The PP1 phosphatase complex coordinates pre-mRNA 3end processing and termination of RNA polymerase II transcription
Phosphorylation plays a central role in coordinating transcription with pre-mRNA maturation, yet the mechanistic understanding of how transcription is regulated by phosphorylation remains limited. The PP1 phosphatase removes CDK9-dependent phosphorylation of Pol II and Spt5, triggering transcription termination. Here, we demonstrate that PNUTS enhances PP1 activity, whereas WDR82 mediates an interaction between PP1 and the pre-mRNA 3' end processing machinery subunit Symplekin. We present the first structure of a PNUTS-WDR82-Symplekin-Ssu72 complex, revealing that PNUTS and Symplekin bind WDR82 through distinct interfaces using conserved short linear motifs. Mutations that inactivate PP1s catalytic activity or disrupt its interaction with either PNUTS or the 3' end processing machinery led to impaired transcription elongation, inefficient cleavage of nascent transcripts, altered poly(A) site selection, and widespread transcription termination defects. Our findings demonstrate that the PP1 complex is essential for coupling transcription to pre-mRNA 3' end maturation, ensuring production of functional mRNA.