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J. Biol. Chem., Vol. 280, Issue 5, 3426-3432, February 4, 2005

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Isolation and Characterization of Bifunctional Escherichia coli TatA Mutant Proteins That Allow Efficient Tat-dependent Protein Translocation in the Absence of TatB^*

Natascha Blaudeck, Peter Kreutzenbeck, Matthias Müller¶, Georg A. Sprenger||, and Roland Freudl**

From the Institut für Biotechnologie 1, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany and the ^¶Institut für Biochemie und Molekularbiologie, Albert-Ludwigs-Universität Freiburg, Hermann-Herder-Strasse 7, 79104 Freiburg, Germany

In Escherichia coli, the Tat system promotes the membrane translocation of a subset of exported proteins across the cytoplasmic membrane. Four genes (tatA, tatB, tatC, and tatE) have been identified that encode the components of the E. coli Tat translocation apparatus. Whereas TatA and TatE can functionally substitute for each other, the TatB and the TatC proteins have been shown to perform distinct functions. In contrast to Tat systems of the ABC(E) type found in E. coli and many other bacteria, some microorganisms possess a TatAC-type translocase that consists of TatA and TatC only, suggesting that, in these systems, TatB is not required or that one of the remaining components (TatA or TatC) additionally takes over the TatB function. We have addressed the molecular basis for the difference in subunit composition between TatABC(E) and TatAC-type systems by using a genetic approach. A plasmid-encoded E. coli minimal Tat translocase consisting solely of TatA and TatC was shown to mediate a low level translocation of a sensitive Tat-dependent reporter protein. Suppressor mutations in the minimal Tat translocase were isolated that compensate for the absence of TatB and that showed substantial increases in translocation activities. All of the mutations mapped to the extreme amino-terminal domain of TatA. No mutations affecting TatC were identified. These results suggest that in TatAC-type systems, the TatA protein represents a bifunctional component fulfilling both the TatA and TatB functions. Furthermore, our results indicate that the structure of the amino-terminal domain of TatA is decisive for whether or not TatB is required.

Received for publication, September 30, 2004 , and in revised form, November 19, 2004.

^* This work was supported by Grants Sp503/2-1 and Sp503/2-2 of the Deutsche Forschungsgemeinschaft. 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.

Present address: Institut für vegetative Physiologie, Universität zu Köln, Robert-Koch-Str. 39, 50931 Köln, Germany.

^|| Present address: Institut für Mikrobiologie, Universität Stuttgart, Allmandring 31, 70569 Stuttgart, Germany.

^** To whom correspondence should be addressed. Tel.: 49-2461-613472; Fax: 49-2461-612710; E-mail: r.freudl{at}fz-juelich.de.

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