Primary structure requirements for correct sorting of the yeast mitochondrial protein ADH III to the yeast mitochondrial matrix space

Labile Low-Molecular-Mass Metal Complexes in Mitochondria: Trials and Tribulations of a Burgeoning Field

Iron, copper, zinc, manganese, cobalt, and molybdenum play important roles in mitochondrial biochemistry, serving to help catalyze reactions in numerous metalloenzymes. These metals are also found in labile "pools" within mitochondria. Although the composition and cellular function of these pools are largely unknown, they are thought to be comprised of nonproteinaceous low-molecular-mass (LMM) metal complexes. Many problems must be solved before these pools can be fully defined, especially problems stemming from the lability of such complexes. This lability arises from inherently weak coordinate bonds between ligands and metals. This is an advantage for catalysis and trafficking, but it makes characterization difficult. The most popular strategy for investigating such pools is to detect them using chelator probes with fluorescent properties that change upon metal coordination. Characterization is limited because of the inevitable destruction of the complexes during their detection. Moreover, probes likely react with more than one type of metal complex, confusing analyses. An alternative approach is to use liquid chromatography (LC) coupled with inductively coupled plasma mass spectrometry (ICP-MS). With help from a previous lab member, the authors recently developed an LC-ICP-MS approach to analyze LMM extracts from yeast and mammalian mitochondria. They detected several metal complexes, including Fe580, Fe1100, Fe1500, Cu5000, Zn1200, Zn1500, Mn1100, Mn2000, Co1200, Co1500, and Mo780 (numbers refer to approximate masses in daltons). Many of these may be used to metalate apo-metalloproteins as they fold inside the organelle. The LC-based approach also has challenges, e.g., in distinguishing artifactual metal complexes from endogenous ones, due to the fact that cells must be disrupted to form extracts before they are passed through chromatography columns prior to analysis. Ultimately, both approaches will be needed to characterize these intriguing complexes and to elucidate their roles in mitochondrial biochemistry.

Alcohol dehydrogenase gene ADH3 activates glucose alcoholic fermentation in genetically engineered Dekkera bruxellensis yeast

Dekkera bruxellensis is a non-conventional Crabtree-positive yeast with a good ethanol production capability. Compared to Saccharomyces cerevisiae, its tolerance to acidic pH and its utilization of alternative carbon sources make it a promising organism for producing biofuel. In this study, we developed an auxotrophic transformation system and an expression vector, which enabled the manipulation of D. bruxellensis, thereby improving its fermentative performance. Its gene ADH3, coding for alcohol dehydrogenase, was cloned and overexpressed under the control of the strong and constitutive promoter TEF1. Our recombinant D. bruxellensis strain displayed 1.4 and 1.7 times faster specific glucose consumption rate during aerobic and anaerobic glucose fermentations, respectively; it yielded 1.2 times and 1.5 times more ethanol than did the parental strain under aerobic and anaerobic conditions, respectively. The overexpression of ADH3 in D. bruxellensis also reduced the inhibition of fermentation by anaerobiosis, the “Custer effect”. Thus, the fermentative capacity of D. bruxellensis could be further improved by metabolic engineering.

The alcohol dehydrogenase system in the xylose-fermenting yeast Candida maltosa

BACKGROUND

The alcohol dehydrogenase (ADH) system plays a critical role in sugar metabolism involving in not only ethanol formation and consumption but also the general "cofactor balance" mechanism. Candida maltosa is able to ferment glucose as well as xylose to produce a significant amount of ethanol. Here we report the ADH system in C. maltosa composed of three microbial group I ADH genes (CmADH1, CmADH2A and CmADH2B), mainly focusing on its metabolic regulation and physiological function.

METHODOLOGY/PRINCIPAL FINDINGS

Genetic analysis indicated that CmADH2A and CmADH2B tandemly located on the chromosome could be derived from tandem gene duplication. In vitro characterization of enzymatic properties revealed that all the three CmADHs had broad substrate specificities. Homo- and heterotetramers of CmADH1 and CmADH2A were demonstrated by zymogram analysis, and their expression profiles and physiological functions were different with respect to carbon sources and growth phases. Fermentation studies of ADH2A-deficient mutant showed that CmADH2A was directly related to NAD regeneration during xylose metabolism since CmADH2A deficiency resulted in a significant accumulation of glycerol.

CONCLUSIONS/SIGNIFICANCE

Our results revealed that CmADH1 was responsible for ethanol formation during glucose metabolism, whereas CmADH2A was glucose-repressed and functioned to convert the accumulated ethanol to acetaldehyde. To our knowledge, this is the first demonstration of function separation and glucose repression of ADH genes in xylose-fermenting yeasts. On the other hand, CmADH1 and CmADH2A were both involved in ethanol formation with NAD regeneration to maintain NADH/NAD ratio in favor of producing xylitol from xylose. In contrast, CmADH2B was expressed at a much lower level than the other two CmADH genes, and its function is to be further confirmed.

Ethanol Reassimilation and Ethanol Tolerance in Pichia stipitis CBS 6054 as Studied by C Nuclear Magnetic Resonance Spectroscopy

Ethanol reassimilation in Pichia stipitis CBS 6054 was studied by using continuous cultures, and the oxidation of [1-C]ethanol was monitored by in vivo and in vitro C nuclear magnetic resonance spectroscopy. Acetate was formed when ethanol was reassimilated. The ATP/ADP ratio and the carbon dioxide production decreased, whereas the malate dehydrogenase activity increased, in ethanol-reassimilating cells. The results are discussed in terms of the low ethanol tolerance in P. stipitis compared with that in Saccharomyces cerevisiae (S. W. Brown, S. G. Oliver, D. E. F. Harrison, and R. C. Righelato, Eur. J. Appl. Microbiol. Biotechnol. 11:151-155, 1981).

Repression of ADH1 and ADH3 during zinc deficiency by Zap1-induced intergenic RNA transcripts

The transcriptional activator Zap1 induces target gene expression in response to zinc deficiency. We demonstrate that during zinc starvation, Zap1 is required for the repression of ADH1 expression. ADH1 encodes the major zinc-dependent alcohol dehydrogenase that is utilized during fermentation. During zinc starvation, Zap1 binds upstream of the activator Rap1 and induces an intergenic RNA transcript, ZRR1. ZRR1 expression leads to the transient displacement of Rap1 from the ADH1 promoter resulting in ADH1 repression. Using a microarray-based approach, we screened for additional genes repressed by Zap1 intergenic transcripts. We found that ADH3, the major mitochondrial alcohol dehydrogenase, is regulated in a manner similar to ADH1. Thus, during zinc deficiency, Zap1 mediates the repression of two of the most abundant zinc-requiring enzymes.

Asymmetric sequence divergence of duplicate genes

Much like humans, gene duplicates may be created equal, but they do not stay that way for long. For four completely sequenced genomes we show that 20%-30% of duplicate gene pairs show asymmetric evolution in the amino acid sequence of their protein products. That is, one of the duplicates evolves much faster than the other. The greater this asymmetry, the greater the ratio Ka/Ks of amino acid substitutions (Ka) to silent substitutions (Ks) in a gene pair. This indicates that most asymmetric divergence may be caused by relaxed selective constraints on one of the duplicates. However, we also find some candidate duplicates where positive (directional) selection of beneficial mutations (Ka/Ks > 1) may play a role in asymmetric divergence. Our analysis rests on a codon-based model of molecular evolution that allows a test for asymmetric divergence in Ka. The method is also more sensitive in detecting positive selection (Ka/Ks > 1) than models relying only on pairwise gene comparisons.

ADEPTs: information necessary for subcellular distribution of eukaryotic sorting isozymes resides in domains missing from eubacterial and archaeal counterparts

Sorting isozymes are encoded by single genes, but the encoded proteins are distributed to multiple subcellular compartments. We surveyed the predicted protein sequences of several nucleic acid interacting sorting isozymes from the eukaryotic taxonomic domain and compared them with their homologs in the archaeal and eubacterial domains. Here, we summarize the data showing that the eukaryotic sorting isozymes often possess sequences not present in the archaeal and eubacterial counterparts and that the additional sequences can act to target the eukaryotic proteins to their appropriate subcellular locations. Therefore, we have named these protein domains ADEPTs (Additional Domains for Eukaryotic Protein Targeting). Identification of additional domains by phylogenetic comparisons should be generally useful for locating candidate sequences important for subcellular distribution of eukaryotic proteins.

Pichia stipitis genes for alcohol dehydrogenase with fermentative and respiratory functions

Two genes coding for isozymes of alcohol dehydrogenase (ADH); designated PsADH1 and PsADH2, have been identified and isolated from Pichia stipitis CBS 6054 genomic DNA by Southern hybridization to Saccharomyces cerevisiae ADH genes, and their physiological roles have been characterized through disruption. The amino acid sequences of the PsADH1 and PsADH2 isozymes are 80.5% identical to one another and are 71.9 and 74.7% identical to the S. cerevisiae ADH1 protein. They also show a high level identity with the group I ADH proteins from Kluyveromyces lactis. The PsADH isozymes are presumably localized in the cytoplasm, as they do not possess the amino-terminal extension of mitochondrion-targeted ADHs. Gene disruption studies suggest that PsADH1 plays a major role in xylose fermentation because PsADH1 disruption results in a lower growth rate and profoundly greater accumulation of xylitol. Disruption of PsADH2 does not significantly affect ethanol production or aerobic growth on ethanol as long as PsADH1 is present. The PsADH1 and PsADH2 isozymes appear to be equivalent in the ability to convert ethanol to acetaldehyde, and either is sufficient to allow cell growth on ethanol. However, disruption of both genes blocks growth on ethanol. P. stipitis strains disrupted in either PsADH1 or PsADH2 still accumulate ethanol, although in different amounts, when grown on xylose under oxygen-limited conditions. The PsADH double disruptant, which is unable to grow on ethanol, still produces ethanol from xylose at about 13% of the rate seen in the parental strain. Thus, deletion of both PsADH1 and PsADH2 blocks ethanol respiration but not production, implying a separate path for fermentation.

Ethanol formation in adh0 mutants reveals the existence of a novel acetaldehyde-reducing activity in Saccharomyces cerevisiae

A strain of Saccharomyces cerevisiae has been constructed which is deficient in the four alcohol dehydrogenase (ADH) isozymes known at present. This strain (adh0), being irreversibly mutated in the genes ADH1, ADH3, and ADH4 and carrying a point mutation in the gene ADH2 coding for the glucose-repressible isozyme ADHII, still produces up to one third of the theoretical maximum yield of ethanol in a homofermentative conversion of glucose to ethanol. Analysis of the glucose metabolism of adh0 cells shows that the lack of all known ADH isozymes results in the formation of glycerol as a major fermentation product, accompanied by a significant production of acetaldehyde and acetate. Treatment of glucose-growing adh0 cells with the respiratory-chain inhibitor antimycin A leads to an immediate cessation of ethanol production, demonstrating that ethanol production in adh0 cells is dependent on mitochondrial electron transport. Reduction of acetaldehyde to ethanol in isolated mitochondria could also be demonstrated. This reduction is apparently linked to the oxidation of acetaldehyde to acetate. Preliminary data suggest that this novel type of ethanol formation in S. cerevisiae is associated with the inner mitochondrial membrane.

Mutant alcohol dehydrogenase (ADH III) presequences that affect both in vitro mitochondrial import and in vitro processing by the matrix protease

Point mutations in the presequence of the mitochondrial alcohol dehydrogerase isoenzyme (ADH III) have been shown to affect either the import of the precursor protein into yeast mitochondria in vivo or its processing within the organelle. In the present work, the behavior of these mutants during in vitro import into isolated mitochondria was investigated. All point mutants tested were imported with a slower initial rate than that of the wild-type precursor. This defect was corrected when the precursors were treated with urea prior to import. Once imported, the extent of processing to the mature form of mutant precursors varied greatly and correlated well with the defects observed in vivo. This result was not affected by prior urea treatment. When matrix extracts enriched for the processing protease were used, this defect was shown to be due to failure of the protease to efficiently recognize or cleave the presequence, rather than to a lack of access to the precursor. The rate of import of two ADH III precursors bearing internal deletions in the leader sequence was similar to those of the point mutants, whereas a deletion leading to the removal of the 15 amino-terminal amino acids was poorly imported. The mature amino terminus of wild-type ADH III was determined to be Gln-25. Mutant m01 (Ser-26 to Phe), which reduced the efficiency of cleavage in vitro by 80%, was cleaved at the correct site.

Transport of the yeast ATP synthase beta-subunit into mitochondria. Effects of amino acid substitutions on targeting

We have isolated the yeast ATP2 gene encoding the beta-subunit of mitochondrial ATP synthase and determined its nucleotide sequence. A fusion between the N-terminal 15 amino acid residues of beta-subunit and the mouse cytosolic protein dihydrofolate reductase (DHFR) was transcribed and translated in vitro and found to be transported into isolated yeast mitochondria. A fusion with the first 35 amino acid residues of beta-subunit attached to DHFR was not only transported but also proteolytically processed by a mitochondrial protease. Amino acid substitutions were introduced into the N-terminal presequence of the beta-subunit by bisulphite mutagenesis of the corresponding DNA. The effects of these mutations on mitochondrial targeting were assessed by transport experiments in vitro using DHFR fusion proteins. All of the mutants, harbourin from one to six amino acid substitutions in the first 14 residues of the presequence, were transported into mitochondria, though at least one of them (I8) was transported and proteolytically processed at a much reduced rate. The I8 mutant beta-subunit also exhibited poor transport and processing in vivo, and expression of this mutant polypeptide failed to complement the glycerol- phenotype of a yeast ATP2 mutant. More remarkably, the expression of I8 beta-subunit induced a more general growth defect in yeast, possibly due to interference with the transport of other, essential, mitochondrial proteins.

Sequence and structural requirements of a mitochondrial protein import signal defined by saturation cassette mutagenesis

The Saccharomyces cerevisiae F1-ATPase beta subunit precursor contains redundant mitochondrial protein import information at its NH2 terminus (D. M. Bedwell, D. J. Klionsky, and S. D. Emr, Mol. Cell. Biol. 7:4038-4047, 1987). To define the critical sequence and structural features contained within this topogenic signal, one of the redundant regions (representing a minimal targeting sequence) was subjected to saturation cassette mutagenesis. Each of 97 different mutant oligonucleotide isolates containing single (32 isolates), double (45 isolates), or triple (20 isolates) point mutations was inserted in front of a beta-subunit gene lacking the coding sequence for its normal import signal (codons 1 through 34 were deleted). The phenotypic and biochemical consequences of these mutations were then evaluated in a yeast strain deleted for its normal beta-subunit gene (delta atp2). Consistent with the lack of an obvious consensus sequence for mitochondrial protein import signals, many mutations occurring throughout the minimal targeting sequence did not significantly affect its import competence. However, some mutations did result in severe import defects. In these mutants, beta-subunit precursor accumulated in the cytoplasm, and the yeast cells exhibited a respiration defective phenotype. Although point mutations have previously been identified that block mitochondrial protein import in vitro, a subset of the mutations reported here represents the first single missense mutations that have been demonstrated to significantly block mitochondrial protein import in vivo. The previous lack of such mutations in the beta-subunit precursor apparently relates to the presence of redundant import information in this import signal. Together, our mutants define a set of constraints that appear to be critical for normal activity of this (and possibly other) import signals. These include the following: (i) mutant signals that exhibit a hydrophobic moment greater than 5.5 for the predicted amphiphilic alpha-helical conformation of this sequence direct near normal levels of beta-subunit import (ii) at least two basic residues are necessary for efficient signal function, (iii) acidic amino acids actively interfere with import competence, and (iv) helix-destabilizing residues also interfere with signal function. These experimental observations provide support for mitochondrial protein import models in which both the structure and charge of the import signal play a critical role in directing mitochondrial protein targeting and import.

Sequence and structural requirements of a mitochondrial protein import signal defined by saturation cassette mutagenesis

The Saccharomyces cerevisiae F1-ATPase beta subunit precursor contains redundant mitochondrial protein import information at its NH2 terminus (D. M. Bedwell, D. J. Klionsky, and S. D. Emr, Mol. Cell. Biol. 7:4038-4047, 1987). To define the critical sequence and structural features contained within this topogenic signal, one of the redundant regions (representing a minimal targeting sequence) was subjected to saturation cassette mutagenesis. Each of 97 different mutant oligonucleotide isolates containing single (32 isolates), double (45 isolates), or triple (20 isolates) point mutations was inserted in front of a beta-subunit gene lacking the coding sequence for its normal import signal (codons 1 through 34 were deleted). The phenotypic and biochemical consequences of these mutations were then evaluated in a yeast strain deleted for its normal beta-subunit gene (delta atp2). Consistent with the lack of an obvious consensus sequence for mitochondrial protein import signals, many mutations occurring throughout the minimal targeting sequence did not significantly affect its import competence. However, some mutations did result in severe import defects. In these mutants, beta-subunit precursor accumulated in the cytoplasm, and the yeast cells exhibited a respiration defective phenotype. Although point mutations have previously been identified that block mitochondrial protein import in vitro, a subset of the mutations reported here represents the first single missense mutations that have been demonstrated to significantly block mitochondrial protein import in vivo. The previous lack of such mutations in the beta-subunit precursor apparently relates to the presence of redundant import information in this import signal. Together, our mutants define a set of constraints that appear to be critical for normal activity of this (and possibly other) import signals. These include the following: (i) mutant signals that exhibit a hydrophobic moment greater than 5.5 for the predicted amphiphilic alpha-helical conformation of this sequence direct near normal levels of beta-subunit import (ii) at least two basic residues are necessary for efficient signal function, (iii) acidic amino acids actively interfere with import competence, and (iv) helix-destabilizing residues also interfere with signal function. These experimental observations provide support for mitochondrial protein import models in which both the structure and charge of the import signal play a critical role in directing mitochondrial protein targeting and import.

Single-amino-acid substitutions within the signal sequence of yeast prepro-alpha-factor affect membrane translocation

We used a genetic approach to identify point mutations in the signal sequence of a secreted eucaryotic protein, yeast alpha-factor. Signal sequence mutants were obtained by selecting for cells that partially mistargeted into mitochondria a fusion protein consisting of the alpha-factor signal sequence fused to the mature portion of an imported mitochondrial protein (Cox IV). The mutations resulted in replacement of a residue in the hydrophobic core of the signal sequence with either a hydrophilic amino acid or a proline. After reassembly into an intact alpha-factor gene, the substitutions were found to decrease up to 50-fold the rate of translocation of prepro-alpha-factor across microsomal membranes in vitro. Two of three mutants tested produced lower steady-state levels of alpha-factor in intact yeast cells, although the magnitude of the effect was less than that in the cell-free system.

Single-amino-acid substitutions within the signal sequence of yeast prepro-alpha-factor affect membrane translocation

We used a genetic approach to identify point mutations in the signal sequence of a secreted eucaryotic protein, yeast alpha-factor. Signal sequence mutants were obtained by selecting for cells that partially mistargeted into mitochondria a fusion protein consisting of the alpha-factor signal sequence fused to the mature portion of an imported mitochondrial protein (Cox IV). The mutations resulted in replacement of a residue in the hydrophobic core of the signal sequence with either a hydrophilic amino acid or a proline. After reassembly into an intact alpha-factor gene, the substitutions were found to decrease up to 50-fold the rate of translocation of prepro-alpha-factor across microsomal membranes in vitro. Two of three mutants tested produced lower steady-state levels of alpha-factor in intact yeast cells, although the magnitude of the effect was less than that in the cell-free system.

The yeast F1-ATPase beta subunit precursor contains functionally redundant mitochondrial protein import information

The NH2 terminus of the yeast F1-ATPase beta subunit precursor directs the import of this protein into mitochondria. To define the functionally important components of this import signal, oligonucleotide-directed mutagenesis was used to introduce a series of deletion and missense mutations into the gene encoding the F1-beta subunit precursor. Among these mutations were three nonoverlapping deletions, two within the 19-amino-acid presequence (delta 5-12 and delta 16-19) and one within the mature protein (delta 28-34). Characterization of the mitochondrial import properties of various mutant F1-beta subunit proteins containing different combinations of these deletions showed that import was blocked only when all three deletions were combined. Mutant proteins containing all possible single and pairwise combinations of these deletions were found to retain the ability to direct mitochondrial import of the F1-beta subunit. These data suggest that the F1-beta subunit contains redundant import information at its NH2 terminus. In fact, we found that deletion of the entire F1-beta subunit presequence did not prevent import, indicating that a functional mitochondrial import signal is present near the NH2 terminus of the mature protein. Furthermore, by analyzing mitochondrial import of the various mutant proteins in [rho-] yeast, we obtained evidence that different segments of the F1-beta subunit import signal may act in an additive or cooperative manner to optimize the import properties of this protein.

The yeast F1-ATPase beta subunit precursor contains functionally redundant mitochondrial protein import information

The NH2 terminus of the yeast F1-ATPase beta subunit precursor directs the import of this protein into mitochondria. To define the functionally important components of this import signal, oligonucleotide-directed mutagenesis was used to introduce a series of deletion and missense mutations into the gene encoding the F1-beta subunit precursor. Among these mutations were three nonoverlapping deletions, two within the 19-amino-acid presequence (delta 5-12 and delta 16-19) and one within the mature protein (delta 28-34). Characterization of the mitochondrial import properties of various mutant F1-beta subunit proteins containing different combinations of these deletions showed that import was blocked only when all three deletions were combined. Mutant proteins containing all possible single and pairwise combinations of these deletions were found to retain the ability to direct mitochondrial import of the F1-beta subunit. These data suggest that the F1-beta subunit contains redundant import information at its NH2 terminus. In fact, we found that deletion of the entire F1-beta subunit presequence did not prevent import, indicating that a functional mitochondrial import signal is present near the NH2 terminus of the mature protein. Furthermore, by analyzing mitochondrial import of the various mutant proteins in [rho-] yeast, we obtained evidence that different segments of the F1-beta subunit import signal may act in an additive or cooperative manner to optimize the import properties of this protein.