Photosynthesis is the process in which solar energy is converted to chemical energy that is stored in energy rich compounds (starch) and is utilised directly or indirectly by all vertebrates on this planet. The by-product of photosynthesis, oxygen, has rendered an atmosphere around our earth and enabled life to move from the oceans to land. How do the sessile plants cope with the constantly changing environmental situations, to be able to optimise the process of photosynthesis? To be able to address this important question we have been working along two lines of research:

In we started to develop a method for the isolation of a highly purified thylakoid lumen using spinach chloroplasts. We carried out the first systematic characterisation of the lumen content and showed that it contained at least 25 proteins (Kieselbach et al ). These data changed the common understanding of the thylakoid lumen being mainly a proton storage for the generation of the proton motive force. Instead we were able to show that the lumen plays an important role in the proper function of the photosynthetic complexes and the integrity of the thylakoid membrane. We characterised several of the novel lumen proteins (Kieselbach et al. , Mant et al. ). As the genome for the model organism Arabidopsis thaliana was getting available we continued our research on this organism and published the first 2D-gels of Arabidopsis in (Kieselbach et al ) and the complete map was published in (Schubert et al. ).

Among the identified lumen proteins, the lumenal peptidyl-prolyl cis-trans isomerases and the PsbP-domain proteins are of particular interest for the assembly and regulation of PSII and PSI complexes. Also many new candidates of the TAT-translocation pathway are included in the list of lumen proteins that will be valuable for future research on chloroplast protein import. Moreover, already before the publication of the Arabidopsis genome, we could show that several proteins such as PsbO, PsbQ and plastocyanin occur in different isoforms. A large program for antibody production, overexpression and anti-sense of these lumen proteins is now in progress.

The published maps of the thylakoid lumen content are now used for analysis of various stress factors such as temperature, light and nutrition. The protein changes are quantified with the Ettan^TM DIGE, and DeCyder^TM image analysis platform (Amersham biosciences). New proteomic maps of other compartments and the whole chloroplast are in progress, with the aim to understand the interaction between chloroplast and nucleus in the eukaryotic plant.

The PSII supracomplex consists of more than 30 different protein subunits. 50% of these proteins have a low molecular mass below 10 kDa. Why does Photosystem II needs all these small proteins? We have so far focused on the PsbW and the PsbX proteins. We have purified the PsbW protein using immuno-affinity chromatography (Irrgang et al. ) and showed its association with PSII via Western blotting of various PSII preparations (Shi and Schröder, ). The orientation of the protein was shown to be opposite to other integral PSII proteins with the N-terminus reaching into the lumen. The rate and mechanism of PsbW degradation under photoinhibitory conditions was found to be similar to the D1-protein (Hagman et al. ). To find the function of the protein transgenic Arabidopsis plants expressing an anti-sense psbW gene were produced. Transgenic plants with a 96% reduction in PsbW content were used for functional studies. From these studies, it became clear that in the absence of the PsbW protein, no stable PSII-dimer complexes are formed, and the monomers were instable compared to the control. As a consequence of the decreased stability the amount of PSII was reduced to roughly 50% in the transgenic plants. These results suggest that the nuclear encoded PsbW protein plays an important role in the stabilisation of the PSII-dimer and in the PSII-damage repair-cycle (Shi et al. ).

PsbX was shown to be associated with the PSII core, but not present in the PSII reaction centre or LHCII. We found the nearest neighbour of PsbX to be the a-subunit of cytochrome b559. The lateral distribution of this protein in the thylakoid membrane was found to be similar to PsbW (80% in the grana regions). The expression of the PsbX was shown to be tightly light regulated, and both the protein and the mRNA are absent in etiolated plants (Shi et al. ). Further analysis of these small proteins and also new transgenic plants of other small proteins in PSII are now in progress to enable us to get a dynamic picture of the PSII function and regulation.

 

Carlberg, I, Hansson, M., Kieselbach, T., Schröder, W.P., Andersson, B. and Vener, A.V. ()  A novel plant protein undergoing light induced phosphorylation and release from the photsynthetic thylakoid membrane. PNAS 100, 757-762.

 

Thidholm, E., Lindström, V., Tissier, C., Robinson, C., Schröder, W.P. and Funk, C. () Novel approach reveals localisation and assembly pathway of the PsbS and PsbW proteins into the Photosystem II dimer. FEBS Lett. 513, 217-222.

 

Schubert, M., Petersson, U.A., Haas, B.J., Funk, C., Schröder, W.P. and Kieselbach, T. () Proteome map of the chloroplast lumen of Arabidopsis thaliana. J. Biol. Chem. 277, .

 

Shi, L-X., Lorkovi´c, Z.J., Oelmüller, R. and Schröder, W.P. () The low molecular mass PsbW protein is involved in the stabilization of the dimeric Photosystem II complex in Arabidopsis thaliana. J. Biol. Chem. 275, .

 

Kieselbach, T., Bystedt, M., Hynds, P., Robinson C., and Schröder, W.P. () A peroxidase homologue and a novel plastocyanin located by proteomics to the Arabidopsis chloroplast thylakoid lumen. FEBS Lett. 480, 271-276.

 

Shi, L-X., Kim, S.J., Marchant, A.,  Robinson, C. and Schröder, W.P. () Characterisation of the PsbX protein from Photosystem II and light regulation of its gene expression in higher plants. Plant. Mol. Biol. 40, 737-744.

 

Mant, A., Kieselbach, T., Schröder, W.P. and Robinson, C. () Characterisation of an Arabidopsis cDNA encoding a novel thylakoid lumen protein imported by the DpH-dependent pathway. Planta 207, 624-627.

 

Kieselbach, T., Mant, A., Robinson, C. and Schröder, W.P. () Characterisation of an Arabidopsis cDNA encoding a thylakoid lumen protein related to a novel "pentapeptide repeat" family of proteins. FEBS Lett. 428, 241-244.

 

Kieselbach, T., Hagman, Å., Andersson, B. and Schröder, W.P. () The thylakoid lumen of chloroplats, Isolation and characterization. J. Biol. Chem. 273, .

 

Shi, L-X. and Schröder, W.P. () Compositional and topological Studies of the PsbW protein in Spinach Thylakoid membranes. Photosynth. Res. 53, 45-53.

 

Hagman, Å., Shi, L-X., Rintamäki, E., Andersson, B. and Schröder, W.P. () The nuclear-encoded PsbW protein subunit of Photosystem II undergoes Light-induced proteolysis. Biochemistry 36, .

 

Lorkovic, Z.J., Schröder, W.P., Pakrasi, H.B., Irrgang, K.-D., Herrmann, R.G. and Oelmüller, R. () Molecular characterization of psbW, a novel and the only nuclear-encoded component of the photosystem II reaction center complex in spinach. Proc. Natl. Acad. Sci. USA 92, .

 

Irrgang, K-D., Shi, L-X., Funk, C.,  and Schröder, W.P. () A nuclear encoded subunit of the Photosystem II reaction centre. J. Biol. Chem. 270, .