| Popis: |
Chloroplasts have evolved from once free living cyanobacteria. During the process of endosymbiosis, chloroplasts transfer the majority of their genes to the nucleus, leaving the genomes of contemporary plastids with only 5-10% as many genes as those of cyanobacteria. Upon completion of translation in the cytosol, the chloroplast destined proteins must be efficiently imported and further sorted. To ensure fidelity of targeting and efficient translocation, most proteins are synthesized with specific amino-terminal cleavable presequence. A hetero-oligomeric molecular complex dubbed the Toc (translocon at the outer envelope of chloroplasts) ensures the specific recognition of precursor proteins and permits their translocation across the outer envelope membrane. Toc works co-ordinately with its counterpart in the inner envelope, the Tic-complex (translocon at the inner envelope of chloroplasts), to jointly facilitate the passage of the polypeptides across both membranes. Once inside the chloroplast, the transit sequence is cleaved off by a stromal processing peptidase. Several lines of evidence suggest that protein translocation into chloroplasts is a highly regulated process involving redox sensors and protein kinases. |
