Abstract
The cotranscriptional mRNA processing and packaging reactions that lead to the formation of export-competent messenger ribonucleoprotein particles (mRNPs) are under the surveillance of quality control steps. Aberrant mRNPs resulting from faulty events are retained in the nucleus with ensuing elimination of their mRNA component. The molecular mechanisms by which the surveillance system recognizes defective mRNPs and stimulates their destruction by the RNA degradation machinery are still not completely elucidated. Using an experimental approach in which mRNP formation in yeast is disturbed by the action of the bacterial Rho helicase, we have shown previously that the targeting of Rho-induced aberrant mRNPs is mediated by Rrp6p, which is recruited cotranscriptionally in association with Nrd1p following Rho action. Here we investigated the specific involvement in this quality control process of different cofactors associated with the nuclear RNA degradation machinery. We show that, in addition to the main hydrolytic action of the exonuclease Rrp6p, the cofactors Rrp47p, Mpp6p as well as the Trf-Air-Mtr4 polyadenylation (TRAMP) components Trf4p, Trf5p, and Air2p contribute significantly by stimulating the degradation process upon their cotranscriptional recruitment. Trf4p and Trf5p are apparently recruited in two distinct TRAMP complexes that both contain Air2p as component. Surprisingly, Rrp47p appears to play an important role in mutual protein stabilization with Rrp6p, which highlights a close association between the two partners. Together, our results provide an integrated view of how different cofactors of the RNA degradation machinery cooperate to target and eliminate aberrant mRNPs.
Highlights
Aberrant messenger ribonucleoprotein particles (mRNPs) are targeted and degraded by an Rrp6p-dependent nuclear quality control system
Using an experimental approach in which mRNP formation in yeast is disturbed by the action of the bacterial Rho helicase, we have shown previously that the targeting of Rho-induced aberrant mRNPs is mediated by Rrp6p, which is recruited cotranscriptionally in association with Nrd1p following Rho action
Rho action along nascent transcripts interferes with normal deposition of mRNA processing and packaging factors, yielding mRNPs that are recognized as defective and eliminated by the nuclear quality control apparatus
Summary
Aberrant mRNPs are targeted and degraded by an Rrp6p-dependent nuclear quality control system. Our results provide an integrated view of how different cofactors of the RNA degradation machinery cooperate to target and eliminate aberrant mRNPs. The expression of protein coding genes in eukaryotic cells is a multistep process in which the genetic message is transcribed from DNA into a pre-mRNA molecule that undergoes numerous modifications such as 5Ј end capping, splicing, 3Ј end cleavage, and polyadenylation while being assembled into a messenger ribonucleoprotein particle (mRNP) that will be exported to the cytoplasm for translation [1, 2]. We reported the implementation of a new assay to study nuclear mRNP surveillance in S. cerevisiae based on perturbation of mRNP biogenesis by the RNA-dependent helicase/ translocase activity of the bacterial Rho factor In this model system, the heterologous expression of Rho in the yeast nucleus induces the production of full-length but aberrant mRNPs that are targeted and eliminated by the Rrp6p-dependent RNA degradation machinery, leading to a growth defect phenotype [27]. Rrp47p appears to intervene primarily by stabilizing Rrp6p, which is consistent with the reported close association between the two proteins [22]
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