Project description:The RNA exosome is fundamental for the degradation of RNA in eukaryotic nuclei. Substrate targeting is facilitated by its co-factor Mtr4p/hMTR4, which links to RNA-binding protein adaptors. One such activity is the human Nuclear EXosome Targeting (NEXT) complex, composed of hMTR4, the Zn-finger protein ZCCHC8 and the RNA-binding factor RBM7. NEXT primarily targets early and unprocessed transcripts, demanding a rationale for how the nuclear exosome recognizes processed RNAs. Here, we describe the PolyA tail eXosome Targeting (PAXT) connection, comprising the hitherto uncharacterized ZFC3H1 Zn-knuckle protein as a central link between hMTR4 and the nuclear polyA binding protein PABPN1. Individual depletion of ZFC3H1 and PABPN1 results in the accumulation of common transcripts, that are generally both longer and more 3’polyadenylated than NEXT substrates. Importantly, ZFC3H1/PABPN1 and ZCCHC8/RBM7 contact hMTR4 in a mutually exclusive manner, revealing that the exosome targets nuclear transcripts of different maturation status by substituting its hMTR4-associating adaptors.
Project description:The eukaryotic mRNA life cycle includes transcription, nuclear mRNA export and degradation. To quantify all these processes simultaneously, we perform thiol-linked alkylation after metabolic labeling of RNA with 4-thiouridine (4sU), followed by sequencing of RNA (SLAM-seq) in the nuclear and cytosolic compartments. We develop a model that reliably quantifies mRNA synthesis, nuclear export, and nuclear and cytosolic degradation rates on a genome-wide scale. We find that nuclear degradation of polyadenylated mRNA is negligible and nuclear mRNA export is slow, while cytosolic mRNA degradation is comparatively fast. Consequently, an mRNA molecule generally spends most of its life in the nucleus. We also observe large differences in the nuclear export rates of different 3’UTR transcript isoforms. Furthermore, we identify genes whose expression is abruptly induced upon metabolic labeling. These transcripts are exported substantially faster than average mRNAs, suggesting the existence of alternative export pathways. Our results highlight nuclear mRNA export as a limiting factor in mRNA metabolism and gene regulation.
Project description:Mammalian genomes are promiscuously transcribed, yielding protein-coding and non-coding products. Many transcripts are short-lived due to their nuclear degradation by the ribonucleolytic RNA exosome. Here, we show that abolished nuclear exosome function causes the formation of distinct nuclear foci, containing polyadenylated (pA+) RNA secluded from nucleocytoplasmic export. We asked whether exosome co-factors could serve such nuclear retention. Co-localization studies revealed the enrichment of pA+ RNA foci with ‘pA-tail exosome targeting (PAXT) connection’ components MTR4, ZFC3H1 and PABPN1, but no overlap with known nuclear structures, such as Cajal bodies, speckles, paraspeckles or nucleoli. Interestingly, ZFC3H1 is required for foci formation, and in its absence selected pA+ RNAs, including coding and non-coding transcripts, are exported to the cytoplasm in a process dependent on the mRNA export factor AlyREF. Our results establish ZFC3H1 as a central nuclear RNA retention factor, counteracting nuclear export activity.
Project description:The nuclear phase of the gene expression pathway culminates in the export of mature mRNAs to the cytoplasm through nuclear pore complexes (NPCs). GANP (Germinal-centre Associated Nuclear Protein) promotes the transfer to NPCs of mRNAs bound to the transport factor NXF1. Here, we demonstrate that GANP, subunit of the TREX-2 mRNA export complex, promotes selective nuclear export of a specific subset of mRNAs whose transport depends on NXF1. Genome-wide gene expression profiling showed that half of the transcripts whose nuclear export was impaired following NXF1 depletion also showed reduced export when GANP was depleted. GANP-dependent transcripts were highly expressed, yet short-lived, and were highly enriched in those encoding central components of the gene expression machinery such as RNA synthesis and processing factors. After injection into Xenopus oocyte nuclei, representative GANP-dependent transcripts showed faster nuclear export kinetics than representative transcripts that were not influenced by GANP depletion. We propose that GANP promotes the nuclear export of specific classes of mRNAs that may facilitate rapid changes in gene expression. We used gene expression profiling to compare the abundance of cytoplasmic RNAs after GANP or NXF1 depletion