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J. Biol. Chem., Vol. 283, Issue 14, 8822-8828, April 4, 2008

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The Chloroplast Tat Pathway Transports Substrates in the Dark^*

From the Department of Plant Biology, University of California, Davis, California 95616

Photosynthetic electron transport pumps protons into the thylakoid lumen, creating an electrochemical potential called the protonmotive force (PMF). The energy of the thylakoid PMF is utilized by such machinery as the chloroplast F₀F₁-ATPase as well as the chloroplast Tat (cpTat) pathway (a protein transporter) to do work. The bulk phase thylakoid PMF decays rapidly after the termination of actinic illumination, and it has been well established via potentiometric measurements that there is no detectable electrical or chemical potential in the thylakoid after a brief time in the dark. Yet, we report herein that cpTat transport can occur for long periods in the dark. We show that the thylakoid PMF is actually present long after actinic illumination of the thylakoids ceases and that this energy is present in physiologically useful quantities. Consistent with previous studies, the dark-persisting thylakoid potential is not detectable by established indicators. We propose that cpTat transport in the dark is dependent on a pool of protons in the thylakoid held out of equilibrium with those in the bulk aqueous phase.

Received for publication, October 31, 2007 , and in revised form, January 9, 2008.

^* This work was supported by a grant from the United States Department of Energy (to S. M. T.). 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: Faculty of Life Sciences, University of Manchester, Manchester M13 9PT, United Kingdom.

² To whom correspondence should be addressed: Dept. of Plant Biology, One Shields Ave., University of California, Davis, CA 95616. Tel.: 530-752-0624; Fax: 530-752-5410; E-mail: smtheg{at}ucdavis.edu.

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