Structural basis of functional cooperation of Tim15/Zim17 with yeast mitochondrial Hsp70

C2H2 proteins: Evolutionary aspects of domain architecture and diversification

The largest group of transcription factors in higher eukaryotes are C2H2 proteins, which contain C2H2-type zinc finger domains that specifically bind to DNA. Few well-studied C2H2 proteins, however, demonstrate their key role in the control of gene expression and chromosome architecture. Here we review the features of the domain architecture of C2H2 proteins and the likely origin of C2H2 zinc fingers. A comprehensive investigation of proteomes for the presence of proteins with multiple clustered C2H2 domains has revealed a key difference between groups of organisms. Unlike plants, transcription factors in metazoans contain clusters of C2H2 domains typically separated by a linker with the TGEKP consensus sequence. The average size of C2H2 clusters varies substantially, even between genomes of higher metazoans, and with a tendency to increase in combination with SCAN, and especially KRAB domains, reflecting the increasing complexity of gene regulatory networks.

The zinc proteome of SARS-CoV-2

Zinc is an essential element for human health. Among its many functions, zinc(II) modulates the immune response to infections and, at high concentrations or in the presence of ionophores, inhibits the replication of various RNA viruses. Structural biology studies on severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) revealed that zinc(II) is the most common metal ion that binds to viral proteins. However, the number of zinc(II)-binding sites identified by experimental methods is far from exhaustive, as metal ions may be lost during protein purification protocols. To better define the zinc(II)-binding proteome of coronavirus, we leveraged the wealth of deposited structural data and state-of-the-art bioinformatics methods. Through this in silico approach, 15 experimental zinc(II) sites were identified and a further 22 were predicted in Spike, open reading frame (ORF)3a/d, ORF8, and several nonstructural proteins, highlighting an essential role of zinc(II) in viral replication. Furthermore, the structural relationships between viral and eukaryotic sites (typically zinc fingers) indicate that SARS-CoV-2 can compete with human proteins for zinc(II) binding. Given the double-edged effect of zinc(II) ions, both essential and toxic to coronavirus, only the complete elucidation of the structural and regulatory zinc(II)-binding sites can guide selective antiviral strategies based on zinc supplementation.

How to get to the other side of the mitochondrial inner membrane – the protein import motor

Biogenesis of mitochondria relies on import of more than 1000 different proteins from the cytosol. Approximately 70% of these proteins follow the presequence pathway – they are synthesized with cleavable N-terminal extensions called presequences and reach the final place of their function within the organelle with the help of the TOM and TIM23 complexes in the outer and inner membranes, respectively. The translocation of proteins along the presequence pathway is powered by the import motor of the TIM23 complex. The import motor of the TIM23 complex is localized at the matrix face of the inner membrane and is likely the most complicated Hsp70-based system identified to date. How it converts the energy of ATP hydrolysis into unidirectional translocation of proteins into mitochondria remains one of the biggest mysteries of this translocation pathway. Here, the knowns and the unknowns of the mitochondrial protein import motor are discussed.

Plant mitochondrial protein import: the ins and outs

The majority of the mitochondrial proteome, required to fulfil its diverse range of functions, is cytosolically synthesised and translocated via specialised machinery. The dedicated translocases, receptors, and associated proteins have been characterised in great detail in yeast over the last several decades, yet many of the mechanisms that regulate these processes in higher eukaryotes are still unknown. In this review, we highlight the current knowledge of mitochondrial protein import in plants. Despite the fact that the mechanisms of mitochondrial protein import have remained conserved across species, many unique features have arisen in plants to encompass the developmental, tissue-specific, and stress-responsive regulation in planta. An understanding of unique features and mechanisms in plants provides us with a unique insight into the regulation of mitochondrial biogenesis in higher eukaryotes.

A folding nucleus and minimal ATP binding domain of Hsp70 identified by single-molecule force spectroscopy

The folding pathways of large proteins are complex, with many of them requiring the aid of chaperones and others folding spontaneously. Along the folding pathways, partially folded intermediates are frequently populated; their role in the driving of the folding process is unclear. The structures of these intermediates are generally not amenable to high-resolution structural techniques because of their transient nature. Here we employed single-molecule force measurements to scrutinize the hierarchy of intermediate structures along the folding pathway of the nucleotide binding domain (NBD) of Escherichia coli Hsp70 DnaK. DnaK-NBD is a member of the sugar kinase superfamily that includes Hsp70s and the cytoskeletal protein actin. Using optical tweezers, a stable nucleotide-binding competent en route folding intermediate comprising lobe II residues (183-383) was identified as a critical checkpoint for productive folding. We obtained a structural snapshot of this folding intermediate that shows native-like conformation. To assess the fundamental role of folded lobe II for efficient folding, we turned our attention to yeast mitochondrial NBD, which does not fold without a dedicated chaperone. After replacing the yeast lobe II residues with stable E. coli lobe II, the obtained chimeric protein showed native-like ATPase activity and robust folding into the native state, even in the absence of chaperone. In summary, lobe II is a stable nucleotide-binding competent folding nucleus that is the key to time-efficient folding and possibly resembles a common ancestor domain. Our findings provide a conceptual framework for the folding pathways of other members of this protein superfamily.

Plasmodium falciparum Hep1 Is Required to Prevent the Self Aggregation of PfHsp70-3

The majority of mitochondrial proteins are encoded in the nucleus and need to be imported from the cytosol into the mitochondria, and molecular chaperones play a key role in the efficient translocation and proper folding of these proteins in the matrix. One such molecular chaperone is the eukaryotic mitochondrial heat shock protein 70 (Hsp70); however, it is prone to self-aggregation and requires the presence of an essential zinc-finger protein, Hsp70-escort protein 1 (Hep1), to maintain its structure and function. PfHsp70-3, the only Hsp70 predicted to localize in the mitochondria of P. falciparum, may also rely on a Hep1 orthologue to prevent self-aggregation. In this study, we identified a putative Hep1 orthologue in P. falciparum and co-expression of PfHsp70-3 and PfHep1 enhanced the solubility of PfHsp70-3. PfHep1 suppressed the thermally induced aggregation of PfHsp70-3 but not the aggregation of malate dehydrogenase or citrate synthase, thus showing specificity for PfHsp70-3. Zinc ions were indeed essential for maintaining the function of PfHep1, as EDTA chelation abrogated its abilities to suppress the aggregation of PfHsp70-3. Soluble and functional PfHsp70-3, acquired by co-expression with PfHep-1, will facilitate the biochemical characterisation of this particular Hsp70 protein and its evaluation as a drug target for the treatment of malaria.

A review of multi-domain and flexible molecular chaperones studies by small-angle X-ray scattering

Intrinsic flexibility is closely related to protein function, and a plethora of important regulatory proteins have been found to be flexible, multi-domain or even intrinsically disordered. On the one hand, understanding such systems depends on how these proteins behave in solution. On the other, small-angle X-ray scattering (SAXS) is a technique that fulfills the requirements to study protein structure and dynamics relatively quickly with few experimental limitations. Molecular chaperones from Hsp70 and Hsp90 families are multi-domain proteins containing flexible and/or disordered regions that play central roles in cellular proteostasis. Here, we review the structure and function of these proteins by SAXS. Our general approach includes the use of SAXS data to determine size and shape parameters, as well as protein shape reconstruction and their validation by using accessory biophysical tools. Some remarkable examples are presented that exemplify the potential of the SAXS technique. Protein structure can be determined in solution even at limiting protein concentrations (for example, human mortalin, a mitochondrial Hsp70 chaperone). The protein organization, flexibility and function (for example, the J-protein co-chaperones), oligomeric status, domain organization, and flexibility (for the Hsp90 chaperone and the Hip and Hep1 co-chaperones) may also be determined. Lastly, the shape, structural conservation, and protein dynamics (for the Hsp90 chaperone and both p23 and Aha1 co-chaperones) may be studied by SAXS. We believe this review will enhance the application of the SAXS technique to the study of the molecular chaperones.

Structural and functional studies of Hsp70-escort protein--Hep1--of Leishmania braziliensis

Hep1 is a mitochondrial Hsp70 (mtHsp70) co-chaperone that presents a zinc finger domain essential for its function. This co-chaperone acts to maintain mtHsp70 in its soluble and functional state. In this work, we have demonstrated that Leishmania braziliensis mtHsp70 (LbmtHsp70) is also dependent on the assistance of Hep1. To understand the L. braziliensis Hep1 (LbHep1) structure-function relationship, we produced LbHep1 and two truncated mutants corresponding to the C-terminal zinc finger domain and the N-terminal region. We observed that LbHep1 is composed of an unfolded N-terminal region and a β-sheet-folded C-terminal domain, which holds the zinc-binding motif. Both LbHep1 and the zinc finger domain construction maintained LbmtHsp70 solubility in co-expression systems after cell lysis. In solution, LbHep1 behaved as a highly elongated monomer, probably due to the unfolded N-terminal region. Furthermore, we also observed that the zinc ion interacted with LbHep1 with high affinity and was critical for LbHep1 structure and stability because its removal from LbHep1 solutions altered the protein structure and stability. In vitro, LbHep1 protected, in sub-stoichiometric fashion, LbmtHsp70 from thermally induced aggregation but did not present intrinsic chaperone activity on model client proteins. Therefore, LbHep1 is a specific chaperone for LbmtHsp70.

Role of membrane contact sites in protein import into mitochondria

Mitochondria import more than 1,000 different proteins from the cytosol. The proteins are synthesized as precursors on cytosolic ribosomes and are translocated by protein transport machineries of the mitochondrial membranes. Five main pathways for protein import into mitochondria have been identified. Most pathways use the translocase of the outer mitochondrial membrane (TOM) as the entry gate into mitochondria. Depending on specific signals contained in the precursors, the proteins are subsequently transferred to different intramitochondrial translocases. In this article, we discuss the connection between protein import and mitochondrial membrane architecture. Mitochondria possess two membranes. It is a long-standing question how contact sites between outer and inner membranes are formed and which role the contact sites play in the translocation of precursor proteins. A major translocation contact site is formed between the TOM complex and the presequence translocase of the inner membrane (TIM23 complex), promoting transfer of presequence-carrying preproteins to the mitochondrial inner membrane and matrix. Recent findings led to the identification of contact sites that involve the mitochondrial contact site and cristae organizing system (MICOS) of the inner membrane. MICOS plays a dual role. It is crucial for maintaining the inner membrane cristae architecture and forms contacts sites to the outer membrane that promote translocation of precursor proteins into the intermembrane space and outer membrane of mitochondria. The view is emerging that the mitochondrial protein translocases do not function as independent units, but are embedded in a network of interactions with machineries that control mitochondrial activity and architecture.

ATP-dependent molecular chaperones in plastids--More complex than expected

Plastids are a class of essential plant cell organelles comprising photosynthetic chloroplasts of green tissues, starch-storing amyloplasts of roots and tubers or the colorful pigment-storing chromoplasts of petals and fruits. They express a few genes encoded on their organellar genome, called plastome, but import most of their proteins from the cytosol. The import into plastids, the folding of freshly-translated or imported proteins, the degradation or renaturation of denatured and entangled proteins, and the quality-control of newly folded proteins all require the action of molecular chaperones. Members of all four major families of ATP-dependent molecular chaperones (chaperonin/Cpn60, Hsp70, Hsp90 and Hsp100 families) have been identified in plastids from unicellular algae to higher plants. This review aims not only at giving an overview of the most current insights into the general and conserved functions of these plastid chaperones, but also into their specific plastid functions. Given that chloroplasts harbor an extreme environment that cycles between reduced and oxidized states, that has to deal with reactive oxygen species and is highly reactive to environmental and developmental signals, it can be presumed that plastid chaperones have evolved a plethora of specific functions some of which are just about to be discovered. Here, the most urgent questions that remain unsolved are discussed, and guidance for future research on plastid chaperones is given. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

The evolution and function of co-chaperones in mitochondria

Mitochondrial chaperones mediate and affect critical organellar processes, essential for cellular function. These chaperone systems have both prokaryotic and eukaryotic features. While some of the mitochondrial co-chaperones have clear homologues in prokaryotes, some are unique to eukaryotes and have no homologues in the chaperone machinery of other cellular compartments. The mitochondrial co-chaperones are required for protein import into the organelle and in enforcing the structure of the main chaperones. In addition to novel types of interaction with their senior partners, unexpected and essential interactions between the co-chaperones themselves have recently been described.

The functional interaction of mitochondrial Hsp70s with the escort protein Zim17 is critical for Fe/S biogenesis and substrate interaction at the inner membrane preprotein translocase

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

Structural and stability studies of the human mtHsp70-escort protein 1: an essential mortalin co-chaperone

Mitochondrial Hsp70 is involved in both protein import and folding process, among other essential functions. In mammalian cells, due to its role in the malignant process, it receives the name of mortalin. Despite its importance in protein and mitochondrial homeostasis, mortalin tends to self-aggregate in vitro and in vivo, the later leads to mitochondrial biogenesis failure. Recently, a zinc-finger protein, named Hsp70-escort protein 1 (Hep1, also called Zim17/TIM15/DNLZ), was described as an essential human mitochondrial mortalin co-chaperone which avoids its self-aggregation. Here, we report structural studies of the human Hep1 (hHep1). The results indicate that hHep1 shares some structural similarities with the yeast ortholog despite the low identity and functional differences. We also observed that hHep1 oligomerizes in a concentration-dependent fashion and that the zinc ion, which is essential for hHep1 in vivo function, has an important protein-structure stabilizing effect.

Biogenesis of the mitochondrial Hsp70 chaperone

Hep1 acts as a specialized chaperone to mediate the de novo folding of yeast mitochondrial Hsp70.

Chaperones mediate protein folding and prevent deleterious protein aggregation in the cell. However, little is known about the biogenesis of chaperones themselves. In this study, we report on the biogenesis of the yeast mitochondrial Hsp70 (mtHsp70) chaperone, which is essential for the functionality of mitochondria. We show in vivo and in organello that mtHsp70 rapidly folds after its import into mitochondria, with its ATPase domain and peptide-binding domain (PBD) adopting their structures independently of each other. Importantly, folding of the ATPase domain but not of the PBD was severely affected in the absence of the Hsp70 escort protein, Hep1. We reconstituted the folding of mtHsp70, demonstrating that Hep1 and ATP/ADP were required and sufficient for its de novo folding. Our data show that Hep1 bound to a folding intermediate of mtHsp70. Binding of an adenine nucleotide triggered release of Hep1 and folding of the intermediate into native mtHsp70. Thus, Hep1 acts as a specialized chaperone mediating the de novo folding of an Hsp70 chaperone.

The Tom40 assembly process probed using the attachment of different intramitochondrial sorting signals

The β-barrel protein Tom40 functions as a protein-conducting channel in the mitochondrial outer membrane. By attaching mitochondrial presequences for various mitochondrial destinations to Tom40, it is possible to follow its sorting process. The results provide insight into the mechanism for the precise delivery of β-barrel proteins to the outer membrane.

The TOM40 complex is a protein translocator in the mitochondrial outer membrane and consists of several different subunits. Among them, Tom40 is a central subunit that constitutes a protein-conducting channel by forming a β-barrel structure. To probe the nature of the assembly process of Tom40 in the outer membrane, we attached various mitochondrial presequences to Tom40 that possess sorting information for the intermembrane space (IMS), inner membrane, and matrix and would compete with the inherent Tom40 assembly process. We analyzed the mitochondrial import of those fusion proteins in vitro. Tom40 crossed the outer membrane and/or inner membrane even in the presence of various sorting signals. N-terminal anchorage of the attached presequence to the inner membrane did not prevent Tom40 from associating with the TOB/SAM complex, although it impaired its efficient release from the TOB complex in vitro but not in vivo. The IMS or matrix-targeting presequence attached to Tom40 was effective in substituting for the requirement for small Tim proteins in the IMS for the translocation of Tom40 across the outer membrane. These results provide insight into the mechanism responsible for the precise delivery of β-barrel proteins to the outer mitochondrial membrane.

The DNLZ/HEP zinc-binding subdomain is critical for regulation of the mitochondrial chaperone HSPA9

Human mitochondrial DNLZ/HEP regulates the catalytic activity and solubility of the mitochondrial hsp70 chaperone HSPA9. Here, we investigate the role that the DNLZ zinc-binding and C-terminal subdomains play in regulating HSPA9. We show that truncations lacking portions of the zinc-binding subdomain (ZBS) do not affect the solubility of HSPA9 or its ATPase domain, whereas those containing the ZBS and at least 10 residues following this subdomain enhance chaperone solubility. Binding measurements further show that DNLZ requires its ZBS to form a stable complex with the HSPA9 ATPase domain, and ATP hydrolysis measurements reveal that the ZBS is critical for full stimulation of HSPA9 catalytic activity. We also examined if DNLZ is active in vivo. We found that DNLZ partially complements the growth of Δzim17 Saccharomyces cerevisiae, and we discovered that a Zim17 truncation lacking a majority of the C-terminal subdomain strongly complements growth like full-length Zim17. These findings provide direct evidence that human DNLZ is a functional ortholog of Zim17. In addition, they implicate the pair of antiparallel β-strands that coordinate zinc in Zim17/DNLZ-type proteins as critical for binding and regulating hsp70 chaperones.

A conserved histidine in human DNLZ/HEP is required for stimulation of HSPA9 ATPase activity

The DNL-type zinc-finger protein DNLZ regulates the activity and solubility of the human mitochondrial chaperone HSPA9. To identify DNLZ residues that are critical for chaperone regulation, we carried out an alanine mutagenesis scan of charged residues in a W115I mutant of human DNLZ and assessed the effect of each mutation on interactions with HSPA9. All mutants analyzed promote the solubility of HSPA9 upon expression in Escherichia coli. However, binding studies examining the effect of DNLZ mutants on chaperone tryptophan fluorescence identified three mutations (R81A, H107A, and D111A) that decrease DNLZ binding affinity for nucleotide-free chaperone. In addition, ATPase measurements revealed that DNLZ-R81A and DNLZ-D111A both stimulate the catalytic activity HSPA9, whereas DNLZ-H107A does not elicit an increase in activity even when present at a concentration that is 10-fold higher than the level required for half-maximal stimulation by DNLZ. These findings implicate a conserved histidine as critical for DNLZ regulation of mitochondrial HSPA9 catalytic activity.

Primary sequence that determines the functional overlap between mitochondrial heat shock protein 70 Ssc1 and Ssc3 of Saccharomyces cerevisiae

The evolutionary diversity of the HSP70 gene family at the genetic level has generated complex structural variations leading to altered functional specificity and mode of regulation in different cellular compartments. By utilizing Saccharomyces cerevisiae as a model system for better understanding the global functional cooperativity between Hsp70 paralogs, we have dissected the differences in functional properties at the biochemical level between mitochondrial heat shock protein 70 (mtHsp70) Ssc1 and an uncharacterized Ssc3 paralog. Based on the evolutionary origin of Ssc3 and a high degree of sequence homology with Ssc1, it has been proposed that both have a close functional overlap in the mitochondrial matrix. Surprisingly, our results demonstrate that there is no functional cross-talk between Ssc1 and Ssc3 paralogs. The lack of in vivo functional overlap is due to altered conformation and significant lower stability associated with Ssc3. The substrate-binding domain of Ssc3 showed poor affinity toward mitochondrial client proteins and Tim44 due to the open conformation in ADP-bound state. In addition to that, the nucleotide-binding domain of Ssc3 showed an altered regulation by the Mge1 co-chaperone due to a high degree of conformational plasticity, which strongly promotes aggregation. Besides, Ssc3 possesses a dysfunctional inter-domain interface thus rendering it unable to perform functions similar to generic Hsp70s. Moreover, we have identified the critical amino acid sequence of Ssc1 and Ssc3 that can "make or break" mtHsp70 chaperone function. Together, our analysis provides the first evidence to show that the nucleotide-binding domain of mtHsp70s plays a critical role in determining the functional specificity among paralogs and orthologs across kingdoms.

Understanding the functional interplay between mammalian mitochondrial Hsp70 chaperone machine components

Mitochondria biogenesis requires the import of several precursor proteins that are synthesized in the cytosol. The mitochondrial heat shock protein 70 (mtHsp70) machinery components are highly conserved among eukaryotes, including humans. However, the functional properties of human mtHsp70 machinery components have not been characterized among all eukaryotic families. To study the functional interactions, we have reconstituted the components of the mtHsp70 chaperone machine (Hsp70/J-protein/GrpE/Hep) and systematically analyzed in vitro conditions for biochemical functions. We observed that the sequence-specific interaction of human mtHsp70 toward mitochondrial client proteins differs significantly from its yeast counterpart Ssc1. Interestingly, the helical lid of human mtHsp70 was found dispensable to the binding of P5 peptide as compared with the other Hsp70s. We observed that the two human mitochondrial matrix J-protein splice variants differentially regulate the mtHsp70 chaperone cycle. Strikingly, our results demonstrated that human Hsp70 escort protein (Hep) possesses a unique ability to stimulate the ATPase activity of mtHsp70 as well as to prevent the aggregation of unfolded client proteins similar to J-proteins. We observed that Hep binds with the C terminus of mtHsp70 in a full-length context and this interaction is distinctly different from unfolded client-specific or J-protein binding. In addition, we found that the interaction of Hep at the C terminus of mtHsp70 is regulated by the helical lid region. However, the interaction of Hep at the ATPase domain of the human mtHsp70 is mutually exclusive with J-proteins, thus promoting a similar conformational change that leads to ATPase stimulation. Additionally, we highlight the biochemical defects of the mtHsp70 mutant (G489E) associated with a myelodysplastic syndrome.

ATPase domain and interdomain linker play a key role in aggregation of mitochondrial Hsp70 chaperone Ssc1

The co-chaperone Hep1 is required to prevent the aggregation of mitochondrial Hsp70 proteins. We have analyzed the interaction of Hep1 with mitochondrial Hsp70 (Ssc1) and the determinants in Ssc1 that make it prone to aggregation. The ATPase and peptide binding domain (PBD) of Hsp70 proteins are connected by a linker segment that mediates interdomain communication between the domains. We show here that the minimal Hep1 binding entity of Ssc1 consists of the ATPase domain and the interdomain linker. In the absence of Hep1, the ATPase domain with the interdomain linker had the tendency to aggregate, in contrast to the ATPase domain with the mutated linker segment or without linker, and in contrast to the PBD. The closest homolog of Ssc1, bacterial DnaK, and a Ssc1 chimera, in which a segment of the ATPase domain of Ssc1 was replaced by the corresponding segment from DnaK, did not aggregate in Delta hep1 mitochondria. The propensity to aggregate appears to be a specific property of the mitochondrial Hsp70 proteins. The ATPase domain in combination with the interdomain linker is crucial for aggregation of Ssc1. In conclusion, our results suggest that interdomain communication makes Ssc1 prone to aggregation. Hep1 counteracts aggregation by binding to this aggregation-prone conformer.

Multiple pathways for mitochondrial protein traffic

Mitochondria are two-membrane bounded organelles consisting of 1000–2000 different proteins, most of which are synthesized in the cytosol and subsequently imported into mitochondria. The imported proteins are further sorted to one of the four compartments, the outer membrane, intermembrane space, inner membrane, and matrix, mostly following one of the five major pathways. Mitochondrial protein import and sorting are mediated by the translocator complexes in the membranes and chaperones in the aqueous compartments operating along the import pathways. Here, we summarize the expanding knowledge on the roles of translocators, chaperones, and related components in the multiple pathways for mitochondrial protein trafficking.

The human escort protein Hep binds to the ATPase domain of mitochondrial hsp70 and regulates ATP hydrolysis

Hsp70 escort proteins (Hep) have been implicated as essential for maintaining the function of yeast mitochondrial hsp70 molecular chaperones (mtHsp70), but the role that escort proteins play in regulating mammalian chaperone folding and function has not been established. We present evidence that human mtHsp70 exhibits limited solubility due to aggregation mediated by its ATPase domain and show that human Hep directly enhances chaperone solubility through interactions with this domain. In the absence of Hep, mtHsp70 was insoluble when expressed in Escherichia coli, as was its isolated ATPase domain and a chimera having this domain fused to the peptide-binding domain of HscA, a soluble monomeric chaperone. In contrast, these proteins all exhibited increased solubility when expressed in the presence of Hep. In vitro studies further revealed that purified Hep regulates the interaction of mtHsp70 with nucleotides. Full-length mtHsp70 exhibited slow intrinsic ATP hydrolysis activity (6.8+/-0.2 x 10(-4) s(-1)) at 25 degrees C, which was stimulated up to 49-fold by Hep. Hep also stimulated the activity of the isolated ATPase domain, albeit to a lower maximal extent (11.5-fold). In addition, gel-filtration studies showed that formation of chaperone-escort protein complexes inhibited mtHsp70 self-association, and they revealed that Hep binding to full-length mtHsp70 and its isolated ATPase domain is strongest in the absence of nucleotides. These findings provide evidence that metazoan escort proteins regulate the catalytic activity and solubility of their cognate chaperones, and they indicate that both forms of regulation arise from interactions with the mtHsp70 ATPase domain.