Protein Transport into “Complex” Diatom Plastids Utilizes Two Different Targeting Signals*
- From the Institut für Biochemie der Pflanzen, Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, d-40225 Düsseldorf, Germany and ‡Martek Biosciences, Columbia, Maryland 21045
Abstract
The plastids found in diatoms and other chromophytic algae are completely enclosed by four membranes in contrast to chloroplasts of higher plants, which are surrounded by only two membranes. The bipartite targeting sequence of diatom nuclear-encoded plastid proteins contains an endoplasmic reticulum signal sequence and, based on sequence comparison, a transit peptide-like domain similar to that which targets proteins into the plastids of higher plants. By performing heterologous import experiments using the precursor of the γ subunit of the chloroplast ATPase from the diatom Odontella sinensis we were able to show that protein import into diatom plastids is at least a two-step event. We demonstrate that the first step involves co-translational transport through endoplasmic reticulum membranes and that there is an additional targeting step which is similar to the import of precursor proteins into chloroplasts of higher plants and green algae indicating that the transit peptide-like domain of the diatom precursor is functionally equivalent to the respective targeting signal of higher plants. Our results suggest that the transit peptide depending targeting mechanism in plastids has apparently remained relatively unchanged over the course of evolution, with only the peptidase cleavage site significantly modified.
Footnotes
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↵* This work was supported by grants from the Deutsche Forschungsgemeinschaft (DFG), Sonderforschungsbereich 189 (Project B3), and Kr1661/1-1 (to P. G. K.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Dedicated to Prof. Dr. Heinrich Strotmann on the occasion of his 60th birthday.
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↵§ To whom correspondence should be addressed: Institut für Biochemie der Pflanzen, Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, D-40225 Düsseldorf, Germany. Tel.: 49-211-81-12343; Fax: 49-211-81-13706; E-mail: Peter.Kroth{at}uni-duesseldorf.de.
- Abbreviations:
- ER
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endoplasmic reticulum
- atpC
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gene for the γ subunit of chloroplast ATPase
- fcp
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genes coding for fucoxanthin chlorophyll a/c-binding proteins (FCP)
- PCR
-
polymerase chain reaction
- PAGE
-
polyacrylamide gel electrophoresis
- Tricine
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N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine.
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- Received February 20, 1998.
- Revision received September 8, 1998.
- The American Society for Biochemistry and Molecular Biology, Inc.
