Studies on the structural and functional role of the extrinsic manganese stabilizing protein in higher plants

Abstract

The PsbO protein acts as a regulatory extrinsic subunit that also stabilizes the manganese cluster of the water oxidizing complex (WOC). This protein often referred to as MSP (manganese stabilizing protein) is present in all oxygen evolving organisms and characterized by striking sequence homologies of the copies from ancient cyanobacteria up to the evolutionary level of higher plants. The overall conformation of the isolated MSP in solution is best described as a thermostable, molten globule state. In order to gather more detailed information on the possible role of the C terminal domain of the MSP comparative studies were performed on the thermal transition of the protein isolated from pea and a synthetic peptide with the C-terminus (KDVKIQGVWYAQLES) sequence of the MSP. Measurements of intrinsic fluorescence emission, circular dichroism in far and near UV and fluorescence polarisation reveal that the cooperativity factor and melting point of MSP depends on pH and the presence of Ca2+ and Mn2+. Furthermore, the reconstitution of oxygen evolution by addition of the solubilized protein to MSP depleted PS II membrane fragment is impaired by the presence of the synthetic peptide thus indicating its interaction with the MSP-binding site of PS II. A possible role of the C-terminus for the structure and function of MSP are discussed.