Components of the protein translocation machinery in the thermophilic cyanobacterium Phormidium laminosum.

Cyanobacteria possess cytoplasmic membranes, which are common to all eubacteria, and an additional internal network of thylakoid membranes. We have investigated the distribution of the protein translocation machinery between the two membrane systems. We have demonstrated that leader peptidase activity exists in both membrane systems, suggesting that newly synthesized protein precursors are targeted directly rather than sorted after translocation. Southern blot analysis suggests that the leader peptidase is likely to be a single enzyme common to both membrane systems. We have also shown that cyanobacteria possess a single SecA homologue. These results indicate that the protein translocation machinery in the two membrane systems is likely to be the same and leaves open the question of what mechanism enables the cell to sort proteins between them.