The chloroplast Tat pathway utilizes the transmembrane
electric potential as an energy source
Nikolai A Braun 1, Andrew W Davis 1 and Steven Theg 1*
1 University of California - Davis
* To whom correspondence should be addressed. E-mail: smtheg{at}ucdavis.edu.
Submitted on October 3, 2006
Revised on November 14, 2006
Accepted on 2 May 2007
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Abstract |
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The thylakoid membrane, located inside the chloroplast, requires proteins transported across it for plastid biogenesis and functional photosynthetic electron transport. The chloroplast Tat translocator found on thylakoids transports proteins from the plastid stroma to the thylakoid lumen. Previous studies have shown that the chloroplast Tat pathway is independent of NTP hydrolysis as an energy source and instead depends on the thylakoid transmembrane proton gradient to power protein translocation. Due to it's localization on the same membrane as the proton motive force dependent F0F1 ATPase, we believed that the chloroplast Tat pathway also made use of the thylakoid electric potential for transporting substrates. By adjusting the rate of photosynthetic proton pumping and by utilizing ionophores, we show that the chloroplast Tat pathway can also utilize the transmembrane electric potential for protein transport. Our findings indicate that the chloroplast Tat pathway is likely dependent on the total protonmotive force (PMF) as an energy source. As a PMF-dependent device, certain predictions can be made about structural features expected to be found in the Tat translocon; specifically, the presence of a proton well, a device in the membrane that converts electrical potential into chemical potential.
Key Words:
Tat pathway, protein transport, protonmotive force, thylakoid, twin arginine
