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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
Umeå University, Faculty of Science and Technology, Department of Plant Physiology.
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2007 (English)In: Biochimica et Biophysica Acta - Bioenergetics, ISSN 0005-2728, E-ISSN 1879-2650, 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
Amsterdam: Elsevier , 2007. Vol. 1767, no 6, 500-508 p.
Keyword [en]
Photosystem II, PsbO protein, GTPase, Oxygen-evolving complex, D1 protein
URN: urn:nbn:se:umu:diva-2870DOI: 10.1016/j.bbabio.2006.10.009PubMedID: 17223069OAI: diva2:141183
Available from: 2008-01-07 Created: 2008-01-07 Last updated: 2015-04-29Bibliographically approved
In thesis
1. Nucleotide-binding Proteins in the Plant Thylakoid Membrane
Open this publication in new window or tab >>Nucleotide-binding Proteins in the Plant Thylakoid Membrane
2006 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Life on Earth is dependent on the oxygen produced through photosynthesis. The thylakoid membrane is the site for the light-driven reactions of photosynthesis, which oxidize water and supply energy in the form of ATP, mainly for carbon fixation. The utilization of ATP in the lumenal space of the thylakoid has not been considered in the past. In the latest years, increasing evidence for nucleotide metabolism in the thylakoid lumen of plant chloroplasts has been presented; ATP transport across the thylakoid membrane, and GTP binding to the PsbO extrinsic subunit of the water-oxidizing photosystem II (PSII) complex.

In this thesis, various methods for prediction, identification, and characterization of novel plant proteins, are described. Nucleotide-binding motifs and nucleotide-dependent processes are reviewed, and the experimental data is discussed. 1) A thylakoid ATP/ADP carrier (TAAC) in Arabidopsis thaliana was identified and functionally characterized, and 2) the spinach PsbO protein was characterized as a GTPase. The Arabidopsis At5g01500 gene product is predicted as a chloroplast protein and to be homologous to the well-studied mitochondrial ADP/ATP carrier. The putative chloroplast localization was confirmed by transient expression of a TAAC-green fluorescent protein fusion construct. Immuno detection with peptide-targeted antibodies and immunogold electron microscopy showed the thylakoid as the main localization of TAAC, with a minor fraction in the chloroplast envelope. TAAC is readily expressed in etiolated seedlings, and its level remains stable throughout the greening process. Its expression is highest in developing green tissues and in leaves undergoing senescence or abiotic stress. It is proposed that the TAAC protein supplies ATP for energy-dependent reactions during thylakoid biogenesis and turnover. Recombinant TAAC protein was functionally integrated in the cytoplasmic membrane of Escherichia coli, and was shown to specifically transport ATP/ADP in a protonophore-sensitive manner, as also reported for mitochondrial AACs.

The PsbO protein stabilizes the oxygen-evolving complex of PSII and is ubiquitous in all oxygenic photosynthetic organisms, including cyanobacteria, green algae, and plants. So far only the 3D-structure of the cyanobacterial PsbO is available. Four GTP-binding motifs in the primary structure of spinach PsbO were predicted from comparison with classic GTP-binding proteins. These motifs were only found in the plant PsbOs, in the -barrel domain of the homologous 3D-structure. Using circular dichroism and intrinsic fluorescence spectroscopy, it was shown that MgGTP induces specific structural changes in the PsbO protein. Spinach PsbO has a low intrinsic GTPase activity, which is considerably stimulated when associated with a dimeric PSII complex. GTP stimulates the dissociation of PsbO from PSII under both inhibitory and non-inhibitory light conditions. A role for PsbO as a GTPase in the function of the oxygen-evolving complex and PSII repair is proposed.

35 p.
Linköping Studies in Health Sciences. Thesis, ISSN 1100-6013 ; 79
plant, Arabidopsis, photosynthesis, thylakoid membrane, nucleotide-binding
National Category
Biochemistry and Molecular Biology
urn:nbn:se:liu:diva-7934 (URN)91-85643-03-3 (ISBN)
2007-01-26, Linden, Health University, entry 65, Linköpings University, Linköping, 13:00 (English)
Available from: 2007-01-15 Created: 2007-01-15 Last updated: 2009-03-10
2. Photosynthetic water oxidation: the function of two extrinsic proteins
Open this publication in new window or tab >>Photosynthetic water oxidation: the function of two extrinsic proteins
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The solar energy accumulated by photosynthesis over billions of years is the sole source of energy available on Earth. Photosystem II (PSII) uses the sunlight to split water, an energetically unfavorable reaction where electrons and protons are extracted from water and oxygen is released as a by-product. Understanding this process is crucial for the future development of clean, renewable and unlimited energy sources, which can use sunlight to split water and produce hydrogen and electricity. In order to do so we need to understand how this is solved in plants.

I have been focusing on the role of two lumenal proteins associated with the thylakoid membrane PsbO and Cah3, in the water oxidation process. Convincing evidences have been presented supporting the hypothesis that bicarbonate acts as a proton acceptor in the water splitting process in PSII and the lumenal carbonic anhydrase, Cah3, supplies bicarbonate required for this function. The PsbO protein, an important constituent of the water-oxidizing complex, however, its function is still unknown. The PsbO protein undergoes a pH dependent conformational change that in turn influences its capacity to bind calcium and manganese, forming a catalytic Mn4Ca cluster in PSII. We propose that light-induced structural dynamics of the PsbO is of functional relevance for the regulation of proton release and for forming a proton sensing - proton transporting pathway. The cluster of conserved glutamic and aspartic acid residues in the PsbO protein acts as buffering antennae providing efficient acceptors of protons derived from substrate water molecules. Both proteins, Cah3 and PsbO have a conserved S-S bridge, required for proper folding and activity; therefore they are potential targets for red-ox regulation in lumen.

Abstract [sv]

Solenergi som omvandlats av fotosyntesen under miljarder av år är basen för nästan all energi på jorden. Fotosystem 2 använder solljuset till att oxidera vatten, ur energisynpunkt en ofördelaktig process, där elektroner och protoner extraheras från vattenmolekyler vilket ger upphov till syrgas som biprodukt. Förståelsen av denna process är viktig för att vi i framtiden skall kunna utveckla rena och förnyelsebara energislag i obegrensad mängd. Genom att efterlikna fotosyntesprocessen skulle vi i framtiden kunna utvecka artificiella system som använder solljuset till att sönderdela vatten för att producera vätgas eller elektrisitet. För att kunna göra det så måste vi kunna förstå hur dessa processer fungerar i växterna.

Min forskning har fokuserat på att förstå funktionen hos två av de proteiner, PsbO och Cah3, som deltar i sönderdelningen av vatten. Jag har visat, för första gången, att ett lumen karboanhydras, Cah3, deltar i regleringen av den process där vatten spjälkas. Jag postulerar att Cah3 underlättar bort transporten av protoner från det vattenoxiderande komplexet genom att generera bikarbonat lokalt, som kan fungera som proton transportör. PsbO proteinet genomgår en pH beroende konformationsförändring vilket påverkar dess kapacitet and binda calcium och mangan som i sin tur formar ett katalytiskt Mn4Ca center i fotosystem 2. Jag föreslår att en ljusberoende strukturförändring av Psbo är av funktionell betydelse för regleringen av protonfrigörandet och formar ett proton-avkännande och proton-transporterande system. Ett kluster av konserverande glutamat- och aspartat-aminosyror i PsbO proteinet fungerar som ett buffrande nätverk för protoner som frigörs vid oxidering av vatten. Båda dessa proteiner innerhåller S-S bryggor ock kan därför vara red-ox reglerade i lumen.

Place, publisher, year, edition, pages
Umeå: Fysiologisk botanik, 2007. 50 p.
photosystem II, Psb0, Cah3, water oxidation
National Category
urn:nbn:se:umu:diva-1476 (URN)978-91-7264-481-6 (ISBN)
Public defence
2008-01-18, KB3A9, KBC, Umeå University, Umeå, 10:00
Available from: 2008-01-07 Created: 2008-01-07 Last updated: 2011-04-01Bibliographically approved

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