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The PsbP protein is an extrinsic subunit of photosystem II (PSII) that is essential for photoautotrophic growth in higher plants. Several crystal structures of PsbP have been reported, but the binding topology of PsbP in PSII has not yet been clarified. In this study, we report that the basic pocket of PsbP, which consists of conserved Arg48, Lys143, and Lys160, is important for the electrostatic interaction with the PSII complex. Our release-reconstitution experiment showed that the binding affinities of PsbP-R48A, -K143A, and -K160A mutated proteins to PSII were lower than that of PsbP-WT, and triple mutations of these residues greatly diminished the binding affinity to PSII. Even when maximum possible binding had occurred, the R48A, K143A, and K160A proteins showed a reduced ability to restore the rate of oxygen evolution at low chloride concentrations. Fourier transform infrared resonance (FTIR) difference spectroscopy results were consistent with the above finding, and suggested that these mutated proteins were not able to induce the normal conformational change around the Mn cluster during S1 to S2 transition. Finally, chemical cross-linking experiments suggested that the interaction between the N-terminus of PsbP with PsbE was inhibited by these mutations. These data suggest that the basic pocket of PsbP is important for proper association and interaction with PSII. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy., (Copyright © 2013 Elsevier B.V. All rights reserved.)

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 Ca(2+) and Cl(-). PsbP induces conformational changes around the catalytic Mn cluster required for Ca(2+) 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 S(1) to S(2) 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 © 2012 Elsevier B.V. All rights reserved.)

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 Q(A) 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, S(2)/Q(A)(-). 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.