Daniel Becker, Anna Greta Hirsch, Lysann Bender, Thomas Lingner, Gabriela Salinas, Heike Krebber
Open Access DOI: https://doi.org/10.1016/j.celrep.2019.05.031
Open Access DOI: https://doi.org/10.1016/j.celrep.2019.05.031
Highlights
• All yeast snRNAs, including U6, shuttle into the cytoplasm
• Export is mediated by Mex67 and Xpo1, and re-import requires Mtr10 and Cse1
• snRNA export prevents an incorporation of immature snRNAs into spliceosomes
• Spliceosomal assembly with immature snRNAs results in genome-wide splicing defects
Summary
Removal of introns from pre-mRNAs is an essential step in eukaryotic gene expression, mediated by spliceosomes that contain snRNAs as key components. Although snRNAs are transcribed in the nucleus and function in the same compartment, all except U6 shuttle to the cytoplasm. Surprisingly, the physiological relevance for shuttling is unclear, in particular because the snRNAs in Saccharomyces cerevisiae were reported to remain nuclear. Here, we show that all yeast pre-snRNAs including U6 undergo a stepwise maturation process after nuclear export by Mex67 and Xpo1. Sm- and Lsm-ring attachment occurs in the cytoplasm and is important for the snRNA re-import, mediated by Cse1 and Mtr10. Finally, nuclear pre-snRNA cleavage and trimethylation of the 5′-cap finalizes shuttling. Importantly, preventing pre-snRNAs from being exported or processed results in faulty spliceosome assembly and subsequent genome-wide splicing defects. Thus, pre-snRNA export is obligatory for functional splicing and resembles an essential evolutionarily conserved quality assurance step.
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