PPR-SMRs: Ancient proteins with enigmatic functions

A small subset of the large pentatricopeptide repeat (PPR) protein family in higher plants contain a C-terminal small MutS-related (SMR) domain. Although few in number, they figure prominently in the chloroplast biogenesis and retrograde signaling literature due to their striking mutant phenotypes. In this review, we summarize current knowledge of PPR-SMR proteins focusing on Arabidopsis and maize proteomic and mutant studies. We also examine their occurrence in other organisms and have determined by phylogenetic analysis that, while they are limited to species that contain chloroplasts, their presence in algae and early branching land plant lineages indicates that the coupling of PPR motifs and an SMR domain into a single protein occurred early in the evolution of the Viridiplantae clade. In addition, we discuss their possible function and have examined conservation between SMR domains from Arabidopsis PPR proteins with those from other species that have been shown to possess endonucleolytic activity.