Immobilization of the three extrinsic proteins in spinach oxygen-evolving Photosystem II membranes: roles of the proteins in stabilization of binding of Mn and Ca2+

The Effect of Removal of External Proteins PsbO, PsbP and PsbQ on Flash-Induced Molecular Oxygen Evolution and Its Biphasicity in Tobacco PSII

The oxygen evolution within photosystem II (PSII) is one of the most enigmatic processes occurring in nature. It is suggested that external proteins surrounding the oxygen-evolving complex (OEC) not only stabilize it and provide an appropriate ionic environment but also create water channels, which could be involved in triggering the ingress of water and the removal of O2 and protons outside the system. To investigate the influence of these proteins on the rate of oxygen release and the efficiency of OEC function, we developed a measurement protocol for the direct measurement of the kinetics of oxygen release from PSII using a Joliot-type electrode. PSII-enriched tobacco thylakoids were used in the experiments. The results revealed the existence of slow and fast modes of oxygen evolution. This observation is model-independent and requires no specific assumptions about the initial distribution of the OEC states. The gradual removal of exogenous proteins resulted in a slowdown of the rapid phase (~ms) of O2 release and its gradual disappearance while the slow phase (~tens of ms) accelerated. The role of external proteins in regulating the biphasicity and efficiency of oxygen release is discussed based on observed phenomena and current knowledge.

Photosynthetic responses to heat treatments at different temperatures and following recovery in grapevine (Vitis amurensis L.) leaves

BACKGROUND

The electron transport chain, Rubisco and stomatal conductance are important in photosynthesis. Little is known about their combined responses to heat treatment at different temperatures and following recovery in grapevines (Vitis spp.) which are often grown in climates with high temperatures.

METHODOLOGY/FINDINGS

The electron transport function of photosystem II, the activation state of Rubisco and the influence of stomatal behavior were investigated in grapevine leaves during heat treatments and following recovery. High temperature treatments included 35, 40 and 45°C, with 25°C as the control and recovery temperature. Heat treatment at 35°C did not significantly (P>0.05) inhibit net photosynthetic rate (P(n)). However, with treatments at 40 and 45°C, P(n) was decreased, accompanied by an increase in substomatal CO(2) concentration (C(i)), decreases in stomatal conductance (g(s)) and the activation state of Rubisco, and inhibition of the donor side and the reaction center of PSII. The acceptor side of PSII was inhibited at 45°C but not at 40°C. When grape leaves recovered following heat treatment, P(n), g(s) and the activation state of Rubisco also increased, and the donor side and the reaction center of PSII recovered. The increase in P(n) during the recovery period following the second 45°C stress was slower than that following the 40°C stress, and these increases corresponded to the donor side of PSII and the activation state of Rubisco.

CONCLUSIONS

Heat treatment at 35°C did not significantly (P>0.05) influence photosynthesis. The decrease of P(n) in grape leaves exposed to more severe heat stress (40 or 45°C) was mainly attributed to three factors: the activation state of Rubisco, the donor side and the reaction center of PSII. However, the increase of P(n) in grape leaves following heat stress was also associated with a stomatal response. The acceptor side of PSII in grape leaves was responsive but less sensitive to heat stress.

Binding and functional properties of five extrinsic proteins in oxygen-evolving photosystem II from a marine centric diatom, Chaetoceros gracilis

Oxygen-evolving photosystem II (PSII) isolated from a marine centric diatom, Chaetoceros gracilis, contains a novel extrinsic protein (Psb31) in addition to four red algal type extrinsic proteins of PsbO, PsbQ', PsbV, and PsbU. In this study, the five extrinsic proteins were purified from alkaline Tris extracts of the diatom PSII by anion and cation exchange chromatographic columns at different pH values. Reconstitution experiments in various combinations with the purified extrinsic proteins showed that PsbO, PsbQ', and Psb31 rebound directly to PSII in the absence of other extrinsic proteins, indicating that these extrinsic proteins have their own binding sites in PSII intrinsic proteins. On the other hand, PsbV and PsbU scarcely rebound to PSII alone, and their effective bindings required the presence of all of the other extrinsic proteins. Interestingly, PSII reconstituted with Psb31 alone considerably restored the oxygen evolving activity in the absence of PsbO, indicating that Psb31 serves as a substitute in part for PsbO in supporting oxygen evolution. A significant difference found between PSIIs reconstituted with Psb31 and with PsbO is that the oxygen evolving activity of the former is scarcely stimulated by Cl(-) and Ca(2+) ions but that of the latter is largely stimulated by these ions, although rebinding of PsbV and PsbU activated oxygen evolution in the absence of Cl(-) and Ca(2+) ions in both the former and latter PSIIs. Based on these results, we proposed a model for the association of the five extrinsic proteins with intrinsic proteins in diatom PSII and compared it with those in PSIIs from the other organisms.

Structures and functions of the extrinsic proteins of photosystem II from different species

This minireview presents a summary of information available on the variety and binding properties of extrinsic proteins that form the oxygen-evolving complex of photosystem II (PSII) of cyanobacteria, red alga, diatom, green alga, euglena, and higher plants. In addition, the structure and function of extrinsic PsbO, PsbV, and PsbU proteins are summarized based on the crystal structure of thermophilic cyanobacterial PSII together with biochemical and genetic studies from various organisms.

Binding and functional properties of the extrinsic proteins in oxygen-evolving photosystem II particle from a green alga, Chlamydomonas reinhardtii having his-tagged CP47

Oxygen-evolving photosystem II (PSII) particles were purified from Chlamydomonas reinhardtii having His-tag extension at the C terminus of the CP47 protein, by a single-step Ni(2+)-affinity column chromatography after solubilization of thylakoid membranes with sucrose monolaurate. The PSII particles consisted of, in addition to intrinsic proteins, three extrinsic proteins of 33, 23 and 17 kDa. The preparation showed a high oxygen-evolving activity of 2,300-2,500 micro mol O(2) (mg Chl)(-1) h(-1) in the presence of Ca(2+) using ferricyanide as the electron acceptor, while its activity was 680-720 micro mol O(2) (mg Chl)(-1) h(-1) in the absence of Ca(2+) and Cl(-) ions. The activity was 710-820 micro mol O(2) (mg Chl)(-1) h(-1) independent of the presence or absence of Ca(2+) and Cl(-) when 2,6-dichloro-p-benzoquinone was used as the acceptor. These activities were scarcely inhibited by DCMU. The kinetics of flash-induced fluorescence decay revealed that the electron transfer from Q(A)(-) to Q(B) was significantly inhibited, and the electron transfer from Q(A)(-) to ferricyanide was largely stimulated in the presence of Ca(2+). These results indicate that the acceptor side, Q(B) site, was altered in the PSII particles but its donor side remained intact. Release-reconstitution experiments revealed that the extrinsic 23 and 17 kDa proteins were released only partially by NaCl-wash, while most of the three extrinsic proteins were removed when treated with urea/NaCl, alkaline Tris or CaCl(2). The 23 and 17 kDa proteins directly bound to PSII independent of the other extrinsic proteins, and the 33 kDa protein functionally re-bound to CaCl(2)-treated PSII which had been reconstituted with the 23 and 17 kDa proteins. These binding properties were largely different from those of the extrinsic proteins in higher plant PSII, and suggest that each of the three extrinsic proteins has their own binding sites independent of the others in the green algal PSII.

A domain of the manganese-stabilizing protein from Synechococcus elongatus involved in functional binding to photosystem II

Site-directed mutagenesis was performed to investigate whether the two protease-sensitive sequences Phe(156)-Gly(163) and Arg(184)-Ser(191), of the manganese-stabilizing protein (MSP) from a thermophilic cyanobacterium, Synechococcus elongatus (Motoki, A., Shimazu, T., Hirano, M., and Katoh, S. (1998) Biochim. Biophys. Acta 1365, 492-502), are involved in functional interaction with photosystem II (PSII). The ability of MSP to bind to its functional site on the PSII complex and to reactivate oxygen evolution was dramatically reduced by the substitution of Arg(152), Asp(158), Lys(160), or Arg(162) with uncharged residues, by insertion of a single residue between Phe(156) and Leu(157), or by deletion of Leu(157). Substitution of each of the four charged residues with an identically charged residue showed that the charges at Asp(158), and possibly Lys(160), are important for the electrostatic interaction with PSII. The reactivating ability was also strongly affected by the alteration of Phe(156) to Leu. Replacement of Lys(188), the only strictly conserved charged residue in the Arg(184)-Ser(191) sequence, by Gln had only a marginal effect on the function of MSP. High affinity binding of MSP to PSII was also affected significantly by mutation at Arg(152), which is located in a region (Val(148)-Arg(152)) strictly conserved among the 14 sequences so far reported. These results imply that the Val(148)-Gly(163) sequence, which is well conserved among MSPs from cyanobacteria to higher plants, is a domain of MSP for functional interaction with PSII.

Substrate water exchange in photosystem II depends on the peripheral proteins

The (18)O exchange rates for the substrate water bound in the S(3) state were determined in different photosystem II sample types using time-resolved mass spectrometry. The samples included thylakoid membranes, salt-washed Triton X-100-prepared membrane fragments, and purified core complexes from spinach and cyanobacteria. For each sample type, two kinetically distinct isotopic exchange rates could be resolved, indicating that the biphasic exchange behavior for the substrate water is inherent to the O(2)-evolving catalytic site in the S(3) state. However, the fast phase of exchange became somewhat slower (by a factor of approximately 2) in NaCl-washed membrane fragments and core complexes from spinach in which the 16- and 23-kDa extrinsic proteins have been removed, compared with the corresponding rate for the intact samples. For CaCl(2)-washed membrane fragments in which the 33-kDa manganese stabilizing protein (MSP) has also been removed, the fast phase of exchange slowed down even further (by a factor of approximately 3). Interestingly, the slow phase of exchange was little affected in the samples from spinach. For core complexes prepared from Synechocystis PCC 6803 and Synechococcus elongatus, the fast and slow exchange rates were variously affected. Nevertheless, within the experimental error, nearly the same exchange rates were measured for thylakoid samples made from wild type and an MSP-lacking mutant of Synechocystis PCC 6803. This result could indicate that the MSP has a slightly different function in eukaryotic organisms compared with prokaryotic organisms. In all samples, however, the differences in the exchange rates are relatively small. Such small differences are unlikely to arise from major changes in the metal-ligand structure at the catalytic site. Rather, the observed differences may reflect subtle long range effects in which the exchange reaction coordinates become slightly altered. We discuss the results in terms of solvent penetration into photosystem II and the regional dielectric around the catalytic site.

Intramolecular cross-linking of the extrinsic 33-kDa protein leads to loss of oxygen evolution but not its ability of binding to photosystem II and stabilization of the manganese cluster

The extrinsic 33-kDa protein of photosystem II (PSII) was intramolecularly cross-linked by a zero-length cross-linker, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. The resulting cross-linked 33-kDa protein rebound to urea/NaCl-washed PSII membranes, which stabilized the binding of manganese as effectively as the untreated 33-kDa protein. In contrast, the oxygen evolution was not restored by binding of the cross-linked protein, indicating that the binding and manganese-stabilizing capabilities of the 33-kDa protein are retained but its reactivating ability is lost by intramolecular cross-linking of the protein. From measurements of CD spectra at high temperatures, the secondary structure of the intramolecularly cross-linked 33-kDa protein was found to be stabilized against heat treatment at temperatures 20 degrees C higher than that of the untreated 33-kDa protein, suggesting that structural flexibility of the 33-kDa protein was much decreased by the intramolecular cross-linking. The rigid structure is possibly responsible for the loss of the reactivating ability of the 33-kDa protein, which implies that binding of the 33-kDa protein to PSII is accompanied by a conformational change essential for the reactivation of oxygen evolution. Peptide mapping, N-terminal sequencing, and mass spectroscopic analysis of protease-digested products of the intramolecularly cross-linked 33-kDa protein revealed that cross-linkings occurred between the amino group of Lys48 and the carboxyl group of Glu246, and between the carboxyl group of Glu10 and the amino group of Lys14. These cross-linked amino acid residues are thus closely associated with each other through electrostatic interactions.

Identification of domains on the 43 kDa chlorophyll-carrying protein (CP43) that are shielded from tryptic attack by binding of the extrinsic 33 kDa protein with photosystem II complex

The structural association of the spinach 33 kDa extrinsic protein with the 43 kDa chlorophyll-carrying protein (CP43) in oxygen-evolving photosystem II (PS II) complexes was investigated by comparing the peptide mappings and N-terminal sequences of the trypsin-digested products of NaCl-washed PS II membranes, which bind the 33 kDa protein, with those of CaCl2-washed PS II membranes, which lack the 33 kDa protein. (1) Peptide from N-terminus to Arg26 of CP43, which is exposed to stromal side, was digested in both PS II membranes, independent of binding of the 33 kDa protein. (2) Peptide bond of Arg357-Phe358 located in the large extrinsic loop E of CP43, which is exposed to lumenal side, was cleaved by trypsin in CaCl2-washed PS II membranes but not in NaCl-washed PS II membranes. This indicates that the region around Arg357-Phe358 in loop E of CP43 is shielded from tryptic attack by binding of the 33 kDa protein to PS II. (3) Trypsin treatment of CaCl2-washed PS II membranes also cleaved peptide bond between Lys457 and Gly458 in C-terminal region of CP43, while no cleavage of this region was detected by trypsin treatment of NaCl-washed PS II membranes. This implies that a conformational change of the C-terminal region of CP43 which is exposed to stromal side occurred upon removal of the 33 kDa protein, which makes the C-terminal region accessible to trypsin. (4) Release of peptide from Gln60 to C-terminus of the alpha-subunit of cytochrome b-559 was detected only in trypsin treatment of CaCl2-washed PS II membranes, indicating that the C-terminal region of this subunit is shielded from tryptic attack by binding of the 33 kDa protein. (5) The PS II membranes, in which Arg357-Phe358, Lys457-Gly458 of CP43 and the C-terminal part of the cytochrome b-559 alpha-subunit had been cleaved by trypsin, was no longer able to bind the 33 kDa protein. This strongly suggests that a domain in loop E of CP43 and/or the C-terminal region of the cytochrome b-559 alpha-subunit are necessary for binding of the extrinsic 33 kDa protein to PS II.

Identification of domains on the extrinsic 33-kDa protein possibly involved in electrostatic interaction with photosystem II complex by means of chemical modification

The extrinsic 33-kDa protein of photosystem II (PSII) was modified with various reagents, and the resulting proteins were checked for the ability to rebind to PSII and to reactivate oxygen evolution. While modification of more than eight carboxyl groups of aspartyl and glutamyl residues with glycine methyl ester did not affect the rebinding and reactivating capabilities, modification of amino groups of lysyl residues with either N-succinimidyl propionate or 2, 4,6-trinitrobenzene sulfonic acid or modification of guanidino groups of arginyl residues with 2,3-butanedione resulted in a loss of rebinding and reactivating capabilities of the 33-kDa protein. Moreover, the number of lysyl and arginyl residues susceptible to modification was significantly decreased when the protein was bound to PSII as compared with when it was free in solution, whereas the number of carboxyl groups modified was little affected. These results suggested that positive charges are important for the electrostatic interaction between the extrinsic 33-kDa protein and PSII intrinsic proteins, whereas negative charges on the protein do not contribute to such interaction. By a combination of protease digestion and mass spectroscopic analysis, the domains of lysyl residues accessible to N-succinimidyl propionate or 2,4, 6-trinitrobenzene sulfonic acid modification only when the 33-kDa protein is free in solution were determined to be Lys4, Lys20, Lys66-Lys76, Lys101, Lys105, Lys130, Lys159, Lys186, and Lys230-Lys236. These domains include those previously reported accessible to N-hydroxysuccinimidobiotin only in solution (Frankel and Bricker (1995) Biochemistry 34, 7492-7497), and may be important for the interaction of the 33-kDa protein with PSII intrinsic proteins.