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Structural Studies of the Manganese Stabilizing Subunit in Photosystem II

Bengt Svensson ^*, David M. Tiede , David R. Nelson ^* and Bridgette A. Barry ^*

^* Department of Biochemistry, Biophysics, and Molecular Biology, University of Minnesota, Gortner Laboratory, St. Paul, Minnesota 55108; and Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439

Correspondence: Address reprint requests to Bridgette A. Barry at her present address, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332. E-mail: bridgette.barry{at}chemistry.gatech.edu.

Photosystem II (PSII) is the plant photosynthetic reaction center that carries out the light driven oxidation of water. The water splitting reactions are catalyzed at a tetranuclear manganese cluster. The manganese stabilizing protein (MSP) of PSII stabilizes the manganese cluster and accelerates the rate of oxygen evolution. MSP can be removed from PSII, with an accompanying decrease in activity. Either an Escherichia coli expressed version of MSP or native, plant MSP can be rebound to the PSII reaction center; MSP reconstitution reverses the deleterious effects associated with MSP removal. We have employed Fourier transform infrared (FTIR) spectroscopy and solution small angle x-ray scattering (SAXS) techniques to investigate the structure of MSP in solution and to define the structural changes that occur before and after reconstitution to PSII. FTIR and SAXS are complementary, because FTIR spectroscopy detects changes in MSP secondary structure and SAXS detects changes in MSP size/shape. From the SAXS data, we conclude that the size/shape and domain structure of MSP do not change when MSP binds to PSII. From FTIR data acquired before and after reconstitution, we conclude that the reconstitution-induced increase in ß-sheet content, which was previously reported, persists after MSP is removed from the PSII reaction center. However, the secondary structural change in MSP is metastable after removal from PSII, which indicates that this form of MSP is not the lowest energy conformation in solution.