Isolation and amino acid sequence of the 9.5 kDa protein of beef heart ubiquinol:cytochrome c reductase

Small single transmembrane domain (STMD) proteins organize the hydrophobic subunits of large membrane protein complexes

The large membrane protein complexes of mitochondrial oxidative phosphorylation are composed of central subunits that are essential for their bioenergetic core function and accessory subunits that may assist in regulation, assembly or stabilization. Although sequence conservation is low, a significant proportion of the accessory subunits is characterized by a common single transmembrane (STMD) topology. The STMD signature is also found in subunits of other membrane protein complexes. We hypothesize that the general function of STMD subunits is to organize the hydrophobic subunits of large membrane protein complexes in specialized environments like the inner mitochondrial membrane.

Structure and function of cytochrome bc complexes

The cytochrome bc complexes represent a phylogenetically diverse group of complexes of electron-transferring membrane proteins, most familiarly represented by the mitochondrial and bacterial bc1 complexes and the chloroplast and cyanobacterial b6f complex. All these complexes couple electron transfer to proton translocation across a closed lipid bilayer membrane, conserving the free energy released by the oxidation-reduction process in the form of an electrochemical proton gradient across the membrane. Recent exciting developments include the application of site-directed mutagenesis to define the role of conserved residues, and the emergence over the past five years of X-ray structures for several mitochondrial complexes, and for two important domains of the b6f complex.

Functional complementation analysis of yeast bc1 mutants. A study of the mitochondrial import of heterologous and hybrid proteins

Previous complementation studies with yeast bc1 mutants, defective in subunit VII or VIII, using heterologous and hybrid subunits, suggested that the requirement for import into mitochondria might significantly restrict the scope of this test for compatible proteins. Prediction algorithms indicate that the N-terminal domain of subunit VII contains all known characteristics of a mitochondrial targeting signal, whereas in subunit VIII such a signal is absent from the N-terminal domain, but possibly present in an internal region of the protein. Despite the fact that the characteristics of a mitochondrial import signal are found in the N-terminus of all known subunit-VII orthologues, in vitro import experiments show that the protein of human origin is not imported into yeast mitochondria. In vitro import can be restored, however, by replacement of the N-terminal part of the human protein by the N-terminus of the Saccharomyces cerevisiae orthologue, indicating a requirement for species-specific elements. Similar experiments were performed with subunit VIII and orthologues thereof, including a hybrid protein in which the N-terminus of the bovine heart orthologue was replaced by that of S. cerevisiae. The ability of yeast mitochondria to import this hybrid protein, in contrast with the bovine subunit-VIII orthologue itself, indicates that for subunit VIII also the N-terminus, in contradiction of theoretical predictions, contributes to the targeting signal, most likely via species-specific elements. Our findings expose the limitations of the currently available criteria for prediction of the presence and location of a mitochondrial targeting sequence and highlight the necessity of performing separate import studies for interpreting complementation studies as long as the species-specific characteristics of the import signals have not been identified.

The role of subunit VIII in the structural stability of the bc1 complex from Saccharomyces cerevisiae studied using hybrid complexes

The QCR8 genes encoding subunit VIII of the bc1 complex from Kluyveromyces lactis and Schizosaccharomyces pombe partially complement the respiratory-deficient phenotype of a S. cerevisiae QCR8-null mutant. This implies that the heterologous Qcr8 subunits can be imported by S. cerevisiae mitochondria and that they assemble to form a hybrid bc1 complex that is sufficiently active to support growth. In contrast, the QCR8 gene from bovine heart, encoding the 9.5-kDa subunit, is not able to restore respiratory function to the S. cerevisiae null mutant. This lack of functional complementation is directly attributable to the inability of S. cerevisiae mitochondria to import this protein as shown by in vitro assays. However, a hybrid gene encoding the N-terminal 26 residues of S. cerevisiae subunit VIII and the rest of the 9.5-kDa bovine heart homologue, was able to functionally complement the QCR8-null mutant, albeit to a very low extent. Successful import into S. cerevisiae mitochondria was confirmed by in vitro import experiments. Surprisingly, although assembly of these hybrid complexes is reduced to an extent that is proportional to the evolutionary distance of the homologue to S. cerevisiae, the specific activities of the assembled complexes are the same as for the wild-type bc1 complex. After solubilisation of the mitochondrial membranes with the mild detergent dodecyl maltoside, the wild-type enzyme can be inactivated by incubation at increased temperature, independent of protease activity. The rate of inactivation can be significantly increased by the addition of o-phenanthroline [Boumans, H., Grivell, L. A. & Berden, J. A. (1997) J. Biol. Chem. 272, 16753-16760]. The hybrid complexes are much more sensitive to both types of treatment. We conclude that substitution of subunit VIII by a homologous counterpart results in a loosening of the structure of the bc1 complex on the intermembrane space side, resulting in a less stable insertion of the Rieske Fe-S protein in vivo and therefore a lower stability of the assembled enzyme under certain in vitro conditions, but without an effect on catalytic activity.

Subunit VII of ubiquinol:cytochrome-c oxidoreductase from Neurospora crassa is functional in yeast and has an N-terminal extension that is not essential for mitochondrial targeting

cDNA clones encoding subunit VII of the Neurospora crassa bc1 complex (ubiquinol:cytochrome-c oxidoreductase), which is homologous with subunit VIII of the complex from yeast (encoded by QCR8), were identified on the basis of functional complementation of a yeast QCR8 deletion strain. The clones contain an open reading frame encoding a protein with a calculated molecular mass of 11.8 kDa. The N-terminal eight residues of the amino acid sequence deduced from the cDNA clones are absent from the mature protein, as revealed by direct sequencing of the isolated protein. To investigate the potential role of the N-terminal octapeptide in mitochondrial targeting, constructs were made encoding the precursor and the mature form of subunit VII from Neurospora. Incubation of isolated mitochondria with the two proteins revealed that the N-terminal extension of the precursor is removed on import. However, the presequence does not encode information for targeting, as the proteins encoded by both constructs can be imported into isolated mitochondria with equal efficiency. In contrast, the octapeptide seems to have functional importance: the defect in the yeast qcr8-null mutant is not complemented on transformation with the construct encoding mature subunit VII from N. crassa in a single-copy plasmid. We therefore speculate that the N-terminal extension plays a role in intramitochondrial sorting of N. crassa subunit VII. This is supported by the fact that the subunit VII precursor is processed by a protease other than the general mitochondrial processing peptidase. Interestingly, the presequence of N. crassa subunit VII has an amino acid composition similar to the octapeptides cleaved off by the mitochondrial intermediate peptidase.

The aromatic domain 66(YWYWW)70 of subunit VIII of the yeast ubiquinol-cytochrome c oxidoreductase is important for both assembly and activity of the enzyme

The aromatic character of the region 66(YWYWW)70 of the 11-kDa subunit VIII of ubiquinol-cytochrome c oxidoreductase (bc1 complex) of the yeast Saccharomyces cerevisiae has previously been demonstrated to be important for assembly of a functional complex [Hemrika et al. (1994) FEBS Lett. 344, 15-19]. Especially the aromatic nature of residue 66 appeared to be relevant, as the very low level (5%) of bc1 complex in the mutant 66(SASAA)70 was restored to nearly 70% of the wild-type level in a phenotypic revertant with the sequence 66(FASAA)70. In the present study, three other site-directed mutants (66(SAYAA)70, 66(SASAW)70 and 66(SWYWW)70) were constructed and analysed. The data indicate that the presence of one aromatic residue is enough for a substantial level of assembly and that its position modulates the level of both assembly and electron transfer activity. The results also confirm the relevance of this region of subunit VIII for the formation of the Q(out) reaction domain, as reported by Hemrika et al. [(1993) Eur. J. Biochem. 215, 601-609]. It is further shown that the lowered specific activity of the mutant described by these authors is not due to the introduction of a cysteine in the sequence of subunit VIII.

Crystallization and preliminary structure of beef heart mitochondrial cytochrome-bc1 complex

The method reported for isolation of ubiquinol-cytochrome-c reductase complex from submitochondrial particles was modified to yield a preparation for crystallization. The cytochrome bc1 complex was first crystallized in large thin plate form and diffracts X-rays to 7 A resolution in the presence of mother liquor. This crystalline complex was enzymatically active and contains ten protein subunits. It had 33 mol phospholipid and 0.6 mol ubiquinone per mol protein. With slightly modified crystallization conditions, different crystal forms were obtained. Crystals grown in the presence of 20% glycerol diffracted X-rays up to 2.9 A resolution using a synchrotron source. Four heavy atom derivatives have been obtained. The 3-D structure of the cytochrome bc1 complex was solved to 3.4 A resolution. Crystalline cytochrome bc1 complex is a dimer: most of the masses of core proteins I and II protrudes from the matrix side of the membrane, whereas the cytochrome b protein is located mainly within the membrane. There are 13 transmembrane helices in each monomer. Most of the mass of cytochrome c1 and iron-sulfur protein including their redox centers are located on the cytoplasmic side of the membrane. The distances between these redox centers have been determined, and several electron transfer inhibitor binding sites in the complex have been located.

Cloning, gene sequencing, and expression of the small molecular mass ubiquinone-binding protein of mitochondrial ubiquinol-cytochrome c reductase

The cDNA encoding QPc-9.5 kDa (subunit VII) of bovine heart mitochondrial ubiquinol-cytochrome c reductase was cloned and sequenced. This cDNA is 665 base pairs long with an open reading frame of 246 base pairs that encodes an 81-amino acid mature QPc-9.5 kDa. The insert contains 395 base pairs of a 3'-noncoding sequence with a poly(A) tail. The amino acid sequence of QPc-9.5 kDa deduced from this nucleotide sequence is the same as that obtained by protein sequencing except that residue 61 is tryptophan instead of cysteine. The QPc-9.5 kDa was overexpressed in Escherichia coli JM109 cells as a glutathione S-transferase fusion protein (GST-QPc) using the expression vector, pGEX/QPc. The yield of soluble active recombinant GST-QPc fusion protein depends on the induction growth time, temperature, and medium. Maximum yield of recombinant fusion protein was obtained from cells harvested 3 h postinduction of growth at 27 degrees C on LB medium containing betaine and sorbitol. QPc-9.5 kDa was released from the fusion protein by proteolytic cleavage with thrombin. Isolated recombinant QPc-9.5 kDa showed one protein band in SDS-polyacrylamide gel electrophroesis corresponding to subunit VII of mitochondrial ubiquinol-cytochrome c reductase. Although the isolated recombinant QPc-9.5 kDa is soluble in aqueous solution, it is in a highly aggregated form, with an apparent molecular mass of over 1 million. Addition of detergent deaggreates the isolated protein to the monomeric state, suggesting that the recombinant protein exists as a hydrophobic aggregation in aqueous solution. The recombinant QPc-9.5 kDa binds ubiquinone and shows a spectral blue shift. Upon titration of the recombinant protein with ubiquinone, a saturation behavior is observed, suggesting that the binding is specific and that the recombinant protein may be in the functionally active state.

The bifunctional cytochrome c reductase/processing peptidase complex from plant mitochondria

Cytochrome c reductase from potato has been extensively studied with respect to its catalytic activities, its subunit composition, and the biogenesis of individual subunits. Molecular characterization of all 10 subunits revealed that the high-molecular-weight subunits exhibit striking homologies with the components of the general mitochondrial processing peptidase (MPP) from fungi and mammals. Some of the other subunits show differences in the structure of their targeting signals or in their molecular composition when compared to their counterparts from heterotrophic organisms. The proteolytic activity of MPP was found in the cytochrome c reductase complexes from potato, spinach, and wheat, suggesting that the integration of the protease into this respiratory complex is a general feature of higher plants.

Identification of additional homologues of subunits VII and VIII of the ubiquinol-cytochrome c oxidoreductase enables definition of consensus sequences

The Candida utilis QCR7 gene encoding subunit VII of the ubiquinol-cytochrome c oxidoreductase was isolated by functional complementation of the Saccharomyces cerevisiae subunit VII-null mutant. Several other subunit VII homologues as well as homologues for subunit VIII were identified by screening the GenBank database. Some of these homologues for subunit VII could only be identified as such using a consensus sequence that was derived from the multiple sequence alignment. Definition of the consensus should facilitate further analysis of structure/function relationships in this protein.

cDNA sequence of subunit VIII of ubiquinol-cytochrome-c oxidoreductase from Schizosaccharomyces pombe

We have cloned a cDNA coding for subunit VIII of the ubiquinol-cytochrome-c oxidoreductase of Schizosaccharomyces pombe by functional complementation of the null mutant in the QCR8 gene of Saccharomyces cerevisiae. DNA sequence analysis reveals an open-reading frame of 276 bp encoding a 10.5 kDa protein with 51% amino acid sequence identity to its counterpart in S. cerevisiae.

Primary structure, cell-free synthesis and mitochondrial targeting of the 8.2 kDa protein of cytochrome c reductase from potato

Cytochrome c reductase from potato comprises ten subunits with apparent molecular sizes between 55 and < 10 kDa. The subunit with the highest electrophoretic mobility on SDS-polyacrylamide gels was isolated and analysed by cyclic Edman degradation. Mixtures of degenerative oligonucleotides were derived from the obtained sequence data and used for the isolation of corresponding cDNA clones. The clones encode a protein of 72 amino acids which exhibits significant sequence identity with a 9.5 kDa subunit of cytochrome c reductase from bovine and a 11 kDa subunit of the enzyme complex from yeast. Comparison between the deduced amino acid sequence of the open reading frame and the sequence of the mature protein reveals that only the initiator methionine is absent in the functional subunit. Hence the protein has a calculated molecular mass of 8.2 kDa. Transcripts of the potato 8.2 kDa protein were not translated in reticulocyte lysates but in vitro translation worked efficiently with wheat germ lysate. Import of the radiolabelled protein into isolated mitochondria from potato seems to depend on a potential across the inner membrane and confirms the absence of a cleavable mitochondrial presequence.

The cDNA sequence of beef heart CII-3, a membrane-intrinsic subunit of succinate-ubiquinone oxidoreductase

We provide the first full-length cDNA and amino acid sequences for beef heart CII-3, one of two hydrophobic subunits that bind succinate dehydrogenase to the mitochondrial inner membrane to form succinate-ubiquinone oxidoreductase (EC 1.3.99.1). Other low molecular weight proteins present in preparations of the isolated complex, including three possible forms of the second anchor polypeptide CII-4, have been identified by amino terminal sequencing.

The aromatic nature of residue 66 of the 11-kDa subunit of ubiquinol-cytochrome c oxidoreductase of the yeast Saccharomyces cerevisiae is important for the assembly of a functional enzyme

Transformation of multi- and single-copy plasmids carrying a mutated version (LTN2, region 66-YWYWW-70 replaced by SASAA) of QCR8, the gene encoding the 11-kDa subunit ubiquinol-cytochrome c oxidoreductase of Saccharomyces cerevisiae, to a QCR8(0) strain indicated the importance of this aromatic region for the assembly of a functional enzyme. Sequencing of plasmids giving spontaneous restoration of growth to some colonies among the single-copy LTN2 transformants showed that changing the sequence SASAA into the sequence FASAA could, to a large extent, overcome the observed assembly defect, indicating the importance of the aromatic nature of residue 66.

Subunit structures of purified beef mitochondrial cytochrome bc1 complex from liver and heart

The existence of tissue-specific isozymes of cytochrome c oxidase has been widely documented. We have now studied if there are differences between subunits of mitochondrial bc1 complexes isolated from liver and heart. For this purpose, we have developed a method for the purification of an active ubiquinol-cytochrome c oxidoreductase from adult bovine liver that includes solubilization of submitochondrial particles with deoxycholate, ammonium acetate fractionation, resolubilization with dodecyl maltoside, and ion exchange chromatography. The electrophoretic pattern of the liver preparation showed the presence of 11 subunits, with apparent molecular weights identical to the ones reported for the heart complex. Western blot analysis and isoelectric focusing followed by two-dimensional gels of bc1 complexes from liver and heart were compared, and no qualitative differences were observed. In addition, the high-molecular-weight subunits of the purified complexes from both tissues, subunits I, II, V, and VI, were isolated by PAGE in the presence of Coomasie Blue and subjected to limited proteolysis and to chemical digestion with cyanogen bromide and BNPS-skatol, and the peptide patterns were compared. Finally, two of the small-molecular-weight subunits from the liver complex were isolated (subunits VII and X), partially analyzed by amino terminal sequencing, and found to be identical with the reported sequence of their heart counterparts. The data suggest that, in contrast to the case of cytochrome c oxidase, bc1 complexes from liver and heart do not exhibit tissue-specific differences.

A region of the C-terminal part of the 11-kDa subunit of ubiquinol-cytochrome-c oxidoreductase of the yeast Saccharomyces cerevisiae contributes to the structure of the Qout reaction domain

QCR8, the gene encoding the 11-kDa subunit of ubiquinol-cytochrome-c oxidoreductase of the yeast Saccharomyces cerevisiae has been resequenced in the course of a search for mutants disturbed in subunit function. Resequencing shows that the previously published sequence [Maarse A.C. & Grivell L.A. (1987) Eur. J. Biochem 155, 419-425] lacks a C at position 185 of the coding sequence. As a result of this extra nucleotide, the reading frame now contains 285 base pairs and it codes for a protein of 94 amino acids with a calculated molecular mass of 11.0 kDa. Despite the altered C-terminus, similarity to the corresponding beef heart subunit is not significantly altered. One mutant (LTN1), arising from hydroxylamine mutagenesis, has been studied in detail: Assembly of the enzyme appears to be normal, as judged from the levels of the subunits observed in Western blots, while spectral analysis showed that only holo-cytochrome b was lowered to 70% of that of the wildtype. Measurement of the specific activity and calculation of the turnover number of the enzyme showed that these were 45% and 56% of that of the wild type, respectively. Further analysis of the mutant showed that the affinity for the inhibitor myxothiazol was decreased, that the 11-kDa subunit stabilises the enzyme once assembly has occurred, and that the reduction of cytochrome b via the Qout site is impaired. Sequence analysis showed that this mutant carries a deletion of 12 nucleotides at position 206-217 of the coding sequence, resulting in the replacement of residues 69-73 (WWKNG) by a cysteine. These results are discussed in terms of the 11-kDa subunit contributing to the conformation of the Qout binding domain.

Mitochondrial ubiquinol-cytochrome c reductase complex: crystallization and protein: ubiquinone interaction

The ubiquinol-cytochrome c reductase complex was crystallized in a thin plate form, which diffracts X-rays to 7 A resolution in the presence of mother liquor. This crystalline complex contains ten protein subunits and 140 nmol phospholipid per milligram protein. Over 90% of the phospholipid and ubiquinone in the reductase can be removed by repeated ammonium sulfate precipitation in the presence of 0.5% sodium cholate. The delipidated complex has no enzymatic activity and shows significant changes in the circular dichroism spectrum in the near UV region and in the EPR characteristics of both cytochromes b. Enzyme activity and spectral characteristics can be restored by replenishing the phospholipid and ubiquinone. The structural requirements of ubiquinone for electron transport were studied by measuring the ability of a variety of synthetic ubiquinone derivatives to restore the enzymatic activity and native spectroscopic signatures to the delipidated complex. Q-binding proteins and binding domains were identified using photoaffinity labeled Q-derivatives and HPLC separation of photolabeled peptides. Interaction between ubiquinol-cytochrome c reductase and succinate-Q reductase was established by differential scanning calorimetry and saturation transfer EPR using spin-labeled ubiquinol-cytochrome c reductase. Involvement of iron-sulfur protein in proton translocation by ubiquinol-cytochrome c reductase was investigated by hematorporphyrin-promoted photoinactivation of the complex. The cDNAs encoding the Rieske iron-sulfur protein and a small molecular mass Q-binding protein (QPc-9.5 kDa) were isolated and their nucleotide sequences determined. These will be useful in future structural and mechanistic studies of ubiquinol-cytochrome c reductase via in vitro reconstitution between an over-expressed, mutated subunit and a specific subunit-depleted reductase.

Core I protein of bovine ubiquinol-cytochrome-c reductase; an additional member of the mitochondrial-protein-processing family. Cloning of bovine core I and core II cDNAs and primary structure of the proteins

Core I and core II proteins are the largest nuclear-encoded subunits of the mitochondrial ubiquinol-cytochrome-c reductase (bc1 complex) lacking redox prosthetic groups. cDNA clones of the two bovine core proteins have been isolated by the screening of lambda ZAP cDNA libraries either with an oligonucleotide probe based on the sequence of an internal peptide or with a polymerase-chain-reaction-amplified fragment. The core I precursor protein consists of 362 amino acids with a 34-amino-acid presequence typical for mitochondrial targeting signals. The mature protein migrates in SDS/polyacrylamide gels with an apparent molecular mass of 47 kDa, which does not correspond to the actual molecular mass of the protein of 35.8 kDa deduced from the cDNA sequence. The core II precursor protein is composed of 453 amino acids having a 14-amino-acid presequence as a targeting sequence. Comparison of the core I amino acid sequence with sequences of the newly discovered protein family [Schulte, U., Arretz, M., Schneider, H., Tropschug, M., Wachter E., Neupert, W. & Weiss, H. (1989) Nature 339, 147 - 149] comprising the processing enhancing protein (PEP), matrix processing peptidase (MPP), and core I and II proteins from Neurospora crassa and Saccharomyces cerevisiae, revealed a remarkable identity of 39% and a high similarity of 49% to N. crassa PEP, which in this fungus is identical to core I. Core II protein is only a distant relative of this protein family. Based on these sequence comparisons and data obtained by genomic Southern blots, we anticipate that the bovine core I subunit, like the N. crassa core I protein, is bifunctional, being responsible for the maintenance of electron transport and processing of proteins during their import into the mitochondrial matrix. The analysis of the primary structure of the two core proteins completes the set of primary structures of all subunits of bovine ubiquinol-cytochrome-c reductase.

Cytochrome bc1 complexes of microorganisms

The cytochrome bc1 complex is the most widely occurring electron transfer complex capable of energy transduction. Cytochrome bc1 complexes are found in the plasma membranes of phylogenetically diverse photosynthetic and respiring bacteria, and in the inner mitochondrial membrane of all eucaryotic cells. In all of these species the bc1 complex transfers electrons from a low-potential quinol to a higher-potential c-type cytochrome and links this electron transfer to proton translocation. Most bacteria also possess alternative pathways of quinol oxidation capable of circumventing the bc1 complex, but these pathways generally lack the energy-transducing, protontranslocating activity of the bc1 complex. All cytochrome bc1 complexes contain three electron transfer proteins which contain four redox prosthetic groups. These are cytochrome b, which contains two b heme groups that differ in their optical and thermodynamic properties; cytochrome c1, which contains a covalently bound c-type heme; and a 2Fe-2S iron-sulfur protein. The mechanism which links proton translocation to electron transfer through these proteins is the proton motive Q cycle, and this mechanism appears to be universal to all bc1 complexes. Experimentation is currently focused on understanding selected structure-function relationships prerequisite for these redox proteins to participate in the Q-cycle mechanism. The cytochrome bc1 complexes of mitochondria differ from those of bacteria, in that the former contain six to eight supernumerary polypeptides, in addition to the three redox proteins common to bacteria and mitochondria. These extra polypeptides are encoded in the nucleus and do not contain redox prosthetic groups. The functions of the supernumerary polypeptides of the mitochondrial bc1 complexes are generally not known and are being actively explored by genetically manipulating these proteins in Saccharomyces cerevisiae.

The C-terminal half of the 11-kDa subunit VIII is not necessary for the enzymic activity of yeast ubiquinol:cytochrome-c oxidoreductase

Inactivation of the gene encoding the 11-kDa subunit VIII of yeast ubiquinol:cytochrome c oxidoreductase leads to an inactive complex, which lacks detectable cytochrome b [Maarse, A. C., De Haan, M., Schoppink, P. J., Berden, J. A. and Grivell, L. A. (1988) Eur. J. Biochem. 172, 179-184] and in which the steady-state levels of the Fe-S protein and the 14-kDa subunit VII are severely reduced. When the 11-kDao mutant is transformed with a gene encoding a protein consisting of the 11-kDa protein minus its last 11 amino acids and fused to a 7-amino-acid sequence encoded by a stop oligonucleotide, the complex is assembled normally. Enzyme activity is similar to that of the wild type, as is also the sensitivity of the complex to antimycin and myxothiazol. Transformation of the mutant with a gene encoding a protein consisting of the 11-kDa protein lacking the last 43 amino acids (i.e. almost half the protein) and fused to the same 7-amino-acid sequence as above, gives partial restoration of the complex. The Fe-S protein and the 14-kDa subunit VII still exhibit low steady-state levels, but cytochrome b is present again, albeit at a strongly reduced level. Electron transport activity is also partially restored and correlates with the level of cytochrome b indicating that the turnover number of the complex is similar to that of wild-type complex III. These findings demonstrate the important role played by the 11-kDa protein in the stabilization of cytochrome b. They also imply that at least the C-terminal half of the 11-kDa protein is not part of an ubiquinol-binding site. Moreover, since the deletion has no effect on the sensitivity of the complex to myxothiazol and antimycin, at least this part of the protein is probably not involved in binding of these inhibitors.

Complexity and tissue specificity of the mitochondrial respiratory chain

There is a renewed interest in the structure and functioning of the mitochondrial respiratory chain with the realization that a number of genetic disorders result from defects in mitochondrial electron transfer. These socalled mitochondrial myopathies include diseases of muscle, heart, and brain. The respiratory chain can be fractionated into four large multipeptide complexes, an NADH ubiquinone reductase (complex I), succinate ubiquinone reductase (complex II), ubiquinol oxidoreductase (complex III), and cytochrome c oxidase (complex IV). Mitochondrial myopathies involving each of these complexes have been described. This review summarizes compositional and structural data on the respiratory chain proteins and describes the arrangement of these complexes in the mitochondrial inner membrane. This biochemical information is provided as a framework for the diagnosis and molecular characterization of mitochondrial diseases.

Nucleotide sequence of the gene encoding the 11-kDa subunit of the ubiquinol-cytochrome-c oxidoreductase in Saccharomyces cerevisiae

The nucleotide sequence of the gene encoding the 11-kDa subunit VIII of the ubiquinol-cytochrome-c oxidoreductase in Saccharomyces cerevisiae has been determined. The coding sequence has a length of 330 bp and is preceded at a distance of 361 bp by another reading frame, coding for a protein of as yet unknown function. The 11-kDa gene is transcribed independently of the URFx gene and transcription of both is sensitive to catabolite repression. Multiple 5' and 3' termini of transcripts of the gene for the 11-kDa subunit were identified by S1 nuclease protection analysis of DNA X RNA hybrids. The 5' termini map 52 +/- 2 and 60 +/- 2 nucleotides upstream of the initiation codon whereas the 3' termini map 336 +/- 2 and 350 +/- 2 nucleotides downstream of the stop codon. The subunit VIII reading frame encodes a protein with a molecular mass of 12.4 kDa and a polarity of 37.6%. It is predicted to contain a high content of beta-sheet segments, which may be capable of forming a barrel-like structure in a lipid bilayer. A comparison of the sequence with those of the small subunits of the beef heart complex reveals similarity with the 9.5-kDa subunit VII (core-linked protein) characterized by Borchart et al. (1986) FEBS Lett. 200, 81-86. The significance of this is discussed.

Analysis of the structures of the subunits of the cytochrome bc1 complex from beef heart mitochondria

The interaction of the protein subunits of the bc1 complex from beef heart is analysed on the basis of protein chemical data and of secondary structure predictions suggesting a large number of amphipathic helices. Electrostatic interactions, i.e. helix-dipole interactions and ionic bonds, may play a major role in the stabilisation of the arrangement of the subunits within the multi-protein complex, formation of subcomplexes and maintenance of the steric strain of cytochrome b. A model of the heme-carrying 'core' of cytochrome b, i.e. of helices II-V, is presented consisting of a twisted '4-alpha-helical' bundle held together by helix-dipole interactions and stabilised by the interaction with other protein subunits of the bc1 complex.