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J. Biol. Chem., Vol. 283, Issue 49, 33838-33845, December 5, 2008

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A Stromal Pool of TatA Promotes Tat-dependent Protein Transport across the Thylakoid Membrane^*

From the Institute of Biology, Plant Physiology, Martin-Luther-University Halle-Wittenberg, Weinbergweg 10, 06120 Halle (Saale), Germany

In chloroplasts and bacteria, the Tat (twin-arginine translocation) system is engaged in transporting folded passenger proteins across the thylakoid and cytoplasmic membranes, respectively. To date, three membrane proteins (TatA, TatB, and TatC) have been identified to be essential for Tat-dependent protein translocation in the plant system, whereas soluble factors seem not to be required. In contrast, in the bacterial system, several cytosolic chaperones were described to be involved in Tat transport processes. Therefore, we have examined whether stromal or peripherally associated membrane proteins also play a role in Tat transport across the thylakoid membrane. Analyzing both authentic precursors as well as the chimeric 16/23 protein, which allows us to study each step of the translocation process individually, we demonstrate that a soluble form of TatA is present in the chloroplast stroma, which significantly improves the efficiency of Tat-dependent protein transport. Furthermore, this soluble TatA is able to reconstitute the Tat transport properties of thylakoid membranes that are transport-incompetent due to extraction with solutions of chaotropic salts.

Received for publication, August 15, 2008 , and in revised form, October 7, 2008.

^* This work was supported by grants from the state Sachsen-Anhalt (Exzellenzcluster Biowissenschaften, Research Cluster B) and the Deutsche Forschungsgemeinschaft (KL 862/2-1). 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 Present address: Institute of Biology, Microbiology, Humboldt-University Berlin, Chausseestrasse 117, 10115 Berlin, Germany.

² To whom correspondence should be addressed. Tel.: 49-345-55-26-200; Fax: 49-345-55-27-285; E-mail: klosgen{at}pflanzenphys.uni-halle.de.

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