Dependence of the folding and import of the precursor to mitochondrial aspartate aminotransferase on the nature of the cell-free translation system

Manipulation of metabolic responses enhances SHetA2 efficacy without toxicity in cervical cancer cell lines and xenografts

OBJECTIVE

The high frequency of cervical cancer recurrence after primary therapy necessitates alternative treatments. High-risk human papillomavirus (HR-HPV) causes cervical cancer and it's continued presence supports elevated metabolism, proliferation and survival of cancer cells. The low-to-no toxicity new investigational drug, SHetA2, counteracts high-risk human papillomavirus (HR-HPV) effects on cell proliferation and survival in cervical cancer cells and xenograft tumors by disrupting heat shock protein 70 chaperone protection of oncogenic proteins. Our objective was to study the involvement of metabolism in SHetA2 effects on cervical cancer cells and tumors.

METHODS

SHetA2-mediated proteomic and metabolic effects were measured in HR-HPV-positive CaSKi and SiHa and HR-HPV-negative C-33 A cervical cancer cell lines. Combined treatment with 2-deoxyglucose (2-DG) was evaluated in cell culture and SiHa xenografts.

RESULTS

SHetA2 inhibited oxidative phosphorylation (OxPhos) and altered levels of proteins involved in metabolism, protein synthesis, and DNA replication and repair. Cervical cancer cells responded by elevating glycolysis. Inhibition of the glycolytic responses using galactose media or 2-DG increased SHetA2 sensitivity of two HR-HPV-positive, but not an HR-HPV-negative cervical cancer cell line. Interaction of 2-DG and SHetA2 was synergistic in HR-HPV positive cell lines in association with augmentation of SHetA2 ATP reduction, but not SHetA2 DNA damage induction. These results were verified in a SiHa xenograft tumor model without evidence of toxicity.

CONCLUSIONS

Compensatory glycolysis counteracts OxPhos inhibition in SHetA2-treated HR-HPV-positive cervical cancer cell lines. Prevention of compensatory glycolysis with 2-DG or another glycolysis inhibitor has the potential to improve SHetA2 therapy without toxicity.

Molecular modeling and functional confirmation of a predicted fatty acid binding site of mitochondrial aspartate aminotransferase

Molecular interactions are necessary for proteins to perform their functions. The identification of a putative plasma membrane fatty acid transporter as mitochondrial aspartate aminotransferase (mAsp-AT) indicated that the protein must have a fatty acid binding site. Molecular modeling suggests that such a site exists in the form of a 500-Å(3) hydrophobic cleft on the surface of the molecule and identifies specific amino acid residues that are likely to be important for binding. The modeling and comparison with the cytosolic isoform indicated that two residues (Arg201 and Ala219) were likely to be important to the structure and function of the binding site. These residues were mutated to determine if they were essential to that function. Expression constructs with wild-type or mutated cDNAs were produced for bacteria and eukaryotic cells. Proteins expressed in Escherichia coli were tested for oleate binding affinity, which was decreased in the mutant proteins. 3T3 fibroblasts were transfected with expression constructs for both normal and mutated forms. Plasma membrane expression was documented by indirect immunofluorescence before [(3)H]oleic acid uptake kinetics were assayed. The V(max) for uptake was significantly increased by overexpression of the wild-type protein but changed little after transfection with mutated proteins, despite their presence on the plasma membrane. The hydrophobic cleft in mAsp-AT can serve as a fatty acid binding site. Specific residues are essential for normal fatty acid binding, without which fatty acid uptake is compromised. These results confirm the function of this protein as a fatty acid binding protein.

Role of low native state kinetic stability and interaction of partially unfolded states with molecular chaperones in the mitochondrial protein mistargeting associated with primary hyperoxaluria

The G170R variant of the alanine:glyoxylate aminotransferase (AGT) is the most common pathogenic allele associated to primary hyperoxaluria type I (PH1), leading to mitochondrial mistargeting when combined with the P11L and I340M polymorphisms (minor allele; AGT(LM)). In this work, we have performed a comparative analysis on the conformation, unfolding energetics and interaction with molecular chaperones between AGT(wt), AGT(LM) and AGT(LRM) (G170R in the minor allele) proteins. Our results show that these three variants share similar conformational and functional properties as folded dimers. However, kinetic stability analyses showed a ≈1,000-fold increased unfolding rate for apo-AGT(LRM) compared to apo-AGT(wt), as well as a reduced folding efficiency upon expression in Escherichia coli. Pyridoxal 5'-phosphate (PLP)-binding provided a 4-5 orders of magnitude enhancement of the kinetic stability for all variants, suggesting a role for kinetic stabilization in pyridoxine-responsive PH1. Conformational studies at mild acidic pH and moderate guanidium concentrations showed the formation of a molten-globule-like unfolding intermediate in all three variants, which do not reactivate to the native state and strongly interact with Hsc70 and Hsp90 chaperones. Additional expression analyses in a mammalian cell-free system at neutral pH showed enhanced interaction of AGT(LRM) with Hsc70 and Hsp90 proteins compared to AGT(wt), suggesting kinetic trapping of the mutant by chaperones along the folding process. Overall, our results suggest that mitochondrial mistargeting of AGT(LRM) may involve the presentation of AGT partially folded states to the mitochondrial import machinery by molecular chaperones, which would be facilitated by the low native state kinetic stability (partially corrected by PLP binding) and kinetic trapping during folding of the AGT(LRM) variant with molecular chaperones.

Cisplatin-induced toxicity is associated with platinum deposition in mouse kidney mitochondria in vivo and with selective inactivation of the alpha-ketoglutarate dehydrogenase complex in LLC-PK1 cells

The anticancer drug cisplatin is nephrotoxic and neurotoxic. Previous data support the hypothesis that cisplatin is bioactivated to a nephrotoxicant. The final step in the proposed bioactivation is the formation of a platinum-cysteine S-conjugate followed by a pyridoxal 5'-phosphate (PLP)-dependent cysteine S-conjugate beta-lyase reaction. This reaction would generate pyruvate, ammonium, and a highly reactive platinum (Pt)-thiol compound in vivo that would bind to proteins. In this work, the cellular location and identity of the PLP-dependent cysteine S-conjugate beta-lyase were investigated. Pt was shown to bind to proteins in kidneys of cisplatin-treated mice. The concentration of Pt-bound proteins was higher in the mitochondrial fraction than in the cytosolic fraction. Treatment of the mice with aminooxyacetic acid (AOAA, a PLP enzyme inhibitor), which had previously been shown to block the nephrotoxicity of cisplatin, decreased the binding of Pt to mitochondrial proteins but had no effect on the amount of Pt bound to proteins in the cytosolic fraction. These data indicate that a mitochondrial enzyme catalyzes the PLP-dependent cysteine S-conjugate beta-lyase reaction. PLP-dependent mitochondrial aspartate aminotransferase (mitAspAT) is a mitochondrial enzyme that catalyzes beta-elimination reactions with cysteine S-conjugates of halogenated alkenes. We reasoned that the enzyme might also catalyze a beta-lyase reaction with the cisplatin-cysteine S-conjugate. In this study, mitAspAT was stably overexpressed in LLC-PK(1) cells. Cisplatin was significantly more toxic in confluent monolayers of LLC-PK(1) cells that overexpressed mitAspAT than in control cells containing vector alone. AOAA completely blocked the cisplatin toxicity in confluent mitAspAT-transfected cells. The Pt-thiol compound could rapidly bind proteins and inactivate enzymes in close proximity of the PLP-dependent cysteine S-conjugate beta-lyase. Treatment with 50 or 100 microM cisplatin for 3 h, followed by removal of cisplatin from the medium for 24 h, resulted in a pronounced loss of alpha-ketoglutarate dehydrogenase complex (KGDHC) activity in both mitAspAT-transfected cells and control cells. Exposure to 100 microM cisplatin resulted in a significantly greater loss of KGDHC activity in the cells overexpressing mitAspAT than in control cells. Aconitase activity was diminished in both cell types, but only at the higher level of exposure to cisplatin. AspAT activity was also significantly decreased by cisplatin treatment. By contrast, several other enzymes (both cytosolic and mitochondrial) involved in energy/amino acid metabolism were not significantly affected by cisplatin treatment in the LLC-PK(1) cells, whether or not mitAspAT was overexpressed. The susceptibility of KGDHC and aconitase to inactivation in kidney cells exposed to cisplatin metabolites may be due to the proximity of mitAspAT to KGDHC and aconitase in mitochondria. These findings support the hypothesis that a mitochondrial cysteine S-conjugate beta-lyase converts the cisplatin-cysteine S-conjugate to a toxicant, and the data are consistent with the hypothesis that mitAspAT plays a role in the bioactivation of cisplatin.

Identification of Hsc70 binding sites in mitochondrial aspartate aminotransferase

Hsc70 binds acid-unfolded mitochondrial aspartate aminotransferase (mAAT), forming either soluble or insoluble complexes depending on the relative concentrations of the proteins. Using partial proteolysis of Hsc70-mAAT complexes in combination with MALDI-TOF mass spectrometry, we have identified several potential Hsc70-binding regions in the mAAT polypeptide. Only one mAAT peptide was found bound to Hsc70 in the insoluble complexes while nine peptides arising from eight sequence regions of mAAT were found associated with Hsc70 in the soluble complexes. Most of these binding sites map to secondary structure elements, particularly alpha-helix, that are partly exposed on the surface of the folded structure. These results suggest that these peptide regions must not only be exposed but still in a flexible extended conformation in the mAAT folding intermediates recognized by Hsc70. Thus, for mAAT the discrimination between native and non-native structures by Hsc70 may rely more on the level of structure of the binding sites than on their degree of exposure to the solvent in the native structure.

Reinvestigation of the requirement of cytosolic ATP for mitochondrial protein import

Protein import into mitochondria requires the energy of ATP hydrolysis inside and/or outside mitochondria. Although the role of ATP in the mitochondrial matrix in mitochondrial protein import has been extensively studied, the role of ATP outside mitochondria (external ATP) remains only poorly characterized. Here we developed a protocol for depletion of external ATP without significantly reducing the import competence of precursor proteins synthesized in vitro with reticulocyte lysate. We tested the effects of external ATP on the import of various precursor proteins into isolated yeast mitochondria. We found that external ATP is required for maintenance of the import competence of mitochondrial precursor proteins but that, once they bind to mitochondria, the subsequent translocation of presequence-containing proteins, but not the ADP/ATP carrier, proceeds independently of external ATP. Because depletion of cytosolic Hsp70 led to a decrease in the import competence of mitochondrial precursor proteins, external ATP is likely utilized by cytosolic Hsp70. In contrast, the ADP/ATP carrier requires external ATP for efficient import into mitochondria even after binding to mitochondria, a situation that is only partly attributed to cytosolic Hsp70.

Roles of cytosolic Hsp70 and Hsp40 molecular chaperones in post-translational translocation of presecretory proteins into the endoplasmic reticulum

Hsp70 molecular chaperones and their co-chaperones work together in various cellular compartments to guide the folding of proteins and to aid the translocation of proteins across membranes. Hsp70s stimulate protein folding by binding exposed hydrophobic sequences thereby preventing irreversible aggregation. Hsp40s stimulate the ATPase activity of Hsp70s and target unfolded proteins to Hsp70s. Genetic and biochemical evidence supports a role for cytosolic Hsp70s and Hsp40s in the post-translational translocation of precursor proteins into endoplasmic reticulum and mitochondria. To gain mechanistic insight, we measured the effects of Saccharomyces cerevisiae Ssa1p (Hsp70) and Ydj1p (Hsp40) on the translocation of histidine-tagged prepro-alpha-factor (ppalphaF6H) into microsomes. Radiolabeled ppalphaF6H was affinity purified from wheat germ translation reactions (or Escherichia coli) to remove endogenous chaperones. We demonstrated that either Ssa1p or Ydj1p stimulates post-translational translocation by preventing ppalphaF6H aggregation. The binding and/or hydrolysis of ATP by Ssa1p were required to maintain the translocation competence of ppalphaF6H. To clarify the contributions of membrane-bound and cytosolic Ydj1p, we compared the efficiency of chaperone-dependent translocation into wild-type and Ydj1p-deficient microsomes. Neither soluble nor membrane-bound Ydj1p was essential for post-translational protein translocation. The ability of Ssa1p, Ydj1p, or both chaperones to restore the translocation competence of aggregated ppalphaF6H was negligible.

Import of yeast mitochondrial transcription factor (Mtf1p) via a nonconventional pathway

The yeast mitochondrial (mt) transcription factor Mtf1p is imported into the mitochondria from the cytoplasm without a conventional mt-targeting presequence. To understand its import the mt translocation of wild type and mutant Mtf1p constructs was investigated in vitro under various assay conditions. We report here that Mtf1p, unlike most mt matrix proteins hitherto studied, is translocated into the mitochondria independent of membrane potential, ATP hydrolysis, and membrane receptor. This unusual import of Mtf1p was also observed on ice (3 degrees C). Sub-mitochondrial fractionation demonstrated that Mtf1p was translocated in vitro to one or more of the same mt sites as the endogenous protein that includes the matrix. To identify the mt-targeting sequence of Mtf1p, various N-terminal, C-terminal, or internally deleted Mtf1p derivatives were generated. The full-length and C-terminal deletions but not the N-terminal truncated Mtf1p were imported into mitochondria, indicating the importance of its N-terminal sequence for mt targeting. However, the internal deletion of Mtf1p revealed that the first 150-amino acid N-terminal sequence alone was not sufficient for mt targeting of Mtf1p, suggesting that an extended rather than a short N-terminal sequence is required for import. We favor a model in which Mtf1p adopts an import-competent conformation during translation. Consistent with this model are three findings: most of the protein sequence appears to be required for optimal import, urea denaturation eliminates its import competence, and the import-competent form of the protein is more resistant to tryptic hydrolysis than is the denatured protein. This represents a novel mechanism for mitochondrial protein import.

Toxic, halogenated cysteine S-conjugates and targeting of mitochondrial enzymes of energy metabolism

Several haloalkenes are metabolized in part to nephrotoxic cysteine S-conjugates; for example, trichloroethylene and tetrafluoroethylene are converted to S-(1,2-dichlorovinyl)-L-cysteine (DCVC) and S-(1,1,2,2-tetrafluoroethyl)-L-cysteine (TFEC), respectively. Although DCVC-induced toxicity has been investigated since the 1950s, the toxicity of TFEC and other haloalkene-derived cysteine S-conjugates has been studied more recently. Some segments of the US population are exposed to haloalkenes either through drinking water or in the workplace. Therefore, it is important to define the toxicological consequences of such exposures. Most halogenated cysteine S-conjugates are metabolized by cysteine S-conjugate beta-lyases to pyruvate, ammonia, and an alpha-chloroenethiolate (with DCVC) or an alpha-difluoroalkylthiolate (with TFEC) that may eliminate halide to give a thioacyl halide, which reacts with epsilon-amino groups of lysine residues in proteins. Nine mammalian pyridoxal 5'-phosphate (PLP)-containing enzymes catalyze cysteine S-conjugate beta-lyase reactions, including mitochondrial aspartate aminotransferase (mitAspAT), and mitochondrial branched-chain amino acid aminotransferase (BCAT(m)). Most of the cysteine S-conjugate beta-lyases are syncatalytically inactivated. TFEC-induced toxicity is associated with covalent modification of several mitochondrial enzymes of energy metabolism. Interestingly, the alpha-ketoglutarate- and branched-chain alpha-keto acid dehydrogenase complexes (KGDHC and BCDHC), but not the pyruvate dehydrogenase complex (PDHC), are susceptible to inactivation. mitAspAT and BCAT(m) may form metabolons with KGDHC and BCDHC, respectively, but no PLP enzyme is known to associate with PDHC. Consequently, we hypothesize that not only do these metabolons facilitate substrate channeling, but they also facilitate toxicant channeling, thereby promoting the inactivation of proximate mitochondrial enzymes and the induction of mitochondrial dysfunction.

Binding to chaperones allows import of a purified mitochondrial precursor into mitochondria

Refolding of the acid-unfolded precursor to mitochondrial aspartate aminotransferase (pmAAT) is inhibited when cytosolic Hsc70 is included in the refolding reaction (Artigues, A., Iriarte, A., and Martinez-Carrion, M. (1997) J. Biol. Chem. 272, 16852-16861). At low molar excess of Hsc70 pmAAT is recovered in insoluble aggregates containing equal amounts of Hsc70. However, in the presence of a large excess of Hsc70, refolding of pmAAT is still arrested, but the enzyme remains in solution. Similar behavior was observed with two other cytosolic chaperones, bovine Hsp90 and yeast Ydj1. Coimmunoprecipitation of pmAAT using Hsc70 antibodies confirmed the formation of soluble Hsc70-pmAAT complexes at high concentrations of the chaperone. Data from analytical centrifugation, sedimentation in glycerol gradients, and partial purification of the soluble complexes indicate that multiple Hsc70 molecules bind per pmAAT polypeptide chain. The absence of catalytic activity together with the protease susceptibility of pmAAT bound to Hsc70, Hsp90, or Ydj1 suggest that these chaperones bind and maintain pmAAT in a partially unfolded state, analogous to the import-competent conformation of the protein synthesized in cell-free extracts. Remarkably, the purified pmAAT bound to Hsc70 or Ydj1, but not to Hsp90, is imported by isolated mitochondria in a reticulocyte lysate-dependent manner. Thus, both Hsc70 and Ydj1 can trap an import-competent folding intermediate of pmAAT, but productive binding and import into mitochondria require the collaboration of additional cytosolic factors from the lysate.

Interaction of the targeting sequence of chloroplast precursors with Hsp70 molecular chaperones

We have analyzed the interaction of DnaK and plant Hsp70 proteins with the wild-type ferredoxin-NADP+ reductase precursor (preFNR) and mutants containing amino-acid replacements in the targeting sequence. Using an algorithm already developed [Rüdiger, S., Germeroth, L., Schneider-Mergener, J. & Bukau, B. (1997) EMBO J. 16, 1501-1507] we observed that 75% of the 727 plastid precursor proteins analyzed contained at least one site with high likelihood of DnaK binding in their transit peptides. Statistical analysis showed a decrease of DnaK binding site frequency within the first 15 amino-acid residues of the transit peptides. Using fusion proteins we detected the interaction of DnaK with the transit peptide of the folded preFNR but not with the mature region of the protein. Discharge of DnaK from the presequence was favored by addition of MgATP. When a putative DnaK binding site was artificially added at the N-terminus of the mature protein, we observed formation of complexes with bacterial and plant Hsp70 molecular chaperones. Reducing the likelihood of DnaK binding by directed mutagenesis of the presequence increased the release of bound DnaK. The Hsp70 proteins from plastids and plant cell cytosol also interacted with the preFNR transit peptide. Overall results are discussed in the context of the proposed models to explain the organelle protein import.

The folding of nascent mitochondrial aspartate aminotransferase synthesized in a cell-free extract can be assisted by GroEL and GroES

At 30 degrees C, the precursor to mitochondrial aspartate aminotransferase (pmAspAT) cannot fold after synthesis in rabbit reticulocyte lysate (RRL), a model for studying intracellular protein folding. However, it folds rapidly once imported into mitochondria. Guanidinium chloride denatured pmAspAT likewise cannot refold at 30 degrees C in a defined in vitro system. However, it refolds rapidly and in good yield in the presence of the intramitochondrial chaperone homologues GroEL and GroES. In this report, we demonstrate that GroEL and GroES can also facilitate the folding of nascent pmAspAT in reticulocyte lysate under conditions where it otherwise would not. When added alone, GroEL arrests the slow folding of nascent pmAspAT and inhibits import into mitochondria. These effects are significantly reversed by adding GroES. These observations suggest that added GroEL participates in an equilibrium with endogenous chaperones in the cytosol which inhibit folding and promote import competence. Native gel electrophoresis suggests that nascent pmAspAT exists in RRL as a heterogeneous population of partially folded species, some of which bind to added GroEL more readily than others. The GroEL-trapped species appear to be among the productive pmAspAT folding intermediates formed in RRL or they at least appear to equilibrate with these intermediates, since they become import competent after GroES-stimulated release from GroEL.

Effect of N-terminal alpha-helix formation on the dimerization and intracellular targeting of alanine:glyoxylate aminotransferase

The unparalleled peroxisome-to-mitochondrion mistargeting of alanine:glyoxylate aminotransferase (AGT) in the hereditary disease primary hyperoxaluria type 1 is caused by the combined presence of a common Pro11 --> Leu polymorphism and a disease-specific Gly170 --> Arg mutation. The Pro11 --> Leu replacement generates a functionally weak N-terminal mitochondrial targeting sequence (MTS), the efficiency of which is increased by the additional presence of the Gly170 --> Arg replacement. AGT dimerization is inhibited in the combined presence of both replacements but not when each is present separately. In this paper we have attempted to identify the structural determinants of AGT dimerization and mitochondrial mistargeting. Unlike most MTSs, the polymorphic MTS of AGT has little tendency to adopt an alpha-helical conformation in vitro. Nevertheless, it is able to target efficiently a monomeric green fluorescent (GFP) fusion protein, but not dimeric AGT, to mitochondria in transfected COS-1 cells. Increasing the propensity of this MTS to fold into an alpha-helix, by making a double Pro11 --> Leu + Pro10 --> Leu replacement, enabled it to target both GFP and AGT efficiently to mitochondria. The double Pro11 --> Leu + Pro10 --> Leu replacement retarded AGT dimerization in vitro as did the disease-causing double Pro11 --> Leu + Gly170 --> Arg replacement. These data suggest that N-terminal alpha-helix formation is more important for maintaining AGT in a conformation (i. e. monomeric) compatible with mitochondrial import than it is for the provision of mitochondrial targeting information. The parallel effects of the Pro10 --> Leu and Gly170 --> Arg replacements on the dimerization and intracellular targeting of polymorphic AGT (containing the Pro11 --> Leu replacement) raise the possibility that they might achieve their effects by the same mechanism.

In vivo mitochondrial import. A comparison of leader sequence charge and structural relationships with the in vitro model resulting in evidence for co-translational import

The positive charges and structural properties of the mitochondrial leader sequence of aldehyde dehydrogenase have been extensively studied in vitro. The results of these studies showed that increasing the helicity of this leader would compensate for reduced import from positive charge substitutions of arginine with glutamine or the insertion of negative charged residues made in the native leader. In this in vivo study, utilizing the green fluorescent protein (GFP) as a passenger protein, import results showed the opposite effect with respect to helicity, but the results from mutations made within the native leader sequence were consistent between the in vitro and in vivo experiments. Leader mutations that reduced the efficiency of import resulted in a cytosolic accumulation of a truncated GFP chimera that was fluorescent but devoid of a mitochondrial leader. The native leader efficiently imported before GFP could achieve a stable, import-incompetent structure, suggesting that import was coupled with translation. As a test for a co-translational mechanism, a chimera of GFP that contained the native leader of aldehyde dehydrogenase attached at the N terminus and a C-terminal endoplasmic reticulum targeting signal attached to the C terminus of GFP was constructed. This chimera was localized exclusively to mitochondria. The import result with the dual signal chimera provides support for a co-translational mitochondrial import pathway.

Interaction of mitochondrial presequences with DnaK and mitochondrial hsp70

Mitochondrial heat shock protein 70 (mt-hsp70) functions as a molecular chaperone in mitochondrial biogenesis. The chaperone in co-operation with its co-proteins acts as a translocation motor pulling the mitochondrial precursor into the matrix. Mt-hsp70s are highly conserved when compared to the bacterial hsp70 homologue, DnaK. Here we have used DnaK as a model to study the interaction of mitochondrial presequences with mt-hsp70 applying a DnaK-binding algorithm, computer modeling and biochemical investigations. DnaK-binding motifs have been analysed on all available, statistically relevant mitochondrial presequences found in the OWL database by running the algorithm. A total of 87 % of mammalian, 97 % of plant, 71 % of yeast and 100 % of Neurospora crassa presequences had at least one DnaK binding site. Based on the prediction, five 13-mer presequence peptides have been synthesized and their inhibitory effect on the molecular chaperone (DnaK/DnaJ/GrpE) assisted refolding of luciferase has been analysed. The peptide with the highest predicted binding likelihood showed the strongest inhibitory effect, whereas the peptide with no predicted binding capacity showed no inhibitory effect. A 3D structure of the pea mt-hsp70 has been constructed using homology modeling. The binding affinities of the 13-mer presequence peptides and additional control peptides to DnaK and pea mt-hsp70 have been theoretically estimated by calculating the buried hydrophobic surface area of the peptides docked to DnaK and to the mt-hsp70 structural model. These results suggest that mitochondrial presequences interact with the mt-hsp70 during or after mitochondrial protein import.

Conformation of aspartate aminotransferase isozymes folding under different conditions probed by limited proteolysis

The partially homologous mitochondrial (mAAT) and cytosolic (cAAT) aspartate aminotransferase have nearly identical three-dimensional structures but differ in their folding rates in cell-free extracts and in their affinity for binding to molecular chaperones. In its native state, each isozyme is protease-resistant. Using limited proteolysis as an index of their conformational states, we have characterized these proteins (a) during the early stages of spontaneous refolding; (b) as species trapped in stable complexes with the chaperonin GroEL; or (c) as newly translated polypeptides in cell-free extracts. Treatment of the refolding proteins with trypsin generates reproducible patterns of large proteolytic fragments that are consistent with the formation of defined folding domains soon after initiating refolding. Binding to GroEL affords considerable protection to both isozymes against proteolysis. The tryptic fragments are similar in size for both isozymes, suggesting a common distribution of compact and flexible regions in their folding intermediates. cAAT synthesized in cell-free extracts becomes protease-resistant almost instantaneously, whereas trypsin digestion of the mAAT translation product produces a pattern of fragments qualitatively akin to that observed with the protein refolding in buffer. Analysis of the large tryptic peptides obtained with the GroEL-bound proteins reveals that the cleavage sites are located in analogous regions of the N-terminal portion of each isozyme. These results suggest that (a) binding to GroEL does not cause unfolding of AAT, at least to an extent detectable by proteolysis; (b) the compact folding domains identified in AAT bound to GroEL (or in mAAT fresh translation product) are already present at the early stages of refolding of the proteins in buffer alone; and (c) the two isozymes seem to bind in a similar fashion to GroEL, with the more compact C-terminal portion completely protected and the more flexible N-terminal first 100 residues still partially accessible to proteolysis.

Opposite behavior of two isozymes when refolding in the presence of non-ionic detergents

GroEL has a greater affinity for the mitochondrial isozyme (mAAT) of aspartate aminotransferase than for its cytosolic counterpart (cAAT) (Mattingly JR Jr, Iriarte A, Martinez-Carrion M, 1995, J Biol Chem 270:1138-1148), two proteins that share a high degree of sequence similarity and an almost identical spatial structure. The effect of detergents on the refolding of these large, dimeric isozymes parallels this difference in behavior. The presence of non-ionic detergents such as Triton X-100 or lubrol at concentrations above their critical micelle concentration (CMC) interferes with reactivation of mAAT unfolded in guanidinium chloride but increases the yield of cAAT refolding at low temperatures. The inhibitory effect of detergents on the reactivation of mAAT decreases progressively as the addition of detergents is delayed after starting the refolding reaction. The rate of disappearance of the species with affinity for binding detergents coincides with the slowest of the two rate-limiting steps detected in the refolding pathway of mAAT. Limited proteolysis studies indicate that the overall structure of the detergent-bound mAAT resembles that of the protein in a complex with GroEL. The mAAT folding intermediates trapped in the presence of detergents can resume reactivation either upon dilution of the detergent below its CMC or by adding beta-cyclodextrin. Thus, isolation of otherwise transient productive folding intermediates for further characterization is possible through the use of detergents.

Aminotransferase variants as probes for the role of the N-terminal region of a mature protein in mitochondrial precursor import and processing

Of the two homologous isozymes of aspartate aminotransferase that are also nearly identical in their folded structures, only the mitochondrial form (mAAT) is synthesized as a precursor (pmAAT). After its in vitro synthesis in rabbit reticulocyte lysate, it can also be efficiently imported into isolated rat liver mitochondria, where it is processed to its native form by removal of the N-terminal presequence. The homologous cytosolic isoenzyme (cAAT) is not imported into mitochondria, even after fusion of the mitochondrial presequence from pmAAT to its N-terminal end. Substitution of the 30-residue N-terminal peptide of the mature portion of pmAAT with the corresponding sequence from the homologous, import-incompetent cytosolic isozyme (pcmAAT) does not prevent import but reduces substantially its processing in the matrix. A detectable amount of the pcmAAT chimera is found associated with the inner mitochondrial membrane. Single and double substitution mutants of Trp-5 and Trp-6 at the N-terminal end of the mature protein are imported into mitochondria with efficiency similar to that of wild type. However, replacement of Trp-5 with proline, or of both tryptophans with either alanine (W5A/W6A mutant) or valine and alanine (W5V/W6A mutant), allows import but interferes with the correct processing of the imported protein despite the presence of an intact cleavage site for the processing peptidase. Similar cleavage results were obtained using newly synthesized proteins and mitochondrial matrix extracts. These results indicate that translocation and processing for a precursor are independent events and that sequences C-terminal to the cleavage site are indeed important for the correct maturation of pmAAT in the matrix, probably because of their contribution to the conformation and flexibility of the peptide region surrounding the cleavage site required for efficient processing. The same region from the mature component of the protein may play a role in the commitment of the passenger protein to complete its translocation into the matrix.

The 5'-untranslated region of GLUT1 glucose transporter mRNA causes differential regulation of the translational rate in plant and animal systems

The blood-brain barrier GLUT1 glucose transporter is under post-transcriptional regulation, and the 5'-untranslated region (5'-UTR) of the GLUT1 mRNA increases its translational rate in mammalian cells. To obtain more insight into the mechanism of translational control of GLUT1, the present investigation studied the translational efficiency of capped full-length synthetic human (h) and rabbit (rab) GLUT1 mRNA and both 5'- and 3'-UTR deleted hGLUT1 mRNAs in both mammalian and plant cell free translation systems. Translation efficiency of both h- and rabGLUT1 mRNA was increased 3- to 6-fold in rabbit retyculocyte lysate (RRL) compared with wheat germ extract (WGE). Confirming previous observations, deletion of 5'- and 5'/-3'-UTR markedly reduced the translation efficiency of the h-GLUT1 transcript in RRL. On the contrary, these deletions markedly increased the translation of GLUT1 in WGE. The present data provide additional evidence suggesting that the 5'-UTR of the GLUT1 mRNA contains cis-acting elements involved in the translational activation of the GLUT1 gene in mammalian cells and that factors involved in this cis/trans-acting interaction are either absent or down-regulated in plant systems.

Differential resistance to proteinase K digestion of the yeast prion-like (Ure2p) protein synthesized in vitro in wheat germ extract and rabbit reticulocyte lysate cell-free translation systems

The Ure2p yeast prion-like protein was translated in vitro in the presence of labeled [35S]methionine in either rabbit reticulocyte lysate (RRL) or wheat germ extract (WGE) cell-free systems. When subjected to proteinase K digestion, the Ure2p protein synthesized in WGE was proteolysed much more slowly compared to that synthesized in RRL; this displays fragments of about 31-34 kDa, persisting over 8 min. Thus, the digestion rate and pattern of the protein synthesized in WGE, unlike that synthesized in RRL, revealed characteristic features of the [URE3] prion-like isoform of the Ure2p protein [Masison, D.C. and Wickner, R.B. (1995) Science 270, 93-95]. Chloramphenicol acetyltransferase, synthesized under the same conditions, differed fundamentally in its proteolytic sensitivity toward proteinase K (PK); in the RRL system it was more slowly digested than in WGE, proving specific PK inhibitors to be absent in both systems. Posttranslational addition of the WGE to the RRL-synthesized Ure2p does not protect Ure2p from efficient PK degradation either. The differences in Ure2p degradation may be ascribed to a specific structure or specific states of association of Ure2p synthesized in WGE; obviously, they yield a protein that mimics the behavior of the Ure2p in [URE3] yeast strains. The present data suggest that particular conditions of the Ure2p protein translation and/or certain cellular components (accessory proteins and extrinsic factors), as well as the nature of the translation process itself, could affect the intracellular folding pathway of Ure2p leading to the de novo formation of the prion [URE3] isoform.

Refolding intermediates of acid-unfolded mitochondrial aspartate aminotransferase bind to hsp70

The cytosolic (cAAT) and mitochondrial (mAAT) isozymes of eukaryotic aspartate aminotransferase share a high degree of sequence identity and almost identical three-dimensional structure. The rat liver proteins can be refolded and reassembled into active dimers after unfolding at low pH. However, refolding of the mitochondrial form after unfolding at pH 2.0 is arrested in the presence of hsp70, whereas this chaperone does not affect the refolding of the cytosolic isozyme unfolded under similar conditions. To elucidate the nature of the differential interaction between hsp70 and the two transaminase forms, we have characterized their refolding from their acid-unfolded states. The recovery of activity of the cytosolic enzyme is monophasic and can be adequately described by a single first-order reaction. By contrast, two sequential first-order rate-limiting steps can be detected for the refolding and reactivation of the mitochondrial protein. The overall refolding pathway of mAAT includes a very fast collapse to an intermediate with 80% of the secondary structure of the active dimer. This is followed by a slow isomerization to form assembly-competent monomers that rapidly associate to form an inactive dimer and a final structural rearrangement of the dimer to the native conformation. Analysis of the interaction of hsp70 with intermediates along the folding pathway of mAAT shows that the polypeptide loses its ability to bind to the chaperone after it has proceeded through the first isomerization/fast dimerization steps. Thus it appears that only the first collapsed intermediate states in the folding of mAAT bind hsp70. By contrast a faster refolding of cAAT from this collapsed state could explain, at least in part, the inability of hsp70 to bind this isozyme.

Mft52, an acid-bristle protein in the cytosol that delivers precursor proteins to yeast mitochondria

We have identified a novel protein, Mft52, in the cytosol of yeast cells. Mft52 has a two-domain structure that includes a receptor-like carboxyl-terminal "acid-bristle" domain, which binds basic, amphipathic mitochondrial targeting sequences. Native Mft52, purified from the cytosol of yeast cells, is found as a large particle eluting in the void volume of a Superose 6 gel filtration column. Fusion proteins, consisting of mitochondrial targeting sequences fused to nonmitochondrial passenger proteins, are targeted to mitochondria in wild-type yeast cells, but defects in the gene encoding Mft52 drastically reduce the delivery of these proteins to the mitochondria. We propose that Mft52 is a subunit of a particle that is part of a system of targeting factors and molecular chaperones mediating the earliest stages of protein targeting to the mitochondria.

N-Ethylmaleimide inactivates a nucleotide-free Hsp70 molecular chaperone

Hsp70 molecular chaperones are ATPases that bind to hydrophobic regions of proteins and guide their folding, assembly, and translocation across membranes. The ability of purified Hsp70s to uncoat clathrin-coated vesicles or to stimulate the post-translational translocation of precursor proteins into the endoplasmic reticulum, mitochondria, and the nucleus was previously shown not to be sensitive to the sulfhydryl-modifying reagent N-ethylmaleimide (NEM). During purification of factors required for protein folding in the cytosol, we found that the ATP-agarose binding activity of the yeast Hsp70 Ssa1p in postribosomal supernatants was inhibited by NEM. We also found that completely removing nucleotides from purified Ssa1p rendered its ATP-agarose binding activity, ATPase activity, and post-translational translocation-stimulating activity sensitive to NEM. We modified nucleotide-free Ssa1p with [14C]NEM and then digested it with proteases. Purification and sequencing of the radiolabeled proteolytic fragments revealed that each of Ssa1p's three cysteine residues (Cys-15, Cys-264, and Cys-303) was modified with [14C]NEM. ADP protected each of the cysteine residues from modification and protected Ssa1p from inactivation. The cysteine residues are the reactive centers of three NEM-reactive sites (NRS1-3). A comparison of Ssa1p's NRSs to sequences of other Hsp70s and actin revealed that Cys-15 of NRS1 is highly conserved and that sensitivity to NEM may be a property of many Hsp70s. Based on the three-dimensional structure of Hsc70, the predicted locations of Ssa1p's cysteine residues suggest that NEM may disrupt the conformation of Ssa1p or interfere with its ability to bind nucleotides. Together the results demonstrate that Ssa1p is an NEM-sensitive factor in cytosolic extracts from yeast that stimulates post-translational translocation of proteins into organelles.

Conformations of the nucleotide and polypeptide binding domains of a cytosolic Hsp70 molecular chaperone are coupled

70-kDa heat shock protein (Hsp70) molecular chaperones are ATPases that participate in protein folding by regulating protein-protein interactions. ATP binds to the highly conserved amino-terminal domain, whereas polypeptides bind to the less conserved carboxyl-terminal domain. These domains are functionally coupled. Polypeptides were previously shown to dissociate from Hsp70s upon ATP binding and to stimulate ATPase activity. We probed the structure of the yeast cytosolic Hsp70 Ssa1p using limited proteolysis to determine whether the conformations of its nucleotide and polypeptide binding domains are also coupled. Ssa1p adopted three distinct conformations, nucleotide-free, ADP-dependent, and ATP-dependent. Complete conformational changes required K+ and Mg2+. Using amino-terminal sequencing, ATP-agarose chromatography, and a carboxyl-terminal-specific antibody, we mapped the locations of the major proteolytic fragments. Nucleotides altered the conformations of both the nucleotide and polypeptide binding domains. Similarly, a polypeptide altered the conformations of both domains. These results indicate that the conformations of the nucleotide and polypeptide binding domains are coupled.

Biogenesis of the covalently flavinylated mitochondrial enzyme dimethylglycine dehydrogenase

Rat dimethylglycine dehydrogenase (Me2GlyDH) was used as model protein to study the biogenesis of a covalently flavinylated mitochondrial enzyme. Here we show that: 1) enzymatically active holoenzyme correlated with trypsin resistance of the protein; 2) folding of the reticulocyte lysate-translated protein into the trypsin-resistant, holoenzyme form was a slow process that was stimulated by the presence of the flavin cofactor and was more efficient at 15 degrees C than at 30 degrees C; 3) the mitochondrial presequence reduced the extent but did not prevent holoenzyme formation; 4) covalent attachment of FAD to the Me2GlyDH apoenzyme proceeded spontaneously and did not require a mitochondrial protein factor; 5) in vitro only the precursor, but not the mature form, of the protein was imported into isolated rat liver mitochondria; in vivo, in stably transfected HepG2 cells, both the precursor and the mature form were imported into the organelle; 6) holoenzyme formation in the cytoplasm did not prevent the translocation of the proteins into the mitochondria in vivo; and 7) lack of vitamin B2 in the tissue culture medium resulted in a reduced recovery of the precursor and the mature form of Me2GlyDH from cell mitochondria, suggesting a decreased efficiency of mitochondrial protein import.

Structural features of the precursor to mitochondrial aspartate aminotransferase responsible for binding to hsp70

The precursor (pmAspAT) and mature (mAspAT) forms of mitochondrial aspartate aminotransferase interact with hsp70 very early during translation when synthesized in either rabbit reticulocyte lysate or wheat germ extract (Lain, B., Iriarte, A., and Martinez-Carrion. (1994) J. Biol. Chem. 269, 15588-15596). The nature of the structural elements responsible for recognition and binding of this protein to hsp70 has been studied by examining the folding and potential association with the chaperone of several engineered forms of this enzyme. Whereas pmAspAT and mAspAT bind hsp70 very early during translation, the cytosolic form of this enzyme (cAspAT) does not interact with hsp70. A fusion protein consisting of the mitochondrial presequence peptide attached to the amino terminus of cAspAT associates with hsp70 only after the protein has acquired its native-like conformation, apparently through binding to the presequence exposed on the surface of the folded protein. Deletion of the amino-terminal segment of mAspAT or its replacement with the corresponding domain from the cytosolic isozyme eliminates the cotranslational binding of hsp70 to the mitochondrial protein. We conclude that both the presequence and NH2-terminal region of pmAspAT represent recognition signals for binding of hsp70 to the newly synthesized mitochondrial precursor. Results from competition studies with synthetic peptides support this conclusion. The ability of hsp70 to discriminate between these two highly homologous proteins probably involves the recognition of specific sequence elements in the NH2-terminal portion of the mitochondrial protein and may relate to their separate localization in the cell. A slower folding rate and higher affinity for cytosolic chaperones may represent evolutionary adaptations of translocated mitochondrial proteins to ensure their efficient importation into the organelle.

Protein-mediated protein maturation in eukaryotes

Eukaryotic cells have developed particular strategies to support the critical steps in protein maturation that starts in the cytosol with the birth of a nascent polypeptide chain, and ends when the protein has reached the appropriate compartment and/or has attained its mature structure. Many of the cellular proteins that have evolved to promote maturation processes are constitutively expressed members of the highly conserved heat shock protein (hsp) family, also known as 'molecular chaperones'. Protein-mediated processes that occur in the cytosol are discussed.

Folding, flavinylation, and mitochondrial import of 6-hydroxy-D-nicotine oxidase fused to the presequence of rat dimethylglycine dehydrogenase

We analyzed the folding, covalent flavinylation, and mitochondrial import of the rabbit reticulocyte lysate-translated bacterial 6-hydroxy-D-nicotine oxidase (6-HDNO) fused to the mitochondrial targeting sequence of rat liver dimethylglycine dehydrogenase. Translation of 6-HDNO in FAD-supplemented reticulocyte lysate resulted in a protein that contained covalently incorporated FAD, exhibited enzyme activity, and was trypsin-resistant, a characteristic of the tight conformation of the holoenzyme. The attached mitochondrial presequence did not prevent folding, binding of FAD, or enzyme activity of the 6-HDNO moiety of the fusion protein (pre-6-HDNO). Pre-6-HDNO was imported into rat liver mitochondria and processed by the mitochondrial processing peptidase. Incubation of the trypsin-resistant pre-holo-6-HDNO protein with deenergized rat liver mitochondria demonstrated that upon contact with mitochondria, the protein was unfolded and became trypsin sensitive. Mitochondrial import assays showed that the unfolded pre-holo-6-HDNO with covalently attached FAD was imported into rat liver mitochondria. Inside the mitochondrion the holo-6-HDNO was refolded into the trypsin-resistant conformation. However, when pre-apo-6-HDNO was imported only part of the protein became trypsin resistant (approximately 20%). Addition of FAD and the allosteric effector glycerol 3-phosphate to apo-6-HDNO containing mitochondrial matrix was required to transform the protein into the trypsin-resistant conformation characteristic of holo-6-HDNO.

Homologous proteins with different affinities for groEL. The refolding of the aspartate aminotransferase isozymes at varying temperatures

The homologous cytosolic and mitochondrial isozymes of aspartate aminotransferase (c- and mAspAT, respectively) seem to follow very different folding pathways after synthesis in rabbit reticulocyte lysate, suggesting that the nascent proteins interact differently with molecular chaperones (Mattingly, J. R., Jr., Iriarte, A., and Martinez-Carrion, M. (1993) J. Biol. Chem. 268, 26320-26327). In an attempt to discern the structural basis for this phenomenon, we have begun to study the effect of temperature on the refolding of the guanidine hydrochloride-denatured, purified proteins and their interaction with the groEL/groES molecular chaperone system from Escherichia coli. In the absence of chaperones, temperature has a critical effect on the refolding of the two isozymes, with mAspAT being more susceptible than cAspAT to diminishing refolding yields at increasing temperatures. No refolding is observed for mAspAT at physiological temperatures. The molecular chaperones groEL and groES can extend the temperature range over which the AspAT isozymes successfully refold; however, cAspAT can still refold at higher temperatures than mAspAT. In the absence of groES and MgATP, the two isozymes interact differently with groEL, groEL arrests the refolding of mAspAT throughout the temperature range of 0-45 degrees C. Adding only MgATP releases very little mAspAT from groEL; both groES and MgATP are required for significant refolding of mAspAT in the presence of groEL. On the other hand, the extent to which groEL inhibits the refolding of cAspAT depends upon the temperature of the refolding reaction, only slowing the reaction at 0 degrees C but arresting it completely at 30 degrees C. MgATP alone is sufficient to effect the release of cAspAT from groEL at any temperature examined; inclusion of groES along with MgATP has no effect on the refolding yield but does increase the refolding rate at temperatures greater than 15 degrees C. These results demonstrate that groEL can have significantly different affinities for proteins with highly homologous final tertiary and quarternary structures and suggest that dissimilarities in the primary sequence of the protein substrates may control the structure of the folding intermediates captured by groEL and/or the composition of the surfaces through which the folding proteins interact with groEL.