Common features of the chloroplastic outer envelope and
the outer membranes of Synechocystis
Bölter, B., Hinnah, S., Wagner, R., Schulz, A., Soll, J.
Universität Kiel, Institut für Botanik, Am Botanischen Garten 1-9, D-24118 Kiel, Germany
Chloroplasts, the photosynthetic active organelles of green plant tissue, are thought to have originated from ancestral cyanobacteria by engulfment and subsequent symbiotic adaption. In course of this process horizontal gene transfer from the endosymbiont to the host nucleus took place. Therefore, many proteins destined for one of the chloroplastic subcompartments have to be translocated across the chloroplast outer and inner envelope membranes. Recently several components of the translocation machinery have been identified: Toc86, Toc75, Toc34, Tic110 and Tic55 (Toc = Translocon at the outer envelope of chloroplasts; Tic = Translocon at the inner envelope of chloroplasts). Homologues to Toc75 and Toc34 have been found on the Synechocystis genome and named synToc75 and synToc34, respectively. These cyanobacterial proteins were overexpressed and used to raise antisera in rabbits. By immuno-blot analysis and immuno-decoration we detected synToc75 in the outer envelope of Synechocystis PCC6803. When synToc75 was reconstituted into liposomes it formed a voltage-dependent channel in vitro. SynToc75 shares this functional characteristic with Toc75, which also represents the translocation pore of the Toc complex. Formerly, the chloroplast outer envelope membrane was thought to have derived from the host’s outer membrane whereas the inner envelope membrane should represent the symbiont’s plasmamembrane. Our results shed new lights on the endosymbiont theory and especially on the origin of the chloroplast outer envelope membrane that shows significant homologies to the gram negative-like outer membrane of Synechocystis.
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