Proteolysis in Plant Organelles

Proteolytic processes are intimately involved in the biogenesis and maintenance of chloroplasts and mitochondria. As such, they influence photosynthesis, respiration and other functions of these organelles. The great majority of the proteins found in these organelles are imported post-translationally. The signal peptides directing the precursor proteins into the respective organelles are removed by specific metallo-processing peptidases to yield mature proteins. The released signal peptide is further degraded by another metalloprotease. Oxidatively damaged proteins are rapidly degraded to allow the incorporation of newly synthesized ones into the respective complexes. The size of the photosynthetic antenna is adjusted by proteolytic degradation due to changes in light intensity. Partially assembled complexes or proteins lacking their prosthetic groups are inactive, and thus are being rapidly turned over. Changes in plastid identity and senescence processes are accompanied by massive the degradation of proteins. Although it is not clear which proteases perform most of these activities, a number of organelle proteases have been identified in recent years. ClpCP is an ATP-dependent serine protease complex that is located in the stroma of chloroplasts. Its catalytic function is performed by the ClpP subunit; ClpC serves as the regulatory subunit, responsible for substrate recognition, unfolding and feeding into the catalytic chamber. Mutations in either ClpP or C affect chloroplast biogenesis and functions, and result in defective growth and development. Mitochondria also contain a Clp protease, but here the regulatory subunit is ClpX, an ATPase with a single ATP-binding domain. FtsH is a membrane-bound ATPdependent metalloprotease found in both chloroplasts and mitochondria. Here the catalytic and the ATPase functions are found on the same polypeptide. This enzyme is involved in the repair cycle of photosystem II from photoinhibition by degrading the oxidatively damaged D1 protein of the reaction center. Mutations in FtsH isozymes lead to leaf variegation. DegP is a serine protease, peripherally attached to both sides of the thylakoid membrane, and expected to reside also in mitochondria. Another ATP-dependent serine protease, Lon protease, was identified in both mitochondria and chloroplasts. In addition, several amino- and endopeptidases are expected to reside in both organelles. Most proteases are encoded by multi-gene families. Specific products are targeted to either one of the organelles. However, the functional significance of this gene multiplication, i.e., whether they perform redundant or specific functions, is not clear yet.