Biomedical subjects

B Guiard

Publications and source records attributed to B Guiard.

At least 55 records · Page 3Linked to original sources

PubMed1993

Evidence for an interaction between the CYP1(HAP1) activator and a cellular factor during heme-dependent transcriptional regulation in the yeast Saccharomyces cerevisiae.

Previously, it was shown that the CYP1(HAP1) gene product mediates the transcription of several oxygen-regulated genes through a metabolic co-effector, heme, in the yeast Saccharomyces cerevisiae. This study investigates the overproduction of the CYP1 protein when the CYP1(HAP1) gene is placed under the control of the GAL10-CYC1 hybrid promoter (either at the locus of the CYP1(HAP1) gene or cloned in a high-copy-number plasmid). In these conditions, the CYP1 protein is detected by Western blot analysis and has a molecular mass in agreement with the open reading frame sequence. Band-shift experiments show that the CYP1(HAP1) protein is able to interact specifically with its target sequences in vitro without addition of hemin, and forms a large complex with one or several unidentified factors denoted as X. Addition of hemin allows the formation of a new complex which has a lower molecular mass. The internal deletion of the seven repeated amino acid sequences containing the KCPVDH motif in the CYP1(HAP1) protein modifies the heme responsiveness phenomenon observed in vitro in the band-shift experiments and in vivo in the transcription of the CYB2, CYC1, CYP3(CYC7) and ERG11 genes. On the basis of these data, we propose a new model for heme-induced activation of the CYP1 protein.

Amino Acid Sequence↗

PubMed1993

Targeting of cytochrome b2 into the mitochondrial intermembrane space: specific recognition of the sorting signal.

UNLABELLED: Cytochrome b2 contains 2-fold targeting information: an amino-terminal signal for targeting to the mitochondrial matrix, followed by a second cleavable sorting signal that functions in directing the precursor into the mitochondrial intermembrane space. The role of the second sorting sequence was analyzed by replacing one, two or all of the three positively charged amino acid residues which are present at the amino-terminal side of the hydrophobic core by uncharged residues or an acidic residue. With a number of these mutant precursor proteins, processing to the mature form was reduced or completely abolished and at the same time targeting to the matrix space occurred. The accumulation in the matrix depended on a high level of intramitochondrial ATP. At low levels of matrix ATP, the mutant proteins were sorted into the intermembrane space like the wild-type precursors. THE RESULTS: (i) suggest the existence of one or more matrix components that specifically recognize the second sorting signal and thereby trigger the translocation into the intermembrane space; (ii) indicate that the mutant signals have reduced ability to interact with the recognition component(s) and then embark on the default pathway into the matrix by interacting with mitochondrial hsp70 in conjunction with matrix ATP; (iii) strongly argue against a mechanism by which the hydrophobic segment of the sorting sequence stops translocation in the hydrophobic phase of the inner membrane.

Adenosine Triphosphate↗

PubMed1993

Presequence and mature part of preproteins strongly influence the dependence of mitochondrial protein import on heat shock protein 70 in the matrix.

To test the hypothesis that 70-kD mitochondrial heat shock protein (mt-hsp70) has a dual role in membrane translocation of preproteins we screened preproteins in an attempt to find examples which required either only the unfoldase or only the translocase function of mt-hsp70. We found that a series of fusion proteins containing amino-terminal portions of the intermembrane space protein cytochrome b2 (cyt. b2) fused to dihydrofolate reductase (DHFR) were differentially imported into mitochondria containing mutant hsp70s. A fusion protein between the amino-terminal 167 residues of the precursor of cyt. b2 and DHFR was efficiently transported into mitochondria independently of both hsp70 functions. When the length of the cyt. b2 portion was increased and included the heme binding domain, the fusion protein became dependent on the unfoldase function of mt-hsp70, presumably caused by a conformational restriction of the heme-bound preprotein. In the absence of heme the noncovalent heme binding domain in the longer fusion proteins no longer conferred a dependence on the unfoldase function. When the cyt. b2 portion of the fusion protein was less than 167 residues, its import was still independent of mt-hsp70 function; however, deletion of the intermembrane space sorting signal resulted in preproteins that ended up in the matrix of wild-type mitochondria and whose translocation was strictly dependent on the translocase function of mt-hsp70. These findings provide strong evidence for a dual role of mt-hsp70 in membrane translocation and indicate that preproteins with an intermembrane space sorting signal can be correctly imported even in mutants with severely impaired hsp70 function.

Biological Transport↗

PubMed1992

Antifolding activity of hsp60 couples protein import into the mitochondrial matrix with export to the intermembrane space.

Cytochrome b2 reaches the intermembrane space of mitochondria by transport into the matrix followed by export across the inner membrane. While in the matrix, the protein interacts with hsp60, which arrests its folding prior to export. The bacterial-type export sequence in pre-cytochrome b2 functions by inhibiting the ATP-dependent release of the protein from hsp60. Release for export apparently requires, in addition to ATP, the interaction of the signal sequence with a component of the export machinery in the inner membrane. Export can occur before import is complete provided that a critical length of the polypeptide chain has been translocated into the matrix. Thus, hsp60 combines two activities: catalysis of folding of proteins destined for the matrix, and maintaining proteins in an unfolded state to facilitate their channeling between the machineries for import and export across the inner membrane. Anti-folding signals such as the hydrophobic export sequence in cytochrome b2 may act as switches between these two activities.

Base Sequence↗

PubMed1992

Targeting of a chemically pure preprotein to mitochondria does not require the addition of a cytosolic signal recognition factor.

To analyze the role of cytosolic cofactors in mitochondrial protein targeting, we prepared a chemically pure mitochondrial preprotein. When diluted out of 7 M urea, this precursor protein was efficiently imported into mitochondria without the addition of cytosolic cofactors. Extensive prewashing of mitochondria (up to 2 M KCl) did not reduce its import. Import of the purified precursor showed the characteristics of authentic mitochondrial import including use of the receptor MOM19, requirement for a membrane potential, and proteolytic processing. When the precursor was preincubated at a low concentration of urea, cytosolic cofactors were needed to preserve its import competence. We conclude that targeting of this preprotein via the mitochondrial master receptor MOM19 does not require a cytosolic signal recognition factor; cytosolic cofactors apparently have chaperone-like functions in mitochondrial protein uptake. Moreover, we found that a cleavable presequence was sufficient to direct protein import via MOM19. Together with the cofactor-independent function of MOM19, it is thus conceivable that MOM19 functions as mitochondrial presequence receptor.

Animals↗

PubMed1991

Protein folding causes an arrest of preprotein translocation into mitochondria in vivo.

With vital yeast cells, a hybrid protein consisting of the amino-terminal third of the precursor to cytochrome b2 and of the entire dihydrofolate reductase was arrested on the import pathway into mitochondria. Accumulation of the protein in the mitochondrial membranes was achieved by inducing a stable tertiary structure of the dihydrofolate reductase domain. Thereby, three salient features of mitochondrial protein uptake in vivo were demonstrated: its posttranslational character; the requirement for unfolding of precursors; and import through translocation contact sites. The permanent occupation of translocation sites by the fusion protein inhibited the import of other precursors; it did, however, not lead to leakage of mitochondrial ions, implying the existence of a channel that is sealed around the membrane spanning polypeptide segment.

Aminopterin↗

PubMed1991

Complex transcriptional regulation of the Saccharomyces cerevisiae CYB2 gene encoding cytochrome b2: CYP1(HAP1) activator binds to the CYB2 upstream activation site UAS1-B2.

Expression of the Saccharomyces cerevisiae gene encoding cytochrome b2 (EC 1.2.2.3), CYB2, was investigated by direct analysis of mRNA transcripts and by measurement of the expression of lacZ fused to the CYB2 control regions. These studies indicated that regulation of the CYB2 gene is subject to several metabolic controls at the transcriptional level: inhibition due to glucose fermentation, induction by lactate, and inhibition in anaerobiosis or in absence of heme biosynthesis. Furthermore, we have shown that the CYB2 promoter contains one cis negative regulatory region and two heme-dependent positive regions, one of which is controlled by the transcriptional regulator CYP1 (HAP1) which is involved in the modulation of the expression of several oxygen-regulated genes. The CYP1 (HAP1)-binding sequence was located by gel retardation and DNase I footprinting experiments and compared with the binding sequences previously characterized in detail (UAS1CYC1, UAS'CYP3 (CYC7), and UASCTT1).

Base Sequence↗

PubMed1990

Polypeptides traverse the mitochondrial envelope in an extended state.

Most mitochondrial proteins are synthesized as precursors in the cytosol and imported through contact sites between outer and inner mitochondrial membranes. The molecular mechanism of membrane translocation of precursor proteins is largely unclear. For this report, various hybrid proteins between portions of the precursor of cytochrome b2 and the entire dihydrofolate reductase (DHFR) were accumulated in mitochondrial contact sites. We unexpectedly found that about 50 amino acid residues of the polypeptide chain in transit were sufficient to span both membranes. This suggests a linear translocation of the polypeptide chain and presents evidence for a high degree of unfolding of polypeptides traversing the mitochondrial membranes.

Biological Transport↗

PubMed1990

Energy requirements for unfolding and membrane translocation of precursor proteins during import into mitochondria.

ATP is involved in conferring transport competence to numerous mitochondrial precursor proteins in the cytosol. Unfolded precursor proteins were found not to require ATP for import into mitochondria, suggesting a role of ATP in the unfolding of precursors. Here we report the unexpected finding that a hybrid protein containing the tightly folded passenger protein dihydrofolate reductase becomes unfolded and specifically translocated across the mitochondrial membranes independently of added ATP. Moreover, interaction of the precursor with the mitochondrial receptor components does not require ATP. The results suggest that ATP is not involved in the actual process of unfolding during membrane translocation of precursors. ATP rather appears to be necessary for preventing the formation of improper structures of precursors in the cytosol and for folding of imported polypeptides on (and release from) chaperone-like molecules in the mitochondrial matrix.

Adenosine Triphosphate↗

PubMed1989

Translocation arrest by reversible folding of a precursor protein imported into mitochondria. A means to quantitate translocation contact sites.

Passage of precursor proteins through translocation contact sites of mitochondria was investigated by studying the import of a fusion protein consisting of the NH2-terminal 167 amino acids of yeast cytochrome b2 precursor and the complete mouse dihydrofolate reductase. Isolated mitochondria of Neurospora crassa readily imported the fusion protein. In the presence of methotrexate import was halted and a stable intermediate spanning both mitochondrial membranes at translocation contact sites accumulated. The complete dihydrofolate reductase moiety in this intermediate was external to the outer membrane, and the 136 amino acid residues of the cytochrome b2 moiety remaining after cleavage by the matrix processing peptidase spanned both outer and inner membranes. Removal of methotrexate led to import of the intermediate retained at the contact site into the matrix. Thus unfolding at the surface of the outer mitochondrial membrane is a prerequisite for passage through translocation contact sites. The membrane-spanning intermediate was used to estimate the number of translocation sites. Saturation was reached at 70 pmol intermediate per milligram of mitochondrial protein. This amount of translocation intermediates was calculated to occupy approximately 1% of the total surface of the outer membrane. The morphometrically determined area of close contact between outer and inner membranes corresponded to approximately 7% of the total outer membrane surface. Accumulation of the intermediate inhibited the import of other precursor proteins suggesting that different precursor proteins are using common translocation contact sites. We conclude that the machinery for protein translocation into mitochondria is present at contact sites in limited number.

Amino Acid Sequence↗

PubMed1988

CYP1 (HAP1) regulator of oxygen-dependent gene expression in yeast. II. Missense mutation suggests alternative Zn fingers as discriminating agents of gene control.

In the accompanying paper, we present and analyse the sequence of a "superactivator" mutant allele of the CYP1 (HAP1) gene. This locus encodes a trans-acting pleiotropic positive regulator of the transcription of both isocytochrome c structural genes. In this paper, we present the genetic localization of the mutation and the sequence of the wild-type fragment that includes the mutation. The mutated phenotype that commutes the expression of the two isocytochrome structural genes (superactivation of CYP3 and inhibition of CYC1) results from a transversion in an AGT codon (serine) in the wild-type to an AGG codon (arginine) in the mutant. Moreover, we show that the missense mutation that affects the amino acid preceding the first cysteine of the "Zn finger" is responsible on its own account for the entire mutated phenotype. In all seven yeast regulatory proteins analysed so far, this position is occupied by a neutral amino acid (serine, alanine or glycine), thus the serine-arginine replacement is a radical one. This result is consistent with the hypothesis of alternative and mutually exclusive Zn fingers, formed either at low or high redox potential, recognizing the target sequences identified in the upstream regions of the CYC1 and CYP3 isocytochrome c structural genes.

Alleles↗

PubMed1987

Successive translocation into and out of the mitochondrial matrix: targeting of proteins to the intermembrane space by a bipartite signal peptide.

We investigated the import and sorting pathways of cytochrome b2 and cytochrome c1, which are functionally located in the intermembrane space of mitochondria. Both proteins are synthesized on cytoplasmic ribosomes as larger precursors and are processed in mitochondria in two steps upon import. The precursors are first translocated across both mitochondrial membranes via contact sites into the matrix. Processing by the matrix peptidase leads to intermediate-sized forms, which are subsequently redirected across the inner membrane. The second proteolytic processing occurs in the intermembrane space. We conclude that the hydrophobic stretches in the presequences of the intermediate-sized forms do not stop transfer across the inner membrane, but rather act as transport signals to direct export from the matrix into the intermembrane space.

Cytochrome c Group↗

PubMed1987

Mitochondrial precursor proteins are imported through a hydrophilic membrane environment.

We have analyzed how translocation intermediates of imported mitochondrial precursor proteins, which span contact sites, interact with the mitochondrial membranes. F1-ATPase subunit beta (F1 beta) was trapped at contact sites by importing it into Neurospora mitochondria in the presence of low levels of nucleoside triphosphates. This F1 beta translocation intermediate could be extracted from the membranes by treatment with protein denaturants such as alkaline pH or urea. By performing import at low temperatures, the ADP/ATP carrier was accumulated in contact sites of Neurospora mitochondria and cytochrome b2 in contact sites of yeast mitochondria. These translocation intermediates were also extractable from the membranes at alkaline pH. Thus, translocation of precursor proteins across mitochondrial membranes seems to occur through an environment which is accessible to aqueous perturbants. We propose that proteinaceous structures are essential components of a translocation apparatus present in contact sites.

Intracellular Membranes↗

PubMed1987

Phenotypic suppression and nuclear accommodation of the mit- oxi1-V25 mutation in isolated yeast mitochondria.

Phenotypic suppression by the antibiotic, paromomycin, of the mitochondrial oxi1- -V25 mutation, a mutation which arrests by premature ochre codon the synthesis of the cox II subunit, was studied in isolated yeast mitochondria competent in translation. This antibiotic is known to suppress the mutation in vivo (Dujardin et al. 1984) and allowed in vitro, at concentrations of 20-1100 micrograms per ml. the synthesis of the cox II subunit. This strongly suggests that phenotypic suppression of mit- mutations is due to the direct action of paromomycin on mitochondrial ribosomes. The effect of paromomycin bears a resemblance to the function of the omnipotent nuclear suppressor mutation R705. The nuclear suppression was expressed in isolated mitochondria; suppressor mutation influenced the structure of the mitoribosome. Therefore, it appears that mitoribosomes are indeed the common target in the phenotypical and genetic nuclear suppression of the oxi1-V25 mutation.

Cell Nucleus↗

PubMed1986

One nuclear gene controls the removal of transient pre-sequences from two yeast proteins: one encoded by the nuclear the other by the mitochondrial genome.

The proteolytic processing of the mitochondrially encoded subunit II of cytochrome oxidase is prevented by the yeast mutation ts2858. We report that the mutant is, in addition, temperature sensitive for the processing of cytochrome b2, a protein encoded by nuclear DNA. Thus the same mutation affects the removal of pre-sequences from a mitochondrially encoded inner membrane protein and from an imported soluble protein located in the intermembrane space. The mutation blocks the second processing step of cytochrome b2. The cytochrome b2 intermediate accumulates in the mutant at 36 degrees C and assumes its enzyme activity. At 23 degrees C the conversion to the mature protein is considerably slower than in wild-type cells. The similarity of the cleavage sites Asn-Asp and Asn-Glu of the precursors for cytochrome oxidase subunit II and cytochrome b2, respectively, suggests a sequence-specific recognition by one protease or a factor activating a protease. On the other hand maturation of cytochrome c peroxidase, another enzyme of the intermembrane space, is not affected by the pet ts2858 mutation.

Cell Nucleus↗

PubMed1985

Structure, expression and regulation of a nuclear gene encoding a mitochondrial protein: the yeast L(+)-lactate cytochrome c oxidoreductase (cytochrome b2).

The yeast L(+)-lactate cytochrome c oxidoreductase or cytochrome b2 is a component of the mitochondrial intermembrane space. The protein is encoded by the nuclear genome, synthesized as a larger precursor in the cytoplasmic compartment, and then proteolytically processed to its mature form during its import into the mitochondria. The structural gene for yeast cytochrome b2 has been cloned. The complete nucleotide sequence of the gene with its 5' and 3' flanking regions was determined. The deduced primary structure of the cytochrome b2 precursor reveals an unusually long amino terminal extension of 80 amino acids. A variety of potentially significant sequences were identified in the region flanking the structural portion of the gene. Transcript mapping with both S1 nuclease and primer extension methods reveals that the site of RNA synthesis is 56-66 bp downstream from a putative TATA box. By Northern blot analysis and gene disruption, it is shown that there is only a single copy of the cytochrome b2 gene per haploid yeast nucleus. The cloned cytochrome b2 gene was used to probe specific mRNA levels and demonstrate that cytochrome b2 expression is transcriptionally repressed by glucose and induced by lactate. The inactivation of the chromosomal cytochrome b2 gene by integrative transformation led to a deficiency in L(+)-lactate dehydrogenase activity and consequently to the inability to use L(+)-lactate as a sole source of carbon. This is the first reported mutation affecting the structural gene of cytochrome b2.

Amino Acid Sequence↗

PubMed1984

Blocking of in vitro translation of Paramecium messenger RNAs is due to messenger RNA primary structure.

Paramecium primaurelia mRNAs were translated in vitro in rabbit reticulocyte lysate and the products of translation were analyzed by their size. We show that the large majority of these products are of short but discrete sizes irrespective of the length of the mRNA which directs their synthesis. An illustrative example is given by the translation of mRNA of G surface antigen which directs the synthesis of a 50 kD polypeptide instead of the complete 250 kD protein. Control experiments suggest that the blocking is due to mRNA primary structure.

Animals↗

PubMed1984

Yeast cytochrome b2 gene: isolation with antibody probes.

An efficient technique was used to clone the gene for yeast cytochrome b2, (a nuclear encoded mitochondrial protein) using the expression vector, lambda gt11 (lac 5 nin 5 c1857 S100). This enables the insertion of yeast DNA into the beta-galactosidase structural gene (lacZ) and promotes synthesis of hybrid proteins. Screening of antigen producing clones in the lambda gt11 recombinant genomic library was achieved using antiserum against cytochrome b2 according to Young and Davis (1983) Two recombinants containing part of the gene coding for cytochrome b2 were isolated and characterized as follows: by their expression in Escherichia coli cells, examined by immuno-blotting with antibodies to pure cytochrome b2. by DNA sequence analysis. One recombinant carries a 3 Kb yeast DNA insert which contains the whole nucleotide sequence encoding cytochrome b2 and a few amino acids of the amino terminal presequence.

Bacteriophage lambda↗