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Arabidopsis PsbOs differ in their GTPase activity
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.
Linköping University, Department of Clinical and Experimental Medicine, Cell Biology. Linköping University, Faculty of Health Sciences.
2008 (English)In: Photosynthesis: Energy from the Sun / [ed] John F. Allen, Elisabeth Gantt, John H. Golbeck, Barry Osmond., Springer , 2008, 729-731 p.Chapter in book (Other academic)
Abstract [en]

Crucial for the optimal function of the oxygen-evolving complex (OEC) is the PsbO subunit of the photosystem II (PSII) complex. Previously we reported the ability of PsbO in spinach to bind and hydrolyze GTP. GTP stimulates the dissociation of PsbO from PSII following illumination and induces the degradation of the D1 protein. We have predicted four plant-specific binding motifs for GTP, which are not conserved in the sequences of the cyanobacteria or green algae PsbO proteins. We have proposed a location of the GTP-binding site inside the β-barrel exposed to the lumenal side. Arabidopsis thaliana has two PsbO isoforms encoded by two different genes: psbO1 and psbO2. Here we have measured and compared the GTPase activities of PSII membranes isolated from Arabidopsis knockouts mutants containing T-DNA insertions in one or the other of the psbO genes. The specific GTPase activity of PsbO2 is three fold higher than that of PsbO1. Furthermore, PsbO2 is more efficiently released than PsbO1 from PSII following light treatment. We conclude that PsbO2 is a better GTPase than Psb.

Place, publisher, year, edition, pages
Springer , 2008. 729-731 p.
Keyword [en]
Photosystem II, PsbO protein, Arabidopsis thaliana, T-DNA insertionO1
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-13006DOI: 10.1007/978-1-4020-6709-9_162ISBN: 978-1-4020-6707-5 (print)OAI: oai:DiVA.org:liu-13006DiVA: diva2:17655
Available from: 2008-03-20 Created: 2008-03-20 Last updated: 2013-06-14Bibliographically 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
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1165
Keyword
plant, chloroplast, thylakoid, nucleotide, PSII, PsbO
National Category
Biological Sciences
Identifiers
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)
Opponent
Supervisors
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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