Ultrastructural changes in chloroplasts resulting from fluctuations in NaCl concentration: freeze-fracture of thylakoid membranes in Dunaliella salina.

The structure of chloroplasts isolated from Dunaliella salina has been studied with respect to changing concentrations of sodium chloride in the culture medium. Freeze-fracture replicas and thin sections of intact chloroplasts do not exhibit any noticeable changes in structure at concentrations ranging between 3.5 and 25% NaCl. Chloroplasts isolated from algal cells that have been acclimatized to the higher salt concentration show a change in the thylakoid membranes. The thylakoid membranes appear compressed over a major portion of the membrane surface, with only the end of the thylakoid membranes unappressed. The number of particles per unit area on the B face is also altered by the salt concentration. The chloroplasts acclimatized to 25% NaCl have about 3 times the number of particles per unit area on a B face of end-membranes as on a comparable face of thylakoid membranes acclimatized to low (3.5% NaCl) salt concentration. These morphological changes can be reversed if the chloroplasts acclimatized to high or low salt concentrations are returned to a medium of different salt concentration prior to freeze-fracturing.     

Developmental heterogeneity of the lipid distribution in the thylakoid membranes of Euglena gracilis

The thylakoid membranes of isolated Euglena chloroplasts were separated into two fractions (appressed and non-appressed membranes) by aqueous two-phase partitioning (mixture of dextran 500 and polyethylene glycol 4000) following press disruption. The lipid composition of these two fractions differ in many respects during most of the cell cycle of this alga in comparison with the thylakoid characteristics of higher plants or green algae. The monogalactosyldiglyceride to digalactosyldiglyceride ratio changes during the cell cycle and the vesicles originating from appressed and nonappressed thylakoid membranes, respectively, differ in this property at the beginning, but tend to be equal at the end of the cell cycle. The levels of sulfoquinovosyldiglyceride and phosphatidylglycerol are highest in appressed membrane regions at about the 6th hour of the cell cycle but are highest in non-appressed membranes near the end of the cell cycle. The insertion and/or assembly of synthesized LHCII is correlated with a high monogalactosyldiglyceride to digalactosyldiglyceride ratio in appressed membrane regions. The heterogeneity of the lipid composition is discussed in relation to the stage-specific development of structure and function of Euglena chloroplasts.     

Unique thylakoid membrane architecture of a unicellular N2-fixing cyanobacterium revealed by electron tomography

Cyanobacteria, descendants of the endosymbiont that gave rise to modern-day chloroplasts, are vital contributors to global biological energy conversion processes. A thorough understanding of the physiology of cyanobacteria requires detailed knowledge of these organisms at the level of cellular architecture and organization. In these prokaryotes, the large membrane protein complexes of the photosynthetic and respiratory electron transport chains function in the intracellular thylakoid membranes. Like plants, the architecture of the thylakoid membranes in cyanobacteria has direct impact on cellular bioenergetics, protein transport, and molecular trafficking. However, whole-cell thylakoid organization in cyanobacteria is not well understood. Here we present, by using electron tomography, an in-depth analysis of the architecture of the thylakoid membranes in a unicellular cyanobacterium, Cyanothece sp. ATCC 51142. Based on the results of three-dimensional tomographic reconstructions of near-entire cells, we determined that the thylakoids in Cyanothece 51142 form a dense and complex network that extends throughout the entire cell. This thylakoid membrane network is formed from the branching and splitting of membranes and encloses a single lumenal space. The entire thylakoid network spirals as a peripheral ring of membranes around the cell, an organization that has not previously been described in a cyanobacterium. Within the thylakoid membrane network are areas of quasi-helical arrangement with similarities to the thylakoid membrane system in chloroplasts. This cyanobacterial thylakoid arrangement is an efficient means of packing a large volume of membranes in the cell while optimizing intracellular transport and trafficking.     

Thylakoid Membrane Protein Kinase Activity as a Signal Transduction Pathway in Chloroplasts

In plants external stimuli are perceived through a cascade of signals and signal transduction pathways. Protein phosphorylation and de-phosphorylation is one of the most important transduction paths for the perception of signals in plants. The highest concentrations of plant phospho-proteins are located in chloroplasts. This facilitates the protection of thylakoid membranes from stress-induced damage and augments adaptive strategies in plants. In this review, the protein kinases associated with phosphorylation of thylakoid membrane protein, and the adaptive changes in thylakoid membrane architecture and developmental cues are given. The presence of membrane bound kinases in thylakoid membranes have evolutionary implications for the signal transduction pathways and the photosynthetic gene expression for thylakoid membrane protein dynamics. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thylakoid membrane remodeling during state transitions in Arabidopsis

Adaptability of oxygenic photosynthetic organisms to fluctuations in light spectral composition and intensity is conferred by state transitions, short-term regulatory processes that enable the photosynthetic apparatus to rapidly adjust to variations in light quality. In green algae and higher plants, these processes are accompanied by reversible structural rearrangements in the thylakoid membranes. We studied these structural changes in the thylakoid membranes of Arabidopsis thaliana chloroplasts using atomic force microscopy, scanning and transmission electron microscopy, and confocal imaging. Based on our results and on the recently determined three-dimensional structure of higher-plant thylakoids trapped in one of the two major light-adapted states, we propose a model for the transitions in membrane architecture. The model suggests that reorganization of the membranes involves fission and fusion events that occur at the interface between the appressed (granal) and nonappressed (stroma lamellar) domains of the thylakoid membranes. Vertical and lateral displacements of the grana layers presumably follow these localized events, eventually leading to macroscopic rearrangements of the entire membrane network.     

Assembly of Newly Imported Oxygen-Evolving Complex Subunits in Isolated Chloroplasts: Sites of Assembly and Mechanism of Binding

We have examined the assembly of the nuclear-encoded subunits of the oxygen-evolving complex (OEC) after their import into isolated intact chloroplasts. We showed that all three subunits examined (OE33, OE23, and OE17) partition between the thylakoid lumen and a site on the inner surface of the thylakoid membrane after import in a homologous system (e.g., pea or spinach subunits into pea or spinach chloroplasts, respectively). Although some interspecies protein import experiments resulted in OEC subunit binding, maize OE17 did not bind thylakoid membranes in chloroplasts isolated from peas. Newly imported OE33 and OE23 were washed from the membranes at the same concentrations of urea and NaCl as the native, indigenous proteins; this observation suggests that the former subunits are bound productively within the OEC. Inhibition of neither chloroplast protein synthesis nor light- or ATP-dependent energization of the thylakoid membrane significantly affected these assembly reactions, and we present evidence suggesting that incoming subunits actively displace those already bound to the thylakoid membrane. Transport of OE33 took place primarily in the stromal-exposed membranes and proceeded through a protease-sensitive, mature intermediate. Initial binding of OE33 to the thylakoid membrane occurred primarily in the stromal-exposed membranes, from where it migrated with measurable kinetics to the granal region. In contrast, OE23 assembly occurred in the granal membrane regions. This information is incorporated into a model of the stepwise assembly of oxygen-evolving photosystem II.     

Cytosolic events involved in chloroplast protein targeting

Chloroplasts are unique organelles that are responsible for photosynthesis. Although chloroplasts contain their own genome, the majority of chloroplast proteins are encoded by the nuclear genome. These proteins are transported to the chloroplasts after translation in the cytosol. Chloroplasts contain three membrane systems (outer/inner envelope and thylakoid membranes) that subdivide the interior into three soluble compartments known as the intermembrane space, stroma, and thylakoid lumen. Several targeting mechanisms are required to deliver proteins to the correct chloroplast membrane or soluble compartment. These mechanisms have been extensively studied using purified chloroplasts in vitro. Prior to targeting these proteins to the various compartments of the chloroplast, they must be correctly sorted in the cytosol. To date, it is not clear how these proteins are sorted in the cytosol and then targeted to the chloroplasts. Recently, the cytosolic carrier protein AKR2 and its associated cofactor Hsp17.8 for outer envelope membrane proteins of chloroplasts were identified. Additionally, a mechanism for controlling unimported plastid precursors in the cytosol has been discovered. This review will mainly focus on recent findings concerning the possible cytosolic events that occur prior to protein targeting to the chloroplasts. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.     

Functional and mutational analysis of the light-harvesting chlorophyll a/b protein of thylakoid membranes

The precursor for a Lemna light-harvesting chlorophyll a/b protein (pLHCP) has been synthesized in vitro from a single member of the nuclear LHCP multigene family. We report the sequence of this gene. When incubated with Lemna chloroplasts, the pLHCP is imported and processed into several polypeptides, and the mature form is assembled into the light-harvesting complex of photosystem II (LHC II). The accumulation of the processed LHCP is enhanced by the addition to the chloroplasts of a precursor and a co-factor for chlorophyll biosynthesis. Using a model for the arrangement of the mature polypeptide in the thylakoid membrane as a guide, we have created mutations that lie within the mature coding region. We have studied the processing, the integration into thylakoid membranes, and the assembly into light-harvesting complexes of six of these deletions. Four different mutant LHCPs are found as processed proteins in the thylakoid membrane, but only one appears to have an orientation in the membrane that is similar to that of the wild type. No mutant LHCP appears in LHC II. The other two mutant LHCPs cannot be detected within the chloroplasts. We conclude that stable complex formation is not required for the processing and insertion of altered LHCPs into the thylakoid membrane. We discuss the results in light of our model.     

Changes of Thylakoid Membrane Stacks and Chl a/b Ratio of Chloroplast from Sacred Lotus (Nelumbo nucifera) Seeds During Their Germination Under Light

Changes of chloroplast thylakoid membrane stacks and Chl a/b ratio in the plumule of sacred lotus (Nelumbo nucifera Gaertn) seeds during their germination under light were as follows: Before germination there were giant grana and very low Chi a/b ratio (0.9) in the chloroplasts. Two days after germination, the thylakoid membranes of the giant grana gradually loosened and even destacked (disintegrated), the Chl a/b ratio was 1.06. Four clays after germination, the newly formed grana thylakoid membranes were 3–5 times shorter than those of the supergrana thylakoid membranes before germination and less grana stacks were seen; the Chl a/b ratio was 1.42. Six days after germination, the stacked thylakoi membranes became more orderly arranged. In addition the grana increased in number, the stroma thylakoid membranes were scarce, the Chl a/b ratio was 2.16. Eiglt days after germination, the thylakoid membranes in each granum decreased, but the total number of grana increased only slightly. In the meantime, some large starch grains and more stroma thylakoid membranes appeared; the Chl a/b ratio was 2.77. Ten days after germination normal thylakoid membrane structure was formed both in grana and stroma lamellae. They were arranged orderly as in the chloroplasts of other higher plants; the Chl a/b ratio was 2.80. The following conclusions could be drawn from the above mentioned results: 1) There was a negative correlation between the degree of stacking of the grana thylakoid membranes and the Chl a/b ratio. This statement further proved that the membranes stacking might mainly be induced by LHCII. 2) Development of the grana thylakoid membranes within chloroplasts from sacred lotus plumule followed that of the stroma thylakoid membranes, and the tendency of changes of their Chl 2/b ratio being from the lowest to the highest and then to normal were quite different from those of other higher plants. The chloroplasts iri the latter plants contain long parallel stacks of nonappressed primary thylakoids at second step, and the changes of their ratio of Chl a/b tend to be from the highest to the lowest and then to normal. There are indications that sacred lotus plumule might employ a distinctive developing pathway. This provides an important basis for Nelumbo to possess an unique position in phylogeny of Angiospermae.     

Isoelectric points of membrane surfaces of three spinach chloroplast classes determined by cross-partition

The isoelectric points of the membranes surrounding three classes of spinach chloroplasts have been determined by partition at different pH values in aqueous two-phase systems where the electrical potential differences at the interface are opposite (cross-partition). Class I chloroplasts, intact chloroplasts, have an isoelectric point at pH 3.8–4.1 and class II chloroplasts, broken chloroplasts or intact thylakoid membranes, have an isoelectric point at pH 4.7–4.9. The third class of particles, class III ‘chloroplasts’, that contain one or more chloroplasts, mitochondria, peroxisomes and some cytoplasm all surrounded by a membrane, probably the plasma membrane, have an isoelectric point at pH 3.4–4.0. The partition technique used presumably yields the isoelectric point of the surface of the membranes exposed to the phase system by the three classes of chloroplasts, i.e., the outer envelope membrane, the thylakoid membrane and the plasma membrane, respectively. The isoelectric points obtained with this technique are suggested to reflect protein to charged-lipid differences in the composition of the membranes.     

Insights into the complex 3-D architecture of thylakoid membranes in the unicellular cyanobacterium Cyanothece sp. ATCC 51142

In cyanobacteria and chloroplasts, thylakoids are the complex internal membrane system where the light reactions of oxygenic photosynthesis occur. In plant chloroplasts, thylakoids are differentiated into a highly interconnected system of stacked grana and unstacked stroma membranes. In contrast, in cyanobacteria, the evolutionary progenitors of chloroplasts, thylakoids do not routinely form stacked and unstacked regions, and the architecture of the thylakoid membrane systems is only now being described in detail in these organisms. We used electron tomography to examine the thylakoid membrane systems in one cyanobacterium, Cyanothece sp. ATCC 51142. Our data showed that thylakoids form a complicated branched network with a rudimentary quasi-helical architecture in this organism. A well accepted helical model of grana-stroma architecture of plant thylakoids describes an organization in which stroma thylakoids wind around stacked granum in right-handed spirals. Here we present data showing that the simplified helical architecture in Cyanothece 51142 is lefthanded in nature. We propose a model comparing the thylakoid membranes in plants and this cyanobacterium in which the system in Cyanothece 51142 is composed of non-stacked membranes linked by fret-like connections to other membrane components of the system in a limited left-handed arrangement.Key words: cyanobacteria, Cyanothece 51142, thylakoid membrane, electron tomography, chloroplast     

The effects of reduced amounts of lipid unsaturation on chloroplast ultrastructure and photosynthesis in a mutant of Arabidopsis

A mutant of Arabidopsis thaliana with reduced content of C_18:3 and C_16:3 fatty acids in membrane lipids exhibited a 45% reduction in the cross-sectional area of chloroplasts and had a decrease of similar magnitude in the amount of chloroplast lamellar membranes. The reduction in chloroplast size was partially compensated by a 45% increase in the number of chloroplasts per cell in the mutant. When expressed on a chlorophyll basis the rates of CO₂-fixation and photosynthetic electron transport were not affected by these changes. Fluorescence polarization measurements indicated that the fluidity of the thylakoid membranes was not significantly altered by the mutation. Similarly, on the basis of temperature-induced fluorescence yield enhancement measurements, there was no significant effect on the thermal stability of chlorophyll-protein complexes in the mutant. These observations suggest that the high content of trienoic fatty acids in chloroplast lipids may be an important factor regulating organelle biogenesis but is not required to support normal levels of the photosynthetic activities associated with the thylakoid membranes.     

Subdiffraction‐resolution live‐cell imaging for visualizing thylakoid membranes

The chloroplast is the chlorophyll‐containing organelle that produces energy through photosynthesis. Within the chloroplast is an intricate network of thylakoid membranes containing photosynthetic membrane proteins that mediate electron transport and generate chemical energy. Historically, electron microscopy (EM) has been a powerful tool for visualizing the macromolecular structure and organization of thylakoid membranes. However, an understanding of thylakoid membrane dynamics remains elusive because EM requires fixation and sectioning. To improve our knowledge of thylakoid membrane dynamics we need to consider at least two issues: (i) the live‐cell imaging conditions needed to visualize active processes in vivo; and (ii) the spatial resolution required to differentiate the characteristics of thylakoid membranes. Here, we utilize three‐dimensional structured illumination microscopy (3D‐SIM) to explore the optimal imaging conditions for investigating the dynamics of thylakoid membranes in living plant and algal cells. We show that 3D‐SIM is capable of examining broad characteristics of thylakoid structures in chloroplasts of the vascular plant Arabidopsis thaliana and distinguishing the structural differences between wild‐type and mutant strains. Using 3D‐SIM, we also visualize thylakoid organization in whole cells of the green alga Chlamydomonas reinhardtii. These data reveal that high light intensity changes thylakoid membrane structure in C. reinhardtii. Moreover, we observed the green alga Chromochloris zofingiensis and the moss Physcomitrella patens to show the applicability of 3D‐SIM. This study demonstrates that 3D‐SIM is a promising approach for studying the dynamics of thylakoid membranes in photoautotrophic organisms during photoacclimation processes.     

MFP1 is a thylakoid-associated,nucleoid-binding protein with a coiled-coil structure

Plastid DNA, like bacterial and mitochondrial DNA, is organized into protein–DNA complexes called nucleoids. Plastid nucleoids are believed to be associated with the inner envelope in developing plastids and the thylakoid membranes in mature chloroplasts, but the mechanism for this re-localization is unknown. Here, we present the further characterization of the coiled-coil DNA-binding protein MFP1 as a protein associated with nucleoids and with the thylakoid membranes in mature chloroplasts. MFP1 is located in plastids in both suspension culture cells and leaves and is attached to the thylakoid membranes with its C-terminal DNA-binding domain oriented towards the stroma. It has a major DNA-binding activity in mature Arabidopsis chloroplasts and binds to all tested chloroplast DNA fragments without detectable sequence specificity. Its expression is tightly correlated with the accumulation of thylakoid membranes. Importantly, it is associated in vivo with nucleoids, suggesting a function for MFP1 at the interface between chloroplast nucleoids and the developing thylakoid membrane system.     

Multiple pathways for protein transport into or across the thylakoid membrane.

Many thylakoid proteins are cytosolically synthesized and have to cross the two chloroplast envelope membranes as well as the thylakoid membrane en route to their functional locations. In order to investigate the localization pathways of these proteins, we over-expressed precursor proteins in Escherichia coli and used them in competition studies. Competition was conducted for import into the chloroplast and for transport into or across isolated thylakoids. We also developed a novel in organello method whereby competition for thylakoid transport occurred within intact chloroplasts. Import of all precursors into chloroplasts was similarly inhibited by saturating concentrations of the precursor to the OE23 protein. In contrast, competition for thylakoid transport revealed three distinct precursor specificity groups. Lumen-resident proteins OE23 and OE17 constitute one group, lumenal proteins plastocyanin and OE33 a second, and the membrane protein LHCP a third. The specificity determined by competition correlates with previously determined protein-specific energy requirements for thylakoid transport. Taken together, these results suggest that thylakoid precursor proteins are imported into chloroplasts on a common import apparatus, whereupon they enter one of several precursor-specific thylakoid transport pathways.     

Import of proteins into chloroplasts. Membrane integration of a thylakoid precursor protein reconstituted in chloroplast lysates

Many of the thylakoid membrane proteins of plant and algal chloroplasts are synthesized in the cytosol as soluble, higher molecular weight precursors. These precursors are post-translationally imported into chloroplasts, incorporated into the thylakoids, and proteolytically processed to mature size. In the present study, the process by which precursors are incorporated into thylakoids was reconstituted in chloroplast lysates using the precursor to the light-harvesting chlorophyll a/b protein (preLHCP) as a model. PreLHCP inserted into thylakoid membranes, but not envelope membranes, if ATP was present in the reaction mixture. Correct integration into the bilayer was verified by previously documented criteria. Integration could also be reconstituted with purified thylakoid membranes if reaction mixtures were supplemented with a soluble extract of chloroplasts. Several other thylakoid precursor proteins in addition to preLHCP, but no stromal precursor proteins, were incorporated into thylakoids under the described assay conditions. These results suggest that the observed in vitro activity represents in vivo events during the biogenesis of thylakoid proteins.     

Events surrounding the early development of Euglena chloroplasts. 15. Origin of plastid thylakoid polypeptides in wild-type and mutant cells.

Techniques are described for the isolation of plastid thylakoid membranes from light-grown and dark-grown cells of Euglena gracilis var. bacillaris, and from mutants affecting plastid development. These membranes, which have minimal contamination with other cell fractions, are localized in sucrose gradients by using the thylakoid membrane sulfolipid as a specific marker. The plastid thylakoid membrane polypeptides isolated from these membranes were separated on SDS polyacrylamide gels and yielded patterns containing 30-40 polypeptides. Light-grown strain Z gave patterns identical with bacillaris. Since the plastid thylakoid polypeptide patterns obtained from dark-grown wild-type cells and from a bleached mutant W3BUL in which plastid DNA is undetectable are identical, it appears that the proplastid thylakoid polypeptides of wild-type cannot be coded in plastid DNA and are probably coded in nuclear DNA. The plastid thylakoid polypeptide patterns obtained from various dark-grown mutants, making large but abnormal chloroplasts, show a correlation between the amount of chlorophyll formed and the amount of a plastid thylakoid polypeptide thought to be associated wtth one of the pigment-protein light-harvesting complexes. Treatment with SAN 9789 (4-chloro-5-(methylamino)-2(alpha, alpha, alpha,-trifluoro-m-tolyl)-3-(2H(pyridazinone) known to block carotenoid synthesis at the level of phytoene, causes a progressive loss of all plastid thylakoid polypeptides during growth in darkness and results in the establishment of a new, lowere steady-state level of sulfolipid. At least ten of the plastid thylakoid polypeptides become labeled when isolated chloroplasts are supplied with radioactive amono acids; of these six are undectable in W3BUL and are, therefore, candidates for coding by plastid DNA.     

Protein translocation across membranes.

Cellular membranes act as semipermeable barriers to ions and macromolecules. Specialized mechanisms of transport of proteins across membranes have been developed during evolution. There are common mechanistic themes among protein translocation systems in bacteria and in eukaryotic cells. Here we review current understanding of mechanisms of protein transport across the bacterial plasma membrane as well as across several organelle membranes of yeast and mammalian cells. We consider a variety of organelles including the endoplasmic reticulum, outer and inner membranes of mitochondria, outer, inner, and thylakoid membranes of chloroplasts, peroxisomes, and lysosomes. Several common principles are evident: (a) multiple pathways of protein translocation across membranes exist, (b) molecular chaperones are required in the cytosol, inside the organelle, and often within the organelle membrane, (c) ATP and/or GTP hydrolysis is required, (d) a proton-motive force across the membrane is often required, and (e) protein translocation occurs through gated, aqueous channels. There are exceptions to each of these common principles indicating that our knowledge of how proteins translocate across membranes is not yet complete.     

Deletion of the chloroplast-localized AtTerC gene product in Arabidopsis thaliana leads to loss of the thylakoid membrane and to seedling lethality

Early seedling development in plants depends on the biogenesis of chloroplasts from proplastids, accompanied by the formation of thylakoid membranes. An Arabidopsis thaliana gene, AtTerC , whose gene product shares sequence similarity with bacterial tellurite resistance C (TerC), is shown to be involved in a critical step required for the normal organization of prothylakoids and transition into mature thylakoid stacks. The AtTerC gene encodes an integral membrane protein, which contains eight putative transmembrane helices, localized in the thylakoid of the chloroplast, as shown by localization of an AtTerC–GFP fusion product in protoplasts and by immunoblot analysis of subfractions of chloroplasts. T-DNA insertional mutation of AtTerC resulted in a pigment-deficient and seedling-lethal phenotype under normal light conditions. Transmission electron microscopic analysis revealed that mutant etioplasts had normal prolamellar bodies (PLBs), although the prothylakoids had ring-like shapes surrounding the PLBs. In addition, the ultrastructures of mutant chloroplasts lacked thylakoids, did not have grana stacks, and showed numerous globular structures of varying sizes. Also, the accumulation of thylakoid membrane proteins was severely defective in this mutant. These results suggest that the AtTerC protein plays a crucial role in prothylakoid membrane biogenesis and thylakoid formation in early chloroplast development.     

Mutations in a signal sequence for the thylakoid membrane identify multiple protein transport pathways and nuclear suppressors

The apparatus that permits protein translocation across the internal thylakoid membranes of chloroplasts is completely unknown, even though these membranes have been the subject of extensive biochemical analysis. We have used a genetic approach to characterize the translocation of Chlamydomonas cytochrome f, a chloroplast-encoded protein that spans the thylakoid once. Mutations in the hydrophobic core of the cytochrome f signal sequence inhibit the accumulation of cytochrome f, lead to an accumulation of precursor, and impair the ability of Chlamydomonas cells to grow photosynthetically. One hydrophobic core mutant also reduces the accumulation of other thylakoid membrane proteins, but not those that translocate completely across the membrane. These results suggest that the signal sequence of cytochrome f is required and is involved in one of multiple insertion pathways. Suppressors of two signal peptide mutations describe at least two nuclear genes whose products likely describe the translocation apparatus, and selected second-site chloroplast suppressors further define regions of the cytochrome f signal peptide.