PPR336 is associated with polysomes in plant mitochondria.

The function of pentatricopeptide repeat (PPR) proteins has been associated with various post-transcriptional steps of organelle gene expression. Among them, translation and its regulation are essential processes. However, in plant mitochondria, they are also the steps of gene expression that are the least understood. In this study, PPR336 was identified as part of a high-molecular-weight complex in Arabidopsis mitochondria. PPR336 is an unusual representative of the large PPR family because it is relatively short and is characterised by a high expression level compared with other PPR proteins. PPR336 defines a small subgroup of eight class P PPR proteins that are similar in terms of motif organization. Among them, PPR336-like is the closest homolog of PPR336. Biochemical analysis has indicated that PPR336 is a strictly mitochondrial protein, extrinsically attached to the inner mitochondrial membrane and part of an RNase-sensitive complex. Sucrose gradients and polysome destabilisation experiments show that PPR336 is associated with ribosomes in plant mitochondria. Moreover, in Ppr336/336-like mutants, mitochondrial polysomes of lower molecular weight accumulate compared with wild-type plants. Polysome association and these unusual features suggest that PPR336 could be involved in a distinctive process, possibly translation in plant mitochondria.