The L motifs of two moss pentatricopeptide repeat proteins are involved in RNA editing but predominantly not in RNA recognition

Pentatricopeptide repeat (PPR) proteins, composed of PPR motifs repeated in tandem, are sequence-specific RNA binding proteins. Recent bioinformatic studies have shown that the combination of polar amino acids at positions 5 and last in each PPR motif recognizes RNA bases, and an RNA recognition code for PPR proteins has been proposed. Subsequent studies confirmed that the P (canonical length) and S (short) motifs bind to specific nucleotides according to this code. However, the contribution of L (long) motifs to RNA recognition is mostly controversial, owing to the presence of a nonpolar amino acid at position 5. The PLS-class PPR protein PpPPR_56 is a mitochondrial RNA editing factor in the moss Physcomitrella patens. Here, we performed in vitro RNA binding and in vivo complementation assays with PpPPR_56 and its variants containing mutated L motifs to investigate their contributions to RNA recognition. In vitro RNA binding assay showed that the original combination of amino acids at positions 5 and last in the L motifs of PpPPR_56 is not required for RNA recognition. In addition, an in vivo complementation assay with RNA editing factors PpPPR_56 and PpPPR_78 revealed the importance of nonpolar amino acids at position 5 of C-terminal L motifs for efficient RNA editing. Our findings suggest that L motifs function as non-binding spacers, not as RNA-binding motifs, to facilitate the formation of a complex between PLS-class PPR protein and RNA. As a result, the DYW domain, a putative catalytic deaminase responsible for C-to-U RNA editing, is correctly placed in proximity to C, which is to be edited.