MORF9 increases the RNA-binding activity of PLS-type pentatricopeptide repeat protein in plastid RNA editing

RNA editing is a post-transcriptional process that modifies the genetic information on RNA molecules. In flowering plants, RNA editing usually alters cytidine to uridine in plastids and mitochondria. The PLS-type pentatricopeptide repeat (PPR) protein and the multiple organellar RNA editing factor (MORF, also known as RNA editing factor interacting protein (RIP)) are two types of key trans-acting factors involved in this process. However, how they cooperate with one another remains unclear. Here, we have characterized the interactions between a designer PLS-type PPR protein (PLS)₃PPR and MORF9, and found that RNA-binding activity of (PLS)₃PPR is drastically increased on MORF9 binding. We also determined the crystal structures of (PLS)₃PPR, MORF9 and the (PLS)₃PPR-MORF9 complex. MORF9 binding induces significant compressed conformational changes of (PLS)₃PPR, revealing the molecular mechanisms by which MORF9-bound (PLS)₃PPR has increased RNA-binding activity. Similarly, increased RNA-binding activity is observed for the natural PLS-type PPR protein, LPA66, in the presence of MORF9. These findings significantly expand our understanding of MORF function in plant organellar RNA editing.