DEAD-box ATPase Dbp2 participates in a specific mRNP assembly checkpoint during RNA biogenesis in S. pombe

Abstract

The largest group of RNA helicases are the DEAD-box helicases of superfamily 2, which are non-processive enzymes that act in a single turnover reaction on short RNA duplexes to trigger RNA unwinding (helicase activity) or protein displacement (RNPase activity). These activities enable them to remodel messenger ribonucleoproteins (mRNPs). The production of functional mRNAs relies on co-transcriptional mRNP assembly that orchestrates highly dynamic and regulated processing events. Remodelling of mRNPs, facilitated by DEAD-box ATPases, is thought to promote RNA maturation by facilitating transitions between different mRNP states. An essential step in eukaryotic mRNA biogenesis is the formation of the 3’-end of the mRNA by endonucleolytic cleavage followed by polyadenylation. The recruitment of cleavage and polyadenylation complex (CPAC) and the subsequent formation of the 3’-end have been extensively investigated. On the contrary, the dismantling of these complex protein assemblies once they have completed their functions is less thoroughly understood. The work here shows that fission yeast Dbp2 is recruited to the 3’-end of genes and interacts with both CPAC components and export factors. It localises to cleavage bodies, subnuclear structures in the nucleus where CPAC factors are also concentrated. Loss of Dbp2 results in the accumulation of 3’-processed, polyadenylated RNAs on chromatin and in cleavage bodies, coinciding with the depletion of CPAC factors from the soluble pool. These observations suggest that in Dbp2-depleted cells, restricted availability of CPAC factors prevents optimal 3’-end processing, resulting in elevated levels of 3′-extended transcripts, along with a delay in transcription termination. Dbp2-dependent remodelling checkpoint shape mRNP assembly at the intersection between RNA 3’-end formation and RNA export, highlighting that transcript release after 3’-end formation as likely an ATP-dependent process.