Evidence of the proximity of ATP synthase subunits 6 (a) in the inner mitochondrial membrane and in the supramolecular forms of Saccharomyces cerevisiae ATP synthase

The involvement of subunit 6 (a) in the interface between yeast ATP synthase monomers has been highlighted. Based on the formation of a disulfide bond and using the unique cysteine 23 as target, we show that two subunits 6 are close in the inner mitochondrial membrane and in the solubilized supramolecular forms of the yeast ATP synthase. In a null mutant devoid of supernumerary subunits e and g that are involved in the stabilization of ATP synthase dimers, ATP synthase monomers are close enough in the inner mitochondrial membrane to make a disulfide bridge between their subunits 6, and this proximity is maintained in detergent extract containing this enzyme. The cross-linking of cysteine 23 located in the N-terminal part of the first transmembrane helix of subunit 6 suggests that this membrane-spanning segment is in contact with its counterpart belonging to the ATP synthase monomer that faces it and participates in the monomer-monomer interface.     

Binding and fluorescence studies of the relationship between neurophysin-peptide interaction and neurophysin self-association: an allosteric system exhibiting minimal cooperativity

The mechanism of peptide-enhanced neurophysin self-association was investigated to address questions raised by the crystal structure of a neurophysin-dipeptide complex. The dependence on protein concentration of the binding of a broad range of peptides to the principal hormone-binding site confirmed that occupancy of this site alone, and not a site that bridges the monomer-monomer interface, is the trigger for enhanced dimerization. For the binding of most peptides to the principal hormone-binding site on bovine neurophysin I, the affinity of each dimer site was at least 10 times that of monomer under the conditions used. No interactions between the two sites of the dimer were evident. Fluorescence polarization studies of pressure-induced dimer dissociation indicated that the volume change for this reaction was almost 4 times greater in the liganded than in the unliganded state, pointing to a significant alteration of the monomer-monomer interface upon peptide binding. Novel conformational changes in the vicinity of the single neurophysin tyrosine, Tyr-49, induced by pressures lower than required for subunit dissociation, were also observed. The bovine neurophysin I dimer therefore appears to represent an allosteric system in which there is thermodynamic and functional communication between each binding site and the monomer-monomer interface, but no communication across the interface to the binding site of the other subunit. A model for the peptide-enhanced dimerization is proposed in which intersubunit contacts between monomers reduce the large unfavorable free energy associated with binding-induced intrasubunit conformational change. Structural origins of the lack of communication across the interface are suggested on the basis of the low volume change associated with dimerization in the unliganded state and monomer-monomer contacts in the crystal structure. Potential roles for the peptide alpha-amino group and position 2 phenyl ring in triggering conformational change are discussed.     

Effect of perfringolysin O on the lateral diffusion constant of membrane proteins of hepatocytes as revealed by fluorescence recovery after photobleaching

Perfringolysin O is a thiol-activated cytolytic exotoxin the primary receptor of which is the membrane cholesterol on the cell surface. The effect of perfringolysin O was tested in various hepatocyte preparations. (i) Smears of fresh liver exposed to a mild H2O2 (1.0 mM) injury for 10 min at 37 degrees C, develop a 'peroxide-induced autofluorescence' (PIAF) on the membrane proteins. PIAF is suitable for measuring the average lateral diffusion constant (D) of the membrane proteins by means of fluorescence recovery after photobleaching technique (FRAP). Incubation for 5 min with 600 or 2000 units/ml of the perfringolysin O resulted in a significant increase (32 and 46%, respectively) of D as compared to the controls of the same age group (13-14 months). Various tests like heat denaturation of cholesterol saturation of perfringolysin O before its application as well as thiol-activation of the smears with dithiothreitol revealed that the increase of D is a specific toxin effect due mot probably to the reaction of perfringolysin O with cholesterol. (ii) Isolated hepatocytes were exposed to perfringolysin O and their viability as well as the release of two cytosolic enzymes (lactate dehydrogenase and glutamic-pyruvic transaminase) were measured; 40-60 units/ml of perfringolysin O in 30 min reduced the viability of the hepatocytes to zero and caused a release of about 70% of both cytosolic enzymes. The significance of the results is discussed from the points of view of both the toxin-effect and the FRAP method.     

Bilin organization in cryptomonad biliproteins

The bilin organization of three cryptomonad biliproteins (phycocyanins 612 and 645 and phycoerythrin 545) was examined in detail. Two others (phycocyanin 630 and phycoerythrin 566) were studied less extensively. Phycocyanin 645 and phycoerythrin 545 were suggested to have one bilin in each monomeric (alphabeta) unit of the dimer (alpha2beta2) isolated from the others, and the remaining six bilins may be in pairs. One pair was found across the monomer-monomer interface of the protein dimer, and two identical pairs were proposed to be within the monomer protein units. For phycocyanin 612, a major surprise was that a pair of bilins was apparently not found across the monomer-monomer interface, but the remaining bilins were distributed as in the other two cryptomonad proteins. The effect of temperature on the CD spectra of phycocyainin 612 demonstrated that two of the bands (one positive and one negative) behaved identically, which is required if they are coupled. The two lowest-energy CD bands of phycocyanin 612 originated from paired bilins, and the two higher-energy bands were from more isolated bilins. The paired bilins within the protein monomers contained the lowest-energy transition for these biliproteins. Using the bilins as naturally occurring reporter groups, phycocyanin 612 was shown to undergo a reversible change in tertiary structure at 40 degrees C. Protein monomers were shown to be functioning biliproteins. A hypothesis is that the coupled pair of bilins within the monomeric units offers important advantages for efficient energy migration, and other bilins transfer energy to this pair, extending the wavelength range or efficiency of light absorption.     

Two structural transitions in membrane pore formation by pneumolysin, the pore-forming toxin of Streptococcus pneumoniae.

The human pathogen Streptococcus pneumoniae produces soluble pneumolysin monomers that bind host cell membranes to form ring-shaped, oligomeric pores. We have determined three-dimensional structures of a helical oligomer of pneumolysin and of a membrane-bound ring form by cryo-electron microscopy. Fitting the four domains from the crystal structure of the closely related perfringolysin reveals major domain rotations during pore assembly. Oligomerization results in the expulsion of domain 3 from its original position in the monomer. However, domain 3 reassociates with the other domains in the membrane pore form. The base of domain 4 contacts the bilayer, possibly along with an extension of domain 3. These results reveal a two-stage mechanism for pore formation by the cholesterol-binding toxins.     

The Cholesterol-dependent Cytolysin Membrane-binding Interface Discriminates Lipid Environments of Cholesterol to Support β-Barrel Pore Insertion

The majority of cholesterol-dependent cytolysins (CDCs) utilize cholesterol as a membrane receptor, whereas a small number are restricted to the GPI-anchored protein CD59 for initial membrane recognition. Two cholesterol-binding CDCs, perfringolysin O (PFO) and streptolysin O (SLO), were found to exhibit strikingly different binding properties to cholesterol-rich natural and synthetic membranes. The structural basis for this difference was mapped to one of the loops (L3) in the membrane binding interface that help anchor the toxin monomers to the membrane after receptor (cholesterol) binding by the membrane insertion of its amino acid side chains. A single point mutation in this loop conferred the binding properties of SLO to PFO and vice versa. Our studies strongly suggest that changing the side chain structure of this loop alters its equilibrium between membrane-inserted and uninserted states, thereby affecting the overall binding affinity and total bound toxin. Previous studies have shown that the lipid environment of cholesterol has a dramatic effect on binding and activity. Combining this data with the results of our current studies on L3 suggests that the structure of this loop has evolved in the different CDCs to preferentially direct binding to cholesterol in different lipid environments. Finally, the efficiency of β-barrel pore formation was inversely correlated with the increased binding and affinity of the PFO L3 mutant, suggesting that selection of a compatible lipid environment impacts the efficiency of membrane insertion of the β-barrel pore.     

Perfringolysin O expression in Clostridium perfringens is independent of the upstream pfoR gene

The pathogenesis of Clostridium perfringens-mediated gas gangrene or clostridial myonecrosis involves the extracellular toxins alpha-toxin and perfringolysin O. Previous studies (T. Shimizu, A. Okabe, J. Minami, and H. Hayashi, Infect. Immun. 59:137-142, 1991) carried out with Escherichia coli suggested that the perfringolysin O structural gene, pfoA, was positively regulated by the product of the upstream pfoR gene. In an attempt to confirm this hypothesis in C. perfringens, a pfoR-pfoA deletion mutant was complemented with isogenic pfoA(+) shuttle plasmids that varied only in their ability to encode an intact pfoR gene. No difference in the ability to produce perfringolysin O was observed for C. perfringens strains carrying these plasmids. In addition, chromosomal pfoR mutants were constructed by homologous recombination in C. perfringens. Again no difference in perfringolysin O activity was observed. Since it was not possible to alter perfringolysin O expression by mutation of pfoR, it was concluded that the pfoR gene product is unlikely to have a role in the regulation of pfoA expression in C. perfringens.     

On the mechanism of bacteriorhodopsin solubilization by surfactants

Purple membrane bacteriorhodopsin can be easily solubilized by Triton X-100 and other detergents, but not by deoxycholate. In order to understand this behavior, we have examined the effects of a variety of surfactants. We show that detergents containing the cholane ring (cholate, taurocholate, 3[(3-cholamidopropyl)diethyl-ammonio]propanesulfonic acid...) are virtually unable to solubilize native bacteriorhodopsin. However, when the protein is reconstituted in dimyristoyl phosphatidylcholine and solubilization is assayed at a temperature such that bacteriorhodopsin is in the form of monomers, solubilization by cholane detergents does occur. We propose that steric factors prevent access of the rigid planar surfactant molecules to the hydrophobic protein regions. These are perhaps located in the monomer-monomer interface, whose solvation by surfactants is essential for solubilization to occur. We note that the capacity of some detergents to solubilize bacteriorhodopsin is always associated within the same range of surfactant concentrations with bleaching (partial or total) of the protein chromophore. The detergent-induced bleaching is at least partially reversible, suggesting that free retinal remains associated to some membrane components. While some surfactant molecules remain tightly bound to the membrane protein, cholane detergents can be completely removed from bacteriorhodopsin. Our results indicate that a structure-function relationship exists for detergents applied to the solubilization of bacteriorhodopsin.     

Monomer-monomer interactions propagate structural transitions necessary for pore formation by the cholesterol-dependent cytolysins

The assembly of the cholesterol-dependent cytolysin (CDC) oligomeric pore complex requires a complex choreography of secondary and tertiary structural changes in domain 3 (D3) of the CDC monomer structure. A point mutation was identified in the archetype CDC, perfringolysin O, that blocks detectable D3 structural changes and traps the membrane-bound monomers in an early and reversible stage of oligomer assembly. Using this and other mutants we show that specific D3 structural changes are propagated from one membrane-bound monomer to another. Propagation of these structural changes results in the exposure of a β-strand in D3 that allows it to pair and form edge-on interactions with a second β-strand of a free membrane-bound monomer. Pairing of these strands establishes the final geometry of the pore complex and is necessary to drive the formation of the β-barrel pore. These studies provide new insights into how structural information is propagated between membrane-bound monomers of a self-assembling system and the interactions that establish the geometry of the final pore complex.     

Structures of perfringolysin O suggest a pathway for activation of cholesterol-dependent cytolysins

Cholesterol-dependent cytolysins (CDCs), a large family of bacterial toxins, are secreted as water-soluble monomers and yet are capable of generating oligomeric pores in membranes. Previous work has demonstrated that large scale structural rearrangements occur during this transition but the detailed mechanism by which these changes take place remains a puzzle. Despite evidence of structural and functional couplings between domains 3 and 4, the crystal structure of the CDC, perfringolysin O (PFO), shows the two domains do not make direct contact. Here, we present crystal structures of PFO that demonstrate movements of domain 4 are sufficient to trigger conformational changes that are transmitted through the molecule to the distant domain 3. These coupled movements result in a loss of many contacts between domain 3 and rest of the molecule that would eventually lead to the exposure of transmembrane regions in preparation for membrane insertion. The structures reveal a detailed molecular pathway that may be the basis for the allosteric transition that occurs on initial membrane binding leading to the exposure of membrane-spanning regions in a domain distant from the initial site of interaction.     

Dimerisation mutants of Lac repressor. II. A single amino acid substitution, D278L, changes the specificity of dimerisation

Assembly of the lactose repressor tetramer involves two subunit interfaces, the C-terminal heptad repeats, and the monomer-monomer interface. Dimerisation between two monomers of Lac repressor of Escherichia coli lacking the two C-terminal heptad repeats occurs through the interactions between three alpha-helices of each monomer, which form a highly hydrophobic interface. Residues possibly involved in specific dimer formation are known from X-ray studies and from the phenotypes of more than 4000 single amino acid substitutions. During the examination of numerous mutants within the dimerisation interface of Lac repressor, we found that substitution of one amino acid, D278 to leucine, is sufficient to change the specificity of dimerisation. Analysis of this single substitution indicates that D278L mutant Lac repressor represses like wild-type. However, it no longer forms heterodimers with wild-type Lac repressor.     

Perfringolysin O, a cholesterol-binding cytolysin, as a probe for lipid rafts

Gaining an understanding of the structural and functional roles of cholesterol in membrane lipid rafts is a critical issue in studies on cellular signaling and because of the possible involvement of lipid rafts in various diseases. We have focused on the potential of perfringolysin O (theta-toxin), a cholesterol-binding cytolysin produced by Clostridium perfringens, as a probe for studies on membrane cholesterol. We prepared a protease-nicked and biotinylated derivative of perfringolysin O (BCtheta) that binds selectively to cholesterol in cholesterol-rich microdomains of cell membranes without causing membrane lesions. Since the domains fulfill the criteria of lipid rafts, BCtheta can be used to detect cholesterol-rich lipid rafts. This is in marked contrast to filipin, another cholesterol-binding reagent, which binds indiscriminately to cell cholesterol. Using BCtheta, we are now searching for molecules that localize specifically in cholesterol-rich lipid rafts. Recently, we demonstrated that the C-terminal domain of perfringolysin O, domain 4 (D4), possesses the same binding characteristics as BCtheta. BIAcore analysis showed that D4 binds specifically to cholesterol with the same binding affinity as the full-size toxin. Cell-bound D4 is recovered predominantly from detergent-insoluble, low-density membrane fractions where raft markers, such as cholesterol, flotillin and Src family kinases, are enriched, indicating that D4 also binds selectively to lipid rafts. Furthermore, a green fluorescent protein-D4 fusion protein (GFP-D4) was revealed to be useful for real-time monitoring of cholesterol in lipid rafts in the plasma membrane. In addition, the expression of GFP-D4 in the cytoplasm might allow the investigations of intracellular trafficking of lipid rafts. The simultaneous visualization of lipid rafts in plasma membranes and inside cells might help in gaining a total understanding of the dynamic behavior of lipid rafts.     

Characterization of domain interfaces in monomeric and dimeric ATP synthase

We disassembled monomeric and dimeric yeast ATP synthase under mild conditions to identify labile proteins and transiently stable subcomplexes that had not been observed before. Specific removal of subunits alpha, beta, oligomycin sensitivity conferring protein (OSCP), and h disrupted the ATP synthase at the gamma-alpha(3)beta(3) rotor-stator interface. Loss of two F(1)-parts from dimeric ATP synthase led to the isolation of a dimeric subcomplex containing membrane and peripheral stalk proteins thus identifying the membrane/peripheral stalk sectors immediately as the dimerizing parts of ATP synthase. Almost all subunit a was found associated with a ring of 10 c-subunits in two-dimensional blue native/SDS gels. We therefore postulate that c10a1-complex is a stable structure in resting ATP synthase until the entry of protons induces a breaking of interactions and stepwise rotation of the c-ring relative to the a-subunit in the catalytic mechanism. Dimeric subunit a was identified in SDS gels in association with two c10-rings suggesting that a c10a2c10-complex may constitute an important part of the monomer-monomer interface in dimeric ATP synthase that seems to be further tightened by subunits b, i, e, g, and h. In contrast to the monomer-monomer interface, the interface between dimers in higher oligomeric structures remains largely unknown. However, we could show that the natural inhibitor protein Inh1 is not required for oligomerization.     

Supramolecular organization of ATP synthase and respiratory chain in mitochondrial membranes

Mitochondrial ATP synthase is mostly isolated in monomeric form, but in the inner mitochondrial membrane it seems to dimerize and to form higher oligomeric structures from dimeric building blocks. Following a period of electron microscopic single particle analyses that revealed an angular orientation of the membrane parts of monomeric ATP synthases in the dimeric structures, and after extensive studies of the monomer-monomer interface, the focus now shifts to the potentially dynamic state of the oligomeric structures, their potential involvement in metabolic regulation of mitochondria and cells, and to newly identified interactions like physical associations of complexes IV and V. Similarly, larger structures like respiratory strings that have been postulated to form from individual respiratory complexes and their supercomplexes, the respirasomes, come into the focus. Progress by structural investigations is paralleled by insights into the functional roles of respirasomes including substrate channelling and stabilization of individual complexes. Cardiolipin was found to be important for the structural stability of respirasomes which in turn is required to maintain cells and tissues in a healthy state. Defects in cardiolipin remodeling cause devastating diseases like Barth syndrome. Novel species-specific roles of respirasomes for the stability of respiratory complexes have been identified, and potential additional roles may be deduced from newly observed interactions of respirasomes with components of the protein import machinery and with the ADP/ATP translocator.     

The Cholesterol-Dependent Cytolysin Signature Motif: A Critical Element in the Allosteric Pathway that Couples Membrane Binding to Pore Assembly

The cholesterol-dependent cytolysins (CDCs) constitute a family of pore-forming toxins that contribute to the pathogenesis of a large number of Gram-positive bacterial pathogens.The most highly conserved region in the primary structure of the CDCs is the signature undecapeptide sequence (ECTGLAWEWWR). The CDC pore forming mechanism is highly sensitive to changes in its structure, yet its contribution to the molecular mechanism of the CDCs has remained enigmatic. Using a combination of fluorescence spectroscopic methods we provide evidence that shows the undecapeptide motif of the archetype CDC, perfringolysin O (PFO), is a key structural element in the allosteric coupling of the cholesterol-mediated membrane binding in domain 4 (D4) to distal structural changes in domain 3 (D3) that are required for the formation of the oligomeric pore complex. Loss of the undecapeptide function prevents all measurable D3 structural transitions, the intermolecular interaction of membrane bound monomers and the assembly of the oligomeric pore complex. We further show that this pathway does not exist in intermedilysin (ILY), a CDC that exhibits a divergent undecapeptide and that has evolved to use human CD59 rather than cholesterol as its receptor. These studies show for the first time that the undecapeptide of the cholesterol-binding CDCs forms a critical element of the allosteric pathway that controls the assembly of the pore complex.     

Assembly and topography of the prepore complex in cholesterol-dependent cytolysins

Cholesterol-dependent cytolysins are a family of poreforming proteins that have been shown to be virulence factors for a large number of pathogenic bacteria. The mechanism of pore formation for these toxins involves a complex series of events that are known to include binding, oligomerization, and insertion of a transmembrane beta-barrel. Several features of this mechanism remain poorly understood and controversial. Whereas a prepore mechanism has been proposed for perfringolysin O, a very different mechanism has been proposed for the homologous member of the family, streptolysin O. To distinguish between the two models, a novel approach that directly measures the dimension of transmembranes pores was used. Pore formation itself was examined for both cytolysins by encapsulating fluorescein-labeled peptides and proteins of different sizes into liposomes. When these liposomes were re-suspended in a solution containing anti-fluorescein antibodies, toxin-mediated pore formation was monitored directly by the quenching of fluorescein emission as the encapsulated molecules were released, and the dyes were bound by the antibodies. The analysis of pore formation determined using this approach reveals that only large pores are produced by perfringolysin O and streptolysin O during insertion (and not small pores that grow in size). These results are consistent only with the formation of a prepore complex intermediate prior to insertion of the transmembrane beta-barrel into the bilayer. Fluorescence quenching experiments also revealed that PFO in the prepore complex contacts the membrane via domain 4, and that the individual transmembrane beta-hairpins in domain 3 are not exposed to the nonpolar core of the bilayer at this intermediate stage.     

The Influence of Natural Lipid Asymmetry upon the Conformation of a Membrane-inserted Protein (Perfringolysin O)

Eukaryotic membrane proteins generally reside in membrane bilayers that have lipid asymmetry. However, in vitro studies of the impact of lipids upon membrane proteins are generally carried out in model membrane vesicles that lack lipid asymmetry. Our recently developed method to prepare lipid vesicles with asymmetry similar to that in plasma membranes and with controlled amounts of cholesterol was used to investigate the influence of lipid composition and lipid asymmetry upon the conformational behavior of the pore-forming, cholesterol-dependent cytolysin perfringolysin O (PFO). PFO conformational behavior in asymmetric vesicles was found to be distinct both from that in symmetric vesicles with the same lipid composition as the asymmetric vesicles and from that in vesicles containing either only the inner leaflet lipids from the asymmetric vesicles or only the outer leaflet lipids from the asymmetric vesicles. The presence of phosphatidylcholine in the outer leaflet increased the cholesterol concentration required to induce PFO binding, whereas phosphatidylethanolamine and phosphatidylserine in the inner leaflet of asymmetric vesicles stabilized the formation of a novel deeply inserted conformation that does not form pores, even though it contains transmembrane segments. This conformation may represent an important intermediate stage in PFO pore formation. These studies show that lipid asymmetry can strongly influence the behavior of membrane-inserted proteins.     

Phospholipid hydrolysis caused by Clostridium perfringens α-toxin facilitates the targeting of perfringolysin O to membrane bilayers

Clostridium perfringens causes gas gangrene and gastrointestinal disease in humans. These pathologies are mediated by potent extracellular protein toxins, particularly α-toxin and perfringolysin O (PFO). While α-toxin hydrolyzes phosphatidylcholine and sphingomyelin, PFO forms large transmembrane pores on cholesterol-containing membranes. It has been suggested that the ability of PFO to perforate the membrane of target cells is dictated by how much free cholesterol molecules are present. Given that C. perfringens α-toxin cleaves the phosphocholine headgroup of phosphatidylcholine, we reasoned that α-toxin may increase the number of free cholesterol molecules in the membrane. Our present studies reveal that α-toxin action on membrane bilayers facilitates the PFO?cholesterol interaction as evidenced by a reduction in the amount of cholesterol required in the membrane for PFO binding and pore formation. These studies suggest a mechanism for the concerted action of α-toxin and PFO during C. perfringens pathogenesis.