A theoretical study on the dynamics of light harvesting in the dimeric photosystem II core complex: regulation and robustness of energy transfer pathways

Coarse-grained model for dimeric PSII core complex reveals robust light harvesting through inter-monomer energy transfer and pooling in CP47s.

The structure, functions and degradation of pigment-binding proteins of photosystem II.

Eleven proteins belonging to photosystem II (PSII) bind photosynthetic pigments in the form of thylakoid membrane-associated pigment-protein complexes. Five of them (PsbA, PsbB, PsbC, PsbD and PsbS) are assigned to PSII core complex while the remaining six (Lhcb1, Lhcb2, Lhcb3, Lhcb4, Lhcb5 and Lhcb6) constitute, along with their pigments, functional complexes situated more distantly with regard to P680 - the photochemical center of PSII. The main function of the pigment-binding proteins is to harvest solar energy and deliver it, in the form of excitation energy, ultimately to P680 although individual pigment-proteins may be engaged in other photosynthesis-related processes as well. The aim of this review is to present the current state of knowledge regarding the structure, functions and degradation of this family of proteins.

Cyanobacterial Small Chlorophyll-binding Protein ScpD (HliB) Is Located on the Periphery of Photosystem II in the Vicinity of PsbH and CP47 Subunits

Cyanobacteria contain several genes coding for small one-helix proteins called SCPs or HLIPs with significant sequence similarity to chlorophyll a/b-binding proteins. To localize one of these proteins, ScpD, in the cells of the cyanobacterium Synechocystis sp. PCC 6803, we constructed several mutants in which ScpD was expressed as a His-tagged protein (ScpDHis). Using two-dimensional native-SDS electrophoresis of thylakoid membranes or isolated Photosystem II (PSII), we determined that after high-light treatment most of the ScpDHis protein in a cell is associated with PSII. The ScpDHis protein was present in both monomeric and dimeric PSII core complexes and also in the core subcomplex lacking CP43. However, the association with PSII was abolished in the mutant lacking the PSII subunit PsbH. In a PSII mutant lacking cytochrome b559, which does not accumulate PSII, ScpDHis is associated with CP47. The interaction of ScpDHis with PsbH and CP47 was further confirmed by electron microscopy of PSII labeled with Ni-NTA Nanogold. Single particle image analysis identified the location of the labeled ScpDHis at the periphery of the PSII core complex in the vicinity of the PsbH and CP47. Because of the fact that ScpDHis did not form any large structures bound to PSII and because of its accumulation in PSII subcomplexes containing CP47 and PsbH we suggest that ScpD is involved in a process of PSII assembly/repair during the turnover of pigment-binding proteins, particularly CP47.

Structural and functional role of the PsbH protein in resistance to light stress in Synechocystis PCC 6803

Four mutants of the cyanobacterium Synechocystis sp. PCC 6803, carrying a modified PsbH subunit on a PSI-less background, were characterized by optically-detected magnetic resonance (ODMR), electron transport kinetics, and oxygen-evolving activity. Their relative tolerance to light stress was measured. Results indicate that: (i) the PsbH protein is deeply involved in determining structural and functional properties of the QB site on the D1 protein, whereas the environment of the primary donor P680 and its acceptors pheophytin and QA are not significantly affected by modifications of this subunit or its deletion; (ii) the charge recombination rate, in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), is reduced by a factor of 2, independently of the particular modification. The same result is found with the strain in which the subunit has been deleted. This result is taken as an indication that PsbH is important in regulating protein dynamics of the entire PSII core complex; (iii)�all investigated mutants display reduced tolerance to light stress, the extent of which depends on the particular modification. In this respect, mutations introduced in the transmembrane portion of the polypeptide are more effective than those involving the extramembrane N-terminal extension.

Effect of MgCl2 and Phosphatidylglycerol on CaCl2-Mediated Recovery of Oxygen Evolution in a Photosystem II Complex Depleted of the 17 and 24 kDa Extrinsic Proteins

Phosphatidylglycerol (PG) is an anionic lipid of the thylakoid membrane of higher plant chloroplasts. PG was shown previously to stimulate the evolution of oxygen in intact pho­tosystem II (PSII) membranes [Fragata, M., Strzałka, K. and Nénonéné, E. K. (1991) J. Photochem. Photobiol. B: Biol 11, 329-342], In this work, a study was undertaken of the effect of MgCl2 and PG on the CaCl2-mediated recovery of oxygen evolution in a PSII complex depleted of the extrinsic proteins (EP) of molecular masses 17 kDa (EP17) and 24 kDa (EP24), hereunder designated d17.24PSII. This molecular system is structurally close to the PSII core complex of cyanobacteria and is therefore useful in the comparative analysis of PSII-PG relationships in cyanobacteria and the higher plants. This work reveals a new aspect of the thylakoid lipids role in the PSII function, namely the PG effect on intact PSII is observed as well in d17.24PSII. The results show that phosphatidylglycerol has the ability to compensate for the loss of EP17 and EP24 in the PSII complex. That is, PG restores the oxygen evolution in d17.24PSII incubated in the presence of MgCl2 and/or CaCl2 to the levels observed in native PSII. Moreover, the site of H2O degradation in d17.24PSII, including most probably the pool of calcium and chloride ions, would seem to be protected by phosphatidyl­glycerol. This suggests that one of the docking sites of PG in the PSII complex is near EP24, inasmuch as this extrinsic protein participates in the regulation of the affinity of the calcium and chloride ions to the water oxidation site. Furthermore, taking into account that in d17.24PSII the PSII core complex is directly exposed to PG, then the phospholipid effect reported here indicates that phosphatidylglycerol might be a functional effector and mem­brane anchor of the D1 protein in the PSII core complex as was shown recently in the cyanobacterium Oscillatoria chalybea [Kruse, O. and Schmid, G. H. (1995) Z. Naturforsch. 50c, 380-390],

Evidence for two-step processing of nuclear-encoded chloroplast proteins during membrane assembly.

A plastome (chloroplast genome) mutant of tobacco, lutescens-1, displays abnormal degradation of the chloroplast-encoded polypeptides which form the core complex of photosystem II (PSII). Two nuclear-encoded proteins (present in polymorphic forms), which normally function in the water oxidation process of PSII, accumulate as larger size-class polypeptides in mutant thylakoid membranes. These accumulated proteins are intermediate in size between the full-length primary protein synthesized in the cytoplasm and the proteolytically processed mature polypeptides. Trypsin treatment of unstacked mutant thylakoids and of inside-out vesicle (PSII-enriched) preparations indicated that the intermediate size forms were correctly localized on the inner surface of the thylakoid membrane, but not surface-exposed in the same way as the mature proteins. Only one of the intermediate size-class proteins could be extracted by salt washes. We interpret these data to be consistent with the idea that the two imported proteins that function in the water oxidation step of photosynthesis and are localized in the loculus (the space within the thylakoid vesicles) undergo two-step processing. The second step in proteolytic processing may be related to transport through a second membrane (the first transport step through the chloroplast envelope having been completed); this step may be arrested in the mutant due to the absence of the PSII core complex.