Mitochondrial RNA binding protein (MRP1/MRP2) - guideRNA complex
Trypanosoma brucei (Africa sleeping sickness parasitic protozoa)
The mitochondrial RNA binding proteins MRP1 and MRP2 function in kinetoplasmide RNA editing (called "kRNA editing"), which occurs in mitocondorion. kRNA editing involves the insertion and/or deletion of uridine (U) nuculeotides in the coding region of mitochondrial mRNA. These events shift the reading frame of coding regions.
Here, we will explain MRP1/MRP2 of trypanosoma within the kinetoplastid flagellates.
In this process, a large number of small RNAs, termed guide RNAs (gRNAs), play an essential roll. When the hybridization between pre-mRNA and gRNAs occurs, a part of the structure of gRNAs takes an unfolded conformation, in which bases are exposed to solvent. MRP1/MRP2 bind to the gRNAs and facilitate the hybridization. That is, MRP1/MRP2 have a roll of matchmaker by staiblizing the unfolded local structures of these RNA molecules for RNA-RNA hybridization.
Hereby shown is the structure of MRP1/MRP2. These structures are distinguished from structures of other RNA binding proteins in that MRP1 and MRP2 combined to form a heterotetramer with C4 symmetry (Fig 1 : MRP1 and MRP2 are colored blue and green, respectively). When they take a heterotetramer, a circular α-helical core is formed.
MRP1 and MRP2 take the same “Whirly” transcription-factor fold despite the proteins have little sequence homology (Fig 2 : Surprisingly, sequence identity is only 18% !). Both MRP1 and MRP2 have an conserved β-β-β-β-α-β-β-β-β-α topology. These topology differ mainly in that MRP1 has a bent α2 helix at C-terminal end while MRP2 has an extra α-helix at N-terminal end.
Fig 3 shows the complex of MRP1/MRP2 and gRNAs. The RNA is bound to the β-sheet surface of MRPs with its phosphate backbone.
Protein Data Bank (PDB)
author: Daisuke Kuroda