Minisymposium 20: Organelle Development
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M2004: Involvement of type I signal peptidase in the biogenesis of chloroplasts
Most chloroplastic proteins are synthesized on cytosolic ribosomes and imported into the organelle posttranslationally. A β-barrel-forming integral membrane protein Toc75 is encoded in the nucleus and postulated to constitute the protein translocation channel at the chloroplast outer envelope. Unlike other outer membrane proteins which do not require cleavable targeting sequences for their targeting, Toc75 is synthesized with a bipartite transit peptide: the first part targets the protein to the chloroplast by a general pathway and is removed by a stromal processing peptidase, whereas the second part is necessary to divert the protein to the envelope membrane by an unknown mechanism and details of its processing had also remained elusive. Recently, based on the similarity of the second cleavage site of Toc75 to the processing sites of type I signal peptidase (SPase I) substrates, we developed and tested a hypothesis that an SPase I-like protein is responsible for the second processing of Toc75 by biochemical and genetic approaches (J Cell Biol 171:425-430). In the end, we identified Plsp1 (plastidic SPase I 1) as the enzyme involved in the complete maturation of Toc75. Interestingly, Plsp1 appears to be located at both the envelope and thylakoid membranes. Furthermore, disruption of the PLSP1 gene resulted in accumulation of intermediate forms of not only Toc75, but also a thylakoidal lumen protein OE33. The mutant chloroplasts that lack Plsp1 show a severe reduction of thylakoid formation although they accumulate major thylakoidal proteins. These results indicate the importance of protein maturation in the biogenesis of chloroplasts. This work has been supported by USDA-CSREES grant number 2003-02860.
