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Splicing is required for rapid and efficient mRNA export in metazoans
Abstract:
Pre-mRNA splicing is among the last known nuclear events before export of mature mRNA to the cytoplasm. At present, it is not known whether splicing and mRNA export are biochemically coupled processes. In this study, we have injected pre-mRNAs containing a single intron or the same mRNAs lacking an intron (Deltai-mRNAs) into Xenopus oocyte nuclei. We find that the spliced mRNAs are exported much more rapidly and efficiently than the identical Deltai-mRNAs. Moreover, competition studies using excess Deltai-mRNA indicate that different factor(s) are involved in the inefficient export of Deltai-mRNA vs. the efficient export of spliced mRNA. Consistent with this conclusion, spliced mRNA and Deltai-mRNA, though identical in sequence, are assembled into different messenger ribonucleoprotein particles (mRNP) in vitro. Strikingly, the mRNA in the spliced mRNP, but not in the Deltai-mRNP, is exported rapidly and efficiently. We conclude that splicing generates a specific nucleoprotein complex that targets mRNA for export. Our results, revealing a link between splicing and efficient mRNA export, may explain the reports that an intron is required for efficient expression of many protein-coding genes in metazoans.
Insights
Messenger RNA (mRNA) splicing is linked to efficient export from the nucleus. Splicing creates a specific complex that targets mRNA for rapid export, unlike unspliced mRNA.
Area of Science:
- Molecular Biology
- Cell Biology
- Gene Expression
Background:
- Pre-mRNA splicing is a critical nuclear process preceding mRNA export.
- The biochemical coupling between mRNA splicing and export remains largely unknown.
- Introns are often required for efficient gene expression in metazoans.
Purpose of the Study:
- To investigate whether pre-mRNA splicing and mRNA export are biochemically coupled.
- To determine if splicing influences the efficiency and mechanism of mRNA export.
Main Methods:
- Injection of pre-mRNAs with and without introns into Xenopus oocyte nuclei.
- Competition assays using intron-less mRNAs (Deltai-mRNAs).
- In vitro assembly of messenger ribonucleoprotein particles (mRNPs) from spliced and unspliced mRNAs.
Main Results:
- Spliced mRNAs were exported more rapidly and efficiently than identical intron-less mRNAs (Deltai-mRNAs).
- Competition studies suggested distinct factors mediate the export of spliced versus Deltai-mRNAs.
- Spliced and Deltai-mRNAs formed different mRNPs in vitro, with only spliced mRNA-containing mRNPs exported efficiently.
Conclusions:
- Splicing generates a specific nucleoprotein complex that targets mRNA for export.
- A direct link exists between mRNA splicing and efficient mRNA export.
- This coupling may explain the requirement of introns for efficient gene expression.