A chloroplast membrane lacking Photosystem II. Thylakoid stacking in the absence of the Photosystem II particle

The polypeptide composition and membrane structure of a variegated mutant of tobacco have been investigated. The pale green mutant leaf regions contain chloroplasts in which the amount of membrane stacking has been reduced (although not totally eliminated). The mutant membranes are almost totally deficient in Photosystem II when compared to wild-type chloroplast membranes, but still show near-normal levels of Photosystem I activity. The pattern of membrane polypeptides separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis shows several differences between mutant and wild-type membranes, although the major chlorophyll-protein complexes described in many other plant species are present in both mutant and wild-type samples. Freeze-fracture analysis of the internal structure of these photosynthetic membranes shows that the Photosystem II-deficient membranes lack the characteristic large particle associated with the E fracture face of the thylakoid. These membranes also lack a tetramer-like particle visible on the inner (ES) surface of the membrane. The other characteristics of the photosynthetic membrane, including the small particles observed on the P fracture faces in both stacked and unstacked regions, and the characteristic changes in the background matrix of the E fracture face which accompany thylakoid stacking, are unaltered in the mutant. From these and other observations we conclude that the large (EF and ES) particle represents an amalgam of many components comprising the Photosystem II reaction complex, that the absence of one or more of its components may prevent the structure from assembling, and that in its absence, Photosystem II activity cannot be observed.     

The light-harvesting chlorpohyll-protein complex of photosystem II. Its location in the photosynthetic membrane

We have investigated the structure of the photosynthetic membrane in a mutant of barley known to lack a chlorophyll-binding protein. This protein is thought to channel excitation energy to photosystem II, and is known as the "light-harvesting chlorophyll-protein complex." Extensive stacking of thylakoids into grana occurs in both mutant and wild-type chloroplasts. Examination of membrane internal structure by freeze-fracturing indicates that only slight differences exist between the fracture faces of mutant and wild-type membranes. These differences are slight reductions in the size of particles visible on the EFs fracture face, and in the number of particles seen on the PFs fracture face. No differences can be detected between mutant and wild-type on the etched out surface of the membrane. In contrast, tetrameric particles visible on the etched inner surface of wild-type thylakoids are extremely difficult to recognize on similar surfaces of the mutant. These particles can be recognized on inner surfaces of the mutant membranes when they are organized into regular lattices, but these lattices show a much closer particle-to-particle spacing than similar lattices in wild-type membranes. Although several interpretations of these data are possible, these observations are consistent with the proposal that the light-harvesting chlorophyll-protein complex of photosystem II is bound to the tetramer (which is visible on the EFs face as a single particle) near the inner surface of the membrane. The large tetramer, which other studies have shown to span the thylakoid membrane, may represent an assembly of protein, lipid, and pigment comprising all the elements of the photosystem II reaction. A scheme is presented which illustrates one possibility for the light reaction across the photosynthetic membrane.     

A chloroplast membrane lacking Photosystem I. Changes in unstacked membrane regions

The structure and polypeptide composition of the photosynthetic membrane of a mutant of maize has been investigated. The thylakoid membranes of the mutant plants are deficient in Photosystem I activity, although Photosystem II is at near normal levels. SDS polyacrylamide gel electrophoresis of thylakoid membranes from the mutant shows them to be deficient in two polypeptide bands which have been associated with Photosystem I. Freeze-fracture studies of the membrane show that the absence of these polypeptides is associated with a measurable reduction in particle diameter on the unstacked protoplasmic fracture face. This fracture face is derived from the splitting of membranes in unstacked regions of the thylakoid membrane system. It is suggested that in membranes stacked by salts in vitro, Photosystem I activity may be confined to this region.     

Chloroplast membrane structure. Intramembranous particles of different sizes make contact in stacked membrane regions.

The supramolecular architecture of stacked thylakoid membrane regions of class II spinach chloroplasts has been investigated by means of freeze-fracture electron microscopy. Such membranes contain two basic types of intramembranous particles: laarge particles, which are found on the fracture face of the lumenal membrane leaflet (Bs face), and smaller ones which are found on the fracture face of the external leaflet (Cs face). By analyzing thylakoid membranes containing geometrical arrangements of intramembranous particles it is shown (a) that within the plane of each membrane approximately two small particles are associated with each large particle, and (b) that normal thylakoid stacking involves the connection of large particles of one membrane to small particles of the other and vice versa. If the two types of particles are related to Photosystems I and II, as suggested by circumstantial evidence, then our observations provide support for the idea that maximum Photosystem I-photosystem II interaction is obtained by intermembrane subunit interaction in grana stacks. To this end, our results suggest that stacking should enhance the quantum yield at very low light intensities.     

The polypeptides of stacked and unstacked Chlamydomonas reinhardi chloroplast membranes and their relation to photosystem II activity

When cells of the ac-5 mutant strain of Chlamydomonas reinhardi are cultured mixotrophically, their chloroplast membranes are unstacked and they lack a group of membrane polypeptides that have been reported to be associated with a membrane fraction enriched for Photosystem II activity. On the other hand, the chloroplast membranes of cells grown phototrophically are stacked and they possess the membrane polypeptides. Since the unstacked membranes possess Photosystem II activity, we suggest that the polypeptides must be present in the chloroplast membrane if stacking is to occur.     

Chloroplast membrane biogenesis in chlamydomonas: Correlation between the formation of membrane components and membrane structure

The y-1 mutant of Chlamydomonas reinhardi, when allowed to green in the presence of chloramphenicol (CAP), an inhibitor of protein synthesis on 70s ribosomes, form photosynthetic membranes which contain somewhat less chlorophyll than those of cells greened in the absence of the drug. Photosystem I and II activities are drastically reduced in the CAP-greened cells, and specific alterations in the polypeptide composition of the thylakoid membranes are also observed. We have examined the internal structure of the thylakoid membranes from cells greened in the presence and absence of CAP, and have found that the large particles observed on the exoplasmic fracture face (EF) are substantially reduced in size and number in the CAP-greened cells. This structural defect seems related to the absence of significant photosystem activities in the CAP-greened cells, despite the presence, of most major membrane polypeptides. We suggest that CAP treatment results in a failure of the cell to organize functional reaction complexes, and is structurally reflected in the absence of large (EF) particles in such membranes. This defect can be repaired by allowing the affected cells to re-green in the absence of the drug, and the large particles reappear, paralleling an increase in photosynthetic activity.     

Partial characterization of the biosynthesis and integration of the Photosystem II reaction centers in the thylakoid membrane of Chlamydomonas reinhardtii

Pulse-labeling of wild-type and a Photosystem II mutant strain of Chlamydomonas reinhardtii was carried out in the presence or absence of inhibitors of either cytoplasmic or chloroplast ribosomes, and their thylakoid membrane polypeptides were analyzed by polyacrylamide gel electrophoresis. A pulse-chase study was also done on the wild-type strain in the presence of anisomycin, an inhibitor of protein synthesis on cytoplasmic ribosomes. The following results were obtained: the Photosystem II reaction center is mainly composed of integral membrane proteins synthesized within the chloroplast. Several of the proteins of the Photosystem II reaction center are post-translationally modified, after they have been inserted in the thylakoid membrane.     

Spatial relationship of photosystem I, photosystem II, and the light- harvesting complex in chloroplast membranes

We have previously demonstrated (Armond, P. A., C. J. Arntzen, J.-M. Briantais, and C. Vernotte. 1976. Arch. Biochem. Biophys. 175:54-63; and Davis, D. J., P. A. Armond, E. L. Gross, and C. J. Arntzen. 1976. Arch. Biochem. Biophys. 175:64-70) that pea seedlings which were exposed to intermittent illumination contained incompletely developed chloroplasts. These plastids were photosynthetically competent, but did not contain grana. We now demonstrate that the incompletely developed plastids have a smaller photosynthetic unit size; this is primarily due to the absence of a major light-harvesting pigment-protein complex which is present in the mature membranes. Upon exposure of intermittent- light seedlings to continuous white light for periods up to 48 h, a ligh-harvesting chlorophyll-protein complex was inserted into the chloroplast membrane with a concomitant appearance of grana stacks and an increase in photosynthetic unit size. Plastid membranes from plants grown under intermediate light were examined by freeze-fracture electron microscopy. The membrane particles on both the outer (PF) and inner (EF) leaflets of the thylakoid membrane were found to be randomly distributed. The particle density of the PF fracture face was approx. four times that of the EF fracture face. While only small changes in particle density were observed during the greening process under continuous light, major changes in particle size were noted, particularly in the EF particles of stacked regions (EFs) of the chloroplast membrane. Both the changes in particle size and an observed aggregation of the EF particles into the newly stacked regions of the membrane were correlated with the insertion of light-harvesting pigment- protein into the membrane. Evidence is presented for identification of the EF particles as the morphological equivalent of a "complete" photosystem II complex, consisting of a phosochemically active "core" complex surrounded by discrete aggregates of the light-harvesting pigment protein. A model demonstrating the spatial relationships of photosystem I, photosystem II, and the light-harvesting complex in the chloroplast membrane is presented.     

Arrangement of photosystem II supercomplexes in crystalline macrodomains within the thylakoid membrane of green plant chloroplasts

The chloroplast thylakoid membrane of green plants is organized in stacked grana membranes and unstacked stroma membranes. We investigated the structural organization of Photosystem II (PSII) in paired grana membrane fragments by transmission electron microscopy. The membrane fragments were obtained by a short treatment of thylakoid membranes with the mild detergent n-dodecyl-alpha, d-maltoside and are thought to reflect the grana membranes in a native state. The membranes frequently show crystalline macrodomains in which PSII is organized in rows spaced by either 26.3 nm (large-spaced crystals) or 23 nm (small-spaced crystals). The small-spaced crystals are less common but better ordered. Image analysis of the crystals by an aperiodic approach revealed the precise positions of the core parts of PSII in the lattices, as well as features of the peripheral light-harvesting antenna. Together, they indicate that the so-called C(2)S(2) and C(2)S(2)M supercomplexes form the basic motifs of the small-spaced and large-spaced crystals, respectively. An analysis of a pair of membranes with a well-ordered large-spaced crystal reveals that many PSII complexes in one layer face only light-harvesting complexes (LHCII) in the other layer. The implications of this type of organization for the efficient transfer of excitation energy from LHCII to PSII and for the stacking of grana membranes are discussed.     

Partition zone penetration by chymotrypsin, and the localization of the chloroplast flavoprotein and photosystem II.

1. Chymotrypsin treatment of chloroplast membranes inactivates Photosystem II. The inactivation is higher when the activity is measured under low intensity actinic light, suggesting that primary photochemistry is preferentially inactivated. 2. Membrane stacking induced by Mg2+ protects Photosystem II against chymotrypsin inactivation. When the membranes are irreversible unstacked by brief treatment with trypsin, Mg2+ protection against chymotrypsin inactivation of Photosystem II is abolished. 3. The kinetics of inactivation by chymotrypsin of Photosystem II indicates that membrane stacking slows down, but does not prevent, the access of chymotrypsin to Photosystem II, which is mostly located within the partition zones. 4. It is concluded that a partition gap exists between stacked membranes of about 45 A, the size of the chymotrypsin molecule. 5. The kinetics of inhibition of the chloroplast flavoprotein, ferredoxin-NADP reductase, bt its specific antibody is not affected by membrane stacking. This indicates that this enzyme is located outside the partition zones.     

Organization of the photosystem II centers and their associated antennae in the thylakoid membranes: a comparative ultrastructural,biochemical, and biophysical study of Chlamydomonas wild type and mutants lacking in photosystem II reaction centers

We investigated the ultrastructure of thylakoid membranes that lacked either some or all of their Photosystem II centers in the F34SU3 and F34 mutants of Chlamydomonas reinhardtii. We obtained the following results: (a) There are no particles of the 160-A size class on the EF faces of the thylakoids in the absence of Photosystem II centers (as in F34); the F34SU3 contains 50% of the wild-type number of PSII centers and EF particles. (b) The density of the particles on the PF faces of the thylakoids is higher in the mutants than in the wild type. (c) The fluorescence analysis shows that the organization of the pigments is the same regardless of whether 50% of the PSII centers are temporarily inactivated (by preilluminating the wild type) or are actually missing from the thylakoid membrane (F34SU3). Our results, therefore, support a model in which: (a) each 160-A EF particle has only one PSII center surrounded by light-harvesting complexes and (b) part of the PSH antenna is associated with 80-A PF particles in both of the mutants and the wild type.     

Freeze-fracture studies on barley plastid membranes. VII. Structural changes associated with phosphorylation of the light-harvesting complex

Chloroplast thylakoid membranes were isolated from barley at room temperature under redox conditions which ensured that the light-harvesting complex was either non-phosphorylated or phosphorylated. The ultrastructural appearance of these membranes was characterised by rotary shadowed, freeze-fracture electron microscopy. Upon phosphorylation, there was a slight (5%) decrease in the extent of thylakoid stacking, as evidenced by an increase in EFu face particle density. It was concluded from detailed measurements of particle density and size distribution that phosphorylation of the light-harvesting complex results in the movement of some of the Photosystem II EFs particles and some of the PFs particles containing the light-harvesting complex from grana to stroma membranes. There was also a slight increase in PFs particle size and the appearance of a population of large particles on this face, which may be due to conformational changes in the light-harvesting complex or to the movement of some Photosystem I particles from stroma to grana membranes.     

Occurrence of a Photosystem II polypeptide in non-photosynthetic membranes of cyanobacteria

Cytoplasmic and thylakoid membranes have been purified from the cyanobacteria Anacystis nidulans R2 and Phormidium laminosum by sucrose density gradient centrifugation. Probing of Western blots of proteins from these purified membrane fractions with antibodies directed against the 33 kDa polypeptide of Photosystem II from pea indicates that this protein is present in both the thylakoid and cytoplasmic membranes, rather than just the thylakoid membranes. This has been confirmed by immunogold labelling of cells. Oxygen evolution assays have been used to show that the 33 kDa polypeptide is not assembled into a functional Photosystem II complex in the cytoplasmic membranes. This may be due to the absence of other Photosystem II components.     

Supramolecular organization of photosystem II in green plants

Green plant photosystem II (PSII) is involved in the light reactions of photosynthesis, which take place in the thylakoid membrane of the chloroplast. PSII is organized into large supercomplexes with variable amounts of membrane-bound peripheral antenna complexes. These supercomplexes are dimeric and contain usually 2-4 copies of trimeric LHCII complexes and have a further tendency to associate into megacomplexes or into crystalline domains, of which several types have been characterized. This review focuses on the overall composition and structure of the PSII supercomplex of green plants and its organization and interactions within the photosynthetic membrane. Further, we present the current knowledge how the thylakoid membrane is three-dimensionally organized within the chloroplast. We also discuss how the supramolecular organization in the thylakoid membrane and the PSII flexibility may play roles in various short-term regulatory mechanisms of green plant photosynthesis. This article is part of a Special Issue entitled: Photosystem II.     

Ultrastructural organization of chloroplast membranes in mutants of Pisum sativum L. with impaired activity in the photosystems

The ultrastructural organization and the photosynthesis reactions of chloroplast membranes were studied in three lethal mutants of Pisum sativum, Chl-1, Chl-19 and Chl-5, all lacking the capacity to evolve oxygen. The rates of 2,6-dichloroindophenol reduction, delayed fluorescence and electron-spin-resonance signal 1 indicate that Chl-1 and Chl-19 have an impaired activity in photosystem II (PS II), while in Chl-5 the electron transport is blocked between PS I and the reactions of CO₂ fixation. Ultrathin sectioning demonstrates the presence of giant grana in the chloroplasts of Chl-1 and Chl-19, while the chloroplast structure of the Chl-5 is very similar to that of the wild-type. The grana of the Chl-19 mutant contain large multilamellar regions of tightly packed membranes. When the chloroplast membranes were studied by freeze-fracture, the exoplasmic and protoplasmic fracture faces (EF and PF, respectively) in both stacked and unstacked membranes were found to show large differences in particle concentrations and relative population area (per m²), and also in particle size distribution, between all mutant chloroplast membranes and the wild-type. A close correlation between increasing k_mt (ratio of particle concentrations on PF/EF) and PS II activity was observed. The differences in particle concentrations on both fracture faces in different regions of the intact chloroplast membranes of the wild-type are the consequence of a rearrangement of existing membrane components by lateral particle movements since quantitative measurements demonstrate almost complete conservation of intramembrane particles in number and size during the stacking of stroma thylakoid membranes. The results indicating particle movements strongly support the concept that the chloroplast membranes have a highly dynamic structure.Abbreviations DPIP 2,6-dichloroindophenol - EF and PF exoplasmic and protoplasmic fracture faces, respectively - PS I and PS II photosystems I and II, respectively     

Statcking and separatio of photosystem I and photosystem II in plant thylak oid membranes: A physico-chemical view

The main objective of this article is to highlight important experimental findings and discuss current theories on photosystem segregation and stacking in thylakoid membranes. The facts are put in a framework of recent theoretical developments in the field of membrane biochemistry and biophysics. Some important experiments not considered by the currently accepted theories are discussed. Modifications of these theories in order to incorporate new results are proposed. The currently accepted theories on formation of membrane domains (probably responsible for segregation of photosystem 1 and II) and on interlamellar interactions (stacking) are discussed. Finally, a scheme is put forward summarising the forces which are responsible for organisation of functional structure of thylakoid membranes.     

Phosphorylation of the light-harvesting chlorophyll-protein regulates excitation energy distribution between photosystem II and photosystem I

The kinetics of thylakoid membrane protein phosphorylation in the presence of light and adenosine triphosphate is correlated to an incease in the 77 °K fluorescence emission at 735 nm (F735) relative to that at 685 nm (F685). Analysis of detergent-derived submembrane fractions indicate phosphorylation only of the polypeptides of Photosystem II, and the light-harvesting chlorophyll-protein complex serving Photosystem II (LHC-II). Although several polypeptides are phosphorylated, only the dephosphorylation kinetics of LHC-II follow the kinetics of the decrease of the $\text{F735}\text{F685}$ fluorescence emission ratios. The relative quantum yield of Photosystem II was significantly lower in phosphorylated membranes compared to dephosphorylated membranes. Reversible LHC-II phosphorylation thus provides the physiological mechanism for the control of the distribution of absorbed excitation energy between the two photosystems.     

Freeze-etched membrane faces and photosynthetic activity in normal and mutantTradescantia chloroplasts

Summary Membrane structure and photosynthetic activity was investigated in normal and mutant plastids ofTradescantia albiflora cv.aureo-vittata. In the stacked membrane regions (the macrograna) of mutant plastids, the B fracture faces lack both 170 Å particles and photosystem II (PS II) activity. The C face has the normal 110 Å particles, and photosystem I (PS I) activity is also similar to that in normal chloroplasts. In dilated macrograna the particle size on the C face significantly decreases, and as progressive plastid destruction occurs so PS I activity also disappears. It has been concluded that the integrity of B face particles is related to PS II activity, rather than for membrane stacking. A similar correlation seems to be valid for C face particles and PS I activity.     

Evidence for a close spatial location of the binding sites for CO2 and for photosystem II inhibitors

1. CO2-depletion of thylakoid membranes results in a decrease of binding affinity of the Photosystem II (PS II) inhibitor atrazine. The inhibitory efficiency of atrazine, expressed as I50-concentration (50% inhibition) of 2,6-dichlorophenolindophenol reduction, is the same in CO2-depleted as well as in control thylakoids. This shows that CO2-depletion results in a complete inactivation of a part of the total number of electron transport chains. 2. A major site of action of CO2, which had previously been located between the two electron acceptor quinone molecule B (or R) and Photosystem II inhibitor atrazine as suggested by the following observations: (a) CO2-depletion results in a shift of the binding constant (kappa b) of [14C]atrazine to thylakoid membranes indicating a decreased affinity of atrazine to membrane; (b) trypsin treatment, which is known to modify the Photosystem II complex at the level of B, strongly diminishes CO2 stimulation of electron transport reactions in CO2-depleted membranes; and (c) thylakoids from atrazine-resistant plants, which contain a Photosystem II complex modified at the inhibitor binding site, show an altered CO2-stimulation of electron flow. 3. CO2-depletion does not produce structural changes in enzyme complexes involved in Photosystem II function of thylakoid membranes, as shown by freeze-fracture studies using electron microscopy.     

Photosynthetic Properties of ac-31, a Mutant Strain of Chlamydomonas reinhardi Devoid of Chloroplast Membrane Stacking

A pale-green mutant strain of Chlamydomonas reinhardi, ac-31, is characterized by the absence of any stacking of its chloroplast membranes. The capacity for photosynthetic electron transport, phosphorylation, and CO₂ fixation in ac-31 is substantial, and it is concluded that these photosynthetic activities occur within the single membrane. The photosynthetic capacities of wild type and ac-31 as a function of increasing light intensity are compared. Saturation is attained at higher light intensities in ac-31, and the kinetics of the 2 sets of curves are distinctly different. The possibility that energy transfer is enhanced by membrane stacking is suggested by these results. The repeatedly-observed correlation between reduced stacking and disfunctional Photosystem II activities is discussed in view of the observation that ac-31 has no stacking but retains a functional Photosystem II.