MicroRNAs (miRNAs), which function as regulators of gene expression in eukaryotes, are processed from larger transcripts by sequential action of nuclear and cytoplasmic ribonuclease III-like endonucleases. We show that Exportin-5 (Exp5) mediates efficient nuclear export of short miRNA precursors (pre-miRNAs) and that its depletion by RNA interference results in reduced miRNA levels. Exp5 binds correctly processed pre-miRNAs directly and specifically, in a Ran guanosine triphosphate-dependent manner, but interacts only weakly with extended pre-miRNAs that yield incorrect miRNAs when processed by Dicer in vitro. Thus, Exp5 is key to miRNA biogenesis and may help coordinate nuclear and cytoplasmic processing steps.

We show that the microRNA miR-155 can be processed from sequences present in BIC RNA, a spliced and polyadenylated but non-protein-coding RNA that accumulates in lymphoma cells. The precursor of miR-155 is likely a transient spliced or unspliced nuclear BIC transcript rather than accumulated BIC RNA, which is primarily cytoplasmic. By using a sensitive and quantitative assay, we find that clinical isolates of several types of B cell lymphomas, including diffuse large B cell lymphoma (

After synthesis and processing in the nucleus, mature transfer RNAs (tRNAs) are exported to the cytoplasm in a Ran.guanosine triphosphate-dependent manner. Export of defective or immature tRNAs is avoided by monitoring both structure and function of tRNAs in the nucleus, and only tRNAs with mature 5' and 3' ends are exported. All tRNAs examined can be aminoacylated in nuclei of Xenopus oocytes, thereby providing a possible mechanism for functional proofreading of newly made tRNAs. Inhibition of aminoacylation of a specific tRNA retards its appearance in the cytoplasm, indicating that nuclear aminoacylation promotes efficient export.

A systematic search for human ribosome biogenesis factors shows conservation of many aspects of eukaryotic ribosome synthesis with the well-studied process in yeast and identifies an export route of 60S subunits that is specific for higher eukaryotes.

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The constitutive transport elements (CTEs) of type D retroviruses are cis-acting elements that promote nuclear export of incompletely spliced mRNAs. Unlike the Rev response element (RRE) of human immunodeficiency virus type 1 (HIV-1), CTEs depend entirely on factors encoded by the host cell genome. We show that an RNA comprised almost entirely of the CTE of Mason-Pfizer monkey virus (CTE RNA) is exported efficiently from Xenopus oocyte nuclei. The CTE RNA and an RNA containing the RRE of HIV-1 (plus Rev) have little effect on export of one another, demonstrating differences in host cell requirements of these two viral mRNA export pathways. Surprisingly, even very low amounts of CTE RNA block export of normal mRNAs, apparently through the sequestration of cellular mRNA export factors. Export of a CTE-containing lariat occurs when wild-type CTE, but not a mutant form, is inserted into the pre-mRNA. The CTE has two symmetric structures, either of which supports export and the titration of mRNA export factors, but both of which are required for maximal inhibition of mRNA export. Two host proteins bind specifically to the CTE but not to non-functional variants, making these proteins candidates for the sequestered mRNA export factors.

The matrix (M) protein of vesicular stomatitis virus (VSV) is a potent inhibitor of bidirectional nuclear transport. Here we demonstrate that inhibition occurs when M protein is in the nucleus of Xenopus laevis oocytes and that M activity is readily reversed by a monoclonal antibody (␣M). We identify a region of M protein, amino acids 51 to 59, that is required both for inhibition of transport and for efficient recognition by ␣M. When expressed in transfected HeLa cells, M protein colocalizes with nuclear pore complexes (NPCs) at the nuclear rim. Moreover, mutation of a single amino acid, methionine 51, eliminates both transport inhibition and targeting to NPCs. We propose that M protein inhibits bidirectional transport by interacting with a component of the NPC or an NPC-associated factor that participates in nucleocytoplasmic transport.Trafficking of large macromolecules (more than 50 kDa) between the nucleus and the cytoplasm occurs through nuclear pore complexes (NPCs) via signal-dependent, carrier-mediated processes (reviewed in references 39 and 54). This transport is subject to control in response to a variety of stimuli such as progression through the cell cycle, exposure to stress, and infection by viruses (reviewed in reference 39). Thus, control of nucleocytoplasmic transport is an important element in the regulation of gene expression.Much of the carrier-mediated movement through NPCs requires cargo-specific transport receptors called importins and exportins (or karyopherins), which are members of the importin ␤ superfamily of proteins (16,21,45,61). Transport receptors can bind their cargoes either directly or via specialized adapter proteins. For example, importin ␤ mediates import of proteins containing basic nuclear localization sequences and small nuclear ribonucleoproteins (snRNPs) using the adapter proteins importin ␣ (1) and snurportin (28, 43), respectively. Importin ␤ can also interact directly with import cargoes, such as cyclin B (40, 53) and certain ribosomal proteins (30). CRM1 (Exportin1) mediates the export of proteins containing leucine-rich nuclear export signals (NES) as well as unspliced viral mRNAs and pre-snRNAs that are bound to specific NEScontaining adapter proteins (14,17,51,57). Exportin-t binds directly to its RNA export cargo, tRNA (3, 33).Directionality of nuclear transport appears to be governed largely by Ran, a small GTPase that is a central component of most known nucleocytoplasmic transport pathways (reviewed in references 9 and 41). Owing to the asymmetric localization of the Ran effector proteins RanGAP (the GTPase activating protein in the cytoplasm) and RCC1 (the guanine nucleotide exchange factor in the nucleus), a steep concentration gradient of RanGTP is presumed to exist across the nuclear envelope (29). This gradient plays a pivotal role in nucleocytoplasmic transport by triggering both assembly and disassembly of receptor-cargo complexes in the appropriate subcellular compartment (60). Thus, import complexes assemble in the cytoplasm in the absence of RanG...

We show that microRNA-427 (miR-427) mediates the rapid deadenylation of maternal mRNAs after the midblastula transition (MBT) of Xenopus laevis embryogenesis. By MBT, the stage when the embryonic cell cycle is remodeled and zygotic transcription of mRNAs is initiated, each embryo has accumulated ;10 9 molecules of miR-427 processed from multimeric primiR-427 transcripts synthesized after fertilization. We demonstrate that the maternal mRNAs for cyclins A1 and B2 each contain a single miR-427 target sequence, spanning less than 30 nucleotides, that is both necessary and sufficient for deadenylation, and that inactivation of miR-427 leads to stabilization of the mRNAs. Although this deadenylation normally takes place after MBT, exogenous miRNAs produced prematurely in vivo can promote deadenylation prior to MBT, indicating that turnover of the maternal mRNAs is limited by the amount of accumulated miR-427. Injected transcripts comprised solely of the cyclin mRNA 39 untranslated regions or bearing a 59 ApppG cap undergo deadenylation, showing that translation of the targeted RNA is not required. miR-427 is not unique in promoting deadenylation, as an unrelated miRNA, let-7, can substitute for miR-427 if the reporter RNA contains an appropriate let-7 target site. We propose that miR-427, like the orthologous miR-430 of zebrafish, functions to down-regulate expression of maternal mRNAs early in development.