FIGURE 1.

Structure of pentatricopeptide repeat (PPR) proteins. Refined classification and motif structure of PPR proteins (modified from Manna, 2015 and Cheng et al., 2016). The PPR family can be further classified into two subfamilies, P and PLS. P subfamily proteins consist of one or more tandem arrays of P motifs; whereas PLS subfamily proteins consist of triplets of P, L, and S motifs, occasionally interspersed with additional S motifs (P, L, and S can be further divided as we described above). A few P-class proteins contain additional motifs, such as the SMR, TGM, TGM CCCH-zinc finger, LAGLIDADG, Mitochondrial RNA polymerase, and PRORP. Almost all PLS-class proteins contain C-terminal E motifs (E1, E2, and E+; E+ motif was defined by Cheng et al., 2016), and many contain, in addition, a DYW motif. Some other examples that belong to the PLS subfamily were shown here. Arrows with different colors represent different motifs. Numbers of PPR motifs shown in each subgroup do not represent all members, and the motifs are not to scale.