Plant and Algal PSII–LHCII Supercomplexes: Structure, Evolution and Energy Transfer

Photosynthesis is the process conducted by plants and algae to capture photons and store their energy into a chemical form. The light-harvesting, excitation transfer, charge separation, and electron transfer in photosystem II (PSII) are the critical initial reactions of photosynthesis and thereby largely determine its overall efficiency. In this review, we outline the rapidly accumulating knowledges about the architectures and assemblies of plant and green algal PSII–light harvesting complex II (LHCII) supercomplexes with a particular focus on new insights provided by the recent high-resolution cryo-electron microscopy (cryo-EM) map of the supercomplexes from a green alga Chlamydomonas reinhardtii. We make pair-wise comparative analyses between the supercomplexes from plants and green algae to gain insights about the evolution of the PSII–LHCII supercomplexes involving the peripheral small PSII subunits that might have been acquired during the evolution, and about the energy transfer pathways that define their light-harvesting and photoprotective properties.

A key role for phosphorylation of PsbH in the biogenesis and repair of photosystem II in Chlamydomonas

ABSTRACTPhosphorylation of the core subunits of photosystem II (PSII) is largely governed by a protein kinase and an antagonistic protein phosphatase. In plants the respective mutants show alterations in the architecture of thylakoid membranes and in the repair of PSII after photo-inhibition. However the protein kinase targets several subunits of PSII, as well as other proteins. To specifically investigate the role of phosphorylation of the different PSII subunits, we used site-directed mutagenesis and chloroplast transformation in Chlamydomonas reinhardtii. Major, evolutionarily-conserved sites of phosphorylation in three components of PSII (CP43, D2 and PsbH) were mutated to replace the corresponding serine or threonine residues with alanine. The alanine substitution mutant of D2 had no apparent phenotype, while the mutant of CP43 presented a minor delay in recovery from photo-inhibition. Alanine substitutions of the phosphorylation sites in PsbH had significant effects on the accumulation of PSII or on its recovery from photo-inhibition. When mutations in two of the target subunits were combined through a second cycle of chloroplast transformation, the strongest phenotype was observed in the mutant lacking phosphorylation of both PsbH and CP43, which showed delayed recovery from photo-inhibition. Surprisingly this phenotype was reversed in the mutant defective for phosphorylation of all three subunits. Our analysis indicates a prominent role for the N-terminus of PsbH in the stable accumulation of PSII and of PsbH phosphorylation in its repair cycle.SIGNIFICANCE STATEMENTTo specifically investigate the role of PSII phosphorylation, alanine-substitution mutants of the major phospho-sites in the subunits of PSII were generated individually or in combinations using chloroplast transformation. PSII assembly was defective in some of the PsbH mutants. PSII repair after photo-inhibition was delayed most strongly in the mutant lacking phosphorylation of both PsbC (CP43) and PsbH.

Structure of Psb29/Thf1 and its association with the FtsH protease complex involved in photosystem II repair in cyanobacteria

One strategy for enhancing photosynthesis in crop plants is to improve their ability to repair photosystem II (PSII) in response to irreversible damage by light. Despite the pivotal role of thylakoid-embedded FtsH protease complexes in the selective degradation of PSII subunits during repair, little is known about the factors involved in regulating FtsH expression. Here we show using the cyanobacterium Synechocystis sp. PCC 6803 that the Psb29 subunit, originally identified as a minor component of His-tagged PSII preparations, physically interacts with FtsH complexes in vivo and is required for normal accumulation of the FtsH2/FtsH3 hetero-oligomeric complex involved in PSII repair. We show using X-ray crystallography that Psb29 from Thermosynechococcus elongatus has a unique fold consisting of a helical bundle and an extended C-terminal helix and contains a highly conserved region that might be involved in binding to FtsH. A similar interaction is likely to occur in Arabidopsis chloroplasts between the Psb29 homologue, termed THF1, and the FTSH2/FTSH5 complex. The direct involvement of Psb29/THF1 in FtsH accumulation helps explain why THF1 is a target during the hypersensitive response in plants induced by pathogen infection. Downregulating FtsH function and the PSII repair cycle via THF1 would contribute to the production of reactive oxygen species, the loss of chloroplast function and cell death. This article is part of the themed issue ‘Enhancing photosynthesis in crop plants: targets for improvement’.

An Activated Glutamate Residue Identified in Photosystem II at the Interface between the Manganese-stabilizing Subunit and the D2 Polypeptide

Photosystem II (PSII) catalyzes the oxidation of water during oxygenic photosynthesis. PSII is composed both of intrinsic subunits, such as D1, D2, and CP47, and extrinsic subunits, such as the manganese-stabilizing subunit (MSP). Previous work has shown that amines covalently bind to amino acid residues in the CP47, D1, and D2 subunits of plant and cyanobacterial PSII, and that these covalent reactions are prevented by the addition of chloride in plant preparations depleted of the 18- and 24-kDa extrinsic subunits. It has been proposed that these reactive groups are carbonyl-containing, post-translationally modified amino acid side chains (Ouellette, A. J. A., Anderson, L. B., and Barry, B. A. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 2204–2209 and Anderson, L. B., Ouellette, A. J. A., and Barry, B. A. (2000) J. Biol. Chem. 275, 4920–4927). To identify the amino acid binding site in the spinach D2 subunit, we have employed a biotin-amine labeling reagent, which can be used in conjunction with avidin affinity chromatography to purify biotinylated peptides from the PSII complex. Multidimensional chromato-graphic separation and multistage mass spectrometry localizes a novel post-translational modification in the D2 subunit to glutamate 303. We propose that this glutamate is activated for amine reaction by post-translational modification. Because the modified glutamate is located at a contact site between the D2 and manganese-stabilizing subunits, we suggest that the modification is important in vivo in stabilizing the interaction between these two PSII subunits. Consistent with this conclusion, mutations at the modified glutamate alter the steady-state rate of photosynthetic oxygen evolution.

Biogenesis, assembly and turnover of photosystem II units

Assembly of photosystem II, a multiprotein complex embedded in the thylakoid membrane, requires stoichiometric production of over 20 protein subunits. Since part of the protein subunits are encoded in the chloroplast genome and part in the nucleus, a signalling network operates between the two genetic compartments in order to prevent wasteful production of proteins. Coordinated synthesis of proteins also takes place among the chloroplast–encoded subunits, thus establishing a hierarchy in the protein components that allows a stepwise building of the complex. In addition to this dependence on assembly partners, other factors such as the developmental stage of the plastid and various photosynthesis–related parameters exert a strict control on the accumulation, membrane targeting and assembly of the PSII subunits. Here, we briefly review recent results on this field obtained with three major approaches: biogenesis of photosystem II during the development of chloroplasts from etioplasts, use of photosystem II–specific mutants and photosystem II turnover during its repair cycle.

Posttranslational events leading to the assembly of photosystem II protein complex: a study using photosynthesis mutants from Chlamydomonas reinhardtii.

We studied the assembly of photosystem II (PSII) in several mutants from Chlamydomonas reinhardtii which were unable to synthesize either one PSII core subunit (P6 [43 kD], D1, or D2) or one oxygen-evolving enhancer (OEE1 or OEE2) subunit. Synthesis of the PSII subunits was analyzed on electrophoretograms of cells pulse labeled with [14C]acetate. Their accumulation in thylakoid membranes was studied on immunoblots, their chlorophyll-binding ability on nondenaturating gels, their assembly by detergent fractionation, their stability by pulse-chase experiments and determination of in vitro protease sensitivity, and their localization by immunocytochemistry. In Chlamydomonas, the PSII core subunits P5 (47 kD), D1, and D2 are synthesized in a concerted manner while P6 synthesis is independent. P5 and P6 accumulate independently of each other in the stacked membranes. They bind chlorophyll soon after, or concomitantly with, their synthesis and independently of the presence of the other PSII subunits. Resistance to degradation increases step by step: beginning with assembly of P5, D1, and D2, then with binding of P6, and, finally, with binding of the OEE subunits on two independent high affinity sites (one for OEE1 and another for OEE2 to which OEE3 binds). In the absence of PSII cores, the OEE subunits accumulate independently in the thylakoid lumen and bind loosely to the membranes; OEE1 was found on stacked membranes, but OEE2 was found on either stacked or unstacked membranes depending on whether or not P6 was synthesized.