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The PsbP protein regulates the binding properties of Ca2+ and Cl-, and stabilizes the Mn cluster of photosystem II (PSII); however, the binding site and topology in PSII have yet to be clarified. Here we report that the structure around His-144 and Asp-165 in PsbP, which is suggested to be a metal binding site, has a crucial role for the functional interaction between PsbP and PSII. The mutated PsbP-H144A protein exhibits reduced ability to retain Cl- anions in PSII, whereas the D165V mutation does not affect PsbP function. Interestingly, H144A/D165V double mutation suppresses the effect of H144A mutation, suggesting that these residues have a role other than metal binding. FTIR difference spectroscopy suggests that H144A/D165V restores proper interaction with PSII and induces the conformational change around the Mn cluster during the S1/S2 transition. Cross-linking experiments show that the H144A mutation affects the direct interaction between PsbP and the Cyt b559 α subunit of PSII (the PsbE protein). However, this interaction is restored in the H144A/D165V mutant. In the PsbP structure, His- 144 and Asp-165 form a salt bridge. H144A mutation is likely to disrupt this bridge and liberate Asp-165, inhibiting the proper PsbP-PSII interaction. Finally, mass spectrometric analysis has identified the cross-linked sites of PsbP and PsbE as Ala-1 and Glu- 57, respectively. Therefore His-144, in the C-terminal domain of PsbP, plays a crucial role in maintaining proper N terminus interaction. These data provide important information about the binding characteristics of PsbP in green plant PSII. [ABSTRACT FROM AUTHOR]

Abstract: The redox potential of QA in Photosystem II (PSII) from Thermosynechococcus elongatus was titrated monitoring chlorophyll fluorescence. A high potential form (E m =+60±25mV) was found in the absence of Mn4Ca, the active site for water oxidation. The low potential form (E m =−60±48mV), which is difficult to measure in conventional titration experiments, could be “locked in” by cross-linking the active enzyme. This indicates that the presence of Mn4Ca is relayed to the quinone site by significant structural changes in the protein. The presence of high and low potential forms agrees with what has been seen in plants, algae from our lab and in T. elongatus (Shibamoto et al., Biochemistry 48 (2009) 10682–10684). In the latter work, the potentials of QA were shifted to lower potentials compared to other measurements. The redox potential of QA in Mn-depleted PSII from spinach was titrated in the presence of redox mediators and the midpoint potential was shifted by 80mV towards a more negative value compared to titrations without mediators. The lower values of the midpoint potential of the () redox couple in the literature could be due to a perturbation due to a specific mediator. [Copyright &y& Elsevier]

Abstract: The PsbP protein is an extrinsic subunit of photosystem II (PSII) specifically found in land plants and green algae. Using PsbP-RNAi tobacco, we have investigated effects of PsbP knockdown on protein supercomplex organization within the thylakoid membranes and photosynthetic properties of PSII. In PsbP-RNAi leaves, PSII dimers binding the extrinsic PsbO protein could be formed, while the light-harvesting complex II (LHCII)–PSII supercomplexes were severely decreased. Furthermore, LHCII and major PSII subunits were significantly dephosphorylated. Electron microscopic analysis showed that thylakoid grana stacking in PsbP-RNAi chloroplast was largely disordered and appeared similar to the stromally-exposed or marginal regions of wild-type thylakoids. Knockdown of PsbP modified both the donor and acceptor sides of PSII; In addition to the lower water-splitting activity, the primary quinone QA in PSII was significantly reduced even when the photosystem I reaction center (P700) was noticeably oxidized, and thermoluminescence studies suggested the stabilization of the charged pair, S2/QA −. These data indicate that assembly and/or maintenance of the functional MnCa cluster is perturbed in absence of PsbP, which impairs accumulation of final active forms of PSII supercomplexes. [Copyright &y& Elsevier]

Abstract: PsbP and PsbQ proteins are extrinsic subunits of photosystem II (PSII) and optimize the oxygen evolution reaction by regulating the binding properties of the essential cofactors Ca2+ and Cl−. PsbP induces conformational changes around the catalytic Mn cluster required for Ca2+ and Cl− retention, and the N-terminal region of PsbP is essential for this reaction. It was reported that PsbQ partially restores the functional defect of N-terminal truncated PsbP [Ifuku and Sato (2002) Plant Cell Physiol. 43, 1244–1249]; however, the mechanism of this restoration is yet to be clarified. In this study, we demonstrate that PsbQ is able to restore the functional binding of mutated PsbPs. In the presence of PsbQ, ∆15-PsbP, a truncated PsbP lacking 15 N-terminal residues, was able to specifically bind to NaCl-washed spinach PSII membranes and significantly restore the oxygen evolving activity. Furthermore, PsbQ was also able to compensate for the impaired ion-retention of H144A-PsbP, in which a conserved histidine at position 144 in the C-terminal domain was substituted with an alanine. Fourier transform infrared (FTIR) difference spectroscopy showed that PsbQ restored the ability of ∆15- and H144A-PsbP to induce proper conformational changes during S1 to S2 transition. These data suggest that the major function of PsbQ is to stabilize PsbP binding, thereby contributing to the maintenance of the catalytic Mn cluster of the water oxidation machinery in higher plant PSII. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial. [Copyright &y& Elsevier]