Trypanosoma brucei 20 S Editosomes Have One RNA Substrate-binding Site and Execute RNA Unwinding Activity

Abstract

Background: Mitochondrial transcripts in African trypanosomes undergo U insertion/deletion-type RNA editing that is catalyzed by a protein complex known as the editosome. Results: Editosomes have a single RNA substrate-binding site and catalyze RNA unwinding. Conclusion: Both U insertion and U deletion are executed within a single, multifunctional reaction center. Significance: RNA binding is followed by RNA unwinding, which represents a novel activity of the editing machinery.

Structure and intrinsic disorder of the proteins of theTrypanosoma brucei editosome

“…Components with a high content of disorder such as the OB-fold proteins TbMP42, TbMP63 and TbMP81 (and to a lesser extent TbMP18, TbMP24 and TbMP46) have as of yet unassigned functions. However, they collectively might be involved in the RNA binding and RNA unwinding function of the editosome [17] since structural flexibility has been shown to be an important trait in many protein-RNA interactions [62]. In the case of the editosome the situation is further complicated by the fact that the complex has to interact with a large variety of differently sized and folded substrate RNAs in addition to literally hundreds of gRNA and partially edited mRNAs.…”

“…In these proteins, IDRs without secondary structure cover 52.4%, 67.2%, and 65.5% residues, respectively. The presence of two C2H2 zinc finger motifs with a similar sequence pattern of (F/T)XCX2CX3FX5WX2HX4H [43], suggests that these proteins take part in protein-protein interactions or act in the process of RNA recognition and binding and perhaps RNA unfolding as suggested by Böhm et al [17]. Moreover, TbMP42 appears to take part in the editosome structure and organization, and may coordinate some steps of reactions cycle [43,44].…”

“…First, as a result of their large IDR content, TbMP81, TbMP63, and TbMP42 may use their flexibility to perform a role in the RNA binding event of the processing cycle, in proteinprotein interactions and perhaps in the documented RNA unwinding activity of the editosome as suggested by Böhm et al [17]. Second, proteins with the smallest fraction of disordered residues (TbMP57, TbMP48, and TbMP52) are engaged in catalytic reactions (U-insertion and RNA ligation) that require less flexibility and more structural rigidity (Table 1).…”

“…Especially, the yeast exosome core protein Rrp44 has been demonstrated to catalyze the unwinding of RNA molecules through a novel mechanism utilizing three OB-fold domains [54,55]. As a consequence, Böhm et al suggested that the inherent RNA chaperone activity of the editosome is executed through the heteromultimeric association of some or all of the editosome OB-fold proteins [17].…”

“…20S complexes do not contain RNA, whereas 35-40S particles share core proteins with 20S particles, contain additional proteins and are associated with endogenous mRNA and gRNA molecules. Editosomes have a single substrate RNA binding site and execute an RNA chaperone-type RNA unwinding activity to partially unfold the different pre-mRNA molecules [17]. Up to 20 proteins were found to be associated with both, the 20S and 35-40S editing complexes, but the exact composition and stoichiometry of the different complexes have not been established.…”