An overview of pentatricopeptide repeat proteins and their applications

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. These proteins facilitate processing, splicing, editing, stability and translation of RNAs. While major advances in PPR research have been achieved with plant PPR proteins, the significance of non-plant PPR proteins is becoming of increasing importance. PPR proteins are classified into different subclasses based on their domain architecture, which is often a reflection of their function. This review provides an overview of the significant findings regarding the functions, evolution and applications of PPR proteins. Horizontal gene transfer appears to have played a major role in the sporadic phylogenetic distribution of different PPR subclasses in both eukaryotes and prokaryotes. Additionally, the use of synthetic biology and protein engineering to create designer PPR proteins to control gene expression in vivo is discussed. This review also highlights some of the aspects of PPR research that require more attention particularly in non-plant organisms. This includes the lack of research into the recently discovered PPR-TGM subclass, which is not only the first PPR subclass absent from plants but present in economically and clinically-relevant pathogens. Investigation into the structure and function of PPR-TGM proteins in these pathogens presents a novel opportunity for the exploitation of PPR proteins as drug targets to prevent disease.