PARENT SESSION
Posters P5A Type II reaction centres : Structure. Abstracts (289-312)

Preliminary structural characterization of the 33-kda protein (PsbO) in solution studied by site-directed mutagenesis and NMR spectroscopy. Marc Nowaczyk^*,1, Carsten Berghaus², Raphael Stoll², Matthias Roegner¹, ¹ Plant Biochemistry, Bochum, Germany² Biomolecular NMR, Bochum, Germany

ABSTRACT- Photosystem 2 (PS2) is located in the thylakoid membrane of chloroplasts and cyanobacteria and performs one of the key reactions on our planet - the light-driven oxidation of water to molecular oxygen. This process occurs in four one-electron oxidation steps (S-states) by the manganese containing water-oxidising complex (WOC). At the lumen side of PS2, the WOC is shielded by several extrinsic proteins. One of them, the 33-kDa protein (PsbO), is of special interest, because it is ubiquitous in all photosynthetic organisms performing water-splitting and its removal results in a strong decrease of the oxygen evolving activity. Therefore, it is often referred to as manganese stabilizing protein (MSP). Site-directed mutagenesis experiments combined with 1D and 2D NMR spectra provide a preliminary insight into the structure and dynamics of the 33-kDa protein (PsbO) from T. elongatus free in solution. The NMR spectra suggest that PsbO rather than forming a 'molten globule' state or being 'natively unfolded', contains both a well folded core and highly flexible regions. The PsbO protein shows a remarkable temperature-, pH-, and long-term-stability being stable for at least four weeks under the conditions tested in this study. Due to this extraordinary stability of PsbO, we characterized four cysteine mutants serving as local probes for structural and dynamic properties of PsbO. The results indicate sites of increased accessibility/flexibility which may be important for the docking to the PS2 core complex.

KEY WORDS: Thermosynechococcus elongatus, photosystem 2, 33-kDa protein/PsbO/MSP, NMR spectroscopy