In this review we examine the structure and function of the extrinsic proteins of Photosystem II. These proteins include PsbO, present in all oxygenic organisms, the PsbP and PsbQ proteins, which are found in higher plants and eukaryotic algae, and the PsbU, PsbV, CyanoQ, and CyanoP proteins, which are found in the cyanobacteria. These proteins serve to optimize oxygen evolution at physiological calcium and chloride concentrations. They also shield the Mn(4)CaO(5) cluster from exogenous reductants. Numerous biochemical, genetic and structural studies have been used to probe the structure and function of these proteins within the photosystem. We will discuss the most recent proposed functional roles for these components, their structures (as deduced from biochemical and X-ray crystallographic studies) and the locations of their proposed binding domains within the Photosystem II complex. This article is part of a Special Issue entitled: Photosystem II.

Recent investigations have provided important new insights into the structures and functions of the extrinsic proteins of Photosystem II. This review is an update of the last major review on the extrinsic proteins of Photosystem II (Bricker et al., Biochemistry 31:4623-4628 2012). In this report, we will examine advances in our understanding of the structure and function of these components. These proteins include PsbO, which is uniformly present in all oxygenic organisms, the PsbU, PsbV, CyanoQ, and CyanoP proteins, found in the cyanobacteria, and the PsbP, PsbQ and PsbR proteins, found in the green plant lineage. These proteins serve to stabilize the Mn4CaO5 cluster and optimize oxygen evolution at physiological calcium and chloride concentrations. The mechanisms used to perform these functions, however, remain poorly understood. Recently, important new findings have significantly advanced our understanding of the structures, locations and functions of these important subunits. We will discuss the biochemical, structural and genetic studies that have been used to elucidate the roles played by these proteins within the photosystem and their locations within the photosynthetic complex. Additionally, we will examine open questions needing to be addressed to provide a coherent picture of the role of these components within the photosystem.

Interfering RNA was used to suppress the expression of two genes that encode the manganese-stabilizing protein of photosystem II in Arabidopsis thaliana, MSP-1 (encoded by psbO-1, At5g66570), and MSP-2 (encoded by psbO-2, At3g50820). A phenotypic series of transgenic plants was recovered that expressed high, intermediate, and low amounts of these two manganese-stabilizing proteins. Chlorophyll fluorescence induction and decay analyses were performed. Decreasing amounts of expressed protein led to the progressive loss of variable fluorescence and a marked decrease in the fluorescence quantum yield (F v /F m ) in both the absence and the presence of dichloromethylurea. This result indicated that the amount of functional photosystem II reaction centers was compromised in the plants that exhibited intermediate and low amounts of the manganese-stabilizing proteins. An analysis of the decay of the variable fluorescence in the presence of dichlorophenyldimethylurea indicated that charge recombination between Q A ؊ and the S 2 state of the oxygen-evolving complex was seriously retarded in the plants that expressed low amounts of the manganesestabilizing proteins. This may have indicated a stabilization of the S 2 state in the absence of the extrinsic component. Immunological analysis of the photosystem II protein complement indicated that significant losses of the CP47, CP43, and D1 proteins occurred upon the loss of the manganese-stabilizing proteins. This indicated that these extrinsic proteins were required for photosystem II core assembly/stability. Additionally, although the quantity of the 24-kDa extrinsic protein was only modestly affected by the loss of the manganese-stabilizing proteins, the 17-kDa extrinsic protein dramatically decreased. The control proteins ribulose bisphosphate carboxylase and cytochrome f were not affected by the loss of the manganese-stabilizing proteins; the photosystem I PsaB protein, however, was significantly reduced in the low expressing transgenic plants. Finally, it was determined that the transgenic plants that expressed low amounts of the manganese-stabilizing proteins could not grow photoautotrophically.In higher plants and cyanobacteria, at least six intrinsic proteins appear to be required for oxygen evolution by photosystem II (PS II) 1 (1-3). These are CP 47, CP 43, the D1 and D2proteins, and the ␣ and ␤ subunits of cytochrome b 559 . Insertional inactivation or deletion of the genes for these components results in the complete loss of oxygen evolution activity. Additionally, a number of low molecular mass, intrinsic membrane protein components are associated with PS II (4 -6), although the functions of many of these proteins remain obscure. Although PS II complexes containing only these intrinsic components can evolve oxygen, they do so at low rates (about 25-40% of control), are extremely susceptible to photoinactivation, and require high, non-physiological levels of calcium and chloride for maximal activity (1, 3). In higher plants, three extrinsic proteins, with apparent...

The Photosystem II reaction center is vulnerable to photoinhibition. The D1 and D2 proteins, lying at the core of the photosystem, are susceptible to oxidative modification by reactive oxygen species that are formed by the photosystem during illumination. Using spin probes and EPR spectroscopy, we have determined that both O 2 . The identification of specific amino acid residues oxidized by reactive oxygen species provides insights into the mechanism of damage to the D1 and D2 proteins under light stress.photosynthesis | Photosystem II | reactive oxygen species | photo inhibition | mass spectrometry P hotosystem II (PSII) functions as a water-plastoquinone oxidoreductase (1, 2) and is a thylakoid membrane pigmentprotein complex present in all oxygenic photosynthetic organisms (cyanobacteria, algae, and higher plants). Current high-resolution structures of thermophilic cyanobacterial PSII (3, 4), and lower resolution structures of the red algal (5) and higher plant photosystems (6), have been critically important in furthering our understanding of the molecular organization of PSII. Structurally, the PSII reaction center core is composed of five proteins: D1, D2, the α-and β-subunits of cytochrome b 559 , and PsbI. These components bind all of the redox-active cofactors of PSII.Excitation energy transfer and electron transport within PSII are unavoidably associated with production of reactive oxygen species (ROS) when the absorption of light by the chlorophyll antenna exceeds the capacity for energy utilization. Many mechanisms for ROS production have been proposed (for reviews, see refs. 7 and 8). Briefly, singlet oxygen ( 1 O 2 ) may be formed by excitation energy transfer from triplet chlorophylls (formed either by the change in orientation of the spin of an excited electron in the PSII antenna complex, or via charge recombination of the primary radical pair 3 [P 680 •+ Pheo •− ]) to O 2 (9, 10). ROS production by electron transport involves either the two-electron oxidation of water or the one-electron reduction of O 2 on the PSII electron donor and acceptor sides, respectively. On the PSII electron donor side, a twoelectron oxidation of water leads to the formation of hydrogen peroxide (H 2 O 2 ), which may be oxidized to the superoxide anion radical (O 2•−

Interfering RNA was used to suppress the expression of the genes At1g06680 and At2g30790 in Arabidopsis thaliana, which encode the PsbP-1 and PsbP-2 proteins, respectively, of photosystem II (PS II In higher plants, algae, and cyanobacteria, at least six intrinsic proteins appear to be required for oxygen evolution by PS II 2 (1-3). These are CP47, CP43, the D1 and D2 proteins, and the ␣ and ␤ subunits of cytochrome b 559 . Insertional inactivation or deletion of the genes for these components results in the disassembly of the PS II complex and the complete loss of oxygen evolution activity (for review, see Ref. 4). Additionally, a number of low molecular mass, intrinsic membrane protein components are associated with PS II (5-7), although the functions of many of these proteins remain obscure. Although PS II complexes containing only these intrinsic components can evolve oxygen in vitro, they do so at low rates (ϳ25-40% of control), are extremely susceptible to photoinactivation, and require high, nonphysiological levels of calcium and chloride for maximal activity (1, 3).)In higher plants and green algae, three extrinsic proteins, with apparent molecular masses of 33, 24, and 17 kDa, are required for high rates of oxygen evolution at physiological inorganic cofactor concentrations. The 33-kDa component, the PsbO protein, has been termed the manganese-stabilizing protein due to its stabilization of the manganese cluster during exposure to low chloride concentrations or to exogenous reductants. In vitro, the 24-and 17-kDa proteins (termed the PsbP and PsbQ proteins, respectively) appear to modulate the calcium and chloride requirements for efficient oxygen evolution. The precise roles of these proteins in oxygen evolution and PS II assembly/stability in vivo, however, remain unclear. These three extrinsic components interact with intrinsic membrane proteins and possibly with each other to yield fully functional oxygen-evolving complexes.The mature PsbP protein is highly conserved (8) in higher plants. In Arabidopsis, there are two putative genes, At1g06680 and At2g30790, which encode PsbP-1 and PsbP-2, respectively. It should be noted that initially only PsbP-1 was observed in Arabidopsis (9, 10) using two-dimensional IEF-SDS-PAGE. Recently, however, PsbP-2 has been detected during two-dimensional difference gel electrophoresis (11). In the cyanobacterium Synechocystis 6803, mutants in which the homologue of the psbP gene had been deleted exhibited reduced photoautotrophic growth as well as decreased water oxidation activity under CaCl 2 -limiting conditions (7, 12), whereas in Chlamydomonas, a mutant which did not accumulate PsbP was deficient in photoactivation (13).RNAi is a post-transcriptional gene-silencing process in which double-stranded RNA induces the degradation of homologous mRNA sequences (14). RNAi has been successfully applied as a powerful gene-silencing tool in a variety of organisms, including Caenorhabditis elegans and Drosophila melanogaster, and in mouse oocytes. It has also become a pop-* T...