A precursor protein partly translocated into yeast mitochondria is bound to a 70 kd mitochondrial stress protein.

We have probed the environment of a precursor protein stuck in mitochondrial import sites using cleavable bifunctional crosslinking reagents. The stuck precursor was crosslinked to a 70 kd protein which, by immunological techniques, was shown to be a matrix protein. The protein was purified to homogeneity by ATP-Sepharose chromatography and partially sequenced. Fourteen of its 15 N-terminal amino acids were identical to residues 24-38 of the protein encoded by the nuclear gene SSC1, which had been proposed to encode a dnaK-like 70 kd mitochondrial stress protein. Our data imply that this mitochondrial hsp70 is made with a cleavable matrix-targeting sequence composed of 23 residues. The complex containing stuck precursor, mitochondrial hsp70, and ISP42 could be solubilized from mitochondria by the non-ionic detergent Triton X-100 even without crosslinking, suggesting tight association of these three components. As the stuck precursor is arrested at an early stage of translocation, mitochondrial hsp70 may initiate the events that lead to refolding of imported precursors in the matrix space.     

The cold sensitivity of a mutant of Saccharomyces cerevisiae lacking a mitochondrial heat shock protein 70 is suppressed by loss of mitochondrial DNA

SSH1, a newly identified member of the heat shock protein (hsp70) multigene family of the budding yeast Saccharomyces cerevisiae, encodes a protein localized to the mitochondrial matrix. Deletion of the SSH1 gene results in extremely slow growth at 23 degrees C or 30 degrees C, but nearly wild-type growth at 37 degrees C. The matrix of the mitochondria contains another hsp70, Ssc1, which is essential for growth and required for translocation of proteins into mitochondria. Unlike SSC1 mutants, an SSH1 mutant showed no detectable defects in import of several proteins from the cytosol to the matrix compared to wild type. Increased expression of Ssc1 partially suppressed the cold- sensitive growth defect of the SSH1 mutant, suggesting that when present in increased amounts, Ssc1 can at least partially carry out the normal functions of Ssh1. Spontaneous suppressors of the cold-sensitive phenotype of an SSH1 null mutant were obtained at a high frequency at 23 degrees C, and were all found to be respiration deficient. 15 of 16 suppressors that were analyzed lacked mitochondrial DNA, while the 16th had reduced amounts. We suggest that Ssh1 is required for normal mitochondrial DNA replication, and that disruption of this process in ssh1 cells results in a defect in mitochondrial function at low temperatures.     

Two-step purification of mitochondrial Hsp70, Ssc1p, using Mge1(His)(6) immobilized on Ni-agarose

The most abundant mitochondrial homolog of Hsp70, Ssc1p, is involved in the import and folding of mitochondrial proteins. We have developed an easy and efficient method for purifying Ssc1p. Following a first step of anion exchange at pH 6.6, a column of Mge1(His)(6) immobilized on Ni(2+)-agarose provides an efficient second dimension that results in highly purified protein. The strong and specific interaction between Ssc1p and its cofactor protein, Mge1, ensures that primarily functional protein is isolated. Ssc1p purified by this method hydrolyzed ATP with a turnover rate of 0.3/min. The ATP hydrolysis was enhanced slightly by Mge1, about 5 times by Mdj1, and 12 times by both cofactors together. The CD spectrum of Ssc1p had a pattern and temperature dependence similar to those shown for other hsp70 homologs, with a midpoint of the major transition at approximately 70 degrees C.     

Characterisation of PHSP1, a cDNA encoding a mitochondrial HSP70 fromPisum sativum

A pea cDNA clone,PHSP1, encoding a member of the HSP70 gene family has been isolated. DNA sequence analysis indicates that the protein encoded byPHSP1 is a homologue of the mitochondrial HSP70 proteins, SSP1 fromSchizosaccharomyces pombe and SSC1 fromS. cerevisiae. It contains an amino-terminal extension of 50 amino acids, rich in basic and hydroxyl amino acids, similar to other plant mitochondrial leader sequences. Western blot analysis indicates that the PHSP1 protein is associated only with mitochondria and not with any other sub-cellular organelle or cytoplasm. Further confirmation of its location within mitochondria was obtained fromin vitro protein translocation experiments into purifiedPisum sativum mitochondria. It was observed that the precursor protein was efficiently imported and that it is processed to produce a protein with anM _r of the anticipated size of the mature protein. Results are discussed with respect to the structure and function of the mitochondrial HSP70 protein.Abbreviations mtHSP70 mitochondrial HSP70 - ER endoplasmic reticulum - nt nucleotide - IgG immunoglobulin G - BiP immunoglobulin-binding protein - hsc heat shock cognate     

Ecm10, a novel hsp70 homolog in the mitochondrial matrix of the yeast Saccharomyces cerevisiae

Members of the heat shock protein 70 (Hsp70) family are found in most of the compartments of eukaryotic cells where they play essential roles in protein metabolism. In yeast mitochondria, two Hsp70 proteins are known: Ssc1 and Ssq1. We identified Ecm10 as a third Hsp70 protein in the mitochondrial matrix. Ecm10 shares 82% amino acid identity with Ssc1 and 54% with Ssq1. Overexpression of Ecm10 mitigates protein import defects in ssc1 mutants suggesting that Ecm10 can play a role in protein translocation. Like Ssc1, Ecm10 interacts with the nucleotide exchange factor Mge1 in an ATP-dependent manner. Deletion of ecm10 leads to synthetic growth defects with ssc1 mutations at low temperature. Our data suggest an overlapping function of Ecm10 and Ssc1.     

The two mitochondrial heat shock proteins 70, Ssc1 and Ssq1, compete for the cochaperone Mge1

Two members of the heat shock protein 70 kDa (Hsp70) family, Ssc1 and Ssq1, perform important functions in the mitochondrial matrix. The essential Ssc1 is an abundant ATP-binding protein required for both import and folding of mitochondrial proteins. The function of Ssc1 is supported by an interaction with the preprotein translocase subunit Tim44, the cochaperone Mdj1, and the nucleotide exchange factor Mge1. In contrast, only limited information is available on Ssq1. So far, a basic characterization of Ssq1 has demonstrated its involvement in the maintenance of mitochondrial DNA, the maturation of the yeast frataxin (Yfh1) after import, and assembly of the mitochondrial Fe/S cluster. Here, we analyzed the biochemical properties and the interaction partners of Ssq1 in detail. Ssq1 showed typical chaperone properties by binding to unfolded substrate proteins in an ATP-regulated manner. Ssq1 was able to form a specific complex with the nucleotide exchange factor Mge1. In particular, complex formation in organello was enhanced significantly when Ssc1 was inactivated selectively. However, even under these conditions, no interaction of Ssq1 with the two other mitochondrial Hsp70-cochaperones, Tim44 and Mdj1, was observed. The Ssq1-Mge1 interaction showed a lower overall stability but the same characteristic nucleotide-dependence as the Ssc1-Mge1 interaction. A quantitative analysis of the interaction properties indicated a competition of Ssq1 with Ssc1 for binding to Mge1. Perturbation of Mge1 function or amounts resulted in direct effects on Ssq1 activity in intact mitochondria. We conclude that mitochondria represent the unique case where two Hsp70s compete for the interaction with one nucleotide exchange factor.     

Differential requirement for the mitochondrial Hsp70-Tim44 complex in unfolding and translocation of preproteins.

The mitochondrial heat shock protein Hsp70 is essential for import of nuclear-encoded proteins, involved in both unfolding and membrane translocation of preproteins. mtHsp70 interacts reversibly with Tim44 of the mitochondrial inner membrane, yet the role of this interaction is unknown. We analysed this role by using two yeast mutants of mtHsp70 that differentially influenced its interaction with Tim44. One mutant mtHsp70 (Ssc1-2p) efficiently bound preproteins, but did not show a detectable complex formation with Tim44; the mitochondria imported loosely folded preproteins with wild-type kinetics, yet were impaired in unfolding of preproteins. The other mutant Hsp70 (Ssc1-3p') bound both Tim44 and preproteins, but the mitochondria did not import folded polypeptides and were impaired in import of unfolded preproteins; Ssc1-3p' was defective in its ATPase domain and did not undergo a nucleotide-dependent conformational change, resulting in permanent binding to Tim44. The following conclusions are suggested. (i) The import of loosely folded polypeptides (translocase function of mtHsp70) does not depend on formation of a detectable Hsp70-Tim44 complex. Two explanations are possible: a trapping mechanism by soluble mtHsp70, or a weak/very transient interaction of Ssc1-2p with Tim44 that leads to a weak force generation sufficient for import of loosely folded, but not folded, polypeptides. (ii) Import of folded preproteins (unfoldase function of mtHsp70) involves a reversible nucleotide-dependent interaction of mtHsp70 with Tim44, including a conformational change in mtHsp70. This is consistent with a model that the dynamic interaction of mtHsp70 with Tim44 generates a pulling force on preproteins which supports unfolding during translocation.     

The hsp90-related protein TRAP1 is a mitochondrial protein with distinct functional properties

The hsp90 family of molecular chaperones was expanded recently due to the cloning of TRAP1 and hsp75 by yeast two-hybrid screens. Careful analysis of the human TRAP1 and hsp75 sequences revealed that they are identical, and we have cloned a similar protein from Drosophila. Immunofluorescence data show that human TRAP1 is localized to mitochondria. This mitochondrial localization is supported by the existence of mitochondrial localization sequences in the amino termini of both the human and Drosophila proteins. Due to the striking homology of TRAP1 to hsp90, we tested the ability of TRAP1 to function as an hsp90-like chaperone. TRAP1 did not form stable complexes with the classic hsp90 co-chaperones p23 and Hop (p60). Consistent with these observations, TRAP1 had no effect on the hsp90-dependent reconstitution of hormone binding to the progesterone receptor in vitro, nor could it substitute for hsp90 to promote maturation of the receptor to its hormone-binding state. However, TRAP1 is sufficiently conserved with hsp90 such that it bound ATP, and this binding was sensitive to the hsp90 inhibitor geldanamycin. In addition, TRAP1 exhibited ATPase activity that was inhibited by both geldanamycin and radicicol. Thus, TRAP1 has functions that are distinct from those of hsp90.     

Sequential action of mitochondrial chaperones in protein import into the matrix.

Translocation and folding of proteins imported into mitochondria are mediated by two matrix-localized chaperones, mhsp70 and hsp60. In order to investigate whether these chaperones act sequentially or in parallel, we studied their interaction with newly imported precursor proteins in isolated yeast mitochondria by coimmunoprecipitation. All precursors bound transiently to mhsp70. Release from mhsp70 required hydrolysis of ATP and did not immediately generate a tightly folded protein. For example, after imported mouse dihydrofolate reductase (a soluble monomeric enzyme) had been released from mhsp70, folding to a protease resistant conformation occurred only after a lag and was much slower than the release. Under standard import conditions, no significant association of DHFR with hsp60 could be detected. Similarly, newly imported hsp60 subunit was released from mhsp70 as an incompletely folded, unassembled intermediate which accumulated at low temperature and assembled to hsp60 14-mer at higher temperature in an ATP-dependent manner. Mas2p (the larger subunit of the MAS-encoded processing protease) first bound to mhsp70, then to hsp60, and only then assembled with its partner subunit, Mas1p. We propose that ATP-dependent release from mhsp70 is insufficient to cause folding of imported proteins and that assembly of hsp60 and Mas2p requires sequential, ATP-dependent interactions with mhsp70 and hsp60.     

The mitochondrial F1ATPase alpha-subunit is necessary for efficient import of mitochondrial precursors.

The mitochondrial import and assembly of the F1ATPase subunits requires, respectively, the participation of the molecular chaperones hsp70SSA1 and hsp70SSC1 and other components operating on opposite sides of the mitochondrial membrane. In previous studies, both the homology and the assembly properties of the F1ATPase alpha-subunit (ATP1p) compared to the groEL homologue, hsp60, have led to the proposal that this subunit could exhibit chaperone-like activity. In this report the extent to which this subunit participates in protein transport has been determined by comparing import into mitochondria that lack the F1ATPase alpha-subunit (delta ATP1) versus mitochondria that lack the other major catalytic subunit, the F1ATPase beta-subunit (delta ATP2). Yeast mutants lacking the alpha-subunit but not the beta-subunit grow much more slowly than expected on fermentable carbon sources and exhibit delayed kinetics of protein import for several mitochondrial precursors such as the F1 beta subunit, hsp60MIF4 and subunits 4 and 5 of the cytochrome oxidase. In vitro and in vivo the F1 beta-subunit precursor accumulates as a translocation intermediate in absence of the F1 alpha-subunit. In the absence of both the ATPase subunits yeast grows at the same rate as a strain lacking only the beta-subunit, and import of mitochondrial precursors is restored to that of wild type. These data indicate that the F1 alpha-subunit likely functions as an "assembly partner" to influence protein import rather than functioning directly as a chaperone. These data are discussed in light of the relationship between the import and assembly of proteins in mitochondria.     

Intramitochondrial sorting of the precursor to yeast cytochrome c oxidase subunit Va

We have continued our studies on the import pathway of the precursor to yeast cytochrome c oxidase subunit Va (pVa), a mitochondrial inner membrane protein. Previous work on this precursor demonstrated that import of pVa is unusually efficient, and that inner membrane localization is directed by a membrane-spanning domain in the COOH- terminal third of the protein. Here we report the results of studies aimed at analyzing the intramitochondrial sorting of pVa, as well as the role played by ancillary factors in import and localization of the precursor. We found that pVa was efficiently imported and correctly sorted in mitochondria prepared from yeast strains defective in the function of either mitochondrial heat shock protein (hsp)60 or hsp70. Under identical conditions the import and sorting of another mitochondrial protein, the precursor to the beta subunit of the F1 ATPase, was completely defective. Consistent with previous results demonstrating that the subunit Va precursor is loosely folded, we found that pVa could be efficiently imported into mitochondria after translation in wheat germ extracts. This results suggests that normal levels of extramitochondrial hsp70 are also not required for import of the protein. The results of this study enhance our understanding of the mechanism by which pVa is routed to the mitochondrial inner membrane. They suggest that while the NH2 terminus of pVa is exposed to the matrix and processed by the matrix metalloprotease, the protein remains anchored to the inner membrane before being assembled into a functional holoenzyme complex.     

The disaggregation activity of the mitochondrial ClpB homolog Hsp78 maintains Hsp70 function during heat stress

Molecular chaperones are important components of mitochondrial protein biogenesis and are required to maintain the organellar function under normal and stress conditions. We addressed the functional role of the Hsp100/ClpB homolog Hsp78 during aggregation reactions and its functional cooperation with the main mitochondrial Hsp70, Ssc1, in mitochondria of the yeast Saccharomyces cerevisiae. By establishing an aggregation/disaggregation assay in intact mitochondria we demonstrated that Hsp78 is indispensable for the resolubilization of protein aggregates generated by heat stress under in vivo conditions. The ATP-dependent disaggregation activity of Hsp78 was capable of reversing the preprotein import defect of a destabilized mutant form of Ssc1. This role in disaggregation of Ssc1 is unique for Hsp78, since the recently identified, Hsp70-specific chaperone Zim17 had no effect on the resolubilization reaction. We observed only a minor effect of the second mitochondrial Hsp100 family member Mcx1 on protein disaggregation. A "holding" activity of the mitochondrial Hsp70 system was a prerequisite for a successful resolubilization of aggregated proteins. We conclude that the protective role of Hsp78 in thermotolerance is mainly based on maintaining the molecular chaperone Ssc1 in a soluble and functional state.     

Role of the mitochondrial Hsp70s, Ssc1 and Ssq1, in the maturation of Yfh1

The mitochondrial matrix of the yeast Saccharomyces cerevisiae contains two molecular chaperones of the Hsp70 class, Ssc1 and Ssq1. We report that Ssc1 and Ssq1 play sequential roles in the import and maturation of the yeast frataxin homologue (Yfh1). In vitro, radiolabeled Yfh1 was not imported into ssc1-3 mutant mitochondria, remaining in a protease-sensitive precursor form. As reported earlier, the Yfh1 intermediate form was only slowly processed to the mature form in Deltassq1 mitochondria (S. A. B. Knight, N. B. V. Sepuri, D. Pain, and A. Dancis, J. Biol. Chem. 273:18389-18393, 1998). However, the intermediate form in both wild-type and Deltassq1 mitochondria was entirely within the inner membrane, as it was resistant to digestion with protease after disruption of the outer membrane. Therefore, we conclude that Ssc1, which is present in mitochondria in approximately a 1,000-fold excess over Ssq1, is required for Yfh1 import into the matrix, while Ssq1 is necessary for the efficient processing of the intermediate to the mature form in isolated mitochondria. However, the steady-state level of mature Yfh1 in Deltassq1 mitochondria is approximately 75% of that found in wild-type mitochondria, indicating that this retardation in processing does not dramatically affect cellular concentrations. Therefore, Ssq1 likely has roles in addition to facilitating the processing of Yfh1. Twofold overexpression of Ssc1 partially suppresses the cold-sensitive growth phenotype of Deltassq1 cells, as well as the accumulation of mitochondrial iron and the defects in Fe/S enzyme activities normally found in Deltassq1 mitochondria. Deltassq1 mitochondria containing twofold-more Ssc1 efficiently converted the intermediate form of Yfh1 to the mature form. This correlation between the observed processing defect and suppression of in vivo phenotypes suggests that Ssc1 is able to carry out the functions of Ssq1, but only when present in approximately a 2,000-fold excess over normal levels of Ssq1.     

Extended N-terminal sequencing of proteins of the large ribosomal subunit from yeast mitochondria

We have determined the N-termini of 26 proteins of the large ribosomal subunit from yeast mitochondria by direct amino acid micro-sequencing. The N-terminal sequences of proteins YmL33 and YmL38 showed a significant similarity to eubacterial ribosomal (r-) proteins L30 and L14, respectively. In addition, several proteins could be assigned to their corresponding yeast nuclear genes. Based on a comparison of the protein sequences deduced from the corresponding DNA regions with the N-termini of the mature proteins, the putative leader peptides responsible for mitochondrial matrix-targeting were compiled. In most leader sequences a relative abundance of aromatic amino acids, preferentially phenylalanine, was found.     

The Functional Interaction of Mitochondrial Hsp70s with the Escort Protein Zim17 Is Critical for Fe/S Biogenesis and Substrate Interaction at the Inner Membrane Preprotein Translocase

The yeast protein Zim17 belongs to a unique class of co-chaperones that maintain the solubility of Hsp70 proteins in mitochondria and plastids of eukaryotic cells. However, little is known about the functional cooperation between Zim17 and mitochondrial Hsp70 proteins in vivo. To analyze the effects of a loss of Zim17 function in the authentic environment, we introduced novel conditional mutations within the ZIM17 gene of the model organism Saccharomyces cerevisiae that allowed a recovery of temperature-sensitive but respiratory competent zim17 mutant cells. On fermentable growth medium, the mutant cells were prone to acquire respiratory deficits and showed a strong aggregation of the mitochondrial Hsp70 Ssq1 together with a concomitant defect in Fe/S protein biogenesis. In contrast, under respiring conditions, the mitochondrial Hsp70s Ssc1 and Ssq1 exhibited only a partial aggregation. We show that the induction of the zim17 mutant phenotype leads to strong import defects for Ssc1-dependent matrix-targeted precursor proteins that correlate with a significantly reduced binding of newly imported substrate proteins to Ssc1. We conclude that Zim17 is not only required for the maintenance of mtHsp70 solubility but also directly assists the functional interaction of mtHsp70 with substrate proteins in a J-type co-chaperone-dependent manner.     

Yeast translational activator Cbs2p: mitochondrial targeting and effect of overexpression

The yeast translational activator protein Cbs2p is imported into mitochondria without obvious proteolytic processing. To test the importance of amino-terminal amino acids for mitochondrial targeting we fused varying portions of the N-terminus with green fluorescent protein and examined the intracellular distribution of the reporter protein. We show that the 25 N-terminal amino acids are sufficient to direct the majority of the fusion protein into mitochondria. Cbs2p derivatives lacking 9 to 35 amino acids from the N-terminus fail to complement the respiratory deficiency of a deltacbs2 strain, but are still imported into mitochondria. Therefore Cbs2p contains at least one independent mitochondrial targeting information in addition to the N-terminal signal. We further analyzed the effect of over-expression of Cbs2p on mitochondrial function. Elevated concentrations of Cbs2p lead to slightly impaired mitochondrial gene expression, probably as the result of the formation of inactive Cbs2p aggregates.     

Importance of two ATP-binding sites for oligomerization, ATPase activity and chaperone function of mitochondrial Hsp78 protein.

The yeast mitochondrial chaperone Hsp78, a homologue of yeast cytosolic Hsp104 and bacterial ClpB, is required for maintenance of mitochondrial functions under heat stress. Here, Hsp78 was purified to homogeneity and shown to form a homo-hexameric complex, with an apparent molecular mass of approximately 440 kDa, in an ATP-dependent manner. Analysis of its ATPase activity reveals that the observed positive cooperativity effect depends both on Hsp78 and ATP concentration. Site-directed mutagenesis of the two putative Hsp78 nucleotide-binding domains suggest that the first nucleotide-binding domain is responsible for ATP hydrolysis and the second one for protein oligomerization. Studies on the chaperone activity of Hsp78 show that its cooperation with the mitochondrial Hsp70 system, consisting of Ssc1p, Mdj1p and Mge1p, is needed for the efficient reactivation of substrate proteins. These studies also suggest that the oligomerization but not the Hsp78 ATPase activity is essential for its chaperone activity.     

A 70-kd protein of the yeast mitochondrial outer membrane is targeted and anchored via its extreme amino terminus

The major 70-kd protein of the yeast mitochondrial outer membrane is made on cytosolic ribosomes and imported into the outer membrane without proteolytic cleavage. We have attempted to identify the sequences which target the protein to the mitochondria and which permanently anchor it to the lipid bilayer of the outer membrane. By manipulating the cloned gene we have deleted 13 different regions throughout the polypeptide; in addition, we have fused amino-terminal regions of different length to beta-galactosidase. Each altered gene was introduced into yeast and the intracellular fate of the corresponding polypeptide product was determined by subcellular fractionation. All the information for targeting and anchoring the 70-kd protein (617 amino acids) was contained within the amino-terminal 41 amino acids. When this entire region was deleted, the protein was recovered with the cytosol fraction. However, several restricted deletions within this amino-terminal region appeared to affect targeting and anchoring differentially: most of the altered protein remained in the cytosol but a small fraction was misrouted into the mitochondrial matrix space. We suggest that targeting is mediated by a region which includes the 11 amino-terminal amino acids whereas the permanent membrane anchor is provided by a typical transmembrane sequence between residues 9 and 38.