA-, CC-, and A-adding enzymes, as well as the related eubacterial poly(A) polymerases, each fall into phylogenetically distinct groups derived from a common ancestor. Intriguingly, the Thermatoga maritima CCA-adding enzyme groups with the A-adding enzymes, suggesting that these distinct tRNA nucleotidyltransferase activities can intraconvert over evolutionary time.

Exportin-5-mediated nuclear export of eukaryotic elongation factor 1A and tRNA

Transport of proteins and RNA into and out of the cell nucleus is mediated largely by a family of RanGTP-binding transport receptors. Export receptors (exportins) need to bind RanGTP for efficient loading of their export cargo. We have identified eukaryotic elongation factor 1A (eEF1A) and tRNA as RanGTP-dependent binding partners of exportin-5 (Exp5). Exp5 stimulates nuclear export of eEF1A when microinjected into the nucleus of Xenopus laevis oocytes. Surprisingly, the interaction between eEF1A and Exp5 is dependent on tRNA that can interact directly with Exp5 and, if aminoacylated, recruits eEF1A into the export complex. These data suggested to us that Exp5 might support tRNA export. Indeed, not only the canonical tRNA export receptor, exportin-t, but also Exp5 can drive nuclear export of tRNA. Taken together, we show that there exists an alternative tRNA export pathway which can be exploited to keep eEF1A out of the cell nucleus.