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BMC Plant Biology

, 10:223

First Online: 19 October 2010Received: 02 December 2009Accepted: 19 October 2010


BackgroundPlastids rely on protein supply by their host cells. In plastids surrounded by two membranes primary plastids targeting of these proteins is facilitated by an N-terminal targeting signal, the transit peptide. In secondary plastids surrounded by three or four membranes, transit peptide-like regions are an essential part of a bipartite topogenic signal sequence BTS, and generally found adjacent to a N-terminally located signal peptide of the plastid pre-proteins. As in primary plastids, for which no wealth of functional information about transit peptide features exists, the transit peptide-like regions used for import into secondary ones show some common features only, which are also poorly characterized.

ResultsWe modified the BTS in the transit peptide-like region of the plastid precursor fucoxanthin-chlorophyll a-c binding protein D FcpD fused to GFP as model substrate for the characterization of pre-protein import into the secondary plastids of diatoms. Thereby we show that i pre-protein import is highly charge dependent. Positive net charge is necessary for transport across the plastid envelope, but not across the periplastid membrane. Acidic net charge perturbs pre-protein import within the ER. Moreover, we show that ii the mature domain of the pre-protein can provide intrinsic transit peptide functions.

ConclusionsOur results indicate important characteristics of targeting signals of proteins imported into secondary plastids surrounded by four membranes. In addition, we show a self-targeting mechanism, in which the mature protein domain contributes to the transit peptide function. Thus, this phenomenon lowers the demand for pre-sequences evolved during the course of endosymbiosis.

Electronic supplementary materialThe online version of this article doi:10.1186-1471-2229-10-223 contains supplementary material, which is available to authorized users.

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Autor: Gregor Felsner - Maik S Sommer - Uwe G Maier


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