Eukaryote membrane genetics: the Fo sector of mitochondrial ATP synthase

Properties of two nuclear pet mutants affecting expression of the mitochondrial oli1 gene of Saccharomyces cerevisiae

This study details the characteristics of two temperature-conditional pet mutants of yeast, strains ts1860 and ts379, which at the non-permissive temperature show deficiencies in the formation of three mitochondrially encoded subunits of the ATP synthase complex. By analysis of mitochondrial translation products, and of mitochondrial transcription in temperature shift experiments from the permissive (22 degrees C) to the non-permissive (36 degrees C) temperature, it was concluded that the nuclear mutations in both mutants primarily inhibit synthesis of ATP synthase subunit 9, and that reductions in subunit 8 and 6 synthesis are secondary pleiotropic effects. Following transfer to 36 degrees C, cells of mutant ts379 display a near complete inhibition of subunit 9 synthesis within 1 h, coincident with a marked reduction in the level of the cognate oli1 mRNA. On the other hand, near complete inhibition of subunit 9 synthesis in strain ts1860 occurs after 3 h at 36 degrees C, at which time there is little change in the level of subunit 9 mRNA. In both mutants the mRNA levels for subunits 6 and 8 are not significantly affected at the time of inhibition of subunit 9 synthesis. Provision of an alternative source of subunit 8, translated extra-mitochondrially for import into the organelle, does not overcome the mutant phenotype of either mutant at 36 degrees C, confirming that subunit 8 is not the sole or primary deficiency in each mutant. The mutants indicate that the products of a least two nuclear genes (designated AEP1 and AEP2) are required for the expression of the mitochondrial oli1 gene and the synthesis of subunit 9. (ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of the ATP synthase subunit 6 and sequence of the mitochondrial ATP6 gene of the yeast Candida parapsilosis

The mitochondrially translated product called subunit 6 was extracted from the yeast Candida parapsilosis mitochondria using an organic solvent mixture and purified by reverse-phase HPLC. The partial N-terminal sequence of subunit 6 reveals a post-translational cleavage site as in Saccharomyces cerevisiae. The structural mitochondrial gene ATP6 was isolated form a mitochondrial DNA library using the oligonucleotide probe procedure. The gene and the surrounding regions were cloned into M13tg130 and M13tg131 phage vectors. The insert contained an open reading frame 738-bp encoding a 246-amino-acid polypeptide. Mature subunit 6 contains 243 amino acid residues and the predicted molecular mass is 26,511 Da. The subunit shows 52% similarity with ATP synthase subunit 6 of the yeast S. cerevisiae. Comparison between protein and DNA sequences shows that the CUN codon family codes for a leucine in C. parapsilosis mitochondria.

Structural similarities between a mitochondrially encoded polypeptide and a family of prokaryotic respiratory toxins involved in plasmid maintenance suggest a novel mechanism for the evolutionary maintenance of mitochondrial DNA

Subunit 8 of mitochondrial ATP synthase (A8), a mitochondrially encoded polypeptide, has no known homologue in any prokaryotic or plastid ATP synthase, suggesting that it has been recruited to its present role in the enzyme from an extraneous source. The polypeptide is poorly conserved at the primary sequence level, but shows a well-conserved hydropathy profile. The hydropathy profiles of A8 from diverse taxa were compared with those of the hok family of prokaryotic respiratory toxins, some of whose members are involved in plasmid maintenance, through postsegregational killing of cells that lose the plasmid at cell division. Such comparisons revealed a highly significant degree of similarity, suggesting a functional relationship. Based on these findings, it is proposed that A8 evolved from a hok-like protein, whose original role was the maintenance of an extrachromosomal replicon in the endosymbiont ancestor of mitochondria. An aggressive mechanism for the evolutionary maintenance of mitochondrial DNA overcomes many of the failings of traditional explanations for its retention as a separate genome.

Protoplast fusion mediated transfer of oligomycin resistance from Nicotiana sylvestris to Solanum tuberosum by intergeneric cybridization

We have successfully bridged the intergeneric barriers between Nicotiana and Solanum with respect to chondriome transfer. To enable this transfer we utilized the donor-recipient protoplast-fusion procedure. Consequently protoplasts of a Nicotiana sylvestris line with putatively oligomycin-resistant mitochondria (line OliR38) were used as irradiated chondriome donors and iodoacetate-treated protoplasts of Solanum tuberosum cv. Desiree served as recipients. The plated fusion products as well as their derived colonies and calli were exposed to gradually increasing levels of oligomycin. The resulting plantlets had potato morphology and were analyzed with respect to their mitochondrial DNA and chloroplast DNA. Fifteen out of 50 regenerated plants were verified as true cybrids. Detailed analyses of one cybrid revealed chondriome components from the oligomycin-resistant donor line, OliR38, but retention of the plastome of potato. This cybrid was oligomycin-resistant as revealed by root-culture analysis. It was thus verified that due to selection, chondriome components could be transferred from a N. sylvestris donor into a cybrid having all the phenotypic features controlled by the nucleus of the recipient fusion partner (S. tuberosum).

cDNA cloning and sequencing for the import precursor of coupling factor 6 in H(+)-ATP synthase from rat liver mitochondria

The nucleotide sequence of the import precursor of coupling factor 6 (factor 6) of rat liver H(+)-ATP synthase has been determined from a recombinant cDNA clone isolated by screening a rat liver cDNA library with a probe DNA. The sequence was composed of 458 nucleotides including a coding region for the import precursor of factor 6 and noncoding regions of both the 5'- and 3'-sides. The import precursor of factor 6 and its mature polypeptide deduced from the open reading frame consisted of 108 and 76 amino acid residues with a molecular weight of 12,494 and 8,927, respectively. The presequence of 32 amino acids could be the import signal peptide which serves to direct the protein into the mitochondrial matrix.

Acquired thermotolerance following heat shock protein synthesis prevents impairment of mitochondrial ATPase activity at elevated temperatures in Saccharomyces cerevisiae

The complex molecular response of cells to sudden temperature changes is a well-characterized phenomenon. Although it is clear that the induction of heat shock proteins provides protection from heat in all of the organisms so far tested, very little is known about the role that this set of proteins plays in cellular homeostasis. Recently, putative roles for hsp60 and hsp70-like proteins have been proposed in Saccharomyces cerevisiae. hsp70-like proteins have been shown to be necessary for translocation of precursor polypeptides into mitochondria and endoplasmic reticulum, while hsp60 is required for the assembly of precursor polypeptides into oligomeric complexes following incorporation into the mitochondrial matrix. In this paper, we report that a brief temperature shock (44 degrees C) impairs coupling of oxidative phosphorylation in S. cerevisiae as measured indirectly by the Cl-CCP/oligomycin assay. Furthermore, at high temperature oligomycin stimulates rather than inhibits oxygen uptake under nonthermotolerant conditions. Pretreatment of cells for a short period of time at 37 degrees C, prior to exposure to higher temperatures rescues the capacity to maintain coupling between oxidative phosphorylation and electron transport. Inhibition of cytoplasmic RNA or protein synthesis during heat shock prevents the protection of this mitochondrial activity. We propose that one of the roles of the induction of heat shock proteins (or related activities) is to protect mitochondrial ATPase activity under conditions of further increase in temperature.

Identification of a 66 KDa protein associated with yeast mitochondrial ATP synthase as heat shock protein hsp60

A 66 kDa protein, denoted P66, not hitherto classified as an integral component of yeast mitochondrial ATPase, is often observed in preparations of this enzyme complex. A physical association exists between P66 and the assembled ATPase complex since both components are coimmunoprecipitated by anti-F1 beta monoclonal antibody. Two recombinant clones expressing proteins immunologically similar to P66 were isolated from a yeast genomic library in lambda gt11 by screening with a polyclonal anti-holo-ATPase antibody. Based on restriction site mapping and partial nucleotide sequence analysis, both clones encompass the gene encoding the yeast heat shock protein hsp60. The identification of P66 with hsp60, taken together with its demonstrated association with the mitochondrial ATPase complex, is consistent with recent suggestions that hsp60 is involved in assembly of the ATP synthase complex.

Assembly of imported subunit 8 into the ATP synthase complex of isolated yeast mitochondria

This study concerns the assembly into a multisubunit enzyme complex of a small hydrophobic protein imported into isolated mitochondria. Subunit 8 of yeast mitochondrial ATPase (normally a mitochondrial gene product) was expressed in vitro as a chimaeric precursor N9L/Y8-1, which includes an N-terminal-cleavable transit peptide to direct its import into mitochondria. Assembly into the enzyme complex of the imported subunit 8 was monitored by immunoadsorption using an immobilized anti-F1-beta monoclonal antibody. Preliminary experiments showed that N9L/Y8-1 imported into normal rho+ mitochondria, with its complement of fully assembled ATPase, did not lead to an appreciable assembly of the exogenous subunit 8. With the expectation that mitochondria previously depleted of subunit 8 could allow such assembly in vitro, target mitochondria were prepared from genetically modified yeast cells in which synthesis of subunit 8 was specifically blocked. Initially, mitochondria were prepared from strain M31, a mit- mutant completely incapable of intramitochondrial biosynthesis of subunit 8. These mit- mitochondria however were unsuitable for assembly studies because they could not import protein in vitro. A controlled depletion strategy was then evolved. An artificial nuclear gene encoding N9L/Y8-1 was brought under the control of a inducible promoter GAL1. This regulated gene construct, in a low copy number yeast expression vector, was introduced into strain M31 to generate strain YGL-1. Galactose control of the expression of N9L/Y8-1 was demonstrated by the ability of strain YGL-1 to grow vigorously on galactose as a carbon source, and by the inability to utilize ethanol alone for prolonged periods of growth. The measurement of bioenergetic parameters in mitochondria from YGL-1 cells experimentally depleted of subunit 8, by transferring growing cells from galactose to ethanol, was consistent with the presence in mitochondria of a mosaic of ATPase, namely fully assembled functional ATPase complexes and partially assembled complexes with defective F0 sectors. These mitochondria demonstrated very efficient import of N9L/Y8-1 and readily incorporated the imported processed subunit 8 protein into ATPase. Comparison of the kinetics of import and assembly of subunit 8 showed that assembly was noticeably delayed with respect to import. These findings open the way to a new systematic analysis of the assembly of imported proteins into multisubunit mitochondrial enzyme complexes.

Organization of the mitochondrial genome of Atlantic cod, Gadus morhua

The mitochondrial DNA (mtDNA) from the Atlantic cod, Gadus morhua, was mapped using 11 different restriction enzymes and cloned into plasmid vectors. Sequence data obtained from more than 10 kilobases of cod mtDNA show that the genome organization, genetic code, and the overall codon usage have been conserved throughout the evolution of vertebrates. Comparison of the derived amino acid sequences of proteins encoded by cod mtDNA to the ones encoded by Xenopus laevis mtDNA revealed that the amino acid identity range from 46% to 93% for the different proteins. ND4L is most divergent while COI is most conserved. GUG was found as the translation initiation codon of the COI gene, indicating a dual coding function for this codon. The sequences of the 997 base pair displacement-loop (D-loop)-containing region and the origin of L-strand replication (oriL), are presented. Only few of the primary and secondary structure features found to be conserved among mammalian mitochondrial D-loops, can be identified in cod. Presence of CSB-2 in the D-loop-containing region and the conserved hairpin structure at oriL, indicates that replication of bony fish mtDNA may follow the same general scheme as described for higher vertebrates.

Orientation of chargerin II (A6L) in the ATP synthase of rat liver mitochondria determined with antibodies against peptides of the protein

Previous studies suggested that the hydrophobic protein chargerin II, which is encoded in the unidentified reading frame A6L of mitochondrial DNA (URFA6L), may have a key role in the energy transduction by mitochondrial ATP synthase because an antibody against chargerin II inhibited ATP synthesis and ATP-Pi exchange, in an energy-dependent fashion. In the present work, the orientation of chargerin II in Fo of the ATP synthase of rat liver mitochondria was examined using antibodies against peptides of chargerin II. Results showed that its N-terminal region (about 8 amino acid residues) was exposed on the surface of the C-side of Fo, but its C-terminal and charge-cluster regions were buried in Fo.

Mitochondrial and nuclear DNA complementation in the respiratory chain function and defects

The 16569 base pairs of the mitochondrial DNA encode with a specific genetic code 13 proteins involved in the respiratory chain complex formation. Nuclear gene products also contribute to the formation of these complexes. In the first point, the organization and expression of the mtDNA are described with the main characteristics of the enzymatic complexes as well as nuclear gene expression. New information concerned with mitochondrial DNA deletions and mutations are described particularly with respect to Kearns-Sayre Syndrome.

The role of subunit 4, a nuclear-encoded protein of the F0 sector of yeast mitochondrial ATP synthase, in the assembly of the whole complex

The yeast nuclear gene ATP4, encoding the ATP synthase subunit 4, was disrupted by insertion into the middle of it the selective marker URA3. Transformation of the Saccharomyces cerevisiae strain D273-10B/A/U produced a mutant unable to grow on glycerol medium. The ATP4 gene is unique since subunit 4 was not present in mutant mitochondria; the hypothetical truncated subunit 4 was never detected. ATPase was rendered oligomycin-insensitive and the F1 sector of this mutant appeared loosely bound to the membrane. Analysis of mitochondrially translated hydrophobic subunits of F0 revealed that subunits 8 and 9 were present, unlike subunit 6. This indicated a structural relationship between subunits 4 and 6 during biogenesis of F0. It therefore appears that subunit 4 (also called subunit b in beef heart and Escherichia coli ATP synthases) plays at least a structural role in the assembly of the whole complex. Disruption of the ATP4 gene also had a dramatic effect on the assembly of other mitochondrial complexes. Thus, the cytochrome oxidase activity of the mutant strain was about five times lower than that of the wild type. In addition, a high percentage of spontaneous rho- mutants was detected.

Modifications of the relative proteolipid composition in the ATP synthase of a respiratory competent mutant of Saccharomyces cerevisiae

A comparative study of the proteolipid composition of the F0-sector of the ATP synthase of wild-type strain of Saccharomyces cerevisiae and of nuclear mutants, modified at the level of the oxidative phosphorylation due to an enhanced proton permeability of the inner membrane, was carried out. Analysis of the crude proteolipid extract by electrophoresis and high liquid performance chromatography showed some differences at the level of mitochondrial DNA encoded proteolipids. Subunit 6 and in particular subunit 8 were present in reduced amounts, whereas subunit 9 was present in equal amounts in both types of strain. However, the phosphate binding affinity of subunit 8 was the same in wild-type and mutant strains. The fact that subunit 6 and subunit 8 are cotranscripted on a single mRNA led to the problem of the regulation of the mitochondrial synthesis of these two proteins by a nuclear gene.

Amino acid substitutions in mitochondrial ATP synthase subunit 9 of Saccharomyces cerevisiae leading to venturicidin or ossamycin resistance

A series of mitochondrially inherited mutants of yeast has been analysed, which were previously identified as showing resistance to the antibiotics venturicidin or ossamycin and whose mutations showed tight linkage to oligomycin-resistance alleles affecting subunit 9 of the mitochondrial ATP synthase. DNA sequence analysis of the oli1 gene of these mutants has been used to define the nature of amino acid substitution in the subunit 9 protein. In the case of the two venturicidin-resistant mutants, mutations affect amino acids on the N-terminal stem of the protein, namely Gly25----Ser (venR ossS oliR) and Ala 27----Gly (venR ossS oliS). The mutations found in the two ossamycin-resistant mutants affect amino acids on the C-terminal stem of the protein; namely Leu53----Phe (vanS ossR oliR) and Leu57----Phe (venS ossR oliS). These results allow us to further develop a fine structure map of domains within the subunit 9 protein involved in antibiotic interaction.

Studies on the import into mitochondria of yeast ATP synthase subunits 8 and 9 encoded by artificial nuclear genes

Direct fusions have been constructed between each of subunits 8 and 9 from mitochondrial ATPase of Saccharomyces cerevisiae, proteins normally encoded inside mitochondria, and the cleavable N-terminal transit peptide from the nuclearly encoded precursor to subunit 9 of Neurospora crassa mitochondrial ATPase. The subunit 8 construct was imported efficiently into isolated yeast mitochondria and was processed at or very near the fusion point. When expressed in vivo from its artificial nuclear gene, this cytoplasmically synthesized form of subunit 8 restored the growth defects of aap 1 mutants unable to produce subunit 8 inside the mitochondria. The subunit 9 construct was, however, unable to be imported into isolated mitochondria and could not, following nuclear expression in vivo, complement growth defects in mitochondrial oli 1 mutants. This behaviour is contrasted with the previously demonstrated import competence of another yeast subunit 9 fusion, bearing the first five residues of mature N. crassa subunit 9 interposed between its own transit peptide and the yeast subunit 9 moiety.

Reprogrammed expression of subunit 9 of the mitochondrial ATPase complex of Saccharomyces cerevisiae. Expression in vitro from a chemically synthesized gene and import into isolated mitochondria

A synthetic gene has been designed and constructed by total chemical synthesis as a first step in the functional relocation from the mitochondrion to the nucleus of a gene encoding subunit 9 of the yeast mitochondrial ATPase complex. This gene (NAP9) incorporates codons frequently used in nuclear genes of Saccharomyces cerevisiae and additionally includes a series of unique restriction enzyme cleavage sites to facilitate future systematic manipulations of the gene and its protein product. Following the expression of the NAP9 gene by transcription and translation in vitro, a radiolabelled protein was produced which displayed a gel electrophoretic mobility and solubility in chloroform/methanol characteristic of the authentic subunit 9 proteolipid encoded in vivo by the mitochondrial oli1 gene. In order to achieve import into mitochondria of yeast subunit 9, a fusion was made between the NAP9 gene and DNA encoding the cleavable presequence of the nuclearly encoded precursor to subunit 9 from Neurospora crassa. Following expression in vitro, the resultant fusion protein was imported and appropriately processed by isolated yeast mitochondria. The import of yeast subunit 9 was less efficient than that observed in parallel import experiments with yeast subunit 8 attached to the same presequence or with the naturally occurring intact N. crassa subunit 9 precursor. Yeast subunit 9 lacking a leader sequence is not imported into mitochondria but, unlike subunit 8, it does not embed itself into the outer membrane, in spite of its highly hydrophobic character.

Assembly of functional proton-translocating ATPase complex in yeast mitochondria with cytoplasmically synthesized subunit 8, a polypeptide normally encoded within the organelle

A mitochondrial gene from Saccharomyces cerevisiae encoding a hydrophobic membrane protein, subunit 8 of the F0/F1-type mitochondrial ATPase complex, has been functionally replaced by an artificial nuclear gene specifying an imported version of this protein. The experiments reported here utilized a multicopy expression vector (pLF1) that replicates in the nucleus of yeast cells and that carries an inserted DNA segment, specifying a precursor protein (N9/Y8) consisting of subunit 8 fused to an N-terminal cleavable transit peptide (the leader sequence from Neurospora crassa ATPase subunit 9). The successful incorporation of the imported subunit 8 into functional ATPase complexes after transformation with pLF1 expressing N9/Y8 was indicated by the efficient genetic complementation of respiratory growth defects of aap1 mit- mutants, which lack endogenous subunit 8. The reconstitution of ATPase function was confirmed by biochemical assays of ATPase performance in mitochondria and by immunochemical analyses that demonstrated the assembly of the cytoplasmically synthesized subunit 8 into the ATPase complex. Reconstitution of ATPase function required the cytoplasmically synthesized subunit to have a transit peptide. The strategy for importation and reconstitution developed for subunit 8 leads to a systematic approach to the directed manipulation of mitochondrially encoded membrane-associated proteins that has general implications for exploring membrane biogenesis mechanistically and evolutionarily.