Divergent Hsc70 binding properties of mitochondrial and cytosolic aspartate aminotransferase. Implications for their segregation to different cellular compartments

Cofilin: a redox sensitive mediator of actin dynamics during T-cell activation and migration

Cofilin is an actin-binding protein that depolymerizes and/or severs actin filaments. This dual function of cofilin makes it one of the major regulators of actin dynamics important for T-cell activation and migration. The activity of cofilin is spatio-temporally regulated. Its main control mechanisms comprise a molecular toolbox of phospho-, phospholipid, and redox regulation. Phosphorylated cofilin is inactive and represents the dominant cofilin fraction in the cytoplasm of resting human T cells. A fraction of dephosphorylated cofilin is kept inactive at the plasma membrane by binding to phosphatidylinositol 4,5-bisphosphate. Costimulation via the T-cell receptor/CD3 complex (signal 1) together with accessory receptors (signal 2) or triggering through the chemokine SDF1α (stromal cell-derived factor 1α) induce Ras-dependent dephosphorylation of cofilin, which is important for immune synapse formation, T-cell activation, and T-cell migration. Recently, it became evident that cofilin is also highly sensitive for microenvironmental changes, particularly for alterations in the redox milieu. Cofilin is inactivated by oxidation, provoking T-cell hyporesponsiveness or necrotic-like programmed cell death. In contrast, in a reducing environment, even phosphatidylinositol 4,5-bisphosphate-bound cofilin becomes active, leading to actin dynamics in the vicinity of the plasma membrane. In addition to the well-established three signals for T-cell activation, this microenvironmental control of cofilin delivers a modulating signal for T-cell-dependent immune reactions. This fourth modulating signal highly impacts both initial T-cell activation and the effector phase of T-cell-mediated immune responses.

Mitochondrial translocation of oxidized cofilin induces caspase-independent necrotic-like programmed cell death of T cells

Oxidative stress leads to T-cell hyporesponsiveness or death. The actin-binding protein cofilin is oxidized during oxidative stress, which provokes a stiff actin cytoskeleton and T-cell hyporesponsiveness. Here, we show that long-term oxidative stress leads to translocation of cofilin into the mitochondria and necrotic-like programmed cell death (PCD) in human T cells. Notably, cofilin mutants that functionally mimic oxidation by a single mutation at oxidation-sensitive cysteins (Cys-39 or Cys-80) predominately localize within the mitochondria. The expression of these mutants alone ultimately leads to necrotic-like PCD in T cells. Accordingly, cofilin knockdown partially protects T cells from the fatal effects of long-term oxidative stress. Thus, we introduce the oxidation and mitochondrial localization of cofilin as the checkpoint for necrotic-like PCD upon oxidative stress as it occurs, for example, in tumor environments.

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.

In vitro detection of differential and cell-specific hepatobiliary toxicity induced by geldanamycin and 17-allylaminogeldanamycin in rats

The differential toxicity of two anticancer agents is described using the in vitro rat liver slice culture model. Liver slices from F-344 rats were cultured for 5 days in Waymouth's-based medium with exposure to a range of geldanamycin (GEL) or 17-allylaminogeldanamycin (17-AAG) concentrations. GEL induced concentration-dependent reduction of alkaline phosphatase and of gamma-glutamyl transferase levels, which are indicators of biliary epithelial cell(s) (BEC) viability, and exhibited hepatocellular toxicity at higher concentrations. Histologically, BEC cell injury was evident at the lowest GEL concentration (0.1 microM) and progressed to overt bile duct necrosis at 5 microM, a level at which hepatocellular damage was also more prominent. Slices exposed to the same concentrations were more sensitive to toxic effects of GEL than of 17-AAG. 17-AAG at the lowest concentration had more slice biomarker retention than GEL, and histological analysis revealed minimal toxic effect on BEC. With increasing concentration, BEC were progressively lost, and BEC proliferation was completely inhibited at 5 microM 17-AAG. Hepatocellular injury was evident only at high dose exposures. This is believed to be the first use of an in vitro liver tissue model to accurately predict the differential and concentration-dependent toxicities of these compounds.

In vitro detection of differential and cell-specific hepatobiliary toxicity induced by geldanamycin and 17-allylaminogeldanamycin using dog liver slices

Experiments on rat liver slices demonstrated the differential hepatobiliary toxic potency of two anticancer agents, geldanamycin (GEL) and 17-allylaminogeldanamycin (17-AAG), over a 5-day period. This report describes the pattern of toxicity of these agents in dog liver tissue, using the in vitro liver slice culture model. Liver slices (200 microm thick) from male beagle dogs were cultured for 5 days in chemically defined culture medium containing a range of GEL and 17-AAG concentrations (0.1-5 microM). Tissues were evaluated using a panel of clinically relevant biomarkers and histological endpoints. GEL-induced reduction of alkaline phosphatase (ALP) and gamma-glutamyl transferase (GGT) slice levels, indicators of biliary epithelial cell (BEC) viability, was supported by histological findings showing an increasing loss of BEC as higher concentrations were applied. At the highest concentrations studied, GEL caused both hepatocellular necrosis and BEC loss. Biomarker pattern results in the medium concurred with those from slice biochemistry measurements and histology. 17-AAG, a less potent compound in vivo, elicited more biomarker retention at higher concentrations than did GEL. Histological analysis revealed higher BEC viability and significant retention of BEC proliferation as compared with GEL. However, at the highest concentration, the toxic insult caused a marked decrease in BEC viability and proliferation. Comparison of responses with both compounds indicated that slices exposed to the same concentrations were more sensitive to GEL than to 17-AAG. Dog liver slices can thus be used to evaluate species-, compound-, and concentration-dependent differences in toxicity.

Precursors with altered affinity for Hsp70 in their transit peptides are efficiently imported into chloroplasts

Protein import into chloroplasts is postulated to occur with the involvement of molecular chaperones. We have determined that the transit peptide of ferredoxin-NADP(H) reductase precursor binds preferentially to an Hsp70 from chloroplast stroma. To investigate the role of Hsp70 molecular chaperones in chloroplast protein import, we analyzed the import into pea chloroplasts of preproteins with decreased Hsp70 binding affinity in their transit peptides. Our results indicate that the precursor with the lowest affinity for Hsp70 molecular chaperones in its transit peptide was imported to chloroplasts with similar apparent Km as the wild type precursor and a 2-fold increase in Vmax. Thus, a strong interaction between chloroplast stromal Hsp70 and the transit peptide seems not to be essential for protein import. These results indicate that in chloroplasts the main unfolding force during protein import may be applied by molecular chaperones other than Hsp70s. Although stromal Hsp70s undoubtedly participate in chloroplast biogenesis, the role of these molecular chaperones in chloroplast protein translocation differs from the one proposed in the mechanisms postulated up to date.

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.

Co-purification of mitochondrial HSP70 and mature protein disulfide isomerase with a functional rat kidney high-M(r) cysteine S-conjugate beta-lyase

S-(1,1,2,2-Tetrafluoroethyl)-L-cysteine (TFEC, the cysteine S-conjugate of tetrafluoroethylene) is an example of a nephrotoxic, halogenated cysteine S-conjugate. Toxicity results in part from the cysteine S-conjugate beta-lyase(s)-catalyzed conversion of TFEC to a thioacylating fragment with the associated production of pyruvate and ammonia. In the present study, we have demonstrated that rat kidney homogenates contain at least three enzyme fractions that are capable of catalyzing a cysteine S-conjugate beta-lyase reaction with TFEC. One of these fractions contains a high-M(r) lyase. At least two proteins co-purify with this high-M(r) complex. N-Terminal analysis (15 cycles) revealed that the smaller species was mature protein disulfide isomerase (M(r) approximately 54,200) from which the 24 amino acid endoplasmic reticulum signal peptide had been removed. Internal amino acid sequencing (15 cycles) revealed that the larger species was mitochondrial HSP70 (mtHSP70; M(r) approximately 75,000). The present findings offer an explanation for the previous observation that mtHSP70 in kidney mitochondria is heavily thioacylated when rats are injected with TFEC (Bruschi et al., J Biol Chem 1993;268:23157-61).

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.

Stress genes and proteins in the archaea

The field covered in this review is new; the first sequence of a gene encoding the molecular chaperone Hsp70 and the first description of a chaperonin in the archaea were reported in 1991. These findings boosted research in other areas beyond the archaea that were directly relevant to bacteria and eukaryotes, for example, stress gene regulation, the structure-function relationship of the chaperonin complex, protein-based molecular phylogeny of organisms and eukaryotic-cell organelles, molecular biology and biochemistry of life in extreme environments, and stress tolerance at the cellular and molecular levels. In the last 8 years, archaeal stress genes and proteins belonging to the families Hsp70, Hsp60 (chaperonins), Hsp40(DnaJ), and small heat-shock proteins (sHsp) have been studied. The hsp70(dnaK), hsp40(dnaJ), and grpE genes (the chaperone machine) have been sequenced in seven, four, and two species, respectively, but their expression has been examined in detail only in the mesophilic methanogen Methanosarcina mazei S-6. The proteins possess markers typical of bacterial homologs but none of the signatures distinctive of eukaryotes. In contrast, gene expression and transcription initiation signals and factors are of the eucaryal type, which suggests a hybrid archaeal-bacterial complexion for the Hsp70 system. Another remarkable feature is that several archaeal species in different phylogenetic branches do not have the gene hsp70(dnaK), an evolutionary puzzle that raises the important question of what replaces the product of this gene, Hsp70(DnaK), in protein biogenesis and refolding and for stress resistance. Although archaea are prokaryotes like bacteria, their Hsp60 (chaperonin) family is of type (group) II, similar to that of the eukaryotic cytosol; however, unlike the latter, which has several different members, the archaeal chaperonin system usually includes only two (in some species one and in others possibly three) related subunits of approximately 60 kDa. These form, in various combinations depending on the species, a large structure or chaperonin complex sometimes called the thermosome. This multimolecular assembly is similar to the bacterial chaperonin complex GroEL/S, but it is made of only the large, double-ring oligomers each with eight (or nine) subunits instead of seven as in the bacterial complex. Like Hsp70(DnaK), the archaeal chaperonin subunits are remarkable for their evolution, but for a different reason. Ubiquitous among archaea, the chaperonins show a pattern of recurrent gene duplication-hetero-oligomeric chaperonin complexes appear to have evolved several times independently. The stress response and stress tolerance in the archaea involve chaperones, chaperonins, other heat shock (stress) proteins including sHsp, thermoprotectants, the proteasome, as yet incompletely understood thermoresistant features of many molecules, and formation of multicellular structures. The latter structures include single- and mixed-species (bacterial-archaeal) types. Many questions remain unanswered, and the field offers extraordinary opportunities owing to the diversity, genetic makeup, and phylogenetic position of archaea and the variety of ecosystems they inhabit. Specific aspects that deserve investigation are elucidation of the mechanism of action of the chaperonin complex at different temperatures, identification of the partners and substitutes for the Hsp70 chaperone machine, analysis of protein folding and refolding in hyperthermophiles, and determination of the molecular mechanisms involved in stress gene regulation in archaeal species that thrive under widely different conditions (temperature, pH, osmolarity, and barometric pressure). These studies are now possible with uni- and multicellular archaeal models and are relevant to various areas of basic and applied research, including exploration and conquest of ecosystems inhospitable to humans and many mammals and plants.