Fragmentation and separation analysis of the photosynthetic membrane from spinach

Membrane vesicles, originating from grana, grana core (appressed grana regions), grana margins and stroma lamellae/end membranes, were analysed by counter current distribution (CCD) using aqueous dextran-polyethylene glycol two-phase systems. Each vesicle population gave rise to distinct peaks in the CCD diagram representing different vesicle subpopulations. The grana vesicles and grana core vesicles each separated into 3 different subpopulations having different chlorophyll a/b ratios and PSI/PSII ratios. Two of the grana core subpopulations had a chlorophyll a/b ratio of 2.0 and PSI/PSII ratio of 0.10 and are among the most PSII enriched thylakoid vesicle preparation obtained so far by a non detergent method. The margin vesicles separated into 3 different populations, with about the same chlorophyll a/b ratios, but different fluorescence emission spectra. The stroma lamellae/end membrane vesicles separated into 4 subpopulations. Plastoglobules, connected to membrane vesicles, were highly enriched in 2 of these subpopulations and it is proposed that these 2 subpopulations originate from stroma lamellae while the 2 others originate from end membranes. Fragmentation and separation analysis shows that the margins of grana constitute a distinct domain of the thylakoid and also allows the estimation of the chlorophyll antenna sizes of PSI and PSII in different thylakoid domains.     

Quantification of photosystem I and II in different parts of the thylakoid membrane from spinach

Electron paramagnetic resonance (EPR) was used to quantify Photosystem I (PSI) and PSII in vesicles originating from a series of well-defined but different domains of the thylakoid membrane in spinach prepared by non-detergent techniques. Thylakoids from spinach were fragmented by sonication and separated by aqueous polymer two-phase partitioning into vesicles originating from grana and stroma lamellae. The grana vesicles were further sonicated and separated into two vesicle preparations originating from the grana margins and the appressed domains of grana (the grana core), respectively. PSI and PSII were determined in the same samples from the maximal size of the EPR signal from P700(+) and Y(D)( .-), respectively. The following PSI/PSII ratios were found: thylakoids, 1.13; grana vesicles, 0.43; grana core, 0.25; grana margins, 1.28; stroma lamellae 3.10. In a sub-fraction of the stroma lamellae, denoted Y-100, PSI was highly enriched and the PSI/PSII ratio was 13. The antenna size of the respective photosystems was calculated from the experimental data and the assumption that a PSII center in the stroma lamellae (PSIIbeta) has an antenna size of 100 Chl. This gave the following results: PSI in grana margins (PSIalpha) 300, PSI (PSIbeta) in stroma lamellae 214, PSII in grana core (PSIIalpha) 280. The results suggest that PSI in grana margins have two additional light-harvesting complex II (LHCII) trimers per reaction center compared to PSI in stroma lamellae, and that PSII in grana has four LHCII trimers per monomer compared to PSII in stroma lamellae. Calculation of the total chlorophyll associated with PSI and PSII, respectively, suggests that more chlorophyll (about 10%) is associated with PSI than with PSII.     

Separation and characterization of stroma and grana membranes — evidence for heterogeneity in antenna size of both Photosystem I and Photosystem II

A rapid procedure to fractionate the thylakoid membrane into two well-separated vesicle populations, one originating from the grana and the other from the stroma-membrane region, has been developed. This was achieved by sonication of thylakoids present in an aqueous two-phase system followed by partitioning either by countercurrent distribution or by a batch procedure in three steps. The membrane populations were analysed according to their composition and photochemical activities. The grana membranes comprise, on chlorophyll basis, about 60% of the thylakoid material and are enriched in PS II, but also contain some PS I, while the stroma membranes comprise about 40% and are enriched in PS I, but also contain some PS II. Cytochrome f was slightly enriched in the grana-derived vesicle fraction. The properties of both PS I and PS II differ between the two populations. The PS I of the grana fraction (PS I) reached half-saturation at about half the light intensity of the PS I in the stroma-membrane fraction (PS I_β). The rate of P-700 photooxidation under low light illumination was higher for PS I than for PS I_β (30% larger rate constant), showing that PS I has a larger antenna. The PS II of the grana fraction (PS II) reached half-saturation at half the light intensity compared to the PS II of the stroma-membrane fraction (PS II_β). The results show that the grana-derived membranes contain PS I and PS II which have larger functional antenna sizes than the corresponding PS I_β and PS II_β of the stroma membranes. The results suggest that the photosystems of the grana are designed to allow effective electron transport both at low and high light intensities, while the stroma-membrane photosystems mainly work at high light intensities as a supplement to the grana systems.     

Quantification of photosystem I and II in different parts of the thylakoid membrane from spinach

Electron paramagnetic resonance (EPR) was used to quantify Photosystem I (PSI) and PSII in vesicles originating from a series of well-defined but different domains of the thylakoid membrane in spinach prepared by non-detergent techniques. Thylakoids from spinach were fragmented by sonication and separated by aqueous polymer two-phase partitioning into vesicles originating from grana and stroma lamellae. The grana vesicles were further sonicated and separated into two vesicle preparations originating from the grana margins and the appressed domains of grana (the grana core), respectively. PSI and PSII were determined in the same samples from the maximal size of the EPR signal from P700⁺ and Y_D, respectively. The following PSI/PSII ratios were found: thylakoids, 1.13; grana vesicles, 0.43; grana core, 0.25; grana margins, 1.28; stroma lamellae 3.10. In a sub-fraction of the stroma lamellae, denoted Y-100, PSI was highly enriched and the PSI/PSII ratio was 13. The antenna size of the respective photosystems was calculated from the experimental data and the assumption that a PSII center in the stroma lamellae (PSIIβ) has an antenna size of 100 Chl. This gave the following results: PSI in grana margins (PSIα) 300, PSI (PSIβ) in stroma lamellae 214, PSII in grana core (PSIIα) 280. The results suggest that PSI in grana margins have two additional light-harvesting complex II (LHCII) trimers per reaction center compared to PSI in stroma lamellae, and that PSII in grana has four LHCII trimers per monomer compared to PSII in stroma lamellae. Calculation of the total chlorophyll associated with PSI and PSII, respectively, suggests that more chlorophyll (about 10%) is associated with PSI than with PSII.     

The structure and function of the chloroplast photosynthetic membrane — a model for the domain organization

Recent work on the domain organization of the thylakoid is reviewed and a model for the thylakoid of higher plants is presented. According to this model the thylakoid membrane is divided into three main domains: the stroma lamellae, the grana margins and the grana core (partitions). These have different biochemical compositions and have specialized functions. Linear electron transport occurs in the grana while cyclic electron transport is restricted to the stroma lamellae. This model is based on the following results and considerations. (1) There is no good candidate for a long-range mobile redox carrier between PS II in the grana and PS I in the stroma lamellae. The lateral diffusion of plastoquinone and plastocyanin is severely restricted by macromolecular crowding in the membrane and the lumen respectively. (2) There is an excess of 14±18% chlorophyll associated with PS I over that of PS II. This excess is assumed to be localized in the stroma lamellae where PS I drives cyclic electron transport. (3) For several plant species, the stroma lamellae account for 20±3% of the thylakoid membrane and the grana (including the appressed regions, margins and end membranes) for the remaining 80%. The amount of stroma lamellae (20%) corresponds to the excess (14–18%) of chlorophyll associated with PS I. (4) The model predicts a quantum requirement of about 10 quanta per oxygen molecule evolved, which is in good agreement with experimentally observed values. (5) There are at least two pools of each of the following components: PS I, PS II, cytochrome bf complex, plastocyanin, ATP synthase and plastoquinone. One pool is in the grana and the other in the stroma compartments. So far, it has been demonstrated that the PS I, PS II and cytochrome bf complexes each differ in their respective pools.Abbreviations PS I and PS II Photosystem I and II - P 700 reaction center of PS I - LHC II light-harvesting complex II     

The constant proportion of grana and stroma lamellae in plant chloroplasts

The relative proportion of stroma lamellae and grana end membranes was determined from electron micrographs of 58 chloroplasts from 21 different plant species. The percentage of grana end membranes varied between 1 and 21% of the total thylakoid membrane indicating a large variation in the size of grana stacks. By contrast the stroma lamellae account for 20.3 ± 2.5 ( sd )% of the total thylakoid membrane. A plot of percentage stroma lamellae against percentage of grana end membranes fits a straight line with a slope of zero showing that the proportion of stroma lamellae is independent of the size of the grana stacks. That stroma lamellae account for about 20% of the thylakoid membrane is in agreement with fragmentation and separation analysis (Gadjieva et al . Biochim. Biophys. Acta 144: 92–100, 1999). Chloroplasts from spinach, grown under high or low light, were fragmented by sonication and separated by countercurrent distribution into two vesicle populations originating from grana and stroma lamellae plus end membranes, respectively. The separation diagrams were very similar lending independent support for the notion that the proportion of stroma lamellae is constant. The results are discussed in relation to the composition and function of the chloroplast in plants grown under different environmental conditions, and in relation to a recent quantitative model for the thylakoid (Albertsson, Trends Plant Sci. 6: 349–354, 2001).     

Movement of a sub-population of the light harvesting complex (LHCII) from grana to stroma lamellae as a consequence of its phosphorylation

Phosphorylation in vitro of the light-harvesting chlorophyll $\text{a}\text{b}$ protein complex associated with Photosystem II (LHC_II) resulted in the lateral migration of a subpopulation of LHC_II from the grana to the stroma lamellae. This movement was characterized by a decrease in the chlorophyll $\text{a}\text{b}$ ratio and an increase in the 77 K fluorescence emission at 681 nm in the stroma lamellae following phosphorylation. Polyacrylamide gel electrophoresis indicated that the principal phosphoproteins under these conditions were polypeptides of 26–27 kDa. These polypeptides increased in relative amount in the stroma lamellae and decreased in the grana during phosphorylation. Pulse/chase experiments confirmed that the polypeptides were labelled in the grana and moved to the stroma lamellae in the subsequent chase period. A fraction at the phospho-LHC_II, however, was unable to move and remained associated with the grana fraction. LHC_II which moved out into the stroma lamellae effectively sensitized Photosystem I (PS I), since the ability to excite fluorescence emission at 735 nm (at 77 K) by chlorophyll b was increased following phosphorylation. These data support the ‘mobile antenna’ hypothesis proposed by Kyle, Staehelin and Arntzen (Arch. Biochem. Biophys. (1983) 222, 527–541) which states that the alterations in the excitation-energy distribution induced by LHC_II phosphorylation are, in part, due to the change in absorptive cross-section of PS II and PS I, resulting specifically from the movement of LHC_II antennae chlorophylls from the PS-II-enriched grana to the PS-I-enriched stroma lamellae.     

Characterization of a non-detergent PS II-cytochrome b/f preparation (BS)

A non-detergent photosystem II preparation, named BS, has been characterized by countercurrent distribution, light saturation curves, absorption spectra and fluorescence at room and at low temperature (–196°C). The BS fraction is prepared by a sonication-phase partitioning procedure (Svensson P and Albertsson P-Å, Photosynth Res 20: 249–259, 1989) which removes the stroma lamellae and the margins from the grana and leaves the appressed partition region intact in the form of vesicles. These are closed structures of inside-out conformation. They have a chlorophyll a/b ratio of 1.8–2.0, have a high oxygen evolving capacity (295 mol O₂ per mg chl h), are depleted in P700 and enriched in the cytochrome b/f complex. They have about 2 Photosystem II reaction centers per 1 cytochrome b/f complex.The plastoquinone pool available for PS II in the BS vesicles is 6–7 quinones per reaction center, about the same as for the whole thylakoid. It is concluded, therefore, that the plastoquinone of the stroma lamellae is not available to the PS II in the grana and that plastoquinone does not act as a long range electron transport shuttler between the grana and stroma lamellae.Compared with Photosystem II particles prepared by detergent (Triton X-100) treatment, the BS vesicles retain more cytochrome b/f complex and are more homogenous in their surface properties, as revealed by countercurrent distribution, and they have a more efficient energy transfer from the antenna pigments to the reaction center.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F_v variable fluorescence - LHC light-harvesting complex - PpBQ phenyl-p-benzoquinone - PQ plastoquinone pool - P700 reaction center of PS I - PS I, PS II Photosystem I, II - Q_A first bound plastoquinone accepter - RC reaction centre     

Lateral mobility of the light-harvesting complex in chloroplast membranes controls excitation energy distribution in higher plants

Chloroplast thylakoid protein phosphorylation produces changes in light-harvesting properties and in membrane structure as revealed by freeze-fracture electron microscopy. Protein phosphorylation resulted in an increase in the 77 °K fluorescence signal at 735 nm relative to that at 685 nm. In addition, a decrease in connectivity between Photosystem II centers (PS II) and a dynamic quenching of the room temperature variable fluorescence was observed upon phosphorylation. Accompanying these fluorescence changes was a 23% decrease in the amount of stacked membranes. Microscopic analyses indicated that 8.0-nm particles fracturing on the P-face moved from the stacked into the unstacked regions upon phosphorylation. The movement of the 8.0-nm particles was accompanied by the appearance of chlorophyll b and 25 to 29 kD polypeptides in isolated stroma lamellae fractions. We conclude that phosphorylation of a population of the light-harvesting chlorophyll $\text{a}\text{b}$ protein complexes (LHC) in grana partitions causes the migration of these pigment proteins from the PS II-rich appressed membranes into the Photosystem I (PS I) enriched unstacked regions. This increases the absorptive cross section of PS I. In addition, we suggest that the mobile population of LHC functions to interconnect PS II centers in grana partitions; removal of this population of LHC upon phosphorylation limits PS II → PS II energy transfer and thereby favors spillover of energy from PS II to PS I.     

Isolation and Characterization of the Cytochrome bf Complex from Whole Thylakoids, Grana, and Stroma Lamellae Vesicles from Spinach Chloroplasts

The cytochrome bf complex was isolated from spinach thylakoids,and also from separated grana and stroma lamellae vesicles,by a procedure involving NaBr washing, detergent treatment andcentrifugation in sucrose gradients. The resulting complex fromall three types of membranes, were almost completely devoidof chlorophyll and carotenoids. The complexes have kinase activitytowards histone III-S and contain a 64 kDa protein claimed tobe a kinase. Electrophoretic analyses indicate that the complexesare in dimeric form and composed of six polypeptides with molecularmasses of 34/33, 23, 20, 17, 12 and 4 kDa. The complexes containtwo moles cytochrome b₆ per mole cytochrome f and one mole RieskeFeS. The 17 kDa and 4 kDa polypeptides are the so called subunit4 and 5 respectively. The 12 kDa protein was identified as plastocyaninby immunoblotting. Plastocyanin and the 4 kDa protein were presentin the cytochrome bf complex even after a second repeated sucrosedensity gradient centrifugation. The sucrose gradient sedimentation pattern was different forthe grana and stroma lamellae complexes. The complex from thestroma lamellae arrives at a higher density than the grana complex.Furthermore, the gradient centrifugation diagram of the stromalamellae consists of one main peak while the diagram of thegrana complex shows two peaks. There is significantly more plastocyaninand 4 kDa protein in the bf complex isolated from stroma lamellaethan from grana. In addition there is a 15 kDa protein in thecomplex isolated from the grana vesicles. Immunoblot analysisafter crosslinking indicated that the 4 kDa protein and theplastocyanin are associated in the cytochrome bf complex. Theoxidoreductase activity is higher (about 50%) in the cytochromebf complex from the grana than from the stroma lamellae fraction.We suggest that a difference in composition of the cytochromebf complex between the two membranes might be important in theregulation of cyclic and non cyclic electron flow. ¹Present address: Department of Plant Physiology II, Universityof Warsaw, 00 927 Warsaw, Poland     

Chlorophyll,glycerolipid and protein ratios in spinach chloroplast grana and stroma lamellae

The relative molar amounts of glycerolipids are similar in grana and stroma lamellae, as are the ratios of total glycerolipid to weight of membrane protein. However the chlorophyll content relative to protein of grana lamellae is about 40% higher than that of stroma lamellae from the same preparation. Previous reports of chemical composition or enzyme activity based on chlorophyll alone can be highly misleading. The large functional and conformational differences between these two membranes may be related to these differences in pigment content, but are likely to result primarily from qualitative protein differences. The data are in accord with a membrane model in which nonpolar regions of membrane protein bind lipid in fairly constant amounts.     

Analysis of the polypeptide composition of grana and stroma thylakoids by two-dimensional gel electrophoresis. Distribution of photosystem II between grana and stroma lamellae

The polypeptide composition of whole thylakoids and membrane subfragments was studied by using a modified two-dimensional gel electrophoresis technique of O'Farrell [J. Biol. Chem. 250, 4007-4021 (1975)]. The modifications were lithium dodecyl sulphate solubilization instead instead of SDS, reverse isofocusing and sensitive silver staining procedure. This high-resolution technique allowed us to separate and identify about 170 polypeptides of thylakoid membranes. After separating grana and stroma thylakoids it was found that both types of lamellae contained nearly equal amounts of polypeptides, but about 70 polypeptides were different in the two preparations. In grana thylakoids, 54 polypeptides out of 95 were found to be mainly present in grana and 31 of them were only present in grana preparations. In stroma membranes, 43 polypeptides out of 99 were mainly present in stroma lamellae and 38 of these polypeptides were exclusively present in stroma lamellae. In a functional photosystem II preparation, 61 individual polypeptides could be distinguished. Most of these polypeptides were present in both grana and stroma lamellae, but 22 of them were more pronounced in grana than in stroma lamellae. 9 polypeptides of photosystem II were distinctly different in grana and stroma lamellae, and these differences may connect closely with the functional differences of photosystem II in the two types of thylakoids.     

Composition of photosynthetic pigments in thylakoid membrane vesicles from spinach

Thylakoid membranes from spinach were fragmented mechanically and separated into vesicles originating from grana and stroma-exposed lamellae (Andreasson et al. (1988) Biochim Biophys Acta 936: 339–350). The grana vesicles were further fragmented and separated into smaller vesicles originating from different parts of the grana (Svensson and Albertsson (1989) Photosynth Res 20: 249–259). All vesicles so obtained were analyzed with respect to chlorophyll and carotenoid composition by reverse phase HPLC. For all fractions the following relations (mole/mole) were found: 1 carotenoid per 4 chlorophyll (a+b), 2 lutein per 5 chlorophyll b and 5 violaxanthin per 100 chlorophyll (a + b). The contents of lutein and neoxanthin were each linearly related to chlorophyll b and -carotene was linearly related to chlorophyll a.     

Functional heterogeneity of photosystem II in domain specific regions of the thylakoid membrane of spinach (Spinacia oleracea L.)

A mild sonication and phase fractionation method has been used to isolate five regions of the thylakoid membrane in order to characterize the functional lateral heterogeneity of photosynthetic reaction centers and light harvesting complexes. Low-temperature fluorescence and absorbance spectra, absorbance cross-section measurements, and picosecond time-resolved fluorescence decay kinetics were used to determine the relative amounts of photosystem II (PSII) and photosystem I (PSI), to determine the relative PSII antenna size, and to characterize the excited-state dynamics of PSI and PSII in each fraction. Marked progressive increases in the proportion of PSI complexes were observed in the following sequence: grana core (BS), whole grana (B3), margins (MA), stroma lamellae (T3), and purified stromal fraction (Y100). PSII antenna size was drastically reduced in the margins of the grana stack and stroma lamellae fractions as compared to the grana. Picosecond time-resolved fluorescence decay kinetics of PSII were characterized by three exponential decay components in the grana fractions, and were found to have only two decay components with slower lifetimes in the stroma. Results are discussed in the framework of existing models of chloroplast thylakoid membrane lateral heterogeneity and the PSII repair cycle. Kinetic modeling of the PSII fluorescence decay kinetics revealed that PSII populations in the stroma and grana margin fractions possess much slower primary charge separation rates and decreased photosynthetic efficiency when compared to PSII populations in the grana stack.     

Reversible particle movements associated with unstacking and restacking of chloroplast membranes in vitro

Freeze-fracture and freeze-etch techniques have been employed to study the supramolecular structure of isolated spinach chloroplast membranes and to monitor structural changes associated with in vitro unstacking and restacking of these membranes. High-resolution particle size histograms prepared from the four fracture faces of normal chloroplast membranes reveal the presence of four distinct categories of intramembranous particles that are nonrandomly distributed between grana and stroma membranes. The large surface particles show a one to one relationship with the EF-face particles. Since the distribution of these particles between grana and stroma membranes coincides with the distribution of photosystem II (PS II) activity, it is argued that they could be structural equivalents of PS II complexes. An interpretative model depicting the structural relationship between all categories of particles is presented. Experimental unstacking of chloroplast membranes in low-salt medium for at least 45 min leads to a reorganization of the lamellae and to a concomitant intermixing of the different categories of membrane particles by means of translational movements in the plane of the membrane. In vitro restacking of such experimentally unstacked chloroplast membranes can be achieved by adding 2-20 mM MgCl2 or 100-200 mM NaCl to the membrane suspension. Membranes allowed to restack for at least 1 h at room temperature demonstrate a resegregation of the EF-face particles into the newly formed stacked membrane regions to yield a pattern and a size distribution nearly indistinguishable from the normally stacked controls. Restacking occurs in two steps: a rapid adhesion of adjoining stromal membrane surfaces with little particle movement, and a slower diffusion of additional large intramembranous particles into the stacked regions where they become trapped. Chlorophyll a:chlorophyll b ratios of membrane fraction obtained from normal, unstacked, and restacked membranes show that the particle movements are paralleled by movements of pigment molecules. The directed and reversible movements of membrane particles in isolated chloroplasts are compared with those reported for particles of plasma membranes.     

From chloroplasts to photosystems: in situ scanning force microscopy on intact thylakoid membranes

Envelope-free chloroplasts were imaged in situ by contact and tapping mode scanning force microscopy at a lateral resolution of 3-5 nm and vertical resolution of approximately 0.3 nm. The images of the intact thylakoids revealed detailed structural features of their surface, including individual protein complexes over stroma, grana margin and grana-end membrane domains. Structural and immunogold-assisted assignment of two of these complexes, photosystem I (PS I) and ATP synthase, allowed direct determination of their surface density, which, for both, was found to be highest in grana margins. Surface rearrangements and pigment- protein complex redistribution associated with salt-induced membrane unstacking were followed on native, hydrated specimens. Unstacking was accompanied by a substantial increase in grana diameter and, eventually, led to their merging with the stroma lamellae. Concomitantly, PS IIalpha effective antenna size decreased by 21% and the mean size of membrane particles increased substantially, consistent with attachment of mobile light-harvesting complex II to PS I. The ability to image intact photosynthetic membranes at molecular resolution, as demonstrated here, opens up new vistas to investigate thylakoid structure and function.     

The grana margins of plant thylakoid membranes

Plant thylakoid membranes contain three structurally distinct domains: the planar appressed membranes of the grana; the planar non-appressed stroma thylakoids; and the highly curved, non-appressed margins of the grana. Evidence is presented to suggest that the grana margins form a significant structural domain, which has hitherto been neglected. If indeed the grana margins contain some of the cytochrome b/f complex, photosystem (PS) I complex and ATP synthase, they form a third functional domain of the laterally heterogeneous continuous thylakoid membrane network. The consequences of grana margins containing complexes are explored with respect to linear electron transport under light-saturating and light-limiting conditions, non-cyclic vs cyclic photophorylation, and the regulation of light energy distribution to both PS I and PS II.     

Further evidence for stroma lamellae as a source of Photosystem 1 fractions from spinach chloroplasts

The mechanism of digitonin action on spinach chloroplasts was investigated by thin sectioning. Evidence is presented which shows that digitonin continues to modify membranes for many minutes after the addition of the fixative glutaraldehyde. However, the action of digitonin can be stopped by simultaneous fixation and dilution of the detergent. Such experiments indicate that the initial action of digitonin is to release stroma lamellae which in turn yield a Photosystem 1 fraction. This interpretation is further supported by a significant correlation between the chlorophyll a/chlorophyll b ratio and the ratio of stroma to grana lamellae in spinach chloroplasts.     

Supramolecular photosystem II organization in grana thylakoid membranes: evidence for a structured arrangement

The distribution of photosystem (PS) II complexes in stacked grana thylakoids derived from electron microscopic images of freeze-fractured chloroplasts are examined for the first time using mathematical methods. These characterize the particle distribution in terms of a nearest neighbor distribution function and a pair correlation function. The data were compared with purely random distributions calculated by a Monte Carlo simulation. The analysis reveals that the PSII distribution in grana thylakoids does not correspond to a random protein mixture but that ordering forces lead to a structured arrangement on a supramolecular level. Neighboring photosystems are significantly more separated than would be the case in a purely random distribution. These results are explained by structural models, in which boundary lipids and light-harvesting complex (LHC) II trimers are arranged between neighboring PSII. Furthermore, the diffusion of PSII was analyzed by a Monte Carlo simulation with a protein density of 80% area occupation (determined for grana membranes). The mobility of the photosystems is severely reduced by the high protein density. From an estimate of the mean migration time of PSII from grana thylakoids to stroma lamellae, it becomes evident that this diffusion contributes significantly to the velocity of the repair cycle of photoinhibited PSII.     

Lateral heterogeneity in the distribution of chlorophyll-protein complexes of the thylakoid membranes of spinach chloroplasts

The lateral distribution of the main chlorophyll-protein complexes between appressed and non-appressed thylakoid membranes has been studied. The reaction centre complexes of Photosystems I and II and the light-harvesting complex have been resolved by an SDS-polyacrylamide gel electrophoretic method which permits most of the chlorophyll to remain protein-bound.

The analyses were applied to subchloroplast fractions shown to be derived from different thylakoid regions. Stroma thylakoids were separated from grana stacks by centrifugation following chloroplast disruption by press treatment or digitonin. Vesicles derived from the grana partitions were isolated by aqueous polymer two-phase partition. A substantial depletion in the amount of Photosystem I chlorophyll-protein complex and an enrichment in the Photosystem II reaction centre complex and the light-harvesting complex occurred in the appressed grana partition region. The high enrichment in this fraction compared to grana stack fractions derived from press or digitonin treatments, suggests that the grana Photosystem I is restricted mainly to the non-appressed grana end membranes and margins, and that the grana partitions possess mainly Photosystem II reaction centre complex and the light-harvesting complex.

In contrast, stroma thylakoids are highly enriched in the Photosystem I reaction centre complex. They possess also some 10–20% of the total Photosystem II reaction centre complex and the light-harvesting complex.

The ratio of light-harvesting complex to Photosystem II reaction centre complex is rather constant in all subchloroplast fractions suggesting a close association between these complexes. This was not so for the ratio of light-harvesting complex and the Photosystem I reaction centre complex.

The lateral heterogeneity in the distribution of the photosystems between appressed and non-appressed membranes must have a profound impact on current understanding of both the distribution of excitation energy and photosynthetic electron transport between the photosystems.