Dynamics in Fip1 regulate eukaryotic mRNA 3'-end processing

Cleavage and polyadenylation factor (CPF/CPSF) is a multiprotein complex essential for mRNA 3-end processing in eukaryotes. It contains an endonuclease that cleaves pre-mRNAs, and a polymerase that adds a poly(A) tail onto the cleaved 3-end. Several CPF subunits, including Fip1, contain intrinsically-disordered regions (IDRs). IDRs within multiprotein complexes can be flexible, or can become ordered upon interaction with binding partners. Here, we show that yeast Fip1 anchors the poly(A) polymerase Pap1 onto CPF via an interaction with zinc finger 4 of another CPF subunit, Yth1. We also reconstitute a fully recombinant 850-kDa CPF. By incorporating selectively-labelled Fip1 into recombinant CPF, we could study the dynamics of this single protein within the megadalton complex using nuclear magnetic resonance spectroscopy (NMR). This reveals that a Fip1 IDR that connects the Yth1- and Pap1-binding sites remains highly dynamic within CPF. Together, our data suggest that Fip1 dynamics mediate conformational transitions within the 3-end processing machinery to coordinate cleavage and polyadenylation.