Little is known about the structures and functions of thylakoid lumen proteins. However, some of these proteins have an essential role in photosynthesis. Photosystem II (PSII) complexes are embedded in the thylakoid membrane of oxygenic photosynthetic organisms and one of the central subunits, the D1 protein, is damaged by light during the light driven water – splitting reaction and must be replaced frequently. One of the thylakoid lumen proteins that is essential for assembly and renewal of PSII complexes is the High Chlorophyll Fluorescence 136 (HCF136) protein. Another important protein for the PSII complex assembly is the Low PSII Accumulation Protein 19 (LPA19). Both proteins, HCF136 and LPA19, were shown to bind to the core subunits of the PSII complex from the lumenal side and LPA19 has been shown to explicitly interact with the soluble C-terminus of the D1 protein, one of the core PSII complex proteins. Prior to the replacement of the damaged D1 protein, the PSII complex needs to be disassembled, which is done with the help of the Maintenance of Photosystem II under High light 2 (MPH2) protein. MPH2, also called TL16, is required during the repair cycle of the PSII complex particularly under increased and fluctuating light conditions.

In this work I have determined the three-dimensional X-ray structures of the HCF136 protein at 1.6 Å resolution and the LPA19 protein at 1.2 Å resolution and have also biochemically analyzed possible interactions of HCF136 with the C-termini of D1 protein. In addition, we have determined the NMR structure of the MPH2 protein.

The protein structures of HCF136, LPA19, and MPH2 determined from A. thaliana provide us with a starting point for further studies to improve our understanding of their functional roles in the assembly, maintenance, disassembly and renewal of the PSII complex. The structures are revealing the molecular details that are particularly important during the design of mutations to study protein-protein interactions and the binding of co-factors.

Furthermore, I have contributed to the characterization of AnPrx6, the 1-Cyx peroxiredoxin from Anabaena sp. 7120. Peroxiredoxins are important caretakers of reactive oxygen species and a homolog PrxQ in A.thaliana is found in the thylakoid lumen. The dimeric AnPrx6 protein revealed different active site residues conformations in each of the dimers, which is probably coupled to its enzymatic activity. Unexpectedly, the protein acted also as a chaperone and showed chaperone activity in its dimeric state, which is a novelty for Prx proteins.

Peroxiredoxins (Prxs) are vital regulators of intracellular reactive oxygen species levels in all living organisms. Their activity depends on one or two catalytically active cysteine residues, the peroxidatic Cys (C-P) and, if present, the resolving Cys (C-R). A detailed catalytic cycle has been derived for typical 2-Cys Prxs, however, little is known about the catalytic cycle of 1-Cys Prxs. We have characterized Prx6 from the cyanobacterium Anabaena sp. strain PCC7120 (AnPrx6) and found that in addition to the expected peroxidase activity, AnPrx6 can act as a molecular chaperone in its dimeric state, contrary to other Prxs. The AnPrx6 crystal structure at 2.3 angstrom resolution reveals different active site conformations in each monomer of the asymmetric obligate homo-dimer. Molecular dynamic simulations support the observed structural plasticity. A FSH motif, conserved in 1-Cys Prxs, precedes the active site PxxxTxxCp signature and might contribute to the 1-Cys Prx reaction cycle.