Stable assembly of PsaE into cyanobacterial photosynthetic membranes is dependent on the presence of other accessory subunits of photosystem I

Photosynthetic hydrogen production by a hybrid complex of photosystem I and [NiFe]-hydrogenase

Nature provides key components for generating fuels from renewable resources in the form of enzymatic nanomachines which catalyze crucial steps in biological energy conversion, for example, the photosynthetic apparatus, which transforms solar power into chemical energy, and hydrogenases, capable of generating molecular hydrogen. As sunlight is usually used to synthesize carbohydrates, direct generation of hydrogen from light represents an exception in nature. On the molecular level, the crucial step for conversion of solar energy into H(2) lies in the efficient electronic coupling of photosystem I and hydrogenase. Here we show the stepwise assembly of a hybrid complex consisting of photosystem I and hydrogenase on a solid gold surface. This device gave rise to light-induced H(2) evolution. Hydrogen production is possible at far higher potential and thus lower energy compared to those of previously described (bio)nanoelectronic devices that did not employ the photosynthesis apparatus. The successful demonstration of efficient solar-to-hydrogen conversion may serve as a blueprint for the establishment of this system in a living organism with the paramount advantage of self-replication.

Photoinduced hydrogen production by direct electron transfer from photosystem I cross-linked with cytochrome c3 to [NiFe]-hydrogenase

The photosynthetic reaction center is an efficient molecular device for the conversion of light energy to chemical energy. In a previous study, we synthesized the hydrogenase/photosystem I (PSI) complex, in which Ralstonia hydrogenase was linked to the cytoplasmic side of Synechocystis PSI, to modify PSI so that it photoproduced molecular hydrogen (H2). In that study, hydrogenase was fused with a PSI subunit, PsaE, and the resulting hydrogenase-PsaE fusion protein was self-assembled with PsaE-free PSI to give the hydrogenase/PSI complex. Although the hydrogenase/PSI complex served as a direct light-to-H2 conversion system in vitro, the activity was totally suppressed by adding physiological PSI partners, ferredoxin (Fd) and ferredoxin-NADP+-reductase (FNR). In the present study, to establish an H2 photoproduction system in which the activity is not interrupted by Fd and FNR, position 40 of PsaE from Synechocystis sp. PCC6803, corresponding to the Fd-binding site on PSI, was selected and targeted for the cross-linking with cytochrome c3 (cytc3) from Desulfovibrio vulgaris. The covalent adduct of cytc3 and PsaE was stoichiometrically assembled with PsaE-free PSI to form the cytc3/PSI complex. The NADPH production by the cytc3/PSI complex coupled with Fd and FNR decreased to approximately 20% of the original activity, whereas the H2 production by the cytc3/PSI complex coupled with hydrogenase from Desulfovibrio vulgaris was enhanced 7-fold. Consequently, in the simultaneous presence of hydrogenase, Fd, and FNR, the light-driven H2 production by the hydrogenase/cytc3/PSI complex was observed (0.30 pmol Hz/mg chlorophyll/h). These results suggest that the cytc3/PSI complex may produce H2 in vivo.

Light-driven hydrogen production by a hybrid complex of a [NiFe]-hydrogenase and the cyanobacterial photosystem I

In order to generate renewable and clean fuels, increasing efforts are focused on the exploitation of photosynthetic microorganisms for the production of molecular hydrogen from water and light. In this study we engineered a 'hard-wired' protein complex consisting of a hydrogenase and photosystem I (hydrogenase-PSI complex) as a direct light-to-hydrogen conversion system. The key component was an artificial fusion protein composed of the membrane-bound [NiFe] hydrogenase from the beta-proteobacterium Ralstonia eutropha H16 and the peripheral PSI subunit PsaE of the cyanobacterium Thermosynechococcus elongatus. The resulting hydrogenase-PsaE fusion protein associated with PsaE-free PSI spontaneously, thereby forming a hydrogenase-PSI complex as confirmed by sucrose-gradient ultracentrifuge and immunoblot analysis. The hydrogenase-PSI complex displayed light-driven hydrogen production at a rate of 0.58 mumol H(2).mg chlorophyll(-1).h(-1). The complex maintained its accessibility to the native electron acceptor ferredoxin. This study provides the first example of a light-driven enzymatic reaction by an artificial complex between a redox enzyme and photosystem I and represents an important step on the way to design a photosynthetic organism that efficiently converts solar energy and water into hydrogen.

Multiple functions for the C terminus of the PsaD subunit in the cyanobacterial photosystem I complex

PsaD subunit of Synechocystis sp PCC 6803 photosystem I (PSI) plays a critical role in the stability of the complex and is part of the docking site for ferredoxin (Fd). In the present study we describe major physiological and biochemical effects resulting from mutations in the accessible C-terminal end of the protein. Four basic residues were mutated: R111, K117, K131, and K135, and a large 36-amino acid deletion was generated at the C terminus. PSI from R111C mutant has a 5-fold decreased affinity for Fd, comparable with the effect of the C terminus deletion, and NADP+ is photoreduced with a 2-fold decreased rate, without consequence on cell growth. The K117A mutation has no effect on the affinity for Fd, but decreases the stability of PsaE subunit, a loss of stability also observed in R111C and the deletion mutants. The double mutation K131A/K135A does not change Fd binding and reduction, but decreases the overall stability of PSI and impairs the cell growth at temperatures above 30 degrees C. Three mutants, R111C, K117A, and the C-terminal deleted exhibit a higher content of the trimeric form of PSI, in apparent relation to the removal of solvent accessible positive charges. Various regions in the C terminus of cyanobacterial PsaD thus are involved in Fd strong binding, PSI stability, and accumulation of trimeric PSI.

Subcellular localization of the BtpA protein in the cyanobacterium Synechocystis sp. PCC 6803

Photosystem I is a large pigment-protein complex embedded in the thylakoid membranes of chloroplasts and cyanobacteria. In the cyanobacterium Synechocystis sp. PCC 6803, the btpA gene encodes a 30-kDa polypeptide. Mutations in this gene significantly affect accumulation of the reaction center proteins of photosystem I in Synechocystis 6803 [Bartsevich, V. V. & Pakrasi, H. B. (1997) J. Biol. Chem. 272, 6372-6378]. We describe here the intracellular localization of the BtpA protein. Immunolocalization in Synechocystis 6803 cells demonstrated that the BtpA protein is tightly associated with the thylakoid membranes. Phase fractionation in the detergent Triton X-114 indicated that BtpA is a peripheral membrane protein. To determine which surface of the thylakoid membrane BtpA is exposed to, we used a two-phase polymer partitioning technique to develop a novel method to isolate inside-out and right-side-out thylakoid vesicles from Synechocystis 6803. Treatments of such vesicles with different salts and protease showed that the BtpA protein is an extrinsic membrane protein which is exposed to the cytoplasmic face of the thylakoid membrane.

Structural features and assembly of the soluble overexpressed PsaD subunit of photosystem I

PsaD is a peripheral protein on the reducing side of photosystem I (PS I). We expressed the psaD gene from the thermophilic cyanobacterium Mastigocladus laminosus in Escherichia coli and obtained a soluble protein with a polyhistidine tag at the carboxyl terminus. The soluble PsaD protein was purified by Ni-affinity chromatography and had a mass of 16716 Da by MALDI-TOF. The N-terminal amino acid sequence of the overexpressed PsaD matched the N-terminal sequence of the native PsaD from M. laminosus. The soluble PsaD could assemble into the PsaD-less PS I. As determined by isothermal titration calorimetry, PsaD bound to PS I with 1.0 binding site per PS I, the binding constant of 7.7x10(6) M-1, and the enthalpy change of -93.6 kJ mol-1. This is the first time that the binding constant and binding heat have been determined in the assembly of any photosynthetic membrane protein. To identify the surface-exposed domains, purified PS I complexes and overexpressed PsaD were treated with N-hydroxysuccinimidobiotin (NHS-biotin) and biotin-maleimide, and the biotinylated residues were mapped. The Cys66, Lys21, Arg118 and Arg119 residues were exposed on the surface of soluble PsaD whereas the Lys129 and Lys131 residues were not exposed on the surface. Consistent with the X-ray crystallographic studies on PS I, circular dichroism spectroscopy revealed that PsaD contains a small proportion of alpha-helical conformation.

Topography of the photosystem I core proteins of the cyanobacterium Synechocystis sp. PCC 6803

PsaA and PsaB are homologous integral membrane proteins that form the heterodimeric core of photosystem I. Domain-specific antibodies were generated to examine the topography of PsaA and PsaB. The purified photosystem I complexes from the wild type strain of Synechocystis sp. PCC 6803 were treated with eight proteases to study the accessibility of cleavage sites in PsaA and PsaB. Proteolytic fragments were identified using the information from N-terminal amino acid sequencing, reactivity to antibodies, apparent mass, and specificity of proteases. The extramembrane loops of PsaA and PsaB differed in their accessibility to proteases, which indicated the folded structure of the loops or their shielding by the small subunits of photosystem I. NaI-treated and mutant photosystem I complexes were used to identify the extramembrane loops that were exposed in the absence of specific small subunits. The absence of PsaD exposed additional proteolytic sites in PsaB, whereas the absence of PsaE exposed sites in PsaA. These studies distinguish PsaA and PsaB in the structural model for photosystem I that has been proposed on the basis of x-ray diffraction studies (Krauss, N., Schubert, W.-D., Klukas, O., Fromme, P., Witt, H. T., and Saenger, W. (1996) Nat. Struct. Biol. 3, 965-973). Using osmotically shocked cells for protease treatments, the N terminus of PsaA was determined to be on the n side of the photosynthetic membranes. Based on these data and available published information, we propose a topological model for PsaA and PsaB.

The precursor of PsaD assembles into the photosystem I complex in two steps

The present study addresses the assembly in the chloroplast thylakoid membranes of PsaD, a peripheral membrane protein of the photosystem I complex. Located on the stromal side of the thylakoids, PsaD was found to assemble in vitro into the membranes in its precursor (pre-PsaD) and also in its mature (PsaD) form. Newly assembled unprocessed pre-PsaD was resistant to NaBr and alkaline wash. Yet it was sensitive to proteolytic digestion. In contradistinction, when the assembled precursor was processed, the resulting mature PsaD was resistant to proteases to the same extent as endogenous [correction of endogeneous] PsaD. The accumulation of protease-resistant PsaD in the thylakoids correlated with the increase of mature-PsaD in the membranes. This protection of mature PsaD from proteolysis could not be observed when PsaD was in a soluble form-i.e. not assembled within the thylakoids. The data suggest that pre-PsaD assembles to the membranes and only in a second step processing takes place. The observation that the assembly of pre-PsaD is affected by salts to a much lesser extent than that of mature-PsaD supports a two-step assembly of pre-PsaD.

Assembly of the chlorophyll-protein complexes

The biogenesis of photosynthetic complexes in plants and algae is a multi-step process that involves intricate coordination of steps in two intracellular compartments, the chloroplast and the cytoplasm. The process initiates with the transcription and translation of the various polypeptide subunits. The nuclear-encoded Chl-binding proteins are translated on cytoplasmic ribosomes as precursors that have a transit (leader) sequence at their amino-terminus. The precursors are post-translationally imported into the chloroplasts, proteolytically processed into their mature forms, inserted into the thylationally imported into the chloroplasts, proteolytically processed into their mature forms, inserted into the thylakoid membrane, and bound to their co-factors (and pigments) and with other subunits to form an active complex. The order and mechanisms by which these events occur, are currently being discovered. Electrostatic interactions, the 'positive inside rule', interhelix interactions, interactions with lipids and chaperone proteins affect the insertion and stabilization of the Chl-proteins in the thylakoids. This review describes the events occurring during the integration and organization of the Chl-proteins.

Function and organization of Photosystem I polypeptides

Photosystem I functions as a plastocyanin:ferredoxin oxidoreductase in the thylakoid membranes of chloroplasts and cyanobacteria. The PS I complex contains the photosynthetic pigments, the reaction center P700, and five electron transfer centers (A0, A1, FX, FA, and FB) that are bound to the PsaA, PsaB, and PsaC proteins. In addition, PS I complex contains at least eight other polypeptides that are accessory in their functions. Recent use of cyanobacterial molecular genetics has revealed functions of the accessory subunits of PS I. Site-directed mutagenesis is now being used to explore structure-function relations in PS I. The overall architecture of PSI complex has been revealed by X-ray crystallography, electron microscopy, and biochemical methods. The information obtained by different techniques can be used to propose a model for the organization of PS I. Spectroscopic and molecular genetic techniques have deciphered interaction of PS I proteins with the soluble electron transfer partners. This review focuses on the recent structural, biochemical and molecular genetic studies that decipher topology and functions of PS I proteins, and their interactions with soluble electron carriers.