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Bryan R Cullen

Publications and source records attributed to Bryan R Cullen.

18 recordsLinked to original sources
PubMed2003

Exon junction complexes mediate the enhancing effect of splicing on mRNA expression.

Intron-containing genes are generally expressed more effectively in human cells than are intronless versions of the same gene. We have asked whether this effect is due directly to splicing or instead reflects the action of components of the exon junction complex (EJC) that is assembled at splice junctions after splicing is completed. Here, we show that intron removal does not enhance gene expression if EJC formation is blocked. Conversely, RNA tethering of the EJC components SRm160 or RNPS1 boosts the expression of intronless mRNAs but not of spliced mRNAs. Splicing and RNPS1 tethering are shown to enhance the same steps in mRNA biogenesis and function, including mRNA 3' end processing and translation. Together, these data argue that the EJC is primarily responsible for the positive effect of splicing on gene expression.

Base Sequence↗
PubMed2003

MicroRNAs and small interfering RNAs can inhibit mRNA expression by similar mechanisms.

MicroRNAs (miRNAs) are endogenously encoded small noncoding RNAs, derived by processing of short RNA hairpins, that can inhibit the translation of mRNAs bearing partially complementary target sequences. In contrast, small interfering RNAs (siRNAs), which are derived by processing of long double-stranded RNAs and are often of exogenous origin, degrade mRNAs bearing fully complementary sequences. Here, we demonstrate that an endogenously encoded human miRNA is able to cleave an mRNA bearing fully complementary target sites, whereas an exogenously supplied siRNA can inhibit the expression of an mRNA bearing partially complementary sequences without inducing detectable RNA cleavage. These data suggest that miRNAs and siRNAs can use similar mechanisms to repress mRNA expression and that the choice of mechanism may be largely or entirely determined by the degree of complementary of the RNA target.

Base Sequence↗
PubMed2003

Human topoisomerase I promotes HIV-1 proviral DNA synthesis: implications for the species specificity and cellular tropism of HIV-1 infection.

Although HIV type 1 (HIV-1) cannot efficiently replicate in simian cells, the mechanism(s) involved in the restriction of virus tropism remain unclear. To investigate this, we have focused on the identification of human cellular factors that can influence the infectivity of HIV-1 derived from African green monkey producer cells. Whereas the infectivity of HIV-1 derived from such cells was only 10-15% of that of human cell-derived virus, expression of human topoisomerase I in the African green monkey cells resulted in a 5-fold increase of the infectivity of progeny HIV-1 virions. Replacement of glutamate-236 and asparagine-237 of human topoisomerase I with the corresponding residues (aspartate and serine, respectively) of the African green monkey enzyme abolished this enhancement of HIV-1 infectivity. This positive effect of human topoisomerase I expression in the African green monkey producer cells seemed to result from the promotion of HIV-1 cDNA synthesis. Thus, human topoisomerase I plays an important role in HIV-1 replication and infectivity, and differences in the species specificity of HIV-1 infection can at least in part be attributed to differences in topoisomerase I activities.

Amino Acid Sequence↗
PubMed2003

Nuclear RNA export.

Eukaryotic cells export several different classes of RNA molecule from the nucleus, where they are transcribed, to the cytoplasm, where the majority participate in different aspects of protein synthesis. It is now clear that these different classes of RNA, including rRNAs, tRNAs, mRNAs and snRNAs, are specifically directed into distinct but in some cases partially overlapping nuclear export pathways. All non-coding RNAs are now known to depend on members of the karyopherin family of Ran-dependent nucleocytoplasmic transport factors for their nuclear export. In contrast, mRNA export is generally mediated by a distinct, Ran-independent nuclear export pathway that is both complex and, as yet, incompletely understood. However, for all classes of RNA molecules, nuclear export is dependent on the assembly of the RNA into the appropriate ribonucleoprotein complex, and nuclear export therefore also appears to function as an important proofreading mechanism.

Active Transport, Cell Nucleus↗
PubMed2003

Nuclear mRNA export: insights from virology.

To maximize the production of progeny virions, several viruses have evolved mechanisms that promote the selective nuclear export of viral mRNA transcripts while, in some cases, inhibiting the export of cellular mRNAs. To achieve this goal, viruses have evolved regulatory proteins and cis-acting RNA elements that selectively interact with key cellular nuclear export factors. Efforts to identify the cellular targets of these viral proteins and RNA elements have led to the identification of Crm1 and Tap as essential human nuclear RNA-export factors and continue to provide insights into how mRNAs are selected for export

Active Transport, Cell Nucleus↗
PubMed2003

Sequence requirements for micro RNA processing and function in human cells.

Most eukaryotes encode a substantial number of small noncoding RNAs termed micro RNAs (miRNAs). Previously, we have demonstrated that miR-30, a 22-nucleotide human miRNA, can be processed from a longer transcript bearing the proposed miR-30 stem-loop precursor and can translationally inhibit an mRNA-bearing artificial target sites. We also demonstrated that the miR-30 precursor stem can be substituted with a heterologous stem, which can be processed to yield novel miRNAs and can block the expression of endogenous mRNAs. Here, we show that a second human miRNA, termed miR-21, can also be effectively expressed when its precursor forms part of a longer mRNA. For both miR-30 and miR-21, mature miRNA production was highly dependent on the integrity of the precursor RNA stem, although the underlying sequence had little effect. In contrast, the sequence of the terminal loop affected miRNA production only moderately. Processing of the initial, miR-30-containing transcript led to the production of not only mature miR-30 but also to the largely nuclear excision of an approximately 65-nucleotide RNA that is likely to represent an important intermediate in miR-30 processing. Consistent with this hypothesis, mutations that affected mature miR-30 production inhibited expression of this miR-30 pre-miRNA to an equivalent degree. Although point mutations could block the ability of both miR-30 and miR-21 to inhibit the translation of mRNAs bearing multiple artificial miRNA target sites, single point mutations only attenuated the miRNA-mediated inhibition of genes bearing single, fully complementary targets. These results suggest that miRNAs, and the closely similar small interfering RNAs, cannot totally discriminate between RNA targets differing by a single nucleotide.

Base Sequence↗
PubMed2003

Analysis of the stimulatory effect of splicing on mRNA production and utilization in mammalian cells.

We have examined how splicing affects the expression of a range of human and nonhuman genes in vertebrate cells. Although our data demonstrate that splicing invariably enhances the level of gene expression, this positive effect is generally moderate. However, in the case of the human beta-globin gene, splicing is essential for significant protein expression. In the absence of introns, 3' end processing is inefficient, and this appears to be causally linked to a significant decrease in the level of both nuclear and cytoplasmic 3' end-processed RNA. In contrast, splicing appears to only modestly enhance nuclear mRNA export. Consistent with this observation, intronless beta-globin gene expression was only partially rescued by the insertion of retroviral nuclear mRNA export elements. Surprisingly, in the case of the highly intron dependent beta-globin gene, the mRNA that did reach the cytoplasm was also only inefficiently translated if it derived from an intronless expression plasmid. Together, these data argue that splicing can increase gene expression by enhancing mRNA 3' end processing, and hence, mRNA production. Moreover, in the case of the highly intron-dependent beta-globin gene, splicing also significantly enhanced the translational utilization of cytoplasmic beta-globin mRNAs.

3T3 Cells↗
PubMed2002

The crystal structure and mutational analysis of a novel RNA-binding domain found in the human Tap nuclear mRNA export factor.

The Tap protein mediates the sequence nonspecific nuclear export of cellular mRNAs as well as the sequence-specific export of retroviral mRNAs bearing the constitutive transport element (CTE). Previously, the structures of individual Tap subdomains, including ribonucleoprotein and leucine-rich repeat domains, have been described. Here, we report the crystal structure of a functional CTE RNA-binding domain of human Tap, including the N-terminal arm of the ribonucleoprotein domain and interdomain linking polypeptide. To identify residues that interact with the CTE, we have introduced 38 alanine substitutions for surface residues in the Tap CTE-binding domain and tested these mutants for their ability to support CTE-dependent nuclear RNA export and CTE binding. Four residues that cluster on a concave surface in the leucine-rich repeat domain were found to be critical for CTE binding and define a CTE-interacting surface on this domain. The second critical CTE-interacting surface on Tap is defined by three previously identified residues on the surface of the ribonucleoprotein domain. The structural and mutational data define a novel RNA-binding site on the Tap protein.

Amino Acid Motifs↗
PubMed2002

Both natural and designed micro RNAs can inhibit the expression of cognate mRNAs when expressed in human cells.

Animal cells have recently been shown to express a range of approximately 22 nucleotide noncoding RNAs termed micro RNAs (miRNAs). Here, we show that the human mir-30 miRNA can be excised from irrelevant, endogenously transcribed mRNAs encompassing the predicted 71 nucleotide mir-30 precursor. Expression of the mir-30 miRNA specifically blocked the translation in human cells of an mRNA containing artificial mir-30 target sites. Similarly, designed miRNAs were also excised from transcripts encompassing artificial miRNA precursors and could inhibit the expression of mRNAs containing a complementary target site. These data indicate that novel miRNAs can be readily produced in vivo and can be designed to specifically inactivate the expression of selected target genes in human cells.

Animals↗
PubMed2002

Both ran and importins have the ability to function as nuclear mRNA export factors.

The Ran protein regulates nucleocytoplasmic transport mediated by the karyopherin family of nuclear transport factors. Ran is converted to the active, GTP bound form in the nucleus and then binds to a conserved domain found in all karyopherins. This interaction induces cargo binding for exportins and cargo release for importins. In either case, the Ran.GTP is then transported to the cytoplasm by the karyopherin, where it is hydrolyzed to Ran.GDP. To ask whether Ran could function as a nuclear mRNA export factor, we fused Ran to the MS2 coat protein and inserted MS2 RNA-binding sites into an unspliced cat mRNA that is normally sequestered in the nucleus. Coexpression of MS2-Ran induced cat mRNA export and CAT enzyme expression as effectively as, for example, an MS2-Rev fusion protein. MS2-Ran dependent nuclear mRNA export was reduced by inhibitors specific for Crm1, but not blocked as was seen with MS2-Rev. Consistent with the hypothesis that Crm1 is not the only karyopherin cofactor for MS2-Ran mediated mRNA export, we show that not only Crm1 but also CAS, transportin, importin beta and exportin t can all export mRNA from the nucleus when tethered via the MS2 RNA-binding domain. In contrast, two shuttling hnRNPs, hnRNP A1 and hnRNP K, proved unable to function as nuclear RNA export factors when expressed as MS2 fusions. Together, these data argue that karyopherins that normally function to transport proteins into or out of the nucleus are also capable of exporting tethered mRNA molecules.

Active Transport, Cell Nucleus↗
PubMed2002

RNA interference in human cells is restricted to the cytoplasm.

RNA interference (RNAi) is an evolutionarily conserved eukaryotic adaptive response that leads to the specific degradation of target mRNA species in response to cellular exposure to homologous double-stranded RNA molecules. Here, we have analyzed the subcellular location at which RNA degradation occurs in human cells exposed to double-stranded short interfering RNAs. To unequivocally determine whether a given mRNA is subject to degradation in the cytoplasm, the nucleus, or both, we have used the retroviral Rev/RRE system to control whether target mRNAs remain sequestered in the nucleus or are exported to the cytoplasm. In the absence of export, we found that the nuclear level of the RRE-containing target mRNA was not affected by activation of RNAi. In contrast, when nuclear export was induced by expression of Rev, cytoplasmic target mRNAs were effectively and specifically degraded by RNAi. Curiously, when the target mRNA molecule was undergoing active export from the nucleus, induction of RNAi also resulted in a reproducible approximately twofold drop in the level of target mRNA present In the nuclear RNA fraction. As this same mRNA was entirely resistant to RNAi when sequestered in the nucleus, this result suggests that RNAi is able to induce degradation of target mRNAs not only in the cytoplasm but also during the process of nuclear mRNA export. Truly nucleoplasmic mRNAs or pre-mRNAs are, in contrast, resistant to RNAi.

Animals↗
PubMed2002

Potent and specific inhibition of human immunodeficiency virus type 1 replication by RNA interference.

Synthetic small interfering RNAs (siRNAs) have been shown to induce the degradation of specific mRNA targets in human cells by inducing RNA interference (RNAi). Here, we demonstrate that siRNA duplexes targeted against the essential Tat and Rev regulatory proteins encoded by human immunodeficiency virus type 1 (HIV-1) can specifically block Tat and Rev expression and function. More importantly, we show that these same siRNAs can effectively inhibit HIV-1 gene expression and replication in cell cultures, including those of human T-cell lines and primary lymphocytes. These observations demonstrate that RNAi can effectively block virus replication in human cells and raise the possibility that RNAi could provide an important innate protective response, particularly against viruses that express double-stranded RNAs as part of their replication cycle.

Cell Line↗
PubMed2002

Recruitment of the Crm1 nuclear export factor is sufficient to induce cytoplasmic expression of incompletely spliced human immunodeficiency virus mRNAs.

Cytoplasmic expression of the incompletely spliced RNA transcripts that encode the late, structural proteins of human immunodeficiency virus type 1 (HIV-1) is dependent on the viral Rev regulatory protein. General agreement exists that Rev acts, at least in part, by recruiting the cellular Crm1 nuclear export factor to HIV-1 transcripts bearing the Rev response element RNA target, and thereby inducing their nuclear egress. However, several groups have argued that Crm1 recruitment may not be sufficient for Rev function. Thus, several additional candidate cofactors for Rev have been proposed, and Rev has also been suggested to also inhibit the nuclear splicing of HIV-1 transcripts and/or to directly enhance their cytoplasmic translation. To examine whether Crm1 recruitment is, instead, sufficient to activate the nuclear export of viral mRNAs, we targeted a leucine-rich Crm1 binding domain, derived from a heterologous protein that normally plays no role in RNA metabolism, to HIV-1 RNAs and showed that this tethered Crm1 binding domain is sufficient to induce the nuclear export and cytoplasmic translation of late HIV-1 mRNA species. More importantly, we show that direct tethering of the Crm1 nuclear export factor to target mRNAs, by fusion to a heterologous RNA binding domain, is in and of itself sufficient to induce the nuclear export and cytoplasmic expression of the unspliced HIV-1 mRNAs that encode the viral Gag proteins.

Alternative Splicing↗
PubMed2002

Formation of Tap/NXT1 heterodimers activates Tap-dependent nuclear mRNA export by enhancing recruitment to nuclear pore complexes.

The Tap protein has been shown to activate the nuclear export of mRNA species bearing retroviral constitutive transport elements and is also believed to play an essential role in the sequence nonspecific export of cellular mRNAs. However, it has remained unclear how Tap activity is regulated in vivo. Here, we report that the small NXT1/p15-1 protein functions as a critical cofactor for Tap-mediated mRNA export in both human and invertebrate cells. In the absence of NXT1 binding, the Tap protein is unable to effectively interact with components of the nuclear pore complex and both Tap nucleocytoplasmic shuttling and the nuclear export of mRNA molecules tethered to Tap are therefore severely attenuated. Formation of a Tap/NXT1 heterodimer enhances nucleoporin binding both in vitro and in vivo and induces the formation of a Tap/NXT1/nucleoporin ternary complex that is likely to be a key intermediate in the process of nuclear mRNA export. The critical importance of NXT1 for the nuclear export of poly(A)(+) RNA is emphasized by the finding that specific inhibition of the expression of the Drosophila homolog of human NXT1, by using RNA interference, results in the nuclear accumulation of poly(A)(+) RNA in cultured insect cells. These data suggest that NXT1 may act as a molecular switch that regulates the ability of Tap to mediate nuclear mRNA export by controlling the interaction of Tap with components of the nuclear pore.

Active Transport, Cell Nucleus↗