The Enigmatic Roles of PPR-SMR Proteins in Plants

Abstract

The pentatricopeptide repeat (PPR) protein family, with more than 400 members, is one of the largest and most diverse protein families in land plants. A small subset of PPR proteins contain a C-terminal small MutS-related (SMR) domain. Although there are relatively few PPR-SMR proteins, they play essential roles in embryo development, chloroplast biogenesis and gene expression, and plastid-to-nucleus retrograde signaling. Here, recent advances in understanding the roles of PPR-SMR proteins and the SMR domain based on a combination of genetic, biochemical, and physiological analyses are described. In addition, the potential of the PPR-SMR protein SOT1 to serve as a tool for RNA manipulation is highlighted.

Keywords: RNA endonuclease; RNA manipulation; pentatricopeptide repeat‐small MutSrelate (PPR‐SMR) proteins.

Figure 1

Proposed models for the possible roles of SVR7 in Arabidopsis and its maize ortholog, ATP4. a) SVR7 is directly or indirectly involved in chloroplast rRNA processing. b) SVR7 and/or ATP4 enhance translational efficiency in atpB/E (SVR7/ATP4), rbcL (SVR7), and atpA (ATP4). c) SVR7 and/or ATP4 enhance the stabilization of rpl16−rpl14 dicistronic RNA (SVR7/ATP4, cooperating with PGR3), psaJ transcripts (ATP4, cooperating with PPR10), and atpF transcripts (ATP4) by blocking the invasion of 3′ to 5′ exonucleases. See related text for details.

Figure 2

Proposed model for the role of SOT1 in chloroplast 23S−4.5S rRNA maturation in Arabidopsis. When SOT1 is present, it binds to the 5′ region of the chloroplast 23S−4.5S rRNA precursor via the PPR motifs to block 5′ to 3′ exonuclease invasion. The SMR domain of SOT1 cleaves the 23S−4.5S rRNA precursor at the −38 site. This cleavage appears to conditional and requires the formation of some form of RNA structure around the cleavage site. SOT1 helps facilitate the processing by mini‐ribonuclease III (Mini‐III) during the maturation of 23S and 4.5S rRNA. It appears that SOT1 binding is crucial to maturation of 23S and 4.5S rRNA. When SOT1 is missing, 5′ to 3′ exonuclease invades the 23S rRNA and the cleavage at the −38 site does not occur, which results in a defect in maturation of 23S and 4.5S rRNA.