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J. Biol. Chem., Vol. 283, Issue 5, 2534-2542, February 1, 2008

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A Minimal Tat System from a Gram-positive Organism

A BIFUNCTIONAL TatA SUBUNIT PARTICIPATES IN DISCRETE TatAC AND TatA COMPLEXES^*

James P. Barnett¹, Robyn T. Eijlander¹, Oscar P. Kuipers, and Colin Robinson²

From the Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, United Kingdom and the Department of Molecular Genetics, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Kerklaan 30, 9751 NN Haren, The Netherlands

The Tat system transports folded proteins across bacterial and thylakoid membranes. In Gram-negative organisms, a TatABC substrate-binding complex and separate TatA complex are believed to coalesce to form an active translocon, with all three subunits essential for translocation. Most Gram-positive organisms lack a tatB gene, indicating major differences in organization and possible differences in mode of action. Here, we have studied Tat complexes encoded by the tatAdCd genes of Bacillus subtilis. Expression of tatAdCd in an Escherichia coli tat null mutant results in efficient export of a large, cofactor-containing E. coli Tat substrate, TorA. We show that the tatAd gene complements E. coli mutants lacking either tatAE or tatB, indicating a bifunctional role for this subunit in B. subtilis. Second, we have identified and characterized two distinct Tat complexes that are novel in key respects: a TatAdCd complex of 230 kDa that is significantly smaller than the analogous E. coli TatABC complex (370 kDa on BN gels) and a separate TatAd complex. The latter is a discrete entity of 270 kDa as judged by gel filtration chromatography, very different from the highly heterogeneous E. coli TatA complex that ranges in size from 50 kDa to over 600 kDa. TatA heterogeneity has been linked to the varying size of Tat substrates being translocated, but the singular nature of the B. subtilis TatAd complex suggests that discrete TatAC and TatA complexes may form a single form of translocon.

Received for publication, October 1, 2007 , and in revised form, November 15, 2007.

^* This work was supported by Bacell Tat Machine Grant LSHC-CT2004-05257 from the European Union (to C. R. and O. P. K.) and by a Biotechnology and Biological Sciences Research Council Studentship (to J. P. B.). 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 These authors contributed equally to this work.

² To whom correspondence should be addressed. Tel.: 44-2476-523557; Fax: 44-2476-523568; E-mail: colin.robinson{at}warwick.ac.uk.

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