Electron crystallographic study of photosystem II of the cyanobacterium Synechococcus elongatus

Identification and Roles of Photosystem II Assembly, Stability, and Repair Factors in Arabidopsis

Photosystem II (PSII) is a multi-component pigment-protein complex that is responsible for water splitting, oxygen evolution, and plastoquinone reduction. Components of PSII can be classified into core proteins, low-molecular-mass proteins, extrinsic oxygen-evolving complex (OEC) proteins, and light-harvesting complex II proteins. In addition to these PSII subunits, more than 60 auxiliary proteins, enzymes, or components of thylakoid protein trafficking/targeting systems have been discovered to be directly or indirectly involved in de novo assembly and/or the repair and reassembly cycle of PSII. For example, components of thylakoid-protein-targeting complexes and the chloroplast-vesicle-transport system were found to deliver PSII subunits to thylakoid membranes. Various auxiliary proteins, such as PsbP-like (Psb stands for PSII) and light-harvesting complex-like proteins, atypical short-chain dehydrogenase/reductase family proteins, and tetratricopeptide repeat proteins, were discovered to assist the de novo assembly and stability of PSII and the repair and reassembly cycle of PSII. Furthermore, a series of enzymes were discovered to catalyze important enzymatic steps, such as C-terminal processing of the D1 protein, thiol/disulfide-modulation, peptidylprolyl isomerization, phosphorylation and dephosphorylation of PSII core and antenna proteins, and degradation of photodamaged PSII proteins. This review focuses on the current knowledge of the identities and molecular functions of different types of proteins that influence the assembly, stability, and repair of PSII in the higher plant Arabidopsis thaliana.

Sparse and incomplete factorial matrices to screen membrane protein 2D crystallization

Electron crystallography is well suited for studying the structure of membrane proteins in their native lipid bilayer environment. This technique relies on electron cryomicroscopy of two-dimensional (2D) crystals, grown generally by reconstitution of purified membrane proteins into proteoliposomes under conditions favoring the formation of well-ordered lattices. Growing these crystals presents one of the major hurdles in the application of this technique. To identify conditions favoring crystallization a wide range of factors that can lead to a vast matrix of possible reagent combinations must be screened. However, in 2D crystallization these factors have traditionally been surveyed in a relatively limited fashion. To address this problem we carried out a detailed analysis of published 2D crystallization conditions for 12 β-barrel and 138 α-helical membrane proteins. From this analysis we identified the most successful conditions and applied them in the design of new sparse and incomplete factorial matrices to screen membrane protein 2D crystallization. Using these matrices we have run 19 crystallization screens for 16 different membrane proteins totaling over 1300 individual crystallization conditions. Six membrane proteins have yielded diffracting 2D crystals suitable for structure determination, indicating that these new matrices show promise to accelerate the success rate of membrane protein 2D crystallization.

Supramolecular organization of thylakoid membrane proteins in green plants

The light reactions of photosynthesis in green plants are mediated by four large protein complexes, embedded in the thylakoid membrane of the chloroplast. Photosystem I (PSI) and Photosystem II (PSII) are both organized into large supercomplexes with variable amounts of membrane-bound peripheral antenna complexes. PSI consists of a monomeric core complex with single copies of four different LHCI proteins and has binding sites for additional LHCI and/or LHCII complexes. PSII supercomplexes are dimeric and contain usually two to four copies of trimeric LHCII complexes. These supercomplexes have a further tendency to associate into megacomplexes or into crystalline domains, of which several types have been characterized. Together with the specific lipid composition, the structural features of the main protein complexes of the thylakoid membranes form the main trigger for the segregation of PSII and LHCII from PSI and ATPase into stacked grana membranes. We suggest that the margins, the strongly folded regions of the membranes that connect the grana, are essentially protein-free, and that protein-protein interactions in the lumen also determine the shape of the grana. We also discuss which mechanisms determine the stacking of the thylakoid membranes and how the supramolecular organization of the pigment-protein complexes in the thylakoid membrane and their flexibility may play roles in various regulatory mechanisms of green plant photosynthesis.

Structural differences in the inner part of photosystem II between higher plants and cyanobacteria

A detailed comparison of key components in the Photosystem II complexes of higher plants and cyanobacteria was carried out. While the two complexes are overall very similar, significant differences exist in the relative orientation of individual components relative to one another. We compared a three-dimensional map of the inner part of plant PS II at 8 A resolution, and a 5.5 A projection map of the same complex determined by electron crystallography, to the recent 3.5-3.8 A X-ray structures of cyanobacterial complexes. The largest differences were found in the rotational alignment of the cyt b(;)559 subcomplex, and of the CP47 core antenna with respect to the D1/D2 reaction centre. Within the D1/D2 proteins, there are clear differences between plants and cyanobacteria at the stromal ends of membrane-spanning helices, even though these proteins are highly homologous. Notwithstanding these differences in the protein scaffold, the distances between the critical photosynthetic pigment cofactors seem to be precisely conserved. The different protein arrangements in the two complexes may reflect an adaptation to the two very different antenna systems, membrane-extrinsic phycobilisomes for cyanobacteria, and membrane-embedded chlorophyll a/b proteins in plants.

The low molecular mass subunits of the photosynthetic supracomplex, photosystem II

The photosystem II (PSII) complex is located in the thylakoid membrane of higher plants, algae and cyanobacteria and drives the water oxidation process of photosynthesis, which splits water into reducing equivalents and molecular oxygen by solar energy. Electron and X-ray crystallography analyses have revealed that the PSII core complex contains between 34 and 36 transmembrane alpha-helices, depending on the organism. Of these helices at least 12-14 are attributed to low molecular mass proteins. However, to date, at least 18 low molecular mass (<10 kDa) subunits are putatively associated with the PSII complex. Most of them contain a single transmembrane span and their protein sequences are conserved among photosynthetic organisms. In addition, these proteins do not have any similarity to any known functional proteins in any type of organism, and only two of them bind a cofactor. These findings raise intriguing questions about why there are so many small protein subunits with single-transmembrane spans in the PSII complex, and their possible functions. This article reviews our current knowledge of this group of proteins. Deletion mutations of the low molecular mass subunits from both prokaryotic and eukaryotic model systems are compared in an attempt to understand the function of these proteins. From these comparisons it seems that the majority of them are involved in stabilization, assembly or dimerization of the PSII complex. The small proteins may facilitate fast dynamic conformational changes that the PSII complex needs to perform an optimal photosynthetic activity.

Interaction of the allophycocyanin core complex with photosystem II

Allophycocyanin core complexes were purified from the thermophilic cyanobacterium Thermosynechococcus elongatus and analysed by negative-stain electron microscopy and single-particle averaging. The purified complex was found to consist of three allophycocyanin cylinders. The single-particle analysis of end-on views of the complex revealed a mirror axis, indicative of two-fold symmetry. This observation allowed the assignment of the allophycocyanin base cylinders and the identification of their potential interaction sites with the thylakoid membrane and with the photosystem II reaction centre in particular. The T. elongatus allophycocyanin core projection map, together with published information on the structure of photosystem II for the same organism, was used to construct a model for the allophycocyanin core-photosystem II dimer supercomplex, from which docking sites between both complexes are suggested. The implications of this association for energy transfer from allophycocyanin to photosystem II are discussed.

Photosystem II: a multisubunit membrane protein that oxidises water

A structure of photosystem II recently determined by X-ray crystallography at 3.8 A resolution complements structural studies using high-resolution electron microscopy and represents a major step towards understanding how photosynthetic organisms use light energy to oxidise water.