Pentatricopeptide repeat (PPR) proteins are a large family of modular RNA-binding proteins which mediate several aspects of gene expression primarily in organelles but also in the nucleus [1]. PPR_long is the region of Arabidopsis protein-only RNase P (PRORP) enzyme that consists of up to eleven alpha-helices [2]. PRORPs are a class of RNA processing enzymes that catalyze maturation of the 5' end of precursor tRNAs in Eukaryotes [3]. All PPR proteins contain tandemly repeated sequence motifs (the PPR motifs) which can vary in number. The series of helix-turn-helix motifs formed by PPR motifs throughout the protein produces a superheros with a central groove that allows the protein to bind RNA [1]. Proteins containing PPR motifs are known to have roles in transcription, RNA processing, splicing, stability, editing, and translation. Over a decade after the discovery of PPR proteins, the super-helical structure was confirmed. The protein-only mitochondrial RNase P crystal structure from Arabidopsis thaliana (PRORP1) confirmed the role of its PPR motifs in pre-tRNA binding and suggest it has evolved independently from other RNase P proteins that rely on catalytic RNA [1].

1: An overview of pentatricopeptide repeat proteins and their applications. Manna S; Biochimie. 2015;113:93-99. PMID:25882680

2: Mitochondrial ribonuclease P structure provides insight into the evolution of catalytic strategies for precursor-tRNA 5' processing. Howard MJ, Lim WH, Fierke CA, Koutmos M; Proc Natl Acad Sci U S A. 2012;109:16149-16154. PMID:22991464

3: Nuclear Protein-Only Ribonuclease P2 Structure and Biochemical Characterization Provide Insight into the Conserved Properties of tRNA 5' End Processing Enzymes. Karasik A, Shanmuganathan A, Howard MJ, Fierke CA, Koutmos M; J Mol Biol. 2016;428:26-40. PMID:26655022