Pentatricopeptide repeat poly(A) binding protein from mitochondria of trypanosomes

Abstract

In Trypanosoma brucei, most mitochondrial mRNAs undergo U-insertion/deletion editing, and 3' adenylation and uridylation. The internal sequence changes and terminal extensions are coordinated: Pre-editing addition of the short (A) tail protects the edited transcript against 3'-5' degradation, while post-editing A/U-tailing renders mRNA competent for ribosome recruitment. Participation of a poly(A) binding protein (PABP) in coupling of editing and 3' modification processes has been inferred, but its identity and mechanism of action remained elusive. We report identification of KPAF4, a pentatricopeptide repeat-containing PABP which sequesters the A-tail and impedes exonucleolytic degradation. Conversely, KPAF4 inhibits uridylation of A-tailed transcripts and, therefore, premature A/U-tailing of partially-edited mRNAs. This quality check point prevents translation of incompletely edited mRNAs. Our findings also implicate the RNA editing substrate binding complex (RESC) in mediating the interaction between the 5' end bound pyrophosphohydrolase MERS1 and 3' end associated KPAF4 to enable mRNA circularization. This event is critical for transcript stability during the editing process.