Structural basis for guide RNA selection by the RESC1-RESC2 complex

Kinetoplastid parasites, such as trypanosomes or leishmania, rely on RNA-templated RNA editing to mature mitochondrial cryptic pre-mRNAs into functional protein-coding transcripts. Processive pan-editing of multiple editing blocks within a single transcript is dependent on the 20-subunit RNA editing substrate binding complex (RESC) that serves as a platform to orchestrate the interactions between pre-mRNA, guide RNAs (gRNAs), the catalytic RNA editing complex (RECC), and a set of RNA helicases. Due to the lack of molecular structures and biochemical studies with purified components, neither the spacio-temporal interplay of these factors nor the selection mechanism for the different RNA components is understood. Here we report the cryo-EM structure of Trypanosoma brucei RESC1-RESC2, a central hub module of the RESC complex. The structure reveals that RESC1 and RESC2 form an obligatory domain-swapped dimer. Although the tertiary structures of both subunits closely resemble each other, only RESC2 selectively binds 5-triphosphate-nucleosides, a defining characteristic of gRNAs. We therefore propose RESC2 as the protective 5-end binding site for gRNAs within the RESC complex. Overall, our structure provides a starting point for the study of the assembly and function of larger RNA-bound kinetoplast RNA editing modules and might aid in the design of anti-parasite drugs. Key findingsO_LIThe kinetoplastid mitochondrial RNA editing factors RESC1 and RESC2 resemble a group of capping enzymes that are only found in protozoans, fungi and viruses. C_LIO_LIRESC1 and RESC2 lack the typical catalytic residues, and only RESC2 can bind a triphosphate-nucleoside. C_LIO_LIWe propose that the RESC1-RESC2 dimer selects guide RNAs based on their 5-triphosphate and serves as a protective 5-end binding complex. C_LI