Structure of the ATP synthase from chloroplasts studied by electron microscopy. Localization of the small subunits

Photosynthesis on the Ultrafast Time Scale within the Confinements of Quantum Nanostructured Photosystems

Fundamental information on the behavior of excited chlorophyll molecules packed within the confinements of nanosized photosystems I and II, following absorption of light, is presented. Using a 100 femtosecond laser with nanojoule (nJ) pulse energy and a one picosecond streak camera, we observed the light emitted from the nanostructured photosystems without oscillations or hops. The fluorescent exponential decay profiles and high efficiency within the nanostructure suggest that light coherently drains out as a unit. This implies that "quantumness" is linked to quantum confinement on the nano scale.

Important subunit interactions in the chloroplast ATP synthase

General structural features of the chloroplast ATP synthase are summarized highlighting differences between the chloroplast enzyme and other ATP synthases. Much of the review is focused on the important interactions between the epsilon and gamma subunits of the chloroplast coupling factor 1 (CF(1)) which are involved in regulating the ATP hydrolytic activity of the enzyme and also in transferring energy from the membrane segment, chloroplast coupling factor 0 (CF(0)), to the catalytic sites on CF(1). A simple model is presented which summarizes properties of three known states of activation of the membrane-bound form of CF(1). The three states can be explained in terms of three different bound conformational states of the epsilon subunit. One of the three states, the fully active state, is only found in the membrane-bound form of CF(1). The lack of this state in the isolated form of CF(1), together with the confirmed presence of permanent asymmetry among the alpha, beta and gamma subunits of isolated CF(1), indicate that ATP hydrolysis by isolated CF(1) may involve only two of the three potential catalytic sites on the enzyme. Thus isolated CF(1) may be different from other F(1) enzymes in that it only operates on 'two cylinders' whereby the gamma subunit does not rotate through a full 360 degrees during the catalytic cycle. On the membrane in the presence of a light-induced proton gradient the enzyme assumes a conformation which may involve all three catalytic sites and a full 360 degrees rotation of gamma during catalysis.

The structure of the H(+)-ATP synthase from chloroplasts and its subcomplexes as revealed by electron microscopy

The electron microscopic data available on CF(0)F(1) and its subcomplexes, CF(0), CF(1), subunit III complex are collected and the CF(1) data are compared with the high resolution structure of MF(1). The data are based on electron microscopic investigation of negatively stained isolated CF(1), CF(0)F(1) and subunit III complex. In addition, two-dimensional crystals of CF(0)F(1) and CF(0)F(1) reconstituted liposomes were investigated by cryo-electron microscopy. Progress in the interpretation of electron microscopic data from biological samples has been made with the introduction of image analysis. Multi-reference alignment and classification of images have led to the differentiation between different conformational states and to the detection of a second stalk. Recently, the calculation of three-dimensional maps from the class averages led to the understanding of the spatial organisation of the enzyme. Such three-dimensional maps give evidence of the existence of a third connection between the F(0) part and F(1) part.

Interrelation between high and low affinity tentoxin binding sites in chloroplast F1-ATPase revealed by synthetic analogues

Eight synthetic analogues of tentoxin (cyclo-(L-N-MeGlu1-L-Leu2-N-MeDeltaZPhe3-Gly4)) modified in residues 1, 2, and 3 were checked for their ability to inhibit and reactivate the ATPase activity of the activated soluble part of chloroplast ATP synthase. The data were consistent with a model involving two binding sites of different affinities for the toxins. The occupancy of the high affinity site (or tight site) gave rise to an inactive complex, whereas filling both sites (tight + loose) gave rise to a complex of variable activity, dependent on the toxin analogue. Competition experiments between tentoxin and nonreactivating analogues allowed discrimination between the absence of binding and a nonproductive binding to the site of lower affinity (or loose site). The affinity for the loose site was not affected significantly by the modifications of the tentoxin molecule, whereas the affinity for the tight site was found notably changed. Increasing the size of side chain 1 or 2 and introducing a net electrical charge both resulted in a decrease of affinity for the tight site, but the second change dominated the first one. The activity of different ternary complexes enzyme-tentoxin-analogue depended on the nature of the toxin bound on each site and not only on that bound on the loose site. This demonstrates that the reactivation process results from an interaction, direct or not, between these two binding sites. Possible molecular mechanisms are discussed.

Proteolytic cleavage within a regulatory region of the gamma subunit of chloroplast coupling factor 1

The gamma subunit of chloroplast coupling factor 1 (CF1) is susceptible to selective proteolysis when the enzyme is in solution and the epsilon subunit is removed [CF1(-epsilon)]. In spinach thylakoid membranes, rapid cleavage of gamma is dependent on the generation of an electrochemical proton potential. The tryptic cleavage sites within the gamma of oxidized CF1 in illuminated thylakoids as well as of reduced CF1(-epsilon) in solution were determined by N-terminal amino acid sequencing. Two large gamma fragments of 27 000 (gamma27) and 10 000 (gammma10) molecular weight were generated by trypsin treatment of membrane-bound CF1. THe N-terminal gamma27 contains amino acids 1-215, and the C-terminal gamma10 contains 232-323. These polypeptides were tightly associated with the trypsin-resistant core of the enzyme. In contrast, three large gamma fragments were produced by trypsinolysis of reduced CF1(-epsilon). These polypeptides, which were also tightly associated with the trypsin-resistant core, have molecular weights of 7 900(gamma8), 14 850 (gamma15), and 10 000 (gamma10). These fragments contain residues 1-70, 71-204, and 232-323, respectively. The C-terminal gamma10 fragment generated by trypsin treatment of membrane-bound and soluble CF1 are identical. These results suggest that the gamma subunit of CF1 in illuminated thylakoids resembles that of CF1(-epsilon) with respect to accessibility to proteolytic cleavage. Cleavage of gamma between residues 215 and 232 is sufficient to fully activate the ATPase activity of the enzyme without reduction of the gamma disulfide. In addition, cutting within this region is responsible for loss of affinity for the inhibitory epsilon subunit.

Electron microscopy of the F1F0 ATP synthase: from structure to function

The F1F0 ATP synthase is the large multisubunit complex which uses the proton gradient of energetically active membranes to synthesize ATP. While biochemical and genetic approaches have characterized the composition of the enzyme and elucidated many details of its mechanism and assembly, electron microscopy has been the tool of primary importance in determining the arrangement of the many subunits which comprise the F1F0. The highly cooperative catalytic mechanism is tightly coupled to transmembrane proton translocation in a separate and rather distant sector of the complex. An understanding of this intricate process and its control requires an appreciation of subunit interactions, starting with their locations relative to one another. Electron microscopy has provided most of the available structural information on the F1F0, and recent applications of cryo-electron microscopy have captured different functionally relevant configurations which may finally address longstanding questions about subunit rearrangements during the catalytic cycle.

Asymmetry and structural changes in ECF1 examined by cryoelectronmicroscopy

The Escherichia coli ATPase (ECF1) has been studied by cryoelectronmicroscopy and an intrinsic asymmetry of the molecule in the hexagonal projection identified. The three beta subunits could be distinguished. One, which we have called beta 1, has a greater density in projection than the other two; the second, beta 2, is of intermediate density in projection, while the third, beta 3, is smeared out in density. These different features of the beta subunits were used to orient images, and the positions of the gamma and epsilon subunits then established. The location of the gamma subunit, as monitored by the central mass, was not fixed. This subunit could be found in positions that followed an arc from close to beta 2 to close to beta 3, a shift of around 10A, with respect to the center of the mass. The location of the epsilon subunit was monitored after reconstituting a complex of epsilon subunit-depleted ECF1 with a mutant epsilon subunit in which His at residue 38 had been replaced by Cys, and this Cys labeled with an approximately 14A gold particle. The epsilon subunit was found in positions described by an arc between an alpha subunit (alpha 1) and the neighboring beta subunit (beta 1), a shift of around 20A, with respect to the center of the gold particle. A nucleotide dependence of the position of the gamma subunit has been established by Gogol, E.P., Johnston, E., Aggeler, R. and Capaldi, R.A. (1990) Proc. Natl. Acad. Sci. USA 87, 9585-9589. A nucleotide dependence of the position of the epsilon subunit is shown here.(ABSTRACT TRUNCATED AT 250 WORDS)

Principles of functional and structural organization in the bacterial cell: 'compartments' and their enzymes

Most bacteria lack obvious compartmentation, i.e., structural partition of the cell into functional entities (organelles) formed by a closed biological membrane. Nevertheless, these organisms exhibit sophisticated regulation and interactions of their catabolic and anabolic pathways; they are able to exploit a great variety of carbon and energy sources, and they conserve and transform energy in an efficient manner. In a less stringent sense, 'compartments' are also present in bacteria if one accepts that bacterial 'compartments' are not necessarily surrounded by a membrane, but are rather defined as mere functional entities characterized by their structural components, their enzymes and other functional proteins such as binding proteins. This view would mean that the bacterial cell can be described as a highly organized structured system comprised of these functional entities. Regulated transport processes within 'compartments' and across boundaries involving low and high molecular mass compounds, solutes, and ions take place within the 'framework' constituted by this structured system. Special emphasis is given to the fact that many of the transport processes take place involving the functional entity 'energized membrane'. This 'framework', the structural basis for the functional potential of a bacterial cell, can be studied by electron microscopy. Advanced sample preparation techniques and imaging modes are available which keep the danger of artefact formation low; they can be applied at cellular and macromolecular levels. Recent developments in immunoelectron microscopy and affinity labelling techniques provide tools which allow to unequivocally locate enzymes and other antigens in the cell and to identify polypeptide chains in enzyme complexes. Application of these approaches in studies on cellular and macromolecular organization of bacteria and their enzyme systems confirmed some old views but also extended our knowledge. This is exemplified by a description of selected enzyme complexes located in the bacterial cytoplasm, in the cytoplasmic membrane or attached to it, in the periplasmic space, and attached to the cell wall or set free into the surrounding medium.

Structure of the Escherichia coli ATP synthase and role of the gamma and epsilon subunits in coupling catalytic site and proton channeling functions

The structure of the Escherichia coli ATP synthase has been studied by electron microscopy and a model developed in which the alpha and beta subunits of the F1 part are arranged hexagonally (in top view) alternating with one another and surrounding a central cavity of around 35 A at its widest point. The alpha and beta subunits are interdigitated in side view for around 60 A of the 90 A length of the molecule. The F1 narrows and has three-fold symmetry at the end furthest from the F0 part. The F1 is linked to F0 by a stalk approximately 45 A long and 25-30 A in diameter. The F0 part is mostly buried in the lipid bilayer. The gamma subunit provides a domain that extends into the central cavity of the F1 part. The gamma and epsilon subunits are in a different conformation when ATP + Mg2+ are present in catalytic sites than when ATP + EDTA are present. This is consistent with these two small subunits switching conformations as a function of whether or not phosphate is bound to the enzyme at the position of the gamma phosphate of ATP. We suggest that this switching is the key to the coupling of catalytic site events with proton translocation in the F0 part of the complex.