Nucleotide-Dependent Processes in the Thylakoid Lumen of Plant Chloroplasts

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…


1. Introduction
1.1. Photosynthesis: an overview
1.2. Photosynthetic energy conversion
1.3. Photosynthetic complexes
2. Functional genomics in plants
2.1. Prediction studies
2.2. Photosynthetic preparations
2.3. Localisation studies
2.4. Structural analyses
2.5. Phenotypic analyses
3. Present investigation
3.1. Aim of this study
3.2. Working model
3.3. Nucleotide-dependent processes in the thylakoid lumen
3.4. Plant PsbO as a GTPase
3.5. ATP and phosphate thylakoid transporters
4. Conclusions

Author: Lundin, Björn

Source: Linkoping University

Download URL 2: Visit Now

Leave a Comment