Metastable proton pools in thylakoids and their importance for the stability of Photosystem II

The Over-expression of the Plastidial Transglutaminase from Maize in Arabidopsis Increases the Activation Threshold of Photoprotection

Plastidial transglutaminase is one of the most promising enzymes in chloroplast bioenergetics due to its link with polyamine pathways and the cross talk with signals such as Ca(2+) and GTP. Here, we show the effect of the increase of transglutaminase activity in Arabidopsis by using genetic transformation techniques. These lines fulfill their biological cycle normally (normal growth in soil, production of viable seeds) and show a relatively mild increase in transglutaminase activity (127%). These overexpressors of transglutaminase (OE TGase) have an extended stroma thylakoid network (71% higher number of PSIIβ centers), similar chlorophyll content (-4%), higher linear electron flow (+13%), and higher threshold of photoprotection activation (∼100%). On the other hand OE TGase showed a reduced maximum photochemistry of PSII (-6.5%), a smaller antenna per photosystem II (-25%), a lower photoprotective "energization" quenching or qE (-77% at 490 μmol photons m(-2) s(-1)) due to a higher threshold of qE activation and slightly lower light induced proton motive force (-17%). The role of the polyamines and of the transglutaminase in the regulation of chemiosmosis and photoprotection in chloroplasts is discussed.

Evidence that the intrinsic membrane protein LHCII in thylakoids is necessary for maintaining localized delta mu H+ energy coupling

This work tested the hypothesis that thylakoid localized proton-binding domains, suggested to be involved in localized delta mu H(+)-driven ATP formation, are maintained with the involvement of several membrane proteins, including the LHCII (Laszlo, J.A., Baker, G.M., and Dilley, R.A. (1984) Biochim. Biophys. Acta 764, 160-169), which comprises about 50% of the total thylakoid protein. The concept we have in mind is that several membrane proteins cooperate to shield a localized proton diffusion pathway from direct contact with the lumen, thus providing a physical barrier to H+ equilibration between the sequestered domains and the lumen. A barely mutant, chlorina f2, that lacks Chl b and does not accumulate some of the LHCII proteins, was tested for its capacity to carry out localized-proton gradient-dependent ATP formation. Two previously developed assays permit clear discrimination between localized and delocalized delta mu H+ gradient-driven ATP formation. Those assays include the effect of a permeable buffer, pyridine, on the number of single-turnover flashes needed to reach the energetic threshold for ATP formation and the more recently developed assay for lumen pH using 8-hydroxy-1,3,6-pyrene trisulfonic acid as a lumenally loaded pH-sensitive fluorescent probe. By those two criteria, the wild-type barley thylakoids revealed either a localized or a delocalized energy coupling mode under low- or high-salt storage conditions, respectively. Addition of Ca++ to the high-salt storage medium caused those thylakoids to maintain a localized energy-coupling response, as previously observed for pea thylakoids.(ABSTRACT TRUNCATED AT 250 WORDS)

Flow-force relationships in lettuce thylakoids. 2. Effect of the uncoupler FCCP on local proton resistances at the ATPase level

The relationship between the steady-state proton gradient (delta pH) and the rate of phosphorylation was investigated in thylakoids under various conditions. Under partial uncoupling by carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP), the rate of ATP synthesis was reduced by less than expected from the decrease of delta pH. This was observed in the case of the pyocyanine-mediated cyclic electron flow around photosystem 1, but not with the H2O-->photosystem 2-->cytochrome b6f-->photosystem 1-->methyl viologen system. In state 4, a unique relation was found between delta pH and the "phosphate potential", delta Gp, regardless of whether the energy level was controlled by light input or FCCP. The anomalous effect of FCCP on the rate of ATP synthesis disappeared when the ATPase was partially blocked by the reversible inhibitor venturicidin, but not in the presence of tentoxin, an irreversible inhibitor. These results are consistent with the existence of a small kinetic barrier for protons, limiting their access to the ATPase. This resistance would be collapsed by FCCP.

Flow-force relationships in lettuce thylakoids. 1. Strict control of electron flow by internal pH

The regulation by the proton gradient of the electron flow from water to ferricyanide was investigated in thylakoids extracted from lettuce leaves. When the transmembrane proton current was varied by an uncoupler or by the ATP synthase activity, a unique relationship was found between the rate of ferricyanide reduction and the proton gradient, restricted here to its delta pH component. This behavior was conserved in CF1-depleted thylakoids where the passive proton flow was varied by the concentration of an Fo inhibitor or by the concentration of an uncoupler after 100% inhibition of Fo. This shows that under our experimental conditions no direct proton transfer exists in steady state between the site of regulation of the redox chain and the ATPase. Studies at two different pH's indicate that the internal pH, and not the transmembrane pH difference, controls the electron transfer between PS2 and PS1. Modeling the data suggests that a single deprotonation step is kinetically limiting.

Protection of photosynthetic O2 evolution against heat inactivation: the role of chloride, pH and coupling status

Heat inactivation of photosynthetic O2 evolution was studied in isolated thylakoids from spinach (Spinacia oleracea) and mangrove (Avicennia marina) leaves. Different temperatures, salt, pH and uncoupler effects were investigated. From these results and others in the literature it was concluced that chloride loss from the membrane and, more specifically, the oxygen-evolving complex of photosystem II, may be the cause of inhibition of oxygen evolution during heat inactivation.

Intrinsic and artifactual pH buffering in chloroplast thylakoids

After an increase in pH of the suspension medium, a pH gradient across the membrane of chloroplast thylakoids stored at pH greater than or equal to 6.5 is often maintained for several minutes. The intrinsic hydrogen ion buffering capacity of the thylakoid membranes between pH 6.5 and 8.5 is about 40 neq/mg chlorophyll, but can be artificially inflated by penetration of the external buffer into the thylakoid vesicle. A delta pH imposed across the thylakoid membrane by an acid/base transition cannot be estimated accurately by the fluorescent probe 9-aminoacridine, especially with osmotically shrunken thylakoids in which 9-aminoacridine appears to become bound or adsorbed to the membrane. This interaction may be related to the existence of the previously demonstrated special pool of slowly equilibrating, "sequestered" protons.

The relation between the chloride status of the photosynthetic water splitting complex and the inhibitory effectiveness of amines

The protective role of chloride ions (Cl(-)) against inhibition of the photosynthetic water splitting complex by amines was investigated with purified photosystem II membrane particles from tobacco chloroplasts. Seemingly competitive interactions occurred between Cl(-) (except at low concentrations) and Tris, but not between Cl(-) and NH3. The rate of Cl(-) release was not increased by the amines but, instead, may have been limited by a labilization under the experimental conditions of the extrinsic 23 kDa polypeptide. An additional detachment of the 18 kDa polypeptide was seen when SO4 (2-) ions were present. Tris induced changes of the thermoluminescence patterns of flash illuminated photosystem II particles were found to be different from those caused by either Cl(-) deficiency or high pH. It is concluded that the protective functions of Cl(-) are brough about not because it is bound to the target site of the inhibitory actions of Lewis bases like amines and hydroxyl ions. Instead, this effect of Cl(-) may be due to its influence on the tertiary and quaternary structures of the water oxidizing protein complex.

Reconciliation of the absorption change at 325 nm and other flash-yield determinations of concentrations of active photosystem II centers

The concentration of photosystem II was determined in thylakoids of dwarf peas by the use of the following methods: absorption change at 325 nm; atrazine binding; and flash yields of oxygen evolution (Emerson-Arnold method), of protons from oxidation of water, and of reduction of DCIP. For the first time all of the flash-yield measurements have been done on the same sample and give equivalent values for the concentration of photosystem II. Agreement of the absorption change measurement at 325 nm with the other measurements was accomplished by the introduction of important improvements to the methods of Melis and co-workers [Proc. Natl. Acad. Sci. USA (1980) 77, 4712-4716]. The atrazine-binding method gave photosystem II values that were twice as large as any of the other photosystem II measurements. Possible reasons are discussed for this discrepancy in terms of the secondary acceptor (Q400) of Ikegami and Katoh [Plant Cell Physiol. (1973) 14, 829-836]. The concentration of photosystem I was measured by absorption change at 705 nm. From the concentration values of photosystem II and I the system II/I stoichiometry was calculated.

The role of chloride ion on the photoreactivation of the oxygen-evolving center of tris-washed, 2,6-dichlorophenol indophenol-treated grana

Chloride ion is found to be an essential factor in photoreactivation of the oxygen-evolving center. Tris-washed, 2,6-dichlorophenol indophenol-treated grana are low in both Mn content and oxygen-evolving activity. These grana can restore high oxygen-evolving activity, however, by incorporating Mn2+ ion under weak light in the presence of chloride and calcium ions with dithiothreitol. This restoration is called photoreactivation. When chloride ion is omitted from the medium for the photoreactivation, the recovery of oxygen-evolving activity is inhibited. Other anions, such as bromide and nitrate anions, could also mediate the reactivation; but, anions of weak acids or polyvalent strong acids were not effective. Chloride ion is also required in the light-induced H+ and Mn2+ uptake of these grana, which are essential partial reactions for the reactivation. It is therefore concluded that chloride ion plays an important role in the photoreactivation.

A minimal hypothesis for membrane-linked free-energy transduction. The role of independent, small coupling units

Experimental data are reviewed that are not in keeping with the scheme of 'delocalized' protonic coupling in membrane-linked free-energy transduction. It turns out that there are three main types of anomalies: (i) rates of electron transfer and of ATP synthesis do not solely depend on their own driving force and on delta mu H, (ii) the ('static head') ratio of delta Gp to delta mu H varies with delta mu H and (iii) inhibition of either some of the electron-transfer chains or some of the H+-ATPases, does not cause an overcapacity in the other, non-inhibited proton pumps. None of the earlier free-energy coupling schemes, alternative to delocalized protonic coupling, can account for these three anomalies. We propose to add a fifth postulate, namely that of the coupling unit, to the four existing postulates of 'delocalized protonic coupling' and show that, with this postulate, protonic coupling can again account for most experimental observations. We also discuss: (i) how experimental data that might seem to be at odds with the 'coupling unit' hypothesis can be accounted for and (ii) the problem of the spatial arrangement of the electrical field in the different free-energy coupling schemes.

Correlation between membrane-localized protons and flash-driven ATP formation in chloroplast thylakoids

Flash-driven ATP formation by spinach chloroplast thylakoids, using the luciferin luminescence assay to detect ATP formed in single turnover flashes, was studied under conditions where a membrane protein amine buffering pool was either protonated or deprotonated before the beginning of the flash trains. The flash number for the onset of ATP formation was delayed by about 10 flashes (from 15 to about 25) when the amine pool was deprotonated as compared to the protonated state. The delay was substantially reversed again by reprotonating the pool upon application of 20-30 single-turnover flashes and 8 min of dark before addition of ADP, Pi, and the luciferin system. In the case of deprotonation by desaspidin, the uncoupler was removed by binding to BSA before the reprotonating flashes were given. Reprotonation was carried out before addition of ADP and Pi, to avoid a possible interference by the ATP-ase, which can energize the system by pumping protons. The reprotonated state, as indicated by an onset lag of about 15 flashes rather than 25 for the deprotonated state, was stable in the dark over extended dark times. The number of protons released by 10 flashes is approximately 30 nmol H+ (mg chl)-1, an amount similar to the size of the reversibly protonated amine group buffering pool. The data are consistent with the hypothesis that the amine buffering groups must be in the protonated state before any protons proceed to the coupling complex and energize ATP formation. Other work has suggested that the amine buffering pool is sequestered within membrane proteins rather than being exposed directly to the inner aqueous bulk phase. Therefore, it is possible that the sequestered amine group array may provide localized association-dissociation sites for proton movement to the coupling complex.

Inhibition of photosynthetic oxygen evolution in non-vesicular preparations releases Mn2+ into a restricted aqueous compartment

Release of Mn2+ following inhibition of the water-splitting enzyme by treatment with Zn2+ was studied in two preparations derived from chloroplasts: oxygen-evolving Photosystem II membrane fragments and Tween-20 treated thylakoid membranes. In both cases the released Mn2+ was retained in the membrane fraction on centrifugation in spite of the fact that the two preparations are known to be non-vesicular. This suggests that upon inhibition Mn2+ is released into an aqueous compartment more restricted than the thylakoid lumen. We propose that this compartment is formed from the various proteins of the Photosystem II complex.

Nonequilibration of membrane-associated protons with the internal aqueous space in dark-maintained chloroplast thylakoids

Isolated spinach thylakoids retain a slowly equilibrating pool of protons in the dark which are predominantly bound to buffering groups, probably amines, with low pKa values. We have measured the effects of permeant buffers, salts, sucrose, and uncouplers on the retention of the proton pool. Acetic anhydride, which reacts with neutral primary amine groups, was used to determine the protonation state of the amine buffering groups. It was previously shown by Baker et al. that the extent of inhibition of photosystem II water-oxidizing capacity by acetic anhydride and the increase in derivatization by the anhydride are proportional to, and dependent on, the deprotonated state of the amine buffering pool. Therefore, acetic anhydride inhibition of water oxidation activity may be used as a measure of the protonation state of the amine buffering pool. By this method it is inferred that protons, in a metastable state, were retained by membranes suspended in high pH buffer for several hours in the dark. When both the internal and external aqueous phases were equilibrated with pH 8.8 buffer, the proton pool was released only upon addition of a protonophore. The osmotic strength of the suspension buffer affected uncoupler-induced proton release while ionic strength had little influence. The acetic anhydride-sensitive buffering group(s) of the water-oxidizing apparatus had an apparent pKa of 7.8. We conclude that an array of protein buffering groups reside either within the membrane matrix, or in proteins at the membrane surface, not in equilibrium with the bulk aqueous phases, and is responsible for the retention of the proton pool in dark maintained chloroplasts.