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J. Biol. Chem., Vol. 282, Issue 41, 30295-30302, October 12, 2007

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Translocation of a Phycoerythrin Subunit across Five Biological Membranes^*

Sven B. Gould¹, Enguo Fan, Franziska Hempel, Uwe-G. Maier, and Ralf Bernd Klösgen

From the Cell Biology, Philipps University Marburg, Karl-von-Frisch Strasse 8, 35032 Marburg, Germany and the Institute of Biology, Plant Physiology, Martin-Luther-University Halle-Wittenberg, Weinbergweg 10, 06120 Halle/Saale

Cryptophytes, unicellular algae, evolved by secondary endosymbiosis and contain plastids surrounded by four membranes. In contrast to cyanobacteria and red algae, their phycobiliproteins do not assemble into phycobilisomes and are located within the thylakoid lumen instead of the stroma. We identified two gene families encoding phycoerythrin and light-harvesting complex proteins from an expressed sequence tag library of the cryptophyte Guillardia theta. The proteins bear a bipartite topogenic signal responsible for the transport of nuclear encoded proteins via the ER into the plastid. Analysis of the phycoerythrin sequences revealed that more than half of them carry an additional, third topogenic signal comprising a twin arginine motif, which is indicative of Tat (twin arginine transport)-specific targeting signals. We performed import studies with several derivatives of one member using a diatom transformation system, as well as intact chloroplasts and thylakoid vesicles isolated from pea. We demonstrated the different targeting properties of each individual part of the tripartite leader and show that phycoerythrin is transported across the thylakoid membrane into the thylakoid lumen and protease-protected. Furthermore, we showed that thylakoid transport of phycoerythrin takes place by the Tat pathway even if the 36 amino acid long bipartite topogenic signal precedes the actual twin arginine signal. This is the first experimental evidence of a protein being targeted across five biological membranes.

Received for publication, March 2, 2007 , and in revised form, August 16, 2007.

New and relevant EST data were deposited using EMBL WebIn under the following accession numbers: AM183804 [GenBank] and AM491779 [GenBank] -AM491800 [GenBank] .

^* This work was supported by grants from the Deutsche Forschungsgemein-schaft (Sonderforschungsbereich 648 and 695) and the state Sachsen-Anhalt (Exzellenzcluster Biowissenschaften). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: School of Botany, University of Melbourne, 3010 Melbourne, Australia. Tel.: 0061383445039; E-mail: sbgould{at}gmail.com.

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