Interaction of Soluble and Immobilized Manganese-Stabilizing PsbO Protein with Manganese Ions and Isolated D1/D2/cyt b559 Complex of the PSII Reaction Center

Interaction of water-soluble and immobilized (on BrCN-activated agarose) manganese-stabilizing protein PsbO with Mn 2+ and Mn 3+ cations and with preparations of D1/D2/cyt b 559 reaction center (RC) of photosystem II was studied. By native electrophoresis, the formation of dimeric and aggregated forms of PsbO protein were found in the presence of Mn 2+ , Mg 2+ , or Fe 2+ ions. The dimerization of PsbO occurred after ultraviolet irradiation of the protein preparation. The presence of protein-bound Mn 3+ cations increased the electrostatic interaction of the immobilized PsbO with the RC. This was evidenced to by higher amounts of CaCl 2 that were required for dissociation of the PsbO–RC complex. It was first demonstrated that the protein exhibited superoxide dismutase (SOD) activity after an electrophoresis in PAAG upon incubation of the gel in an Mn 2+ -containing solution. Tetrazolium-reductase activity was also ascertained in the protein after its electrophoresis in a mixture with preparations of the oxygen-evolving complex (OEC). It is suggested that the protein interaction with Mn ions and superoxide radicals, as well as short-term UV irradiation, reduces tyrosine and a disulfide bond in the PsbO protein. This yields tyrosil radical and SH-groups participating in redox reactions with ETC components. The interactions of PsbO with Mn cations and UV light, taking place in the chloroplast thylakoids, may regulate the protein binding to RC, modify structural organization of the protein, and promote its participation in alternative pathways of electron transport under the influence of stress factors. The hypothetical scheme of interaction of the immobilized PsbO protein with Mn ions and RC is discussed.