Poly(A) binding KPAF4/5 complex stabilizes kinetoplast mRNAs in Trypanosoma brucei

In Trypanosoma brucei, mitochondrial pre-mRNAs undergo 3'-5' exonucleolytic processing, 3' adenylation and uridylation, 5' pyrophosphate removal, and, often, internal U-insertion/deletion editing. The 3' modification processes are modulated by pentatricopeptide repeat (PPR) RNA binding proteins termed Kinetoplast Polyadenylation Factors (KPAFs). We have shown that KPAF3 binding to the 3' region stabilizes properly trimmed pre-mRNA and stimulates pre-editing A-tailing. Conversely, poly(A) binding PPR factor KPAF4 shields the nascent A-tail from uridylation and decay thereby protecting pre-mRNA after KPAF3 displacement by editing events. Finally, KPAF1/2 induces post-editing A/U-tailing to activate translation of fully edited mRNA. Remarkably, the PPR-factor directed 5' end recognition and modification by the PPsome pyrophosphohydrolase complex is also required for mRNA stabilization. Here, we demonstrate that KPAF4 functions as a heterodimer with KPAF5, a protein lacking discernable motifs and similarities beyond Kinetoplastea. Thus, KPAF4 and KPAF4 constitute a poly(A) binding complex that stabilizes mRNA during the editing process and impedes A/U-tailing, hence spontaneous translational activation, of partially edited transcripts. We present evidence that RNA editing substrate binding complex (RESC) bridges the PPsome and polyadenylation complexes. This interaction likely enables mRNA circularization, an apparently critical element of mitochondrial mRNA stability and quality control.