Characterization of the roles of yeast nuclear exosome cofactor TRAMP complex in pre-mRNA splicing

Abstract

In budding yeast, the Trf4/5p-Air1/2p-Mtr4p polyadenylation (TRAMP) complex recognizes unwanted RNA transcripts in the nucleus and then targets them to the nuclear exosome for rapid degradation, constituting an important pathway of nuclear RNA quality control. Each pre-mRNA splicing event unavoidably generates a RNA side-product that should be recognized by TRAMP and then removed by the nuclear exosome to prevent the potentially harmful sequestration of splicing factors and/or ribonucleotides. While successful pre-mRNA splicing inevitably produces a spliced-out intron, errors in pre-mRNA splicing lead to the emergence of either an abnormal splicing intermediate, or a splicing-incompetent pre-mRNA that cannot be properly spliced. However, it remains unclear how and when these RNA side-products of pre-mRNA splicing are recognized by TRAMP. In this study, chromatin immunoprecipitation (ChIP) was applied to demonstrate that both TRAMP and the nuclear exosome component Rrp6p are cotranscriptionally recruited to nascent RNA transcripts, particularly to intronic sequences, indicating that splicing side-products are recognized by TRAMP and committed to subsequent nuclear-exosome-mediated degradation in a cotranscriptional manner. Deletion of TRF4, of both AIR1 and AIR2, or of RRP6, resulted in accumulation of unspliced pre-mRNAs. Surprisingly, while such pre-mRNAs accumulated in rrp6 cells owing to defects in pre-mRNA degradation, the same phenotype in trf4 and air1air2 cells involved splicing defects, demonstrating that only TRAMP, but not the nuclear exosome, contributes to optimal pre-mRNA splicing. Consistent with a direct stimulatory role for TRAMP in pre-mRNA splicing, negative genetic interactions and physical interactions between Trf4p and several splicing factors were observed, and that Trf4p was further shown to be required for optimal recruitment of the splicing factor Msl5p. The direct facilitation of pre-mRNA splicing by TRAMP may act as a fail-safe mechanism to ensure the cotranscriptional recruitment of TRAMP to nascent intron-containing transcripts before or during pre-mRNA splicing, such that the subsequently generated spliced-out introns, abnormal splicing intermediates, or splicing-incompetent pre-mRNAs can be recognized immediately by TRAMP, and then targeted to the nuclear exosome for prompt degradation before their potentially harmful accumulation. Since most TRAMP and nuclear exosome components found in budding yeast also contain functional human homologs, this work provides important insights into how splicing side-products are rapidly degraded by the nuclear RNA quality control system in human cells, which have a much higher frequency of introns within their genome, and certainly require a much more efficient pathway for the removal of an increased amount of splicing side-products due to the greater number of splicing events.