A member of the Hsp70 family is localized in mitochondria and resembles Escherichia coli DnaK

Mitochondrial HSP70 Chaperone System-The Influence of Post-Translational Modifications and Involvement in Human Diseases

Since their discovery, heat shock proteins (HSPs) have been identified in all domains of life, which demonstrates their importance and conserved functional role in maintaining protein homeostasis. Mitochondria possess several members of the major HSP sub-families that perform essential tasks for keeping the organelle in a fully functional and healthy state. In humans, the mitochondrial HSP70 chaperone system comprises a central molecular chaperone, mtHSP70 or mortalin (HSPA9), which is actively involved in stabilizing and importing nuclear gene products and in refolding mitochondrial precursor proteins, and three co-chaperones (HSP70-escort protein 1-HEP1, tumorous imaginal disc protein 1-TID-1, and Gro-P like protein E-GRPE), which regulate and accelerate its protein folding functions. In this review, we summarize the roles of mitochondrial molecular chaperones with particular focus on the human mtHsp70 and its co-chaperones, whose deregulated expression, mutations, and post-translational modifications are often considered to be the main cause of neurological disorders, genetic diseases, and malignant growth.

Folding the Mitochondrial UPR into the Integrated Stress Response

Eukaryotic cells must accurately monitor the integrity of the mitochondrial network to overcome environmental insults and respond to physiological cues. The mitochondrial unfolded protein response (UPRmt) is a mitochondrial-to-nuclear signaling pathway that maintains mitochondrial proteostasis, mediates signaling between tissues, and regulates organismal aging. Aberrant UPRmt signaling is associated with a wide spectrum of disorders, including congenital diseases as well as cancers and neurodegenerative diseases. Here, we review recent research into the mechanisms underlying UPRmt signaling in Caenorhabditis elegans and discuss emerging connections between the UPRmt signaling and a translational regulation program called the 'integrated stress response'. Further study of the UPRmt will potentially enable development of new therapeutic strategies for inherited metabolic disorders and diseases of aging.

An essential thioredoxin-type protein of Trypanosoma brucei acts as redox-regulated mitochondrial chaperone

Most known thioredoxin-type proteins (Trx) participate in redox pathways, using two highly conserved cysteine residues to catalyze thiol-disulfide exchange reactions. Here we demonstrate that the so far unexplored Trx2 from African trypanosomes (Trypanosoma brucei) lacks protein disulfide reductase activity but functions as an effective temperature-activated and redox-regulated chaperone. Immunofluorescence microscopy and fractionated cell lysis revealed that Trx2 is located in the mitochondrion of the parasite. RNA-interference and gene knock-out approaches showed that depletion of Trx2 impairs growth of both mammalian bloodstream and insect stage procyclic parasites. Procyclic cells lacking Trx2 stop proliferation under standard culture conditions at 27°C and are unable to survive prolonged exposure to 37°C, indicating that Trx2 plays a vital role that becomes augmented under heat stress. Moreover, we found that Trx2 contributes to the in vivo infectivity of T. brucei. Remarkably, a Trx2 version, in which all five cysteines were replaced by serine residues, complements for the wildtype protein in conditional knock-out cells and confers parasite infectivity in the mouse model. Characterization of the recombinant protein revealed that Trx2 can coordinate an iron sulfur cluster and is highly sensitive towards spontaneous oxidation. Moreover, we discovered that both wildtype and mutant Trx2 protect other proteins against thermal aggregation and preserve their ability to refold upon return to non-stress conditions. Activation of the chaperone function of Trx2 appears to be triggered by temperature-mediated structural changes and inhibited by oxidative disulfide bond formation. Our studies indicate that Trx2 acts as a novel chaperone in the unique single mitochondrion of T. brucei and reveal a new perspective regarding the physiological function of thioredoxin-type proteins in trypanosomes.

Mortalin (GRP75/HSPA9) Promotes Survival and Proliferation of Thyroid Carcinoma Cells

We previously reported that upregulation of mortalin (HSPA9/GRP75), the mitochondrial HSP70 chaperone, facilitates tumor cell proliferation and survival in human medullary thyroid carcinoma (MTC), proposing mortalin as a novel therapeutic target for MTC. In this report, we show that mortalin is also upregulated in other thyroid tumor types, including papillary thyroid carcinoma (PTC), follicular thyroid carcinoma (FTC), and anaplastic thyroid carcinoma (ATC), and that mortalin depletion can effectively induce growth arrest and cell death in human PTC (TPC-1), FTC (FTC133), and ATC (8505C and C643) cells in culture. Intriguingly, mortalin depletion induced varied effects on cell cycle arrest (G0/G1 phase arrest in TPC-1 and C643, G2/M phase arrest in 8505C, and mild G2/M phase arrest with increased sub-G0/G1 population in FTC133) and on the levels of TP53, E2F-1, p21^CIP1, p27^KIP1, and poly (ADP-ribose) polymerase cleavage in these cells, suggesting that thyroid tumor cells respond to mortalin depletion in a cell type-specific manner. In these cells, we also determined the efficacy of triphenyl-phosphonium-carboxy-proxyl (Mito-CP) because this mitochondria-targeted metabolism interfering agent exhibited similar tumor suppressive effects as mortalin depletion in MTC cells. Indeed, Mito-CP also induced robust caspase-dependent apoptosis in PTC and ATC cell lines in vitro, exhibiting IC₅₀ lower than PLX4032 in 8505C cells and IC₅₀ lower than vandetanib and cabozantinib in TPC-1 cells. Intriguingly, Mito-CP-induced cell death was partially rescued by mortalin overexpression, suggesting that Mito-CP may inactivate a mechanism that requires mortalin function. These findings support the significance of mortalin and mitochondrial activity in a broad spectrum of thyroid cancer.

Genome-Wide Characterization of Heat-Shock Protein 70s from Chenopodium quinoa and Expression Analyses of Cqhsp70s in Response to Drought Stress

Heat-shock proteins (HSPs) are ubiquitous proteins with important roles in response to biotic and abiotic stress. The 70-kDa heat-shock genes (Hsp70s) encode a group of conserved chaperone proteins that play central roles in cellular networks of molecular chaperones and folding catalysts across all the studied organisms including bacteria, plants and animals. Several Hsp70s involved in drought tolerance have been well characterized in various plants, whereas no research on Chenopodium quinoa HSPs has been completed. Here, we analyzed the genome of C. quinoa and identified sixteen Hsp70 members in quinoa genome. Phylogenetic analysis revealed the independent origination of those Hsp70 members, with eight paralogous pairs comprising the Hsp70 family in quinoa. While the gene structure and motif analysis showed high conservation of those paralogous pairs, the synteny analysis of those paralogous pairs provided evidence for expansion coming from the polyploidy event. With several subcellular localization signals detected in CqHSP70 protein paralogous pairs, some of the paralogous proteins lost the localization information, indicating the diversity of both subcellular localizations and potential functionalities of those HSP70s. Further gene expression analyses revealed by quantitative polymerase chain reaction (qPCR) analysis illustrated the significant variations of Cqhsp70s in response to drought stress. In conclusion, the sixteen Cqhsp70s undergo lineage-specific expansions and might play important and varied roles in response to drought stress.

Promoting Neuronal Tolerance of Diabetic Stress: Modulating Molecular Chaperones

The etiology of diabetic peripheral neuropathy (DPN) involves an interrelated series of metabolic and vascular insults that ultimately contribute to sensory neuron degeneration. In the quest to pharmacologically manage DPN, small-molecule inhibitors have targeted proteins and pathways regarded as "diabetes specific" as well as others whose activity are altered in numerous disease states. These efforts have not yielded any significant therapies, due in part to the complicating issue that the biochemical contribution of these targets/pathways to the progression of DPN does not occur with temporal and/or biochemical uniformity between individuals. In a complex, chronic neurodegenerative disease such as DPN, it is increasingly appreciated that effective disease management may not necessarily require targeting a pathway or protein considered to contribute to disease progression. Alternatively, it may prove sufficiently beneficial to pharmacologically enhance the activity of endogenous cytoprotective pathways to aid neuronal tolerance to and recovery from glucotoxic stress. In pursuing this paradigm shift, we have shown that modulating the activity and expression of molecular chaperones such as heat shock protein 70 (Hsp70) may provide translational potential for the effective medical management of insensate DPN. Considerable evidence supports that modulating Hsp70 has beneficial effects in improving inflammation, oxidative stress, and glucose sensitivity. Given the emerging potential of modulating Hsp70 to manage DPN, the current review discusses efforts to characterize the cytoprotective effects of this protein and the benefits and limitations that may arise in drug development efforts that exploit its cytoprotective activity.

Mitochondrial peroxiredoxin functions as crucial chaperone reservoir in Leishmania infantum

Cytosolic eukaryotic 2-Cys-peroxiredoxins have been widely reported to act as dual-function proteins, either detoxifying reactive oxygen species or acting as chaperones to prevent protein aggregation. Several stimuli, including peroxide-mediated sulfinic acid formation at the active site cysteine, have been proposed to trigger the chaperone activity. However, the mechanism underlying this activation and the extent to which the chaperone function is crucial under physiological conditions in vivo remained unknown. Here we demonstrate that in the vector-borne protozoan parasite Leishmania infantum, mitochondrial peroxiredoxin (Prx) exerts intrinsic ATP-independent chaperone activity, protecting a wide variety of different proteins against heat stress-mediated unfolding in vitro and in vivo. Activation of the chaperone function appears to be induced by temperature-mediated restructuring of the reduced decamers, promoting binding of unfolding client proteins in the center of Prx's ringlike structure. Client proteins are maintained in a folding-competent conformation until restoration of nonstress conditions, upon which they are released and transferred to ATP-dependent chaperones for refolding. Interference with client binding impairs parasite infectivity, providing compelling evidence for the in vivo importance of Prx's chaperone function. Our results suggest that reduced Prx provides a mitochondrial chaperone reservoir, which allows L. infantum to deal successfully with protein unfolding conditions during the transition from insect to the mammalian hosts and to generate viable parasites capable of perpetuating infection.

Mortalin (GRP75/HSPA9) upregulation promotes survival and proliferation of medullary thyroid carcinoma cells

Medullary thyroid carcinoma (MTC) is a neuroendocrine tumor mainly caused by mutations in the rearranged during transfection (RET) proto-oncogene. For therapy of advanced MTC, the Food and Drug Administration recently approved vandetanib and cabozantinib, the tyrosine kinase inhibitors targeting RET, vascular endothelial growth factor receptor, epidermal growth factor receptor and/or c-MET. Nevertheless, not all patients respond to these drugs, demanding additional therapeutic strategies. We found that mortalin (HSPA9/GRP75), a member of HSP70 family, is upregulated in human MTC tissues and that its depletion robustly induces cell death and growth arrest in MTC cell lines in culture and in mouse xenografts. These effects were accompanied by substantial downregulation of RET, induction of the tumor-suppressor TP53 and altered expression of cell cycle regulatory machinery and apoptosis markers, including E2F-1, p21(CIP1), p27(KIP1) and Bcl-2 family proteins. Our investigation of the molecular mechanisms underlying these effects revealed that mortalin depletion induces transient MEK/ERK (extracellular signal-regulated kinase) activation and altered mitochondrial bioenergetics in MTC cells, as indicated by depolarized mitochondrial membrane, decreased oxygen consumption and extracellular acidification and increased oxidative stress. Intriguingly, mortalin depletion induced growth arrest partly via the MEK/ERK pathway, whereas it induced cell death by causing mitochondrial dysfunction in a Bcl-2-dependent manner. However, TP53 was not necessary for these effects except for p21(CIP1) induction. Moreover, mortalin depletion downregulated RET expression independently of MEK/ERK and TP53. These data demonstrate that mortalin is a key regulator of multiple signaling and metabolic pathways pivotal to MTC cell survival and proliferation, proposing mortalin as a novel therapeutic target for MTC.

Molecular Chaperones HscA/Ssq1 and HscB/Jac1 and Their Roles in Iron-Sulfur Protein Maturation

Genetic and biochemical studies have led to the identification of several cellular pathways for the biosynthesis of iron-sulfur proteins in different organisms. The most broadly distributed and highly conserved system involves an Hsp70 chaperone and J-protein co-chaperone system that interacts with a scaffold-like protein involved in [FeS]-cluster preassembly. Specialized forms of Hsp70 and their co-chaperones have evolved in bacteria (HscA, HscB) and in certain fungi (Ssq1, Jac1), whereas most eukaryotes employ a multifunctional mitochondrial Hsp70 (mtHsp70) together with a specialized co-chaperone homologous to HscB/Jac1. HscA and Ssq1 have been shown to specifically bind to a conserved sequence present in the [FeS]-scaffold protein designated IscU in bacteria and Isu in fungi, and the crystal structure of a complex of a peptide containing the IscU recognition region bound to the HscA substrate binding domain has been determined. The interaction of IscU/Isu with HscA/Ssq1 is regulated by HscB/Jac1 which bind the scaffold protein to assist delivery to the chaperone and stabilize the chaperone-scaffold complex by enhancing chaperone ATPase activity. The crystal structure of HscB reveals that the N-terminal J-domain involved in regulation of HscA ATPase activity is similar to other J-proteins, whereas the C-terminal domain is unique and appears to mediate specific interactions with IscU. At the present time the exact function(s) of chaperone-[FeS]-scaffold interactions in iron-sulfur protein biosynthesis remain(s) to be established. In vivo and in vitro studies of yeast Ssq1 and Jac1 indicate that the chaperones are not required for [FeS]-cluster assembly on Isu. Recent in vitro studies using bacterial HscA, HscB and IscU have shown that the chaperones destabilize the IscU[FeS] complex and facilitate cluster delivery to an acceptor apo-protein consistent with a role in regulating cluster release and transfer. Additional genetic and biochemical studies are needed to extend these findings to mtHsp70 activities in higher eukaryotes.

Comparison of Intra-organellar Chaperone Capacity for Dealing with Stress-induced Protein Unfolding

Molecular chaperones are essential for cells to prevent that partially unfolded proteins form non-functional, toxic aggregates. This requirement is increased when cells experience protein unfolding stresses and such could affect all compartments in the eukaryotic cell. Whether all organelles are equipped with comparable chaperone capacities is largely unknown, mainly due to the lack of suitable reporters that allow such a comparison. Here we describe the development of fluorescent luciferase reporters that are sorted to various cellular locations (nucleus, cytoplasm, endoplasmic reticulum, and peroxisomes) and that differ minimally in their intrinsic thermal stability properties. When heating living cells, the rate of inactivation was most rapid for the nuclear-targeted luciferase, indicating that the nucleus is the most sensitive organelle toward heat-induced denaturing stress. Post-heat re-activation, however, occurred at equal kinetics irrespective of luciferase localization. Also, induction of thermotolerance by a priming heat treatment, that coordinately up-regulates all heat-inducible chaperones, resulted in a transient heat resistance of the luciferase in all organelles in a comparable manner. Overexpression of the main heat-inducible Hsp70 family member, HspA1A, protected only the cytosolic and nuclear, but not the other luciferases. Together, our data suggest that in each compartment investigated, including the peroxisome in which so far no chaperones could be detected, chaperone machines are present and can be induced with activities similar to those present in the cytosolic/nuclear compartment.

Histidine 89 is an essential residue for Hsp70 in the phosphate transfer reaction

Autophosphorylation of Hsp70 is detected in the process of substrate refolding in the presence of adenosine triphosphate (ATP) in the reaction mixture. But to date, the role and mechanism of Hsp70 autophosphorylation have not been elucidated. In this study we determined the site of histidine phosphorylation of Hsp70 as an intermediate in the process of phosphate transfer reaction by site-directed mutagenesis. We selected two possible sites (ie, His89 and His227) of intermediate histidine phosphorylation based on our hypothesis of the transfer of gamma-phosphoryl groups and replacement by glycine and serine. Although an acid labile autophosphorylation intermediate of Hsp70 and its cytidine diphosphate-dependent dephosphorylation were detected in wild-type Hsp70, they were markedly suppressed in the H89S mutation of Hsp70, but not on the H227S mutation. The ATPase activity and ATP synthesis activity of Hsp70 were almost completely suppressed in the H89S and H89G mutations. The role of His89 in the phosphate transfer reaction of Hsp70 is discussed.

Two-dimensional gel analysis of the heat-shock response in marine snails(genus Tegula): interspecific variation in protein expression and acclimation ability

The degree to which temperature acclimation modifies the acute synthesis of the entire heat-shock protein (Hsp) complement is still unknown, but it may constitute an important mechanism for understanding the differences in acclimation ability among closely related ectothermic species that occupy widely varying thermal environments. In general, eurythermal (heat-tolerant)species modify physiological function in response to an increase in acclimation temperature to a greater extent than stenothermal (heat-sensitive)species. In the present work I used 35S-labelled amino acids and two-dimensional gel electrophoresis to test this assumption for how acclimation affects acute Hsp expression (referred to as phenotypic plasticity) in two heat-sensitive, low-intertidal to subtidal zone turban snails, Tegula brunnea and T. montereyi, in comparison to a heat-tolerant, mid- to low-intertidal zone congener, T. funebralis. I was able (i) to detect the synthesis of over 30 proteins in gill tissue,primarily in the 70 kDa range, in response to an increase in temperature(13°C, 24°C, 27°C and 30°C), (ii) to assess the effect of acclimation (13°C vs 22°C) on acute Hsp synthesis, and (iii)to compare this effect among the three Tegula congeners. After increasing acclimation temperature from 13°C to 22°C, synthesis of the most highly expressed Hsps decreased more in T. brunnea and T. montereyi than in T. funebralis. Two highly expressed proteins of molecular mass 71 and 74 kDa, however, were also synthesized constitutively at 13°C and changed with increasing acclimation temperature in all three species. Although similar in phenotypic plasticity, T. brunnea and T. montereyi synthesized either a 76 or a 72 kDa cluster of proteins,respectively, and differed in how acclimation affected the acute synthesis of several 77 kDa proteins. Thus, in Tegula, the effect of acclimation on Hsp expression is (i) Hsp-specific, (ii) dependent on a protein's expression pattern (constitutive and inducible vs only inducible),(iii) and is actually limited in the more eurythermal mid- to low-intertidal congener. These results contradict the general assumption that greater heat tolerance correlates with an increased ability to modify physiological function in response to acclimation.

The role of molecular chaperones in mitochondrial protein import and folding

Molecular chaperones play a critical role in many cellular processes. This review concentrates on their role in targeting of proteins to the mitochondria and the subsequent folding of the imported protein. It also reviews the role of molecular chaperons in protein degradation, a process that not only regulates the turnover of proteins but also eliminates proteins that have folded incorrectly or have aggregated as a result of cell stress. Finally, the role of molecular chaperones, in particular to mitochondrial chaperonins, in disease is reviewed. In support of the endosymbiont theory on the origin of mitochondria, the chaperones of the mitochondrial compartment show a high degree of similarity to bacterial molecular chaperones. Thus, studies of protein folding in bacteria such as Escherichia coli have proved to be instructive in understanding the process in the eukaryotic cell. As in bacteria, the molecular chaperone genes of eukaryotes are activated by a variety of stresses. The regulation of stress genes involved in mitochondrial chaperone function is reviewed and major unsolved questions regarding the regulation, function, and involvement in disease of the molecular chaperones are identified.

Molecular comparison of the mitochondrial and cytoplasmic hsp70 of Trypanosoma cruzi, Trypanosoma brucei and Leishmania major

We compared the expression and localization of the mitochondrial and cytoplasmic hsp70 of the protozoans Trypanosoma cruzi, Trypanosoma brucei and Leishmania major. The mitochondrial protein is encoded by multiple mRNA in all species, while the cytoplasmic protein is encoded by a single mRNA. In all three species, the mitochondrial hsp70 is concentrated in the kinetoplast, a submitochondrial structure that houses the unusual DNA (kDNA) that characterizes this group of organisms, while the cytoplasmic protein is distributed throughout the cell. These results suggest that, in all kinetoplastid species, mt-hsp70 has a specific function in kDNA biology, possibly in the processes of kDNA replication, RNA editing or kinetoplast structure.

cDNA cloning and efficient mitochondrial import of pre-mtHSP70 from rat liver

Members of the 70-kD heat shock protein family have been found in all free-living organisms investigated and in major compartments of eukaryotic cells where they are essential to a wide range of functions, including protein folding and targeting. We have isolated a mitochondrial homolog (mtHSP70) from rat liver using ATP agarose affinity chromatography. Its identity was confirmed on the basis of immunological analysis and Ca(2+)-dependent autophosphorylation. Using protein sequence obtained from the amino termius and nine endo Lys-C peptide fragments, we have employed oligonucleotides to isolate a full-length cDNA clone. The open reading frame encodes a protein of 679 amino acids and calculated M(r) 73,913 daltons. The sequence has a high degree of identity with other members of the HSP70 family, including Escherichia coli DnaK (51%), Saccharomyces cerevisiae SSC1p (65%), the constitutive cytosolic HSP70 from rat, HSC70 (46%), and the rat endoplasmic reticulum isoform, BiP, (49%). The cDNA encodes a precursor protein with a 46-amino-acid signal peptide that is absent from the protein isolated from rat liver. The protein also shows a high degree of identity (98%) with a protein isolated from mouse and human tissues (PBP74, Domanico et al., 1993; mortalin, Wadhwa et al., 1993a; CSA, Michikawa et al., 1993a); however, the intracellular localization of these proteins is uncertain. We show that the precursor of mtHSP70 is efficiently imported into isolated mitochondria from rat liver and processed from 74 kD to the mature 69-kD protein.

Genetic aspects of the hsp70 multigene family in vertebrates

The family of genes encoding heat shock proteins of about 70 kDa (hsp70) in vertebrates is reviewed under genetic aspects. After a detailed description of the various hsp70 genes more general characteristics of the organization and evolution of the multigene family are discussed.

Heat shock response in the central nervous system

The heat shock response is induced in nervous tissue in a variety of clinically significant experimental models including ischemic brain injury (stroke), trauma, thermal stress and status epilepticus. Excessive excitatory neurotransmission or the inability to metabolically support normal levels of excitatory neurotransmission may contribute to neuronal death in the nervous system in many of the same pathophysiologic circumstances. We demonstrated that in vitro glutamate-neurotransmitter induced excitotoxicity is attenuated by the prior induction of the heat shock response. A short thermal stress induced a pattern of protein synthesis characteristic of the highly conserved heat shock response and increased the expression of heat shock protein (HSP) mRNA. Protein synthesis was necessary for the neuroprotective effect. The study of the mechanisms of heat shock mediated protection may lead to important clues as to the basic mechanisms underlying the molecular actions of the HSP and the factors important for excitotoxic neuronal injury. The clinical relevance of these findings in vitro is suggested by experiments performed by others in vivo demonstrating that pretreatment of animals with a submaximal thermal or ischemic stress confers protection from a subsequent ischemic insult.

PBP74, a new member of the mammalian 70-kDa heat shock protein family, is a mitochondrial protein

The cloning of a cDNA encoding a new member of the highly conserved mammalian 70-kDa heat shock protein (hsp 70) family termed PBP74 was recently reported. Critical to an understanding of the function of this new hsp 70 is delineating its subcellular localization. Here we use a variety of immunological and biochemical approaches both in vitro and in vivo to demonstrate that PBP74 is imported into and resides in mitochondria. By confocal immunofluorescence microscopy PBP74 is detected in mitochondria, colocalizing with the mitochondrial 60-kDa heat shock protein. To address the inherent problem of serological cross-reactivity among the hsp70 family members, an influenza virus hemagglutinin epitope tag was introduced into the PBP74 cDNA. The epitope-tagged PBP74 protein transiently expressed in L cells localized to mitochondria. Moreover, deletion of the N-terminal 46-amino acid presequence results in a cytosolic localization of the epitope-tagged protein. Cell fractionation studies demonstrated PBP74 in purified mitochondria in a protease-protected location. After coupled transcription-translation the precursor of PBP74 is imported into isolated yeast mitochondria, where it becomes processed to the mature protein. According to a subfractionation of the mitochondria, the imported protein was found to be localized in the matrix space. Import in vitro is time- and temperature-dependent, requires matrix ATP, and is abolished upon depletion of the membrane potential across the mitochondrial inner membrane. Similarly, in mammalian cells PBP74 is synthesized as a pre-protein that requires membrane potential-dependent import into mitochondria for its maturation. Taken together, our data demonstrate that PBP74 is a mammalian mitochondrial hsp70.

Heat-shock proteins as molecular chaperones

Functional proteins within cells are normally present in their native, completely folded form. However, vital processes of protein biogenesis such as protein synthesis and translocation of proteins into intracellular compartments require the protein to exist temporarily in an unfolded or partially folded conformation. As a consequence, regions buried when a polypeptide is in its native conformation become exposed and interact with other proteins causing protein aggregation which is deleterious to the cell. To prevent aggregation as proteins become unfolded, heat-shock proteins protect these interactive surfaces by binding to them and facilitating the folding of unfolded or nascent polypeptides. In other instances the binding of heat-shock proteins to interactive surfaces of completely folded proteins is a crucial part of their regulation. As heat shock and other stress conditions cause cellular proteins to become partially unfolded, the ability of heat-shock proteins to protect cells against the adverse effects of stress becomes a logical extension of their normal function as molecular chaperones.

Heat shock proteins: molecular chaperones of protein biogenesis

Heat shock proteins (Hsps) were first identified as proteins whose synthesis was enhanced by stresses such as an increase in temperature. Recently, several of the major Hsps have been shown to be intimately involved in protein biogenesis through a direct interaction with a wide variety of proteins. As a reflection of this role, these Hsps have been referred to as molecular chaperones. Hsp70s interact with incompletely folded proteins, such as nascent chains on ribosomes and proteins in the process of translocation from the cytosol into mitochondria and the endoplasmic reticulum. Hsp60 also binds to unfolded proteins, preventing aggregation and facilitating protein folding. Although less well defined, other Hsps such as Hsp90 also play important roles in modulating the activity of a number of proteins. The function of the proteolytic system is intertwined with that of molecular chaperones. Several components of this system, encoded by heat-inducible genes, are responsible for the degradation of abnormal or misfolded proteins. The budding yeast Saccharomyces cerevisiae has proven very useful in the analysis of the role of molecular chaperones in protein maturation, translocation, and degradation. In this review, results of experiments are discussed within the context of experiments with other organisms in an attempt to describe the current state of understanding of these ubiquitous and important proteins.

HSP68--a DnaK-like heat-stress protein of plant mitochondria

A 68-kDa heat-stress protein (HSP68) has been purified from cell-suspension cultures of tomato (Lycopersicon peruvianum L.). Antibodies raised against HSP68 cross-react with the Escherichia coli heat-stress protein DnaK. HSP68 was found to be a hydrophilic, ATP-binding protein. Immunological analysis of subcellular fractions and immunogold-labelling of ultrathin sections showed consistently that HSP68 is localized in the mitochondrial matrix. In-vitro translation experiments indicated that HSP68 is synthesized as a precursor protein. Immunoscreening of cDNA libraries from tomato and potato (Solanum tuberosum L.) led to the isolation of corresponding cDNA clones. The deduced amino-acid sequences show strong relationships to the DnaK-like proteins from bacteria and organelles of eukaryotic cells. The protein HSP68 is constitutively expressed, but its synthesis is increased during heat stress in all cells of higher plants investigated so far.

Autophosphorylation of the pea mitochondrial heat-shock protein homolog

Highly purified mitochondria isolated from 14-day-old pea (Pisum sativum L., cv Little Marvel) seedlings contain a homolog of the 70,000 molecular weight heat-shock protein. The amount of this heat-shock cognate (Hsc70) was not reduced by limited proteolysis of intact mitochondria or by preparation of mitoplasts, indicating that the protein is located within the matrix compartment. Pea mitochondrial Hsc70 binds to immobilized ATP and reacts on western blots with anti-tomato Hsc70 antiserum. When a mitochondrial matrix fraction was incubated with [gamma-(32)P]ATP, there was phosphorylation of Hsc70. The extent of phosphorylation was increased by including calcium chloride in the reactions. Phospho amino acid analysis of purified mitochondrial Hsc70, phosphorylated in the calcium-stimulated reaction, revealed only phosphothreonine. Pea mitochondrial Hsc70, purified by a combination of ATP-agarose affinity chromatography and gel permeation chromatography, was labeled when incubated with ATP plus calcium, suggesting autophosphorylation rather than phosphorylation by an associated kinase. In analogy to mammalian cells and yeast, it is likely that mitochondrial Hsc70 acts as a molecular chaperone, and it is possible that phosphorylation plays a role in chaperone function.

Precursor of mitochondrial aspartate aminotransferase synthesized in Escherichia coli is complexed with heat-shock protein DnaK

On expression of the cDNA encoding the precursor of chicken mitochondrial aspartate aminotransferase (pmAspAT) in Escherichia coli, the bulk of pmAspAT was found to be associated with the 70-kDa heat-shock protein DnaK which is closely related to mitochondrial 70-kDa heat-shock protein (HSP70). Purification protocols for the DnaK/pmAspAT complex and its individual components were elaborated. The complex dissociated on treatment with MgATP or at pH 5.5. Like the mature enzyme, pmAspAT is a dimer (2 x 47 kDa) and exhibits about a third of its enzyme activity. In the DnaK/pmAspAT complex, one DnaK molecule is bound to each subunit of pmAspAT; this tetramer may further aggregate to an octamer. The complex is catalytically almost as active as free pmAspAT. It could be reconstituted from isolated DnaK and pmAspAT. No complex was formed with mAspAT. Apparently, DnaK binds to the solvent-exposed presequence of folded pmAspAT without significantly changing the structure and functional properties of its mature moiety.

Homology modelling of integrin EF-hands. Evidence for widespread use of a conserved cation-binding site

Integrin alpha-subunits contain three or four peptide sequences that are similar to the EF-hand, a 13-residue bivalent cation-binding motif found in calmodulin and parvalbumin. The integrin sequences differ from classical EF-hands in that they lack a co-ordinating residue at position 12. One hypothesis to explain integrin-ligand binding is that aspartate-containing recognition sequences in integrin ligands, which bind at or near to the EF-hand-like sequences, may take the place of the missing residue and co-ordinate directly to the bound cation. In this report, homology modelling of integrin EF-hand-like sequences has been performed using the X-ray structure of calmodulin as a template in order to assess the functional activity of the integrin sequences. In the calmodulin-integrin hybrid structures, integrin EF-hand-like sequences were able to retain cations whereas control sequences did not. Structural analyses demonstrated that the integrin sequences in the hybrid proteins closely resembled conventional EF-hands. The integrin sequences are therefore highly likely to bind Ca2+ ions in vivo, a prerequisite for the ligand-binding model. Database searching with a matrix derived from known integrin EF-hand-like sequences has been used to identify other proteins containing the integrin EF-hand-like motif. Annexin V (anchorin CII), atrial natriuretic peptide receptors and the 70 kDa heat-shock protein were identified by the matrix; the functions of these proteins are known from previous studies to be bivalent cation-dependent. These findings suggest that the integrin EF-hand-like sequence may be a more common motif than originally thought.

Isolation and characterization of extragenic suppressors of mutations in the SSA hsp70 genes of Saccharomyces cerevisiae

Saccharomyces cerevisiae strains that contain null alleles of two hsp70 genes, SSA1 and SSA2, are temperature sensitive for growth. In this study, extragenic suppressors of ssa1 ssa2 have been isolated. Suppression is due to mutations at nuclear loci designated EXA1, EXA2 and EXA3 for EXtragenic suppressor hsp70 subfamily A. Two of the four EXA1 alleles are dominant as is EXA3-1. The other two EXA1 alleles as well as the sole EXA2 allele are recessive. EXA1 mutations lead to accumulation of a previously uncharacterized form of hsp70. EXA2 and EXA3 mutations affect the regulation of the stress response. In exa2-1 ssa1 ssa2 strains the gene products of the remaining SSA hsp70 genes, SSA3 and SSA4 (Ssa3/4p), accumulate to higher levels. The EXA3-1 mutation results in increased accumulation of both Ssa3/4p and the hsp70s encoded by the SSB1 and SSB2 genes (Ssb1/2p), suggesting that the EXA3 gene product plays a central role in the yeast stress response. Consistent with this hypothesis, EXA3-1 is tightly linked to HSF1, the gene encoding the transcriptional regulatory protein known as "heat shock factor." All of the genes identified in this study seem to be involved in regulating the expression of SSA3 and SSA4 or the activity of their protein products.

Protein folding in the cell

In the cell, as in vitro, the final conformation of a protein is determined by its amino-acid sequence. But whereas some isolated proteins can be denatured and refolded in vitro in the absence of other macromolecular cellular components, folding and assembly of polypeptides in vivo involves other proteins, many of which belong to families that have been highly conserved during evolution.

DnaK as a thermometer: threonine-199 is site of autophosphorylation and is critical for ATPase activity

DnaK, the sole Escherichia coli member of the highly conserved 70-kDa heat shock protein (HSP70) family of proteins, autophosphorylates when incubated with ATP in vitro. We show that threonine-199 is the amino acid that becomes phosphorylated and we demonstrate that threonine-199 is critical for the ATPase activity of DnaK. We also report that both the ATPase and autophosphorylating activities of DnaK increase very strongly over the range of temperatures that is physiologically relevant for E. coli growth. The temperature dependence of either or both of these activities could be of significance with respect to the postulated role of DnaK as a molecular chaperone in helping cells ameliorate the deleterious consequences of elevated temperature. Furthermore, we postulate that DnaK plays a key role in regulation of the heat shock response by serving as a cellular thermometer that directly senses the environmental temperature.

Characterization of HSP-70 cognate proteins from wheat

Animal and plant cells contain a family of constitutively expressed HSP-70 cognate proteins that are localized in different subcellular locations and are presumed to play a role in protein folding and transport. Utilizing antibodies raised against the yeast endoplasmicreticulum-localized HSP-70 cognate termed BiP/GRP-78, as well as antibodies raised against the Escherichia coli HSP-70 protein DnaK, we have identified and characterized a large family of closely related proteins in wheat. One protein band of 78 kDa that is apparently closely related to yeast BiP was localized in the endoplasmic reticulum. This band cross-reacted with the yeast BiP but not with the DnaK-specific antibodies. The yeast BiP antibodies also recognized a cytoplasmic protein of 70 kDa that is probably related to the HSC-70 cognate proteins. These two proteins were further confirmed as HSP-70 cognates by their ability to bind to an ATP-agarose column. Probing of proteins from purified wheat mitochondrial preparations with the yeast BiP and DnaK-specific antibodies showed that this organelle contained a family of HSP-70-related proteins. The yeast BiP antibodies recognized two mitochondrial proteins of 60 and 58 kDa, but failed to detect any protein in the size rang of 70 to 80 kDa. However, the presence of immunologically distinct proteins of 90 and 78 kDa, as well as of lower molecular weight from this family in the mitochondria, was shown by probing with the DnaK-specific antibodies. A new protein of 30 kDa, cross-reacting with anti-yeast BiP antibodies, was detected only in developing seeds, close to their maturity. The evolution of HSP-70 cognate proteins in wheat as shown in this study is discussed.

Functional analysis of a conserved amino-terminal region of HSP70 by site-directed mutagenesis

Hsp70 proteins have been highly conserved throughout evolution. As a first step in a structure-function analysis of hsp70, we constructed and analysed the consequences of mutations in a portion of the SSA1 gene, a member of the Saccharomyces cerevisiae HSP70 multigene family, that encodes a nearly invariant region near the amino terminus. Analysis of strains expressing SSA1 proteins with alterations at positions 8, 11 and 15 showed that these conserved residues within this region are critical for normal functioning of the protein. SSA1 protein containing either of two changes at position 15 was able to slightly complement the inviability of an ssa1ssa2ssa4 strain, but was inactive in other complementation assays. The other mutant proteins tested were unable to complement any tested phenotype. Effective interallelic complementation of several phenotypes was observed when a mutant protein substituted at position 8 was expressed in the same cell with either of two proteins carrying substitutions at position 15, suggesting that hsp70 acts as a multimer. Evidence from previous studies suggests that hsp70 proteins engage in ATP-driven cycles of binding and release from peptides. The ability of the mutant proteins to bind ATP and a peptide was tested. The Ssa1p carrying a substitution at position 8, which inhibits growth of cells carrying wild-type SSA proteins, showed a defect in release from a peptide relative to wild type. Two mutations, one each at position 8 and 15, resulted in accumulation of phosphorylated isoforms which may be a normal, transient hsp70 intermediate.

GroEL-related molecular chaperones are present in the cytosol of oat cells

In eukaryotic cells GroEL-related molecular chaperones (cpn 60) are considered to be restricted to plastids and mitochondria. Re-evaluation of the intracellular localization of chaperonins by electron microscopy, using two different anti-chaperonin antisera, revealed additionally their presence in the cytosol of oat primary leaf and coleoptile cells. The distribution of cpn 60 is not influenced by heat or light treatments.

The two mammalian mitochondrial stress proteins, grp 75 and hsp 58, transiently interact with newly synthesized mitochondrial proteins

In mammalian cells, two of the so-called heat shock (hsp) or stress proteins are components of the mitochondria. One of these, hsp 58, is a member of the bacterial GroEL family, whereas the other, glucose-regulated protein (grp) 75, represents a member of the hsp 70 family of stress proteins. Owing to previous studies implicating a role for both the hsp 70 and GroEL families in facilitating protein maturation events, we used the method of native immunoprecipitation to examine whether hsp 58 and grp 75 might interact with other proteins of the mitochondria. In cells pulse-labeled with [35S]-methionine, a significant number of newly synthesized mitochondrial proteins co-precipitated with either hsp 58 or grp 75. Such interactions appeared transient. For example, providing the pulse-labeled cells a subsequent chase period in the absence of radiolabel resulted in a reduction of co-precipitating proteins. If the pulse-chase labeling experiments were performed in the presence of an amino acid analogue, somewhat different results were obtained. Specifically, although many of the newly synthesized and analogue-containing proteins again were observed to co-precipitate with grp 75, the interactions did not appear transient, but instead were stable. Under steady-state labeling conditions, we also observed a portion of hsp 58 and grp 75 in an apparent complex with one another. On addition of ATP, the complex was dissociated. Accompanying this dissociation was the concomitant autophosphorylation of grp 75. On the basis of these observations, as well as previous studies examining the structure/function of the hsp 70 and GroEL proteins, we suspect that both hsp 58 and grp 75 interact with and facilitate the folding and assembly of proteins as they enter into the mitochondria.

Mitochondrial protein import

Most polypeptides of mitochondria are imported from the cytosol. Precursor proteins contain targeting and sorting information, often in the form of amino-terminal presequences. Precursors first bind to receptors in the outer membrane. Two putative import receptors have been identified: a 19-kilodalton protein (MOM19) in Neurospora mitochondria, and a 70-kilodalton protein (MAS70) in yeast. Some precursors integrate directly into the outer membrane, but the majority are translocated through one or both membranes. This process requires an electrochemical potential across the inner membrane. Import appears to occur through a hydrophilic pore, although the inner and outer membranes may contain functionally separate translocation machineries. In yeast, a 42-kilodalton protein (ISP42) probably forms part of the outer membrane channel. After import, precursors interact with "chaperonin" ATPases in the matrix. Presequences then are removed by the matrix protease. Finally, some proteins are retranslocated across the inner membrane to the intermembrane space.

Requirement for hsp70 in the mitochondrial matrix for translocation and folding of precursor proteins

By analysis of a temperature-sensitive yeast mutant, a heat-shock protein in the matrix of mitochondria, mitochondrial hsp70 (Ssc1p), is found to be involved both in translocation of nuclear-encoded precursor proteins across the mitochondrial membranes and in (re)folding of imported proteins in the matrix.

Identification of a groES-like chaperonin in mitochondria that facilitates protein folding

Mitochondria contain a polypeptide that is functionally equivalent to Escherichia coli chaperonin 10 (cpn10; also known as groES). This mitochondrial cpn10 has been identified in beef and rat liver and is able to replace bacterial cpn10 in the chaperonin-dependent reconstitution of chemically denatured ribulose-1,5-bisphosphate carboxylase. Thus, like the bacterial homologue, mitochondrial cpn10 facilitates a K(+)- and Mg.ATP-dependent discharge of unfolded (or partially folded) ribulose bisphosphate carboxylase from bacterial chaperonin 60 (cpn60; also known as groEL). Instrumental to its identification, mitochondrial cpn10 and bacterial cpn60 form a stable complex in the presence of Mg.ATP. Bacterial and mitochondrial cpn10 compete for a common saturable site on bacterial cpn60. As a result of complex formation, with either mitochondrial or bacterial cpn10, the "uncoupled ATPase" activity of bacterial cpn60 is virtually abolished. The most likely candidate for mitochondrial cpn10 is an approximately 45-kDa oligomer composed of approximately 9-kDa subunits. We propose that, like the protein-folding machinery of prokaryotes, mitochondrial cpn60 requires a cochaperonin for full biological function.

A review of the role of 70 kDa heat shock proteins in protein translocation across membranes

The compartmentalization of essential hsp70 proteins indicates that hsp70s carry out crucial functions in several compartments of the cell. The use of conditional mutants has allowed study of the cellular processes that require hsp70 function. For efficient translocation of proteins across membranes hsp70s are required in the cytoplasm, as well as in the matrix of mitochondria and in the lumen of the endoplasmic reticulum.

Three-dimensional structure of the ATPase fragment of a 70K heat-shock cognate protein

The three-dimensional structure of the amino-terminal 44K ATPase fragment of the 70K bovine heat-shock cognate protein has been solved to a resolution of 2.2 A. The ATPase fragment has two structural lobes with a deep cleft between them; ATP binds at the base of the cleft. Surprisingly, the nucleotide-binding 'core' of the ATPase fragment has a tertiary structure similar to that of hexokinase, although the remainder of the structures of the two proteins are completely dissimilar, suggesting that both the phosphotransferase mechanism and the substrate-induced conformational change intrinsic to the hexokinases may be used by the 70K heat shock-related proteins.