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Subsequent events to GTP binding by the plant PsbO protein: structural changes, GTP hydrolysis and dissociation from the photosystem II complex
Linköping University, Department of Clinical and Experimental Medicine, Cell Biology. Linköping University, Faculty of Health Sciences.
Linköping University, Department of Clinical and Experimental Medicine, Cell Biology. Linköping University, Faculty of Health Sciences.
Department of Plant Physiology, Umeå Plant Science Center, Umeå University, Umeå, Sweden.
Linköping University, Department of Clinical and Experimental Medicine, Cell Biology. Linköping University, Faculty of Health Sciences.
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2007 (English)In: Biochimica et Biophysica Acta - Bioenergetics, ISSN 0005-2728, Vol. 1767, no 6, 500-508 p.Article in journal (Refereed) Published
Abstract [en]

Besides an essential role in optimizing water oxidation in photosystem II (PSII), it has been reported that the spinach PsbO protein binds GTP [C. Spetea, T. Hundal, B. Lundin, M. Heddad, I. Adamska, B. Andersson, Proc. Natl. Acad. Sci. U.S.A. 101 (2004) 1409–1414]. Here we predict four GTP-binding domains in the structure of spinach PsbO, all localized in the β-barrel domain of the protein, as judged from comparison with the 3D-structure of the cyanobacterial counterpart. These domains are not conserved in the sequences of the cyanobacterial or green algae PsbO proteins. MgGTP induces specific changes in the structure of the PsbO protein in solution, as detected by circular dichroism and intrinsic fluorescence spectroscopy. Spinach PsbO has a low intrinsic GTPase activity, which is enhanced fifteen-fold when the protein is associated with the PSII complex in its dimeric form. GTP stimulates the dissociation of PsbO from PSII under light conditions known to also release Mn2+ and Ca2+ ions from the oxygen-evolving complex and to induce degradation of the PSII reaction centre D1 protein. We propose the occurrence in higher plants of a PsbO-mediated GTPase activity associated with PSII, which has consequences for the function of the oxygen-evolving complex and D1 protein turnover.

Place, publisher, year, edition, pages
2007. Vol. 1767, no 6, 500-508 p.
Keyword [en]
Photosystem II, PsbO protein, GTPase, Oxygen-evolving complex, D1 protein
National Category
Medical and Health Sciences
URN: urn:nbn:se:liu:diva-13004DOI: 10.1016/j.bbabio.2006.10.009OAI: diva2:17653
Available from: 2008-03-20 Created: 2008-03-20 Last updated: 2009-05-07Bibliographically approved
In thesis
1. Nucleotide-Dependent Processes in the Thylakoid Lumen of Plant Chloroplasts
Open this publication in new window or tab >>Nucleotide-Dependent Processes in the Thylakoid Lumen of Plant Chloroplasts
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Plants, algae and photosynthetic bacteria are able to harvest the sunlight and use its energy to transform water and carbon dioxide to carbohydrate molecules and oxygen, both important to sustain life on Earth. This process is called photosynthesis and is the route by which almost all energy enters the biosphere. As most simple things in life, the process of photosynthesis is easily explained but unfortunately not that easy to reproduce. If we could, we would be living in a much different world with almost unlimited energy. Light energy is harvested by chlorophyll molecules, bound to proteins in the chloroplast thylakoid membrane and drives the oxygen-evolving complex, to extract electrons from water. Electrons are then transferred to NADPH through photosystem II (PSII) to cytochrome b6f and photosystem I, the major photosynthetic protein complexes. The cytochrome b6f complex also transfers protons into the lumenal space of the thylakoid. These protons together with those from water oxidation create an electrochemical gradient across the thylakoid membrane, which fuels the ATP synthase to produce ATP. ATP, NADPH and carbon dioxide are used during the dark reactions to produce sugars in the chloroplast stroma. The thylakoid lumenal space where the water oxidation occurs has until recently been viewed as a proton sink with very few proteins. With the publication of the genome of Arabidopsis thaliana it seems to be a much more complex compartment housing a wide variety of biochemical processes.

ATP is a nucleotide and the major energy currency, but there are also other nucleotides such as AMP, ADP, GMP, GDP and GTP. Chloroplast metabolism has mostly been associated with ATP, but GTP has been shown to have a role in integration of light harversting complexes into the thylakoid. In this work, we have demonstrated the occurrence of nucleotide-dependent processes in the lumenal space of spinach by bringing evidence first for nucleotide (ATP) transport across the thylakoid membrane, second for nucleotide inter-conversion (ATP to GTP) by a nucleoside diphosphate kinase, and third the discovery that the PsbO extrinsic subunit of PSII complex can bind and hydrolyse GTP to GDP. The active PSII complex functions as a dimer but following light-induced damage, it is monomerised allowing for repair of its reaction center D1 protein. PsbO is ubiquitous in all oxygenic photosynthetic organisms and together with other extrinsic proteins stabilises the oxygen-evolving complex. We have modelled the GTP-binding site in the PsbO structure and showed that the GTPase activity of spinach PsbO induces changes in the protein structure, dissociation from the complex and stimulates the degradation of the D1 protein, possibly by inducing momerisation of damaged PSII complexes. As compared to spinach, Arabidopsis has two isoforms of PsbO, PsbO1 and PsbO2, expressed in a 4:1 ratio. A T-DNA insertion knockout mutant of PsbO1 showed a retarded growth rate, pale green leaves and a decrease in the oxygen evolution while a PsbO2 knockout mutant did not show any visual phenotype as compared to wild type. Unexpectedly, during growth under high light conditions the turnover rate of the D1 protein was impaired in the PsbO2 knockout, whereas it occurred faster in the PsbO1 knockout as compared to wild type. We concluded that the PsbO1 protein mainly functions in stabilizing the oxygen evolving complex, whereas the PsbO2 protein regulates the turnover of the D1 protein. The two PsbO proteins also differ in their GTPase-activity (PsbO2 >> PsbO1). Although their amino acid sequences are 90% identical, they differ in the GTP-binding region which could explain the difference in their GTPase activity. Based on these data, we propose that the GTPase activity of PsbO(2) leads to structural changes in interacting loops and plays a role in the initial steps of D1 turnover such as the PSII monomerisation step.

The nucleotide-dependent processes we discovered in the thylakoid lumen raise questions of transporters to facilitate these processes. As stated earlier, we provided biochemical evidence of an ATP thylakoid transporter, and most recently have identified a transporter that may be important for the export of lumenal phosphate back to the stroma. More transporters for GDP, metal ions and others solutes have still to be identified.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi, 2008
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1165
plant, chloroplast, thylakoid, nucleotide, PSII, PsbO
National Category
Biological Sciences
urn:nbn:se:liu:diva-11244 (URN)978-91-7393-965-2 (ISBN)
Public defence
2008-04-04, Linden, Hus 421, Ingång 65; HU, Campus US, Linköpings universitet, Linköping, 09:30 (English)
Available from: 2008-03-20 Created: 2008-03-20 Last updated: 2009-05-07

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Lundin, BjörnThuswaldner (Heurtel), SophieSpetea (Wiklund), Cornelia
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