Trypanosoma brucei RBP16 is a mitochondrial Y-box family protein with guide RNA binding activity

Trypanosoma brucei mitochondria possess a unique mechanism of mRNA maturation called RNA editing. In this process, uridylate residues are inserted and deleted posttranscriptionally into pre-mRNA to create translatable messages. The genetic information for RNA editing resides in small RNA molecules called guide RNAs (gRNAs). Thus, proteins in direct contact with gRNA are likely to catalyze or influence RNA editing. Herein we characterize an abundant gRNA-binding protein from T. brucei mitochondria. This protein, which we term RBP16 (for RNA-binding protein of 16 kDa), binds to different gRNA molecules. The major determinant of this interaction is the oligo(U) tail, present on the 3'-ends of gRNAs. RBP16 forms multiple, stable complexes with gRNA in vitro, and immunoprecipitation experiments provide evidence for an association between RBP16 and gRNA within T. brucei mitochondria. Mature RBP16 contains a cold shock domain at the N terminus and a C-terminal region rich in arginine and glycine. The presence of the cold shock domain places RBP16 as the first organellar member of the highly conserved Y-box protein family. The arginine and glycine rich C terminus in combination with the cold shock domain predicts that RBP16 will be involved in the regulation of gene expression at the posttranscriptional level.