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J. Biol. Chem., Vol. 281, Issue 9, 5476-5483, March 3, 2006

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Oligomers of Tha4 Organize at the Thylakoid Tat Translocase during Protein Transport^*

Carole Dabney-Smith, Hiroki Mori, and Kenneth Cline¹

From the Horticultural Sciences Department and the Plant Molecular and Cellular Biology Program, University of Florida, Gainesville, Florida 32611 and the School of Pharmacy, Shujitsu University, 1-6-1 Nishigawara, Okayama 703-8516, Japan

The Tat (twin arginine translocation) systems of thylakoids and bacteria transport fully folded protein substrates without breaching the permeability barrier of the membrane. Two components of the thylakoid system, cpTatC and Hcf106, compose a precursor-bound receptor complex. The third component, Tha4, assembles with the precursor-bound receptor complex for the translocation step and is thought to compose at least part of the protein-conducting channel. Here, we used two different cross-linking approaches to explore the organization of Tha4 in the translocase. These cross-linking techniques showed that transition to an active protein transport state resulted in an alignment of the Tha4 amphipathic helix and C-terminal tail domains to form Tha4 oligomers. Oligomerization required functional Tha4, a twin arginine signal peptide, and an active cpTatC-Hcf106 receptor complex. The spectrum of oligomers obtained was independent of the mature folded domain of the precursor. We propose a trapdoor mechanism for translocation whereby aligned oligomers of Tha4 amphipathic helices fold into the membrane to allow formfitting passage of precursor proteins.

Received for publication, November 21, 2005 , and in revised form, December 30, 2005.

^* This work was supported in part by National Institutes of Health Grant R01 GM48179 (to K. C.). 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.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1–S3.

¹ To whom correspondence should be addressed: Horticultural Sciences Dept., University of Florida, 1109 Fifield Hall, P. O. Box 110690, Gainesville, FL 32611. Tel.: 352-392-4711 (ext. 219); Fax: 352-392-5653; E-mail: kcline{at}ufl.edu.

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