Shredding the signal: targeting peptide degradation in mitochondria and chloroplasts

• The targeting of most proteins to mitochondria and chloroplasts is dependent on N-terminal targeting signals.
• N-terminal targeting signals are cleaved off after import, generating free targeting peptides.
• Oligopeptidases PreP and OOP constitute a complementary pathway for targeting peptide degradation in Arabidopsis thaliana.
• The structures of OOP and PreP reveal the molecular basis for substrate length restriction.
• Degradation of peptides generated in mitochondria and chloroplasts is important for proper plant development.

The biogenesis and functionality of mitochondria and chloroplasts depend on the constant turnover of their proteins. The majority of mitochondrial and chloroplastic proteins are imported as precursors via their N-terminal targeting peptides. After import, the targeting peptides are cleaved off and degraded. Recent work has elucidated a pathway involved in the degradation of targeting peptides in mitochondria and chloroplasts, with two proteolytic components: the presequence protease (PreP) and the organellar oligopeptidase (OOP). PreP and OOP are specialized in degrading peptides of different lengths, with the substrate restriction being dictated by the structure of their proteolytic cavities. The importance of the intraorganellar peptide degradation is highlighted by the fact that elimination of both oligopeptidases affects growth and development of Arabidopsis thaliana.