Nuclear Pre-snRNA Export Is an Essential Quality Assurance Mechanism for Functional Spliceosomes

Abstract

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. : Becker et al. show snRNA maturation in yeast involves nuclear export and re-import mediated by Mex67-Mtr2 and Xpo1/Crm1 and by Mtr10 and Cse1, respectively. They propose a model for obligatory shuttling in eukaryotes by showing spliceosomes assemble with immature snRNAs when export is prevented, resulting in defective spliceosomes and genome-wide splicing defects. Keywords: snRNA, splicing, mRNA, U6, Mex67, Xpo1/Crm1, Mtr10, Cse1, RNA processing, RNA quality control, ncRNA export