The orientation of iron-sulfur clusters and a spin-coupled ubiquinone pair in the mitochondrial membrane

Five decades of research on mitochondrial NADH-quinone oxidoreductase (complex I)

NADH-quinone oxidoreductase (complex I) is the largest and most complicated enzyme complex of the mitochondrial respiratory chain. It is the entry site into the respiratory chain for most of the reducing equivalents generated during metabolism, coupling electron transfer from NADH to quinone to proton translocation, which in turn drives ATP synthesis. Dysfunction of complex I is associated with neurodegenerative diseases such as Parkinson’s and Alzheimer’s, and it is proposed to be involved in aging. Complex I has one non-covalently bound FMN, eight to 10 iron-sulfur clusters, and protein-associated quinone molecules as electron transport components. Electron paramagnetic resonance (EPR) has previously been the most informative technique, especially in membrane in situ analysis. The structure of complex 1 has now been resolved from a number of species, but the mechanisms by which electron transfer is coupled to transmembrane proton pumping remains unresolved. Ubiquinone-10, the terminal electron acceptor of complex I, is detectable by EPR in its one electron reduced, semiquinone (SQ) state. In the aerobic steady state of respiration the semi-ubiquinone anion has been observed and studied in detail. Two distinct protein-associated fast and slow relaxing, SQ signals have been resolved which were designated SQNf and SQNs. This review covers a five decade personal journey through the field leading to a focus on the unresolved questions of the role of the SQ radicals and their possible part in proton pumping.

Defining a direction: electron transfer and catalysis in Escherichia coli complex II enzymes

There are two homologous membrane-bound enzymes in Escherichia coli that catalyze reversible conversion between succinate/fumarate and quinone/quinol. Succinate:ubiquinone reductase (SQR) is a component of aerobic respiratory chains, whereas quinol:fumarate reductase (QFR) utilizes menaquinol to reduce fumarate in a final step of anaerobic respiration. Although, both protein complexes are capable of supporting bacterial growth on either minimal succinate or fumarate media, the enzymes are more proficient in their physiological directions. Here we evaluate factors that may underlie this catalytic bias. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease.

EPR detection of two protein-associated ubiquinone components (SQ(Nf) and SQ(Ns)) in the membrane in situ and in proteoliposomes of isolated bovine heart complex I

The success of Sazanov's group in determining the X-ray structure of the whole bacterial complex I is a great contribution to the progress of complex I research. In this mini-review of 35years' history of my laboratory and collaborators, we characterized the function of protein-associated semiquinone molecules in the proton-pumping mechanism in complex I (NADH-quinone oxidoreductase). We have constructed most of the frame work of our hypothesis, utilizing EPR techniques before the X-ray structures of complex I were reported by Sazanov's and Brandt's groups. One of the semiquinones (SQ(Nf)) is extremely sensitive to a proton motive force imposed on the energy-transducing membrane, while the other (SQ(Ns)) is insensitive. Their sensitivity to rotenone inhibition also differs. These differences were exploited using tightly coupled bovine heart submitochondrial particles with a high respiratory control ratio (>8). We determined the distance between SQ(Nf) and iron-sulfur cluster N2 on the basis of their direct spin-spin interaction. We are extending this line of work using reconstituted bovine heart complex I proteoliposomes which shows a respiratory control ratio >5. Two frontier research groups support our view point based on their mutagenesis studies. High frequency (33.9GHz; Q-band) EPR experiments appear to favor our two-semiquinone model. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

Direct localization of the 51 and 24 kDa subunits of mitochondrial complex I by three-dimensional difference imaging

Complex I is the largest complex in the respiratory chain, and the least understood. We have determined the 3D structure of complex I from Yarrowia lipolytica lacking the flavoprotein part of the N-module, which consists of the 51 kDa (NUBM) and the 24 kDa (NUHM) subunits. The reconstruction was determined by 3D electron microscopy of single particles. A comparison to our earlier reconstruction of the complete Y. lipolytica complex I clearly assigns the two flavoprotein subunits to an outer lobe of the peripheral arm of complex I. Localizing the two subunits allowed us to fit the X-ray structure of the hydrophilic fragment of complex I from Thermus thermophilus. The fit that is most consistent with previous immuno-electron microscopic data predicts that the ubiquinone reducing catalytic center resides in the second peripheral lobe, while the 75 kDa subunit is placed near the previously seen connection between the peripheral arm and the membrane arm protrusions.

Conformation-driven and semiquinone-gated proton-pump mechanism in the NADH-ubiquinone oxidoreductase (complex I)

A novel mechanism for proton/electron transfer is proposed for NADH-quinone oxidoreductase (complex I) based on the following findings: (1) EPR signals of the protein-bound fast-relaxing semiquinone anion radicals (abbreviated as Q(Nf)-) are observable only in the presence of proton-transmembrane electrochemical potential; (2) Iron-sulfur cluster N2 and Q(Nf)- are directly spin-coupled; and (3) The projection of the interspin vector extends only 5A along the membrane normal [Yano, T., Dunham, W.R. and Ohnishi, T. (2005) Biochemistry, 44, 1744-1754]. We propose that the proton pump is operated by redox-driven conformational changes of the quinone binding protein. In the input state, semiquinone is reduced to quinol, acquiring two protons from the N (matrix) side of the mitochondrial inner membrane and an electron from the low potential (NADH) side of the respiratory chain. A conformational change brings the protons into position for release at the P (inter-membrane space) side of the membrane via a proton-well. Concomitantly, an electron is donated to the quinone pool at the high potential side of the coupling site. The system then returns to the original state to repeat the cycle. This hypothesis provides a useful frame work for further investigation of the mechanism of proton translocation in complex I.

EPR characterization of ubisemiquinones and iron-sulfur cluster N2, central components of the energy coupling in the NADH-ubiquinone oxidoreductase (complex I) in situ

The proton-translocating NADH-ubiquinone oxidoreductase (complex I) is the largest and least understood respiratory complex. The intrinsic redox components (FMN and iron-sulfur clusters) reside in the promontory part of the complex. Ubiquinone is the most possible key player in proton-pumping reactions in the membrane part. Here we report the presence of three distinct semiquinone species in complex I in situ, showing widely different spin relaxation profiles. As our first approach, the semiquinone forms were trapped during the steady state NADH-ubiquinone-1 (Q1) reactions in the tightly coupled, activated bovine heart submitochondrial particles, and were named SQNf (fast-relaxing component), SQNS (slow-relaxing), and SQNx (very slow relaxing). This indicates the presence of at least three different quinone-binding sites in complex I. In the current study, special attention was placed on the SQNf, because of its high sensitivities to DeltamicroH+ and to specific complex I inhibitors (rotenone and piericidin A) in a unique manner. Rotenone inhibits the forward electron transfer reaction more strongly than the reverse reaction, while piericidine A inhibits both reactions with a similar potency. Rotenone quenched the SQNf signal at a much lower concentration than that required to quench the slower relaxing components (SQNs and SQNx). A close correlation was shown between the line shape alteration of the g// = 2.05 signal of the cluster N2 and the quenching of the SQNf signal, using two different experimental approaches: (1) changing the DeltamicroH+ poise by the oligomycin titration which decreases proton leak across the SMP membrane; (2) inhibiting the reverse electron transfer with different concentrations of rotenone. These new experimental results further strengthen our earlier proposal that a direct spin-coupling occurs between SQNf and cluster N2. We discuss the implications of these findings in connection with the energy coupling mechanism in complex .

The [2Fe-2S] cluster E(m) as an indicator of the iron-sulfur subunit position in the ubihydroquinone oxidation site of the cytochrome bc1 complex

Recent crystallographic and kinetic data have revealed the crucial role of the large scale domain movement of the iron-sulfur subunit [2Fe-2S] cluster domain during the ubihydroquinone oxidation reaction catalyzed by the cytochrome bc(1) complex. Previously, the electron paramagnetic resonance signature of the [2Fe-2S] cluster and its redox midpoint potential (E(m)) value have been used extensively to characterize the interactions of the [2Fe-2S] cluster with the occupants of the ubihydroquinone oxidation (Q(o)) catalytic site. In this work we analyze these interactions in various iron-sulfur subunit mutants that carry mutations in its flexible hinge region. We show that the E(m) increases of the iron-sulfur subunit [2Fe-2S] cluster induced either by these mutations or by the addition of stigmatellin do not act synergistically. Moreover, the E(m) increases disappear in the presence of class I inhibitors like myxothiazol. Because various inhibitors are known to affect the location of the iron-sulfur subunit cluster domain, the measured E(m) value of the [2Fe-2S] cluster therefore reflects its equilibrium position in the Q(o) site. We also demonstrate the existence in this site of a location where the E(m) of the cluster is increased by about 150 mV and discuss its possible implications in term of Q(o) site catalysis and energetics.

Structure-function studies of iron-sulfur clusters and semiquinones in the NADH-Q oxidoreductase segment of the respiratory chain

Our recent experimental data on iron-sulfur clusters and semiquinones in the complex I segment of the respiratory chain is presented, focusing on the Paracoccus (P.) denitrificans and bovine heart studies. The iron-sulfur cluster N2 has attracted the attention of investigators in the research field of complex I, since the mid-point redox potential of this cluster is the highest among all clusters in complex I, and is pH dependent (60 mV/pH). It is known that this cluster is located either in the NQO6 (NuoB in E. coli/PSST in bovine heart nomenclature) or in the NQO9 (NuoI/TYKY) subunit in the amphipathic domain of complex I. Our preliminary data indicate that the cluster N2 is located in the NuoB rather than the long-advocated NuoI subunit, and may have a unique ligand structure. We previously reported spin-spin interactions between cluster N2 and two distinct species of semiquinone (designated SQNf and SQNs) associated with complex I. A parallel intensity change was observed between the SQNf (g = 2.00) signal and the N2 split g parallel signal, further supporting our proposed interaction between SQNf and N2 spins.

Iron-sulfur clusters/semiquinones in complex I

NADH-quinone 1 oxidoreductase (Complex I) isolated from bovine heart mitochondria was, until recently, the major source for the study of this most complicated energy transducing device in the mitochondrial respiratory chain. Complex I has been shown to contain 43 subunits and possesses a molecular mass of about 1 million. Recently, Complex I genes have been cloned and sequenced from several bacterial sources including Escherichia coli, Paracoccus denitrificans, Rhodobacter capsulatus and Thermus thermophilus HB-8. These enzymes are less complicated than the bovine enzyme, containing a core of 13 or 14 subunits homologous to the bovine heart Complex I. From this data, important clues concerning the subunit location of both the substrate binding site and intrinsic redox centers have been gleaned. Powerful molecular genetic approaches used in these bacterial systems can identify structure/function relationships concerning the redox components of Complex I. Site-directed mutants at the level of bacterial chromosomes and over-expression and purification of single subunits have allowed detailed analysis of the amino acid residues involved in ligand binding to several iron-sulfur clusters. Therefore, it has become possible to examine which subunits contain individual iron-sulfur clusters, their location within the enzyme and what their ligand residues are. The discovery of g=2.00 EPR signals arising from two distinct species of semiquinone (SQ) in the activated bovine heart submitochondrial particles (SMP) is another line of recent progress. The intensity of semiquinone signals is sensitive to DeltamicroH+ and is diminished by specific inhibitors of Complex I. To date, semiquinones similar to those reported for the bovine heart mitochondrial Complex I have not yet been discovered in the bacterial systems. This mini-review describes three aspects of the recent progress in the study of the redox components of Complex I: (A) the location of the substrate (NADH) binding site, flavin, and most of the iron-sulfur clusters, which have been identified in the hydrophilic electron entry domain of Complex I; (B) experimental evidence indicating that the cluster N2 is located in the amphipathic domain of Complex I, connecting the promontory and membrane parts. Very recent data is also presented suggesting that the cluster N2 may have a unique ligand structure with an atypical cluster-ligation sequence motif located in the NuoB (NQO6/PSST) subunit rather than in the long advocated NuoI (NQO9/TYKY) subunit. The latter subunit contains the most primordial sequence motif for two tetranuclear clusters; (C) the discovery of spin-spin interactions between cluster N2 and two distinct Complex I-associated species of semiquinone. Based on the splitting of the g1 signal of the cluster N2 and concomitant strong enhancement of the semiquinone spin relaxation, one semiquinone species was localized 8-11 A from the cluster N2 within the inner membrane on the matrix side (N-side). Spin relaxation of the other semiquinone species is much less enhanced, and thus it was proposed to have a longer distance from the cluster N2, perhaps located closer to the other side (P-side) surface of the membrane. A brief introduction of EPR technique was also described in Appendix A of this mini-review.

Angular dependence of electron paramagnetic resonances of an azide-NO complex of cytochrome c oxidase: orientation of the haem-copper axis in cytochrome aa3 from ox heart

The orientation dependence of the EPR signals arising from the azide-nitric oxide complex of cytochrome oxidase was investigated using oriented multilayers of mitochondrial membranes from ox heart. Variations in line shape of the DeltaMS=1 signal of the triplet state were apparent, whilst the DeltaMS=2 transitions between g=4.7 and 3.9 varied in intensity as the angle of the applied magnetic field was varied. These half-field signals were maximal with the field parallel to the membrane plane. A model of the bi-liganded azide-nitric oxide complex has been constructed, in which the nitric oxide is bound to the high-spin haem in a bent configuration, with the Fe-N=O plane at 60-90 degrees to the membrane plane and the azide bound to the copper, distal from the haem. In addition, angular variations of the signals at g'=11 and g' around 3.5, derived from an integer-spin complex, were also observed.

Nitric oxide prevents oxidative damage produced by tert-butyl hydroperoxide in erythroleukemia cells via nitrosylation of heme and non-heme iron. Electron paramagnetic resonance evidence

We studied protective effects of NO against tert-butylhydroperoxide (t-BuOOH)-induced oxidations in a subline of human erythroleukemia K562 cells with different intracellular hemoglobin (Hb) concentrations. t-BuOOH-induced formation of oxoferryl-Hb-derived free radical species in cells was demonstrated by low temperature EPR spectroscopy. Intensity of the signals was proportional to Hb concentrations and was correlated with cell viability. Peroxidation of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and cardiolipin metabolically labeled with oxidation-sensitive cis-parinaric acid was induced by t-BuOOH. An NO donor, (Z)-1-[N-(3-ammoniopropyl)-N-(n-propyl)amino]-diazen-1-iu m-1, 2-diolate], produced non-heme iron dinitrosyl complexes and hexa- and pentacoordinated Hb-nitrosyl complexes in the cells. Nitrosylation of non-heme iron centers and Hb-heme protected against t-BuOOH-induced: (a) formation of oxoferryl-Hb-derived free radical species, (b) peroxidation of cis-parinaric acid-labeled phospholipids, and (c) cytotoxicity. Since NO did not inhibit peroxidation induced by an azo-initiator of peroxyl radicals, 2, 2'-azobis(2,4-dimethylvaleronitrile), protective effects of NO were due to formation of iron-nitrosyl complexes whose redox interactions with t-BuOOH prevented generation of oxoferryl-Hb-derived free radical species.

Functional implications of the structure of the 'Rieske' iron-sulfur protein of bovine heart mitochondrial cytochrome bc1 complex

Recently, we have determined the structure of the catalytic domain of the 'Rieske' iron-sulfur protein of bovine heart mitochondrial bc1 complex at 1.5 A resolution (Iwata, S., Saynovits, M., Link, T.A. and Michel, H. (1996) Structure, 4, 567-579). This is the first structure of a bis-histidine coordinated [2Fe-2S] cluster. The spectroscopic, electrochemical, and functional implications of the structure will be discussed.

Energy-dependent Complex I-associated ubisemiquinones in submitochondrial particles

Two distinct species of Complex I-associated ubisemiquinones (SQNf and SQNs) were detected by cryogenic EPR analysis of tightly coupled submitochondrial particles oxidizing NADH or succinate under steady-state conditions. The g = 2.00 signals from both fast-relaxing SQNf (P1/2 = 170 mW at 40 K) and slow-relaxing SQNs (P1/2 = 0.7 mW) are sensitive to uncouplers, rotenone and thermally induced deactivation of Complex I. At higher temperatures the SQNf signal is broadened and only the SQNs signal is seen (P1/2 = 7 mW at 105 K). The spin-spin interaction between SQNf and the iron-sulfur cluster N2 was detected as split peaks of the g parallel 2.5 signal with a coupling constant of 1.65 mT, revealing their mutual distance of 8-11 A. The data obtained are consistent with a model in which N2 and two interacting bound ubisemiquinone species are spatially arranged within the hydrophobic domain of Complex I, participating in the vectorial proton translocation.

The bc1 complexes of Rhodobacter sphaeroides and Rhodobacter capsulatus

Photosynthetic bacteria offer excellent experimental opportunities to explore both the structure and function of the ubiquinol-cytochrome c oxidoreductase (bc1 complex). In both Rhodobacter sphaeroides and Rhodobacter capsulatus, the bc1 complex functions in both the aerobic respiratory chain and as an essential component of the photosynthetic electron transport chain. Because the bc1 complex in these organisms can be functionally coupled to the photosynthetic reaction center, flash photolysis can be used to study electron flow through the enzyme and to examine the effects of various amino acid substitutions. During the past several years, numerous mutations have been generated in the cytochrome b subunit, in the Rieske iron-sulfur subunit, and in the cytochrome c1 subunit. Both site-directed and random mutagenesis procedures have been utilized. Studies of these mutations have identified amino acid residues that are metal ligands, as well as those residues that are at or near either the quinol oxidase (Qo) site or the quinol reductase (Qi) site. The postulate that these two Q-sites are located on opposite sides of the membrane is supported by these studies. Current research is directed at exploring the details of the catalytic mechanism, the nature of the subunit interactions, and the assembly of this enzyme.

Structure and function of the mitochondrial bc1 complex. A mutational analysis of the yeast Rieske iron-sulfur protein

Respiratory-defective mutants of Saccharomyces cerevisiae assigned to a single complementation group (G12) have been determined to have lesions in the iron-sulfur protein (Rieske protein) of ubiquinol: cytochrome c reductase. Mutants capable of expressing the protein were chosen for further studies. The genes from 13 independent isolates were cloned and their mutations sequenced. Twelve mutations were ascertained to cause single amino acid substitutions in the carboxyl-terminal regions of the protein between residues 127 and 173. This region is proposed to be part of the catalytic domain with the ligands responsible for co-ordinating the two irons of the 2Fe-2S cluster. Based on the catalytic properties of the ubiquinol: cytochrome c reductase complex and the electron paramagnetic resonance (e.p.r.) signals of the iron-sulfur protein, the mutants describe two different phenotypes. A subset of mutants have no detectable iron-sulfur cluster and are completely deficient in ubiquinol: cytochrome c reductase activity. These strains identify mutations in residues considered to be essential for binding of the iron or for maintaining a proper tertiary structure of the catalytic domain. A second group of mutants have reduced levels of enzymatic activity and exhibit e.p.r. spectra characteristic of the Rieske iron-sulfur cluster. The mutations in the latter strains have been ascribed to residues that influence the redox properties of the cluster by distorting the iron-binding pocket. A secondary and tertiary structure model is presented of the carboxyl-terminal 65 residues constituting the catalytic domain of the iron-sulfur protein. It is postulated that the two irons of the cluster are co-ordinated by three cysteine and a single histidine residue located in a loop structure. The catalytic domain also contains two short alpha-helices and three beta-strands that form a partial beta-barrel. Most of the hydrophilic amino acids are present in turns that map to one pole of the domain. When viewed in the context of the model, mutations that abolish the iron-sulfur cluster are mostly in residues defining the boundaries of the alpha-helices and beta-strands. The notable exception is a cysteine residue that has been assigned to the loop with the iron ligands. This cysteine residue is proposed to co-ordinate one iron of the cluster. Mutations that reduce ubiquinol: cytochrome c reductase activity and alter the redox potential of the cluster occur in residues located in the loop that contains the ligands of the cluster.

Steroidogenic electron transport in adrenal cortex mitochondria

The flavoprotein NADPH-adrenodoxin reductase and the iron sulfur protein adrenodoxin function as a short electron transport chain which donates electrons one-at-a-time to adrenal cortex mitochondrial cytochromes P-450. The soluble adrenodoxin acts as a mobile one-electron shuttle, forming a complex first with NADPH-reduced adrenodoxin reductase from which it accepts an electron, then dissociating, and finally reassociating with and donating an electron to the membrane-bound cytochrome P-450 (Fig. 9). Dissociation and reassociation with flavoprotein then allows a second cycle of electron transfers. A complex set of factors govern the sequential protein-protein interactions which comprise this adrenodoxin shuttle mechanism; among these factors, reduction of the iron sulfur center by the flavin weakens the adrenodoxin-adrenodoxin reductase interaction, thus promoting dissociation of this complex to yield free reduced adrenodoxin. Substrate (cholesterol) binding to cytochrome P-450scc both promotes the binding of the free adrenodoxin to the cytochrome, and alters the oxidation-reduction potential of the heme so as to favor reduction by adrenodoxin. The cholesterol binding site on cytochrome P-450scc appears to be in direct communication with the hydrophobic phospholipid milieu in which this substrate is dissolved. Specific effects of both phospholipid headgroups and fatty acyl side-chains regulate the interaction of cholesterol with its binding side. Cardiolipin is an extremely potent positive effector for cholesterol binding, and evidence supports the existence of a specific effector lipid binding site on cytochrome P.450scc to which this phospholipid binds.

Studies on the stabilized ubisemiquinone species in the succinate-cytochrome c reductase segment of the intact mitochondrial membrane system

1. Evidence is presented for the presence of a stable ubisemiquinone pair in the vicinity of iron-sulphur centre S-3, based on its thermodynamic and spin relaxation properties. 2. These semiquinones are coupled by dipolar interaction; quantitative analysis of the signals of the spin-coupled semiquinones (at pH 7.4) gives midpoint redox potentials E1 (oxidized to semiquinone state) and E2 (semiquinone to fully reduced state) of 140 and 80mV, respectively, for individual ubiquinones. 3. Values of pKS (pK of the semiquinone form) below 6.5 and pKR (pK of the fully reduced ubiquinone) of about 8.0 or above were estimated from the pH-dependence of the midpoint potentials of the spin coupled signals. Thus the ubisemiquinone associated with succinate dehydrogenase (designated as SQS) functions mostly in the anionic form of the physiological pH range. 4. Theonyltrifluoroacetone, a specific inhibitor of the succinate-ubiquinone reductase segment of the respiratory chain, destabilized the intermediate redox state; thus it quenches both the g = 2.00 signal and ubisemiquinone (SQS) and split signals from the spin coupled pair. This inhibitor has no significant effect on another bound ubisemiquinone species present in the cytochrome bc1 region (designated as SQC). 5. The possible function and location of these stabilized ubisemiquinone species were discussed in connection with Site-II energy transduction.

The orientation of the magnetic axes of the membrane-bound iron-sulfur clusters of spinach chloroplasts

Spinach chloroplast membranes were oriented onto mylar sheets by partial dehydration, and the orientation of the magnetic axes of membrane-bound paramagnetic clusters determined by electron paramagnetic resonance (EPR) spectroscopy. Our results indicate that the reduced Rieske iron-sulfur cluster signal is of orthorhombic symmetry oriented with th gy = 1.90 axis orthogonal to the membrane plane and with the gz = 2.03 axis in the membrane plane; the gx-axis is undetectable, presumably due to its broadness. If the Rieske center is a two-iron iron-sulfur cluster, we conclude that the iron-iron axis lies in the plane of the membrane. Illumination reduces the two bound chloroplast iron-sulfur proteins known as Clusters A and B. Center A is oriented such that gx = 1.86 and gy = 1.94 lie at an angle of about 40, and gz = 2.05 is at approximately 25, to the membrane plane. There are two possible orientations of Cluster B depending on the set of g-values assigned to this cluster. For one set of g-values, gz = 2.04 and gx = 1.89 are oriented in the plane of the membrane while gy = 1.92 is orthogonal to the plane. Alternatively, gz = 2.07 and gy = 1.94 are oriented approximately 50 and 40 to the membrane plane respectively, and gx = 1.80 is in the plane of the membrane. An additional light-induced signal at g = 2.15 oriented orthogonal to the plane is currently unexplained, as are other membrane perpendicular signals seen at g = 2.3 and g = 1.73 in dark-adapted samples.

The orientation of iron-sulphur clusters in membrane multilayers prepared from aerobically-grown Escherichia coli K12 and a cytochrome-deficient mutant

1. Membrane particles prepared from ultrasonically-disrupted, aerobically-grown Escherichia coli were centrifuged on to a plastic film that was supported perpendicular to the centrifugal field to yield oriented membrane multilayers. In such preparations, there is a high degree of orientation of the planes of the membranes such that they lie parallel to each other and to the supporting film. 2. When dithionite- or succinate-reduced multilayers are rotated in the magnetic field of an e.p.r. spectrometer, about an axis lying in the membrane plane, angular-dependent signals from an iron-sulphur cluster at g(x)=1.92, g(y)=1.93 and g(z)=2.02 are seen. The g=1.93 signal has maximal amplitude when the plane of the multilayer is perpendicular to the magnetic field. Conversely, the g=2.02 signal is maximal when the plane of the multilayer is parallel with the magnetic field. 3. Computer simulations of the experimental data show that the cluster lies in the cytoplasmic membrane with the g(y) axis perpendicular to the membrane plane and with the g(x) and g(z) axes lying in the membrane plane. 4. In partially-oxidized multilayers, a signal resembling the mitochondrial high-potential iron-sulphur protein (Hipip) is seen whose g(z)=2.02 axis may be deduced as lying perpendicular to the membrane plane. 5. Appropriate choice of sample temperature and receiver gain reveals two further signals in partially-reduced multilayers: a g=2.09 signal arises from a cluster with its g(z) axis in the membrane plane, whereas a g=2.04 signal is from a cluster with the g(z) axis lying along the membrane normal. 6. Membrane particles from a glucose-grown, haem-deficient mutant contain dramatically-lowered levels of cytochromes and exhibit, in addition to the iron-sulphur clusters seen in the parental strain, a major signal at g=1.90. 7. Only the latter may be demonstrated to be oriented in multilayer preparations from the mutant. 8. Comparisons are drawn between the orientations of the iron-sulphur proteins in the cytoplasmic membrane of E. coli and those in mitochondrial membranes. The effects of diminished cytochrome content on the properties of the iron-sulphur proteins are discussed.