Pentatricopeptide repeat (PPR) proteins represent a large family of sequence-specific RNA-binding proteins that are involved in multiple aspects ofRNA metabolism. PPRproteins, which are found in exceptionally large numbers in the mitochondria and chloroplasts of terrestrial plants, recognize single-strandedRNA (ssRNA) in a modular fashion. The maize chloroplast protein PPR10 binds to two similar RNA sequences from the ATPI-ATPH and PSAJ-RPL33 intergenic regions, referred to as ATPH and PSAJ, respectively. By protecting the target RNA elements from 5' or 3' exonucleases, PPR10 defines the corresponding 5' and 3' messenger RNA termini. Despite rigorous functional characterizations, the structuralbasis of sequence-specific ssRNA recognition by PPRproteins remains to be elucidated. Here we report the crystal structures of PPR10 in RNA-free and RNA-bound states at resolutions of 2.85 and 2.45 Å, respectively. In the absence of RNA binding, the nineteen repeats of PPR10 are assembled into a right-handed superhelical spiral. PPR10 forms an antiparallel, intertwined homodimer and exhibits considerable conformational changes upon binding to its target ssRNA, an 18-nucleotide PSAJ element. Six nucleotides of PSAJ are specifically recognized by six corresponding PPR10 repeats following the predicted code. The molecular basis for the specific and modularrecognition of RNA bases A, G and U is revealed. The structural elucidation of RNArecognition by PPRproteins provides an important framework for potential biotechnological applications of PPRproteins in RNA-related research areas.

Yin P, Li QX, Yan CY, Liu Y, Liu JJ, Yu F, Wang Z, Long JF, He JH, Wang HW, Wang JW, Zhu JK, Shi YG, Yan NE*. (2013) Structural basis for the modular recognition of single-stranded RNA by PPR proteins. Nature, 504(7478):168-171.