TRYPANOSOMA-BRUCEI GBP21 - AN ARGININE-RICH MITOCHONDRIAL PROTEIN THAT BINDS TO GUIDE RNA WITH HIGH-AFFINITY

RNA editing in Trypanosoma brucei is a mitochondrial RNA processing re action that results in the insertion and deletion of uridylate residue s into otherwise untranslatable mRNAs. The process is directed by guid e RNAs which function to specify the edited sequence, RNA editing in v itro requires mitochondrial protein extracts and guide RNAs have been identified as part of high molecular weight ribonucleoprotein complexe s. Within the complexes, the RNAs are in close contact with several mi tochondrial proteins and here we describe the isolation and cloning of a gRNA-interacting poly-peptide from Trypanosoma brucei. The protein was named gBP21 for guide RNA-binding protein of 21 kDa. gBP21 shows n o homology to proteins in other organisms, it is arginine-rich and bin ds to gRNA molecules with a dissociation constant in the nanomolar ran ge. The protein does not discriminate for differences in the primary s tructures of gRNAs and thus likely binds to higher order structural fe atures common to all gRNA molecules. gBP21 binding does not perturb th e overall structure of gRNAs but the gRNA/gBP21 ribonucleoprotein comp lex is more stable than naked guide RNAs. Although the protein is argi nine-rich, the free amino acid or an arginine-rich peptide were not ab le to inhibit the association to the RNAs, In contrast, the gRNA-gBP21 complex formation was sensitive to potassium and ammonium cations, th us indicating a contribution of ionic contacts to the binding.