Cytochrome c1 from potato: a protein with a presequence for targeting to the mitochondrial intermembrane space

Dual targeting of a mitochondrial protein: the case study of cytochrome c1

As a result of the endosymbiotic gene transfer, the majority of proteins of mitochondria and chloroplasts is encoded in the nucleus and synthesized in the cytosol as precursor molecules carrying N-terminal transit peptides for the transport into the respective target organelle. In most instances, transport takes place into either mitochondria or chloroplasts, although a few examples of dual targeting into both organelles have been described. Here, we show by a combination of three different experimental strategies that also cytochrome c(1) of potato, a component of the respiratory electron transport chain, is imported not only into mitochondria, but also into plastids. In organello import experiments with isolated mitochondria and chloroplasts, which were analyzed in both single and mixed organelle assays, demonstrate that the processing products accumulating after import within the two endosymbiotic organelles are different in size. Dual targeting of cytochrome c(1) is observed also in vivo, after biolistic transformation of leaf epidermal cells with suitable reporter constructions. Finally, Western analyses employing cytochrome c(1)-specific antiserum provide evidence that the protein accumulates in significant amounts in mitochondria and chloroplasts of both pea and spinach. The possible consequences of our findings on the relevance of the dual targeting phenomenon are discussed.

Simultaneous isolation of intact mitochondria and chloroplasts from a single pulping of plant tissue

Isolated organelles are suitable tools for the investigation of organelle function. However, if the properties of different organelles are to be compared, analysis is generally impeded by the fact that the organelles are isolated independently from each other from different specimens, different tissues or even different plants, i.e. the organelles have been exposed to different conditions during growth and development. Here we describe a method to isolate intact chloroplasts and mitochondria simultaneously from a single pulping of pea leaves, which results in organelles with an essentially identical physiological background. The functionality of the isolated chloroplasts and mitochondria is demonstrated by protein transport experiments, which yield results identical to those obtained with independently isolated organelles. With slight modifications, the method is also successfully applied to organelles from potato and spinach, which implies that it may be generally applicable to organelles from many different species.

A structural investigation of complex I and I+III2 supercomplex from Zea mays at 11-13 A resolution: assignment of the carbonic anhydrase domain and evidence for structural heterogeneity within complex I

The projection structures of complex I and the I+III2 supercomplex from the C4 plant Zea mays were determined by electron microscopy and single particle image analysis to a resolution of up to 11 A. Maize complex I has a typical L-shape. Additionally, it has a large hydrophilic extra-domain attached to the centre of the membrane arm on its matrix-exposed side, which previously was described for Arabidopsis and which was reported to include carbonic anhydrase subunits. A comparison with the X-ray structure of homotrimeric gamma-carbonic anhydrase from the archaebacterium Methanosarcina thermophila indicates that this domain is also composed of a trimer. Mass spectrometry analyses allowed to identify two different carbonic anhydrase isoforms, suggesting that the gamma-carbonic anhydrase domain of maize complex I most likely is a heterotrimer. Statistical analysis indicates that the maize complex I structure is heterogeneous: a less-abundant "type II" particle has a 15 A shorter membrane arm and an additional small protrusion on the intermembrane-side of the membrane arm if compared to the more abundant "type I" particle. The I+III2 supercomplex was found to be a rigid structure which did not break down into subcomplexes at the interface between the hydrophilic and the hydrophobic arms of complex I. The complex I moiety of the supercomplex appears to be only of "type I". This would mean that the "type II" particles are not involved in the supercomplex formation and, hence, could have a different physiological role.

Mitochondrial cytochrome c oxidase and succinate dehydrogenase complexes contain plant specific subunits

Respiratory oxidative phosphorylation represents a central functionality in plant metabolism, but the subunit composition of the respiratory complexes in plants is still being defined. Most notably, complex II (succinate dehydrogenase) and complex IV (cytochrome c oxidase) are the least defined in plant mitochondria. Using Arabidopsis mitochondrial samples and 2D Blue-native/SDS-PAGE, we have separated complex II and IV from each other and displayed their individual subunits for analysis by tandem mass spectrometry and Edman sequencing. Complex II can be discretely separated from other complexes on Blue-native gels and consists of eight protein bands. It contains the four classical SDH subunits as well as four subunits unknown in mitochondria from other eukaryotes. Five of these proteins have previously been identified, while three are newly identified in this study. Complex IV consists of 9-10 protein bands, however, it is more diffuse in Blue-native gels and co-migrates in part with the translocase of the outer membrane (TOM) complex. Differential analysis of TOM and complex IV reveals that complex IV probably contains eight subunits with similarity to known complex IV subunits from other eukaryotes and a further six putative subunits which all represent proteins of unknown function in Arabidopsis . Comparison of the Arabidopsis data with Blue-native/SDS-PAGE separation of potato and bean mitochondria confirmed the protein band complexity of these two respiratory complexes in plants. Two-dimensional Blue-native/Blue-native PAGE, using digitonin followed by dodecylmaltoside in successive dimensions, separated a diffusely staining complex containing both TOM and complex IV. This suggests that the very similar mass of these complexes will likely prevent high purity separations based on size. The documented roles of several of the putative complex IV subunits in hypoxia response and ozone stress, and similarity between new complex II subunits and recently identified plant specific subunits of complex I, suggest novel biological insights can be gained from respiratory complex composition analysis.

Identification and characterization of respirasomes in potato mitochondria

Plant mitochondria were previously shown to comprise respiratory supercomplexes containing cytochrome c reductase (complex III) and NADH dehydrogenase (complex I) of I(1)III(2) and I(2)III(4) composition. Here we report the discovery of additional supercomplexes in potato (Solanum tuberosum) mitochondria, which are of lower abundance and include cytochrome c oxidase (complex IV). Highly active mitochondria were isolated from potato tubers and stems, solubilized by digitonin, and subsequently analyzed by Blue-native (BN) polyacrylamide gel electrophoresis (PAGE). Visualization of supercomplexes by in-gel activity stains for complex IV revealed five novel supercomplexes of 850, 1,200, 1,850, 2,200, and 3,000 kD in potato tuber mitochondria. These supercomplexes have III(2)IV(1), III(2)IV(2), I(1)III(2)IV(1), I(1)III(2)IV(2), and I(1)III(2)IV(4) compositions as shown by two-dimensional BN/sodium dodecyl sulfate (SDS)-PAGE and BN/BN-PAGE in combination with activity stains for cytochrome c oxidase. Potato stem mitochondria include similar supercomplexes, but complex IV is partially present in a smaller version that lacks the Cox6b protein and possibly other subunits. However, in mitochondria from potato tubers and stems, about 90% of complex IV was present in monomeric form. It was suggested that the I(1)III(2)IV(4) supercomplex represents a basic unit for respiration in mammalian mitochondria termed respirasome. Respirasomes also occur in potato mitochondria but were of low concentrations under all conditions applied. We speculate that respirasomes are more abundant under in vivo conditions.

New insights into the respiratory chain of plant mitochondria. Supercomplexes and a unique composition of complex II

A project to systematically investigate respiratory supercomplexes in plant mitochondria was initiated. Mitochondrial fractions from Arabidopsis, potato (Solanum tuberosum), bean (Phaseolus vulgaris), and barley (Hordeum vulgare) were carefully treated with various concentrations of the nonionic detergents dodecylmaltoside, Triton X-100, or digitonin, and proteins were subsequently separated by (a) Blue-native polyacrylamide gel electrophoresis (PAGE), (b) two-dimensional Blue-native/sodium dodecyl sulfate-PAGE, and (c) two-dimensional Blue-native/Blue-native PAGE. Three high molecular mass complexes of 1,100, 1,500, and 3,000 kD are visible on one-dimensional Blue native gels, which were identified by separations on second gel dimensions and protein analyses by mass spectrometry. The 1,100-kD complex represents dimeric ATP synthase and is only stable under very low concentrations of detergents. In contrast, the 1,500-kD complex is stable at medium and even high concentrations of detergents and includes the complexes I and III(2). Depending on the investigated organism, 50% to 90% of complex I forms part of this supercomplex if solubilized with digitonin. The 3,000-kD complex, which also includes the complexes I and III, is of low abundance and most likely has a III(4)I(2) structure. The complexes IV, II, and the alternative oxidase were not part of supercomplexes under all conditions applied. Digitonin proved to be the ideal detergent for supercomplex stabilization and also allows optimal visualization of the complexes II and IV on Blue-native gels. Complex II unexpectedly was found to be composed of seven subunits, and complex IV is present in two different forms on the Blue-native gels, the larger of which comprises additional subunits including a 32-kD protein resembling COX VIb from other organisms. We speculate that supercomplex formation between the complexes I and III limits access of alternative oxidase to its substrate ubiquinol and possibly regulates alternative respiration. The data of this investigation are available at http://www.gartenbau.uni-hannover.de/genetik/braun/AMPP.

Structure, organization and expression of the genes encoding mitochondrial cytochrome c(1) and the Rieske iron-sulfur protein in Chlamydomonas reinhardtii

The sequence and organization of the Chlamydomonas reinhardtii genes encoding cytochrome c(1) ( Cyc1) and the Rieske-type iron-sulfur protein ( Isp), two key nucleus-encoded subunits of the mitochondrial cytochrome bc(1) complex, are presented. Southern hybridization analysis indicates that both Cyc1 and Isp are present as single-copy genes in C. reinhardtii. The Cyc1 gene spans 6404 bp and contains six introns, ranging from 178 to 1134 bp in size. The Isp gene spans 1238 bp and contains four smaller introns, ranging in length from 83 to 167 bp. In both genes, the intron/exon junctions follow the GT/AG rule. Internal conserved sequences were identified in only some of the introns in the Cyc1 gene. The levels of expression of Isp and Cyc1 genes are comparable in wild-type C. reinhardtii cells and in a mutant strain carrying a deletion in the mitochondrial gene for cytochrome b (dum-1). Nevertheless, no accumulation of the nucleus-encoded cytochrome c(1) or of core proteins I and II was observed in the membranes of the respiratory mutant. These data show that, in the green alga C. reinhardtii, the subunits of the cytochrome bc(1) complex fail to assemble properly in the absence of cytochrome b.

Multiple losses and transfers to the nucleus of two mitochondrial succinate dehydrogenase genes during angiosperm evolution

Unlike in animals, the functional transfer of mitochondrial genes to the nucleus is an ongoing process in plants. All but one of the previously reported transfers in angiosperms involve ribosomal protein genes. Here we report frequent transfer of two respiratory genes, sdh3 and sdh4 (encoding subunits 3 and 4 of succinate dehydrogenase), and we also show that these genes are present and expressed in the mitochondria of diverse angiosperms. Southern hybridization surveys reveal that sdh3 and sdh4 have been lost from the mitochondrion about 40 and 19 times, respectively, among the 280 angiosperm genera examined. Transferred, functional copies of sdh3 and sdh4 were characterized from the nucleus in four and three angiosperm families, respectively. The mitochondrial targeting presequences of two sdh3 genes are derived from preexisting genes for anciently transferred mitochondrial proteins. On the basis of the unique presequences of the nuclear genes and the recent mitochondrial gene losses, we infer that each of the seven nuclear sdh3 and sdh4 genes was derived from a separate transfer to the nucleus. These results strengthen the hypothesis that angiosperms are experiencing a recent evolutionary surge of mitochondrial gene transfer to the nucleus and reveal that this surge includes certain respiratory genes in addition to ribosomal protein genes.

Molecular cloning and expression analyses of mitochondrial and plastidic isoforms of cysteine synthase (O-acetylserine(thiol)lyase) from Arabidopsis thaliana

Cysteine synthase, the key enzyme for fixation of inorganic sulfide, catalyses the formation of cysteine from O-acetylserine and inorganic sulfide. Here we report the cloning of cDNAs encoding cysteine synthase isoforms from Arabidopsis thaliana. The isolated cDNA clones encode for a mitochondrial and a plastidic isoform of cysteine synthase (O-acetylserine (thiol)-lyase, EC 4.2.99.8), designated cysteine synthase C (AtCS-C, CSase C) and B (AtCS-B; CSase B), respectively. AtCS-C and AtCS-B, having lengths of 1569-bp and 1421-bp, respectively, encode polypeptides of 430 amino acids (approximately 45.8 kD) and of 392 amino acids (approximately 41.8 kD), respectively. The deduced amino acid sequences of the mitochondrial and plastidic isoforms exhibit high homology even with respect to the presequences. The predicted presequence of AtCS-C has a N-terminal extension of 33 amino acids when compared to the plastidic isoform. Northern blot analysis showed that AtCS-C is higher expressed in roots than in leaves whereas the expression of AtCS-B is stronger in leaves. Furthermore, gene expression of both genes was enhanced by sulfur limitation which in turn led to an increase in enzyme activity in crude extracts of plants. Expression of the AtCS-B gene is regulated by light. The mitochondrial, plastidic and cytosolic (Hesse and Altmann, 1995) isoforms of cysteine synthase of Arabidopsis are able to complement a cysteine synthase-deficient mutant of Escherichia coli unable to grow on minimal medium without cysteine, indicating synthesis of functional plant proteins in the bacterium. Two lines of evidence proved that AtCS-C encodes a mitochondrial form of cysteine synthase; first, import of in vitro translation products derived from AtCS-C in isolated intact mitochondria and second, Western blot analysis of mitochondria isolated from transgenic tobacco plants expressing AtCS-C cDNA/c-myc DNA fusion protein.

Unique composition of the preprotein translocase of the outer mitochondrial membrane from plants

Transport of most nuclear encoded mitochondrial proteins into mitochondria is mediated by heteropolymeric translocases in the membranes of the organelles. The translocase of the outer mitochondrial membrane (TOM) was characterized in fungi, and it was shown that TOM from yeast comprises nine different subunits. This publication is the first report on the preparation of the TOM complex from plant mitochondria. The protein complex from potato was purified by (a) blue native polyacrylamide gel electrophoresis and (b) by immunoaffinity chromatography. On blue native gels, the potato TOM complex runs close to cytochrome c oxidase at 230 kDa and hence only comprises about half of the size of fungal TOM complexes. Analysis of the TOM complex from potato by SDS-polyacrylamide gel electrophoresis allows separation of seven different subunits of 70, 36, 23, 9, 8, 7, and 6 kDa. The 23-kDa protein is identical to the previously characterized potato TOM20 receptor, as shown by in vitro assembly of this protein into the 230-kDa complex, by immunoblotting and by direct protein sequencing. Partial amino acid sequence data of the other subunits allowed us to identify sequence similarity between the 36-kDa protein and fungal TOM40. Sequence analysis of cDNAs encoding the 7-kDa protein revealed significant sequence homology of this protein to TOM7 from yeast. However, potato TOM7 has a N-terminal extension, which is very rich in basic amino acids. Counterparts to the TOM22 and TOM37 proteins from yeast seem to be absent in the potato TOM complex, whereas an additional low molecular mass subunit occurs. Functional implications of these findings are discussed.

Molecular mechanisms of cytochrome c biogenesis: three distinct systems

The past 10 years have heralded remarkable progress in the understanding of the biogenesis of c-type cytochromes. The hallmark of c-type cytochrome synthesis is the covalent ligation of haem vinyl groups to two cysteinyl residues of the apocytochrome (at a Cys-Xxx-Yyy-Cys-His signature motif). From genetic, genomic and biochemical studies, it is clear that three distinct systems have evolved in nature to assemble this ancient protein. In this review, common principles of assembly for all systems and the molecular mechanisms predicted for each system are summarized. Prokaryotes, plant mitochondria and chloroplasts use either system I or II, which are each predicted to use dedicated mechanisms for haem delivery, apocytochrome ushering and thioreduction. Accessory proteins of systems I and II co-ordinate the positioning of these two substrates at the membrane surface for covalent ligation. The third system has evolved specifically in mitochondria of fungi, invertebrates and vertebrates. For system III, a pivotal role is played by an enzyme called cytochrome c haem lyase (CCHL) in the mitochondrial intermembrane space.

New insights into the co-evolution of cytochrome c reductase and the mitochondrial processing peptidase

The mitochondrial processing peptidase (MPP) is a heterodimeric enzyme that forms part of the cytochrome c reductase complex from higher plants. Mitochondria from mammals and yeast contain two homologous enzymes: (i) an active MPP within the mitochondrial matrix and (ii) an inactive MPP within the cytochrome c reductase complex. To elucidate the evolution of MPP, the cytochrome c reductase complexes from lower plants were isolated and tested for processing activity. Mitochondria were prepared from the staghorn fern Platycerium bifurcatum, from the horsetail Equisetum arvense, and from the colorless algae Polytomella, and cytochrome c reductase complexes were purified by a micro-isolation procedure based on Blue-native polyacrylamide gel electrophoresis and electroelution. This is the first report on the subunit composition of a respiratory enzyme complex from a fern or a horsetail. The cytochrome c reductase complexes from P. bifurcatum and E. arvense are shown to efficiently process mitochondrial precursor proteins, whereas the enzyme complex from Polytomella lacks proteolytic activity. An evolutionary model is suggested that assumes a correlation between the presence of an active MPP within the cytochrome c reductase complex and the occurrence of chloroplasts.

The 23-kDa light-stress-regulated heat-shock protein of chenopodium rubrum L. is located in the mitochondria

The 23-kDa nuclear-encoded heat-shock protein (HSP) of Chenopodium rubrum L. is regulated by light at the posttranslational level. Higher light intensities are more effective in inducing the accumulation of the mature protein under heat-shock conditions. Based on this and other properties the protein was considered to belong to the group of small chloroplastic HSPs. However, we have now obtained the following evidence that this 23-kDa HSP is localized in the mitochondria: (i) Immunogold-labelled protein was almost exclusively restricted to the mitochondria in electron microscope thin sections. (ii) Using purified, isolated mitochondria from potato tubers the in-vitro-synthesized translation product of 31 kDa was readily transported into mitochondria where it was processed to the 23-kDa product. (iii) The protein could be detected by Western blotting in a preparation of washed mitochondria of Chenopodium, while under the same conditions no signal could be obtained in a preparation of isolated chloroplasts. (iv) Finally, sequence comparison with the published sequences of mitochondrial proteins by Lenne et al. (1995, Biochem J 311:805-813) and LaFayette et al. (1996, Plant Mol Biol 30:159-169) showed clearly that the 23-kDa protein is considerably more similar to these two proteins than to the group of plastid small HSPs. From these data we infer that mitochondria are involved in the response of the plants to high light stress under heat-shock conditions.

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.

Molecular structure of the 8.0 kDa subunit of cytochrome-c reductase from potato and its delta psi-dependent import into isolated mitochondria

The cytochrome-c reductase (EC 1.10.2.2) of the mitochondrial respiratory chain couples electron transport from ubiquinol to cytochrome c with proton translocation across the inner mitochondrial membrane. The enzyme from potato was shown to be composed of 10 subunits. Isolation and characterization of cDNA clones for the second smallest subunit reveal an open reading frame of 216 bp encoding a protein of 8.0 kDa. The protein exhibits similarities to a 7.2/7.3 kDa subunit of cytochrome-c reductase from bovine and yeast, that is localized on the intermembrane space side of the enzyme complex. It also shows similarity to a previously unidentified 7.8 kDa protein of cytochrome-c reductase from Euglena. The potato 8.0 kDa protein has a segmental structure, as its sequence can be divided into four parts, each comprising a central Arg-(Xaa)5-Val motif. N-terminal sequencing of the mature 8.0 kDa proteins indicates the absence of a cleavable mitochondrial targeting sequence. Import of the in vitro synthesized 8.0 kDa protein into isolated potato mitochondria confirms the lack of a presequence and reveals a dependence of the transport on the membrane potential delta psi across the inner mitochondrial membrane. These features are unique among the intermembrane space proteins known so far.

orf250 encodes a second subunit of an ABC-type heme transporter in Oenothera mitochondria

A highly transcribed region in Oenothera mitochondria codes for an open reading frame comprising 250 condons (orf250). This open reading frame shows high sequence similarity to the helC gene of Rhodobacter capsulatus which encodes a subunit of a proposed ABC-type heme transporter. Transcripts of orf250 are edited by cytidine to uridine transitions at 29 sites, altering 10% of all encoded amino acids. Genes homologous to helC have also been found in the bacteria Bradyrhizobium japonicum and Escherichia coli, and are conserved in mitochondria of Marchantia polymorpha, Daucus carota, and Arabidopsis thaliana. In bacteria these genes belong to operons that are involved in the biogenesis of c-type cytochromes. The bacterial gene organization is partly conserved in Marchantia, but altered in the mitochondrial genome of Oenothera.

A gene proposed to encode a transmembrane domain of an ABC transporter is expressed in wheat mitochondria

In a study of transcribed regions of the wheat mitochondrial genome, we identified an open reading frame of 720 bp, which was consequently designated orf240. The amino acid sequence deduced from orf240 shows a high level of similarity with HelC, a protein essential for c-type cytochrome biogenesis in the photosynthetic purple bacterium Rhodobacter capsulatus. HelC is part of a putative heme ABC transporter. An open reading frame homologous to orf240 is present in the mitochondrial genome of Marchantia polymorpha. The wheat gene is expressed as an mRNA of 2.8 kb, which is further processed to smaller transcripts. The transcript is highly edited, with 36 C to U modifications found in the coding region of all cDNAs sequenced. RNA editing is responsible for changes in 14% of the amino acids specified by the transcript.

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 mitochondrial processing peptidase from potato: a self-processing enzyme encoded by two differentially expressed genes

Cytochrome c reductase from potato is a bifunctional protein complex located in the inner mitochondrial membrane, which is involved in respiratory electron transport and processing of mitochondrial precursor proteins. The three largest subunits of the complex share the highest degree of sequence identity with the alpha- and beta-subunits of the soluble processing peptidase (MPP) from fungi and mammals. Evidence is provided that another substoichiometric polypeptide of the cytochrome c reductase complex resembles the alpha-subunit of MPP. A cDNA clone corresponding to the second alpha-MPP protein (alpha-II MPP) encodes a polypeptide of 504 amino acids which is 84% identical to alpha-I MPP. The two different alpha-MPP polypeptides have similar sizes on SDS-polyacrylamide gels but can be distinguished with an antibody raised against a decapeptide that is specific for alpha-II MPP. The presequences of both alpha-subunits of MPP are proteolytically removed by the integrated processing enzyme complex indicating that it acts on the targeting signals of its own precursor proteins. Gene-specific oligonucleotides reveal that the genes encoding alpha-subunit I and alpha-subunit II of MPP are differentially expressed in all tissues analysed but the transcript levels do not vary between tissues.

Primary structure and expression of a gene encoding the cytosolic ribosomal protein S4 from potato

The primary structure of a cDNA clone encoding the S4 protein from the small subunit of 80S ribosomes from potato was determined. Cytosolic ribosomal protein S4 is hydrophilic and has a prevalence for positively charged residues. In potato it is 264 amino acids long and contains a putative nuclear targeting signal close to the N-terminus. Having 65-69% identical amino acids cytosolic ribosomal protein S4 from mammals, fungi and plants belongs to the highly conserved proteins. The S4 gene is transcribed in all potato tissues analysed and has a relatively high expression level in comparison to nuclear genes encoding mitochondrial proteins.

The 'Hinge' protein of cytochrome c reductase from potato lacks the acidic domain and has no cleavable presequence

The 'Hinge' protein of cytochrome c reductase from fungi and mammals is thought to support electron transport from cytochrome c1 to cytochrome c and was reported to be one of the most acidic proteins known. Isolation and analysis of cDNA clones of the first 'Hinge' protein from a plant source reveals that it has a surplus of basic residues in potato. While the overall identity between the deduced amino acid sequence of the potato 'Hinge' protein and the proteins from yeast and bovine is in the range of 40%, the characteristic acidic domain is lacking. Therefore the numerous theories on the function of the mitochondrial 'Hinge' protein seem not to apply for the protein from potato. Also the atypical acidic presequence of the 'Hinge' protein from fungi and mammals is absent as revealed by N-terminal sequencing of the isolated potato 'Hinge' protein. Functional implications of these results for the 'Hinge' proteins from other organisms are discussed.

Molecular features, processing and import of the Rieske iron-sulfur protein from potato mitochondria

The mitochondrial iron-sulfur protein (also termed Rieske iron-sulfur protein) of cytochrome c reductase was purified from potato tubers and identified with heterologous antibodies. The sequences of the N-terminus of this 25 kDa protein and of an internal peptide were determined to design oligonucleotide mixtures for screening a cDNA library. One class of cDNA clones containing an open reading frame of 265 amino acids was isolated. The encoded protein contains the peptide sequences of the 25 kDa protein and shares about 50% sequence identity with the Rieske iron-sulfur proteins from fungi and around 43% with those from mammals. In vitro transcription and translation of the cDNA reveals that the iron-sulfur protein is made as a larger precursor of 30 kDa which is processed by the cytochrome c reductase/processing peptidase complex from potato. The processing product obtained after in vitro processing has the same size as the mature protein imported into isolated mitochondria. The presequence, which targets the protein to the organelle, is 53 amino acids long and has molecular features different from those found in presequences of fungal iron-sulfur proteins, which are processed in two steps. Our results indicate that, unlike in yeast and Neurospora, the presequence of the iron-sulfur protein from potato is removed by a single processing enzyme in one step.

A gene involved in the biogenesis of c-type cytochromes is co-transcribed with a ribosomal protein gene in wheat mitochondria [corrected]

Sequence analysis of a transcribed region of the wheat mitochondrial (mt) genome revealed two open reading frames (orfs) coding for proteins of 589 and 174 amino acids. Both genes are co-transcribed in a 2.6-kb RNA. The largest orf codes for a hydrophobic protein which bears similarity to a bacterial protein involved in the biogenesis of c-type cytochromes. Its corresponding RNA sequence is fully edited at 34 positions. The second orf encodes a protein homologous to the amino-terminal third of E. coli ribosomal protein S1, corresponding to the ribosome-binding domain of this protein. Its RNA sequence is edited at four positions, one of the edits creating a stop codon. The presence of both proteins in wheat mitochondria was demonstrated using specific antibodies raised against fusion proteins obtained in E. coli from the corresponding cDNAs.

The presequence of cytochrome c1 from potato mitochondria is encoded on four exons

The structural organization of a nuclear gene encoding cytochrome c1 from potato was determined. The gene spans 5.1 kb and contains eight introns. All intron/exon junctions follow the GT/AG rule. Functional domains of the mature cytochrome c1 protein are located on separate exons. The presequence, which targets the cytochrome c1 precursor to the mitochondrion and to the correct intra-mitochondrial location, is encoded on the first four exons. The largest intron (2.8 kb) separates the information for mitochondrial targeting from the "intra-mitochondrial sorting domain" of the cytochrome c1 protein. In contrast to other organellar precursor proteins, there is no intron between the DNA sequence encoding the presequence and the mature protein. This may indicate that during evolution the genetic information for the prokaryotic cytochrome c1 was transferred to the nucleus together with the bacterial secretion signal which is structurally and functionally related to "intramitochondrial sorting domains".

Purification and sequencing of cytochrome b from potato reveals methionine cleavage of a mitochondrially encoded protein

Several mitochondrial genes from a large number of different fungi, mammals and plants have been sequenced but little is known about the corresponding translation products. We have affinity purified cytochrome c reductase from potato mitochondria and isolated the mitochondrially encoded cytochrome b protein. Amino-terminal sequencing reveals that the polypeptide does not start with a methionine. Comparison of the amino acid sequence with the recently published sequence of the gene encoding the cytochrome b apoprotein suggests that the N-formylmethionine is removed. This result provides the first evidence for the presence of a deformylase and a methionine aminopeptidase in mitochondria.

The mitochondrial gene encoding ribosomal protein S12 has been translocated to the nuclear genome in Oenothera

The Oenothera mitochondrial genome contains only a gene fragment for ribosomal protein S12 (rps12), while other plants encode a functional gene in the mitochondrion. The complete Oenothera rps12 gene is located in the nucleus. The transit sequence necessary to target this protein to the mitochondrion is encoded by a 5'-extension of the open reading frame. Comparison of the amino acid sequence encoded by the nuclear gene with the polypeptides encoded by edited mitochondrial cDNA and genomic sequences of other plants suggests that gene transfer between mitochondrion and nucleus started from edited mitochondrial RNA molecules. Mechanisms and requirements of gene transfer and activation are discussed.

Affinity purification of cytochrome c reductase from potato mitochondria

Ubiquinol-cytochrome-c oxidoreductase has been isolated from potato (Solanum tuberosum L.) mitochondria by cytochrome-c affinity chromatography and gel-filtration chromatography. The procedure, which up to now only proved applicable to Neurospora, yields a highly pure and active protein complex in monodisperse state. The molecular mass of the purified complex is about 650 kDa, indicating that potato cytochrome c reductase occurs as a dimer. Upon reconstitution into phospholipid membranes, the dimeric enzyme catalyzes electron transfer from a synthetic ubiquinol to equine cytochrome c with a turnover number of 50 s-1. The activity is inhibited by antimycin A and myxothiazol. A myxothiazol-insensitive and antimycin-sensitive transhydrogenation reaction, with a turnover number of 16 s-1, can be demonstrated as well. The protein complex consists of ten subunits, most of which have molecular masses similar to those of the nine-subunit fungal enzyme. Individual subunits were identified immunologically and spectral properties of b and c cytochromes were monitored. Interestingly, an additional 'core' polypeptide which is not present in other cytochrome bc1 complexes forms part of the enzyme from potato. Antibodies raised against individual polypeptides reveal that the core proteins are clearly immuno-distinguishable. The additional subunit may perform a specific function and contribute to the high molecular mass which exceeds those reported for other cytochrome-c-reductase dimers.