Destabilization of peptide binding and interdomain communication by an E543K mutation in the bovine 70-kDa heat shock cognate protein, a molecular chaperone

Neutron crystallographic analysis of the nucleotide-binding domain of Hsp72 in complex with ADP

The 70 kDa heat-shock proteins (Hsp70s) are ATP-dependent molecular chaperones that contain an N-terminal nucleotide-binding domain (NBD) and a C-terminal substrate-binding domain. Hsp70s bind to misfolded/unfolded proteins and thereby prevent their aggregation. The ATP hydrolysis reaction in the NBD plays a key role in allosteric control of the binding of substrate proteins. In the present study, the neutron crystal structure of the NBD of Hsp72, a heat-inducible Hsp70 family member, was solved in complex with ADP in order to study the structure-function relationship with a focus on hydrogens. ADP bound to Hsp72 was fully deprotonated, and the catalytically important residues, including Asp10, Asp199 and Asp206, are also deprotonated. Neutron analysis also enabled the characterization of the water clusters in the NBD. Enzymatic assays and X-ray crystallographic analysis revealed that the Y149A mutation exhibited a higher ATPase activity and caused disruption of the water cluster and incorporation of an additional magnesium ion. Tyr149 was suggested to contribute to the low intrinsic ATPase activity and to stabilize the water cluster. Collectively, these structural studies will help to elucidate the molecular basis of the function of Hsp72.

Human Stress-inducible Hsp70 Has a High Propensity to Form ATP-dependent Antiparallel Dimers That Are Differentially Regulated by Cochaperone Binding

Eukaryotic protein homeostasis (proteostasis) is largely dependent on the action of highly conserved Hsp70 molecular chaperones. Recent evidence indicates that, apart from conserved molecular allostery, Hsp70 proteins have retained and adapted the ability to assemble as functionally relevant ATP-bound dimers throughout evolution. Here, we have compared the ATP-dependent dimerization of DnaK, human stress-inducible Hsp70, Hsc70 and BiP Hsp70 proteins, showing that their dimerization propensities differ, with stress-inducible Hsp70 being predominantly dimeric in the presence of ATP. Structural analyses using hydrogen/deuterium exchange mass spectrometry, native electrospray ionization mass spectrometry and small-angle X-ray scattering revealed that stress-inducible Hsp70 assembles in solution as an antiparallel dimer with the intermolecular interface closely resembling the ATP-bound dimer interfaces captured in DnaK and BiP crystal structures. ATP-dependent dimerization of stress-inducible Hsp70 is necessary for its efficient interaction with Hsp40, as shown by experiments with dimerization-deficient mutants. Moreover, dimerization of ATP-bound Hsp70 is required for its participation in high molecular weight protein complexes detected ex vivo, supporting its functional role in vivo As human cytosolic Hsp70 can interact with tetratricopeptide repeat (TPR) domain containing cochaperones, we tested the interaction of Hsp70 ATP-dependent dimers with Chip and Tomm34 cochaperones. Although Chip associates with intact Hsp70 dimers to form a larger complex, binding of Tomm34 disrupts the Hsp70 dimer and this event plays an important role in Hsp70 activity regulation. In summary, this study provides structural evidence of robust ATP-dependent antiparallel dimerization of human inducible Hsp70 protein and suggests a novel role of TPR domain cochaperones in multichaperone complexes involving Hsp70 ATP-bound dimers.

Disrupted Hydrogen-Bond Network and Impaired ATPase Activity in an Hsc70 Cysteine Mutant

The ATPase domain of members of the 70 kDa heat shock protein (Hsp70) family shows a high degree of sequence, structural, and functional homology across species. A broadly conserved residue within the Hsp70 ATPase domain that captured our attention is an unpaired cysteine, positioned proximal to the site of nucleotide binding. Prior studies of several Hsp70 family members show this cysteine is not required for Hsp70 ATPase activity, yet select amino acid replacements of the cysteine can dramatically alter ATP hydrolysis. Moreover, post-translational modification of the cysteine has been reported to limit ATP hydrolysis for several Hsp70s. To better understand the underlying mechanism for how perturbation of this noncatalytic residue modulates Hsp70 function, we determined the structure for a cysteine-to-tryptophan mutation in the constitutively expressed, mammalian Hsp70 family member Hsc70. Our work reveals that the steric hindrance produced by a cysteine-to-tryptophan mutation disrupts the hydrogen-bond network within the active site, resulting in a loss of proper catalytic magnesium coordination. We propose that a similarly altered active site is likely observed upon post-translational oxidation. We speculate that the subtle changes we detect in the hydrogen-bonding network may relate to the previously reported observation that cysteine oxidation can influence Hsp70 interdomain communication.

Mutations in the Yeast Hsp70, Ssa1, at P417 Alter ATP Cycling, Interdomain Coupling, and Specific Chaperone Functions

The major cytoplasmic Hsp70 chaperones in the yeast Saccharomyces cerevisiae are the Ssa proteins, and much of our understanding of Hsp70 biology has emerged from studying ssa mutant strains. For example, Ssa1 catalyzes multiple cellular functions, including protein transport and degradation, and to this end, the ssa1-45 mutant has proved invaluable. However, the biochemical defects associated with the corresponding Ssa1-45 protein (P417L) are unknown. Consequently, we characterized Ssa1 P417L, as well as a P417S variant, which corresponds to a mutation in the gene encoding the yeast mitochondrial Hsp70. We discovered that the P417L and P417S proteins exhibit accelerated ATPase activity that was similar to the Hsp40-stimulated rate of ATP hydrolysis of wild-type Ssa1. We also found that the mutant proteins were compromised for peptide binding. These data are consistent with defects in peptide-stimulated ATPase activity and with results from limited proteolysis experiments, which indicated that the mutants' substrate binding domains were highly vulnerable to digestion. Defects in the reactivation of heat-denatured luciferase were also evident. Correspondingly, yeast expressing P417L or P417S as the only copy of Ssa were temperature sensitive and exhibited defects in Ssa1-dependent protein translocation and misfolded protein degradation. Together, our studies suggest that the structure of the substrate binding domain is altered and that coupling between this domain and the nucleotide binding domain is disabled when the conserved P417 residue is mutated. Our data also provide new insights into the nature of the many cellular defects associated with the ssa1-45 allele.

Spatial Control of Epsin-induced Clathrin Assembly by Membrane Curvature

Epsins belong to the family of highly conserved clathrin-associated sorting proteins that are indispensable for clathrin-mediated endocytosis, but their precise functions remain unclear. We have developed an assay system of budded supported membrane tubes displaying planar and highly curved membrane surfaces to analyze intrinsic membrane curvature preference shown by clathrin-associated sorting proteins. Using real-time fluorescence microscopy, we find that epsin preferentially partitions to and assembles clathrin on highly curved membrane surfaces. Sorting of epsin to regions of high curvature strictly depends on binding to phosphatidylinositol 4,5-bisphosphate. Fluorescently labeled clathrins rapidly assemble as foci, which in turn cluster epsin, while maintaining tube integrity. Clathrin foci grow in intensity with a typical time constant of ∼75 s, similar to the time scales for coated pit formation seen in cells. Epsin therefore effectively senses membrane curvature to spatially control clathrin assembly. Our results highlight the potential role of membrane curvature in orchestrating the myriad molecular interactions necessary for the success of clathrin-mediated membrane budding.

Fine tuning of a biological machine: DnaK gains improved chaperone activity by altered allosteric communication and substrate binding

DnaK is a member of the Hsp70 family of molecular chaperones. This molecular machine couples the binding and hydrolysis of ATP to binding and release of substrate proteins. The switches that are involved in allosteric communication within this multidomain protein are mostly unknown. Previous insights were largely obtained by mutants, which displayed either wild-type activity or reduced folding assistance of substrate proteins. With a directed evolution approach for improved folding assistance we selected a DnaK variant characterized by a glycine to alanine substitution at position 384 (G384A); this resulted in a 2.5-fold higher chaperone activity in an in vitro DnaK-assisted firefly luciferase refolding assay. Quantitative biochemical characterization revealed several changes of key kinetic parameters compared to the wild type. Most pronounced is a 13-fold reduced rate constant for substrate release in the ATP-bound state, which we assume, in conjunction with the resulting increase in substrate affinity, to be related to improved chaperone activity. As the underlying mechanistic reason for this change we propose an altered interface of allosteric communication of mutant G384A, which is notably located at a hinge position between nucleotide and substrate binding domain.

High affinity binding between Hsp70 and the C-terminal domain of the measles virus nucleoprotein requires an Hsp40 co-chaperone

The major inducible 70 kDa heat shock protein (hsp70) binds the measles virus (MeV) nucleocapsid with high affinity in an ATP-dependent manner, stimulating viral transcription and genome replication, and profoundly influencing virulence in mouse models of brain infection. Binding is mediated by two hydrophobic motifs (Box-2 and Box-3) located within the C-terminal domain (N(TAIL)) of the nucleocapsid protein, with N(TAIL) being an intrinsically disordered domain. The current work showed that high affinity hsp70 binding to N(TAIL) requires an hsp40 co-chaperone that interacts primarily with the hsp70 nucleotide binding domain (NBD) and displays no significant affinity for N(TAIL). Hsp40 directly enhanced hsp70 ATPase activity in an N(TAIL)-dependent manner, and formation of hsp40-hsp70-N(TAIL) intracellular complexes required the presence of N(TAIL) Box-2 and 3. Results are consistent with the functional interplay between hsp70 nucleotide and substrate binding domains (SBD), where ATP hydrolysis is rate limiting to high affinity binding to client proteins and is enhanced by hsp40. As such, hsp40 is an essential variable in understanding the outcome of MeV-hsp70 interactions.

DnaJ recruits DnaK to protein aggregates

Thermal stress might lead to protein aggregation in the cell. Reactivation of protein aggregates depends on Hsp100 and Hsp70 chaperones. We focus in this study on the ability of DnaK, the bacterial representative of the Hsp70 family, to interact with different aggregated model substrates. Our data indicate that DnaK binding to large protein aggregates is mediated by DnaJ, and therefore it depends on its affinity for the cochaperone. Mutations in the structural region of DnaK known as the "latch" decrease the affinity of the chaperone for DnaJ, resulting in a defective activity as protein aggregate-removing agent. As expected, the chaperone activity is recovered when DnaJ concentration is raised to overcome the lower affinity of the mutant for the cochaperone, suggesting that a minimum number of aggregate-bound DnaK molecules is necessary for its efficient reactivation. Our results provide the first experimental evidence of DnaJ-mediated recruiting of ATP-DnaK molecules to the aggregate surface.

Crystal structure of the C-terminal three-helix bundle subdomain of C. elegans Hsp70

Hsp70 chaperones are composed of two domains; the 40 kDa N-terminal nucleotide-binding domain (NDB) and the 30 kDa C-terminal substrate-binding domain (SBD). Structures of the SBD from Escherichia coli homologues DnaK and HscA show it can be further divided into an 18 kDa beta-sandwich subdomain, which forms the hydrophobic binding pocket, and a 10 kDa C-terminal three-helix bundle that forms a lid over the binding pocket. Across prokaryotes and eukaryotes, the NBD and beta-sandwich subdomain are well conserved in both sequence and structure. The C-terminal subdomain is, however, more evolutionary variable and the only eukaryotic structure from rat Hsc70 revealed a diverged helix-loop-helix fold. We have solved the crystal structure of the C-terminal 10 kDa subdomain from Caenorhabditis elegans Hsp70 which forms a helical-bundle similar to the prokaryotic homologues. This provides the first confirmation of the structural conservation of this subdomain in eukaryotes. Comparison with the rat structure reveals a domain-swap dimerisation mechanism; however, the C. elegans subdomain exists exclusively as a monomer in solution in agreement with the hypothesis that regions out with the C-terminal subdomain are necessary for Hsp70 self-association.

Primate chaperones Hsc70 (constitutive) and Hsp70 (induced) differ functionally in supporting growth and prion propagation in Saccharomyces cerevisiae

Hsp70's are highly conserved essential protein chaperones that assist protein folding and prevent protein aggregation. They have modular structures consisting of ATPase, substrate-binding, and C-terminal domains. Substrate binding and release is regulated by ATP hydrolysis and nucleotide exchange, which in turn are regulated by cochaperones. Eukaryotes have constitutive (Hsc70) and stress-inducible (iHsp70) isoforms, but their functions have not been systematically compared. Using a yeast system to evaluate heterologous Hsp70's we find that primate Hsc70 supported growth but iHsp70 did not. Plant Hsc70 and iHsp70 counterparts behaved similarly, implying evolutionary conservation of this distinction. Swapping yeast and primate Hsp70 domains showed that (i) the Hsc70-iHsp70 distinction resided in the ATPase domain, (ii) substrate-binding domains of Hsp70's within and across species functioned similarly regarding growth, (iii) C-terminal domain function was important for growth, and (iv) Hsp70 functions important for cell growth and prion propagation were separable. Enzymatic analysis uncovered a correlation between substrate affinity and prion phenotype and showed that ATPase and protein-folding activities were generally similar. Our data support a view that intrinsic activities of Hsp70 isoforms are comparable, and functional differences in vivo lie mainly in complex interactions of Hsp70 with cochaperones.

Ionic contacts at DnaK substrate binding domain involved in the allosteric regulation of lid dynamics

To gain further insight into the interactions involved in the allosteric transition of DnaK we have characterized wild-type (wt) protein and three mutants in which ionic interactions at the interface between the two subdomains of the substrate binding domain, and within the lid subdomain have been disrupted. Our data show that ionic contacts, most likely forming an electrically charged network, between the N-terminal region of helix B and an inner loop of the beta-sandwich are involved in maintaining the position of the lid relative to the beta-subdomain in the ADP state but not in the ATP state of the protein. Disruption of the ionic interactions between the C-terminal region of helix B and the outer loops of the beta-sandwich, known as the latch, does not have the same conformational consequences but results equally in an inactive protein. This indicates that a variety of mechanisms can inactivate this complex allosteric machine. Our results identify the ionic contacts at the subdomain and interdomain interfaces that are part of the hinge region involved in the ATP-induced allosteric displacement of the lid away from the peptide binding site. These interactions also stabilize peptide-Hsp70 complexes at physiological (37 degrees C) and stress (42 degrees C) temperatures, a requirement for productive substrate (re)folding.

Structural basis of interdomain communication in the Hsc70 chaperone

Hsp70 family proteins are highly conserved chaperones involved in protein folding, degradation, targeting and translocation, and protein complex remodeling. They are comprised of an N-terminal nucleotide binding domain (NBD) and a C-terminal protein substrate binding domain (SBD). ATP binding to the NBD alters SBD conformation and substrate binding kinetics, but an understanding of the mechanism of interdomain communication has been hampered by the lack of a crystal structure of an intact chaperone. We report here the 2.6 angstroms structure of a functionally intact bovine Hsc70 (bHsc70) and a mutational analysis of the observed interdomain interface and the immediately adjacent interdomain linker. This analysis identifies interdomain interactions critical for chaperone function and supports an allosteric mechanism in which the interdomain linker invades and disrupts the interdomain interface when ATP binds.

Chaperones in preventing protein denaturation in living cells and protecting against cellular stress

A variety of cellular internal and external stress conditions can be classified as proteotoxic stresses. Proteotoxic stresses can be defined as stresses that increase the fraction of proteins that are in an unfolded state, thereby enhancing the probability of the formation of intracellular aggregates. These aggregates, if not disposed, can lead to cell death. In response to the appearance of damaged proteins, cells induce the expression of heat shock proteins. These can function as molecular chaperones to prevent protein aggregation and to keep proteins in a state competent for either refolding or degradation. Most knowledge of the function and regulation (by co-factors) of individual heat shock proteins comes from cell free studies on refolding of heat- or chemically denatured, purified proteins. Unlike the experimental situation in a test tube, cells contain multiple chaperones and co-factors often moving in and out different subcompartments that contain a variety of protein substrates at different folding states. Also, within cells folding competes with the degradative machinery. In this chapter, an overview will be provided on how the main cytosolic/nuclear chaperone Hsp70 is regulated, what is known about its interaction with other main cytosolic/nuclear chaperone families (Hsp27, Hsp90, and Hsp110), and how it may function as a molecular chaperone in living mammalian cells to protect against proteotoxic stresses.

Crystal structure of the molecular chaperone HscA substrate binding domain complexed with the IscU recognition peptide ELPPVKIHC

HscA, a specialized bacterial Hsp70-class molecular chaperone, interacts with the iron-sulfur cluster assembly protein IscU by recognizing a conserved LPPVK sequence motif. We report the crystal structure of the substrate-binding domain of HscA (SBD, residues 389-616) from Escherichia coli bound to an IscU-derived peptide, ELPPVKIHC. The crystals belong to the space group I222 and contain a single molecule in the asymmetric unit. Molecular replacement with the E.coli DnaK(SBD) model was used for phasing, and the HscA(SBD)-peptide model was refined to Rfactor=17.4% (Rfree=21.0%) at 1.95 A resolution. The overall structure of HscA(SBD) is similar to that of DnaK(SBD), although the alpha-helical subdomain (residues 506-613) is shifted up to 10 A relative to the beta-sandwich subdomain (residues 389-498) when compared to DnaK(SBD). The ELPPVKIHC peptide is bound in an extended conformation in a hydrophobic cleft in the beta-subdomain, which appears to be solvent-accessible via a narrow passageway between the alpha and beta-subdomains. The bound peptide is positioned in the reverse orientation of that observed in the DnaK(SBD)-NRLLLTG peptide complex placing the N and C termini of the peptide on opposite sides of the HscA(SBD) relative to the DnaK(SBD) complex. Modeling of the peptide in the DnaK-like forward orientation suggests that differences in hydrogen bonding interactions in the binding cleft and electrostatic interactions involving surface residues near the cleft contribute to the observed directional preference.

Hsp70 and Hsp90--a relay team for protein folding

Molecular chaperones are a functionally defined set of proteins which assist the structure formation of proteins in vivo. Without certain protective mechanisms, such as binding nascent polypeptide chains by molecular chaperones, cellular protein concentrations would lead to misfolding and aggregation. In the mammalian system, the molecular chaperones Hsp70 and Hsp90 are involved in the folding and maturation of key regulatory proteins, like steroid hormone receptors, transcription factors, and kinases, some of which are involved in cancer progression. Hsp70 and Hsp90 form a multichaperone complex, in which both are connected by a third protein called Hop. The connection of and the interplay between the two chaperone machineries is of crucial importance for cell viability. This review provides a detailed view of the Hsp70 and Hsp90 machineries, their cofactors and their mode of regulation. It summarizes the current knowledge in the field, including the ATP-dependent regulation of the Hsp70/Hsp90 multichaperone cycle and elucidates the complex interplay and their synergistic interaction.

Crystal structure of the C-terminal 10-kDa subdomain of Hsc70

The 70-kDa heat shock proteins (Hsp70), including the cognates (Hsc70), are molecular chaperones that prevent misfolding and aggregation of polypeptides in cells under both normal and stressed conditions. They are composed of two major structural domains: an N-terminal 44-kDa ATPase domain and a C-terminal 30-kDa substrate binding domain. The 30-kDa domain can be divided into an 18-kDa subdomain and a 10-kDa subdomain. Here we report the crystal structure of the 10-kDa subdomain of rat Hsc70 at 3.45 A. Its helical region adopted a helix-loop-helix fold. This conformation is different from the equivalent subdomain of DnaK, the bacterial homologue of Hsc70. Moreover, in the crystalline state, the 10-kDa subdomain formed dimers. The results of gel filtration chromatography further supported the view that this subdomain was self-associated. Upon gel filtration, Hsc70 was found to exist as a mixture of monomers, dimers, and oligomers, but the 60-kDa fragment was predominantly found to exist as monomers. These findings suggest that the alpha-helical region of the 10-kDa subdomain dictates the chaperone self-association.

Dependence of endoplasmic reticulum-associated degradation on the peptide binding domain and concentration of BiP

ER-associated degradation (ERAD) removes defective and mis-folded proteins from the eukaryotic secretory pathway, but mutations in the ER lumenal Hsp70, BiP/Kar2p, compromise ERAD efficiency in yeast. Because attenuation of ERAD activates the UPR, we screened for kar2 mutants in which the unfolded protein response (UPR) was induced in order to better define how BiP facilitates ERAD. Among the kar2 mutants isolated we identified the ERAD-specific kar2-1 allele (Brodsky et al. J. Biol. Chem. 274, 3453-3460). The kar2-1 mutation resides in the peptide-binding domain of BiP and decreases BiP's affinity for a peptide substrate. Peptide-stimulated ATPase activity was also reduced, suggesting that the interdomain coupling in Kar2-1p is partially compromised. In contrast, Hsp40 cochaperone-activation of Kar2-1p's ATPase activity was unaffected. Consistent with UPR induction in kar2-1 yeast, an ERAD substrate aggregated in microsomes prepared from this strain but not from wild-type yeast. Overexpression of wild-type BiP increased substrate solubility in microsomes obtained from the mutant, but the ERAD defect was exacerbated, suggesting that simply retaining ERAD substrates in a soluble, retro-translocation-competent conformation is insufficient to support polypeptide transit to the cytoplasm.

Saccharomyces cerevisiae Hsp70 mutations affect [PSI+] prion propagation and cell growth differently and implicate Hsp40 and tetratricopeptide repeat cochaperones in impairment of [PSI+]

We previously described an Hsp70 mutant (Ssa1-21p), altered in a conserved residue (L483W), that dominantly impairs yeast [PSI(+)] prion propagation without affecting growth. We generated new SSA1 mutations that impaired [PSI(+)] propagation and second-site mutations in SSA1-21 that restored normal propagation. Effects of mutations on growth did not correlate with [PSI(+)] phenotype, revealing differences in Hsp70 function required for growth and [PSI(+)] propagation and suggesting that Hsp70 interacts differently with [PSI(+)] prion aggregates than with other cellular substrates. Complementary suppression of altered activity between forward and suppressing mutations suggests that mutations that impair [PSI(+)] affect a similar Hsp70 function and that suppressing mutations similarly overcome this effect. All new mutations that impaired [PSI(+)] propagation were located in the ATPase domain. Locations and homology of several suppressing substitutions suggest that they weaken Hsp70's substrate-trapping conformation, implying that impairment of [PSI(+)] by forward mutations is due to altered ability of the ATPase domain to regulate substrate binding. Other suppressing mutations are in residues important for interactions with Hsp40 or TPR-containing cochaperones, suggesting that such interactions are necessary for the impairment of [PSI(+)] propagation caused by mutant Ssa1p.

Characterization of the L399P and R447G mutants of hsc70: the decrease in refolding activity is correlated with an increase in the rate of substrate dissociation

It is known that 70-kDa heat-shock cognate protein (hsc70) is capable of forming complexes with unfolded polypeptide substrates and works with DnaJ homologues to refold denatured proteins. Herein, we characterized two hsc70 mutants, hsc70(L399P) and hsc70(R447G). They retained the capability of restoring the activity of denatured luciferase, but their activity was decreased to 40% and 20%, respectively, of that of hsc70. The rate of dissociation for the heptapeptide substrate FYQLALT from the mutants was increased, and the R447G mutant had the faster rate of peptide dissociation. Thus, the reduction in the ability of these mutants to refold denatured proteins was correlated with an increase in the rate of substrate dissociation.

Bag-1M accelerates nucleotide release for human Hsc70 and Hsp70 and can act concentration-dependent as positive and negative cofactor

The cytosol of mammalian cells contains several Hsp70 chaperones and an arsenal of cochaperones, including the anti-apoptotic Bag-1M protein, which regulate the activities of Hsp70s by controlling their ATPase cycles. To elucidate the regulatory function of Bag-1M, we determined its influence on nucleotide exchange, substrate release, ATPase rate, and chaperone activity of the housekeeping Hsc70 and stress-inducible Hsp70 homologs of humans. Bag-1M and a C-terminal fragment of it are potent nucleotide exchange factors as they stimulated the ADP dissociation rate of Hsc70 and Hsp70 up to 900-fold. The N-terminal domain of Bag-1M decreased the affinity of Bag-1M for Hsc70/Hsp70 by 4-fold, indicating a modulating role of the N terminus in Bag-1M action as nucleotide exchange factor. Bag-1M inhibited Hsc70/Hsp70-dependent refolding of luciferase in the absence of P(i). Surprisingly, under physiological conditions, i.e. low Bag-1M concentrations and presence of P(i), Bag-1M activates the chaperone action of Hsc70/Hsp70 in luciferase refolding. Bag-1M accelerated ATP-triggered substrate release by Hsc70/Hsp70. We propose that Bag-1M acts as substrate discharging factor for Hsc70 and Hsp70.

The effect of mutating arginine-469 on the substrate binding and refolding activities of 70-kDa heat shock cognate protein

On the basis of the X-ray structure of DnaK, we obtained an energy-minimized model for the C-terminal domain of rat 70-kDa heat shock cognate protein (hsc70). The model suggests that Arg-469 may play an important role in maintaining the substrate-bound conformation of hsc70. To verify this hypothesis, we substituted cysteine for Arg-469 and generated the hsc70(R469C) mutant. Compared to the wild-type hsc70, the mutant was more accessible to cleavage by endopeptidase Lys-C, implying that the overall structure of hsc70(R469C) is relatively loose. Moreover, hsc70(R469C) did not form tightly associated complexes with S-carboxymethyl-alpha-lactalbumin, an unfolded protein. The amount of heptapeptide FYQLALT bound to hsc70(R469C) was also decreased as determined by gel filtration. Thus, the affinity of hsc70(R469C) for polypeptide substrates is reduced. In the presence of DnaJ, the capability of hsc70(R469C) to refold the denatured luciferase was decreased by 50%. Therefore, for hsc70, reduction in affinity for substrates may affect its DnaJ-dependent refolding activity.

Kinetic characterization of the ATPase cycle of the molecular chaperone Hsc66 from Escherichia coli

Hsc66 from Escherichia coli is a constitutively expressed hsp70 class molecular chaperone whose activity is coupled to ATP binding and hydrolysis. To better understand the mechanism and regulation of Hsc66, we investigated the kinetics of ATP hydrolysis and the interactions of Hsc66 with nucleotides. Steady-state experiments revealed that Hsc66 has a low affinity for ATP (K(m)(ATP) = 12.7 microM) compared with other hsp70 chaperones. The kinetics of nucleotide binding were determined by analyzing changes in the Hsc66 absorbance spectrum using stopped-flow methods at 23 degrees C. ATP binding results in a rapid, biphasic increase of Hsc66 absorbance at 280 nm; this is interpreted as arising from a two-step process in which ATP binding (k(a)(ATP) = 4.2 x 10(4) M(-1) s(-1), k(d)(ATP) = 1.1 s(-1)) is followed by a slow conformational change (k(conf) = 0. 1 s(-1)). Under single turnover conditions, the ATP-induced transition decays exponentially with a rate (k(decay) = 0.0013 s(-1)) similar to that observed in both steady-state and single turnover ATP hydrolysis experiments (k(hyd) = 0.0014 s(-1)). ADP binding to Hsc66 results in a monophasic transition in the absence (k(a)(ADP) = 7 x 10(5) M(-1) s(-1), k(d)(ADP) = 60 s(-1)) and presence of physiological levels of inorganic phosphate (k(a)(ADP(P(i)) = 0.28 x 10(5) M(-1) s(-1), k(d)(ADP(P(i)) = 9.1 s(-1)). These results indicate that ATP hydrolysis is the rate-limiting step under steady-state conditions and is >10(3)-fold slower than the rate of ADP/ATP exchange. Thus, in contrast to DnaK and eukaryotic forms of hsp70 that have been characterized to date, the R if T equilibrium balance for Hsc66 is shifted in favor of the low peptide affinity T state, and regulation of the reaction cycle is expected to occur at the ATP hydrolysis step rather than at nucleotide exchange.

Hsc40, a new member of the hsp40 family, exhibits similar expression profile to that of hsc70 in mammalian cells

Here, we report the identification and characterization of a new hsp40 family member, heat shock cognate 40 (hsc40), which may be a specific functional partner for hsc70. The hsc40 gene consists of five exons and four introns. The previously characterized hsp40 gene contains only three exons and two introns. Despite this difference in the numbers of exons and introns, the structures of these two genes are actually quite similar. If the first two exons and introns of the hsc40 gene are excluded, the rest is essentially the same as the whole hsp40 gene. The intron/exon boundaries of the hsc40 and hsp40 genes are conserved with respect to amino acid coding, indicating that the hsp40 gene might have arisen from a duplicated hsc40 gene, which developed a new transcription-regulatory mechanism in the second intron. Hsc40 is evolutionally conserved. There is over 95% sequence identity between the putative mouse and human hsc40 proteins. However, there is only 60% sequence identity between the hsc40 and hsp40 proteins from either human or mouse cells. Northern blot analysis of various tissues and cells in culture revealed that this gene was expressed under normal conditions, and its expression was further increased after various stress treatments. The expression pattern of the hsc40 gene is very similar to that of the hsc70 gene under both normal and stress conditions, whereas the hsp40 gene exhibited an expression pattern more similar to that of hsp70 genes on the same Northern blots. These results, considered along with the high rate of sequence conservation between the same proteins from different species and high rate of variation between the two different proteins from the same species, strongly suggest that hsc40 and hsp40 may perform different functions and/or have different specificities in the cell.

N-Ethylmaleimide-modified Hsp70 inhibits protein folding

Hsp70 molecular chaperones facilitate protein folding and translocation by binding to hydrophobic regions of nascent or unfolded proteins, thereby preventing their aggregation. N-Ethylmaleimide (NEM) inhibits the ATPase and protein translocation-stimulating activities of the yeast Hsp70 Ssa1p by modifying its three cysteine residues, which are located in its ATPase domain. NEM alters the conformation of Ssa1p and disrupts the coupling between its nucleotide- and polypeptide-binding domains. Ssa1p and the yeast DnaJ homolog Ydj1p constitute a protein folding machinery of the yeast cytosol. Using firefly luciferase as a model protein to study chaperone-dependent protein refolding, we have found that NEM also inhibits the protein folding activity of Ssa1p. Interestingly, the NEM-modified protein (NEM-Ssa1p) is a potent inhibitor of protein folding. NEM-Ssa1p can prevent the aggregation of luciferase and stimulate the ATPase activity of Ssa1p suggesting that it acts as an inhibitor by binding to nonnative forms of luciferase and by competing with them for the polypeptide binding site of Ssa1p. NEM-Ssa1p inhibits Ssa1p/Ydj1p-dependent protein refolding at different stages indicating that the chaperones bind and release nonnative forms of luciferase multiple times before folding is completed.

Tissue-specific expression of dominant negative mutant Drosophila HSC70 causes developmental defects and lethality

The Drosophila melanogaster HSC3 and HSC4 genes encode Hsc70 proteins homologous to the mammalian endoplasmic reticulum (ER) protein BiP and the cytoplasmic clathrin uncoating ATPase, respectively. These proteins possess ATP binding/hydrolysis activities that mediate their ability to aid in protein folding by coordinating the sequential binding and release of misfolded proteins. To investigate the roles of HSC3 (Hsc3p) and HSC4 (Hsc4p) proteins during development, GAL4-targeted gene expression was used to analyze the effects of producing dominant negatively acting Hsc3p (D231S, K97S) and Hsc4p (D206S, K71S) proteins, containing single amino acid substitutions in their ATP-binding domains, in specific tissues of Drosophila throughout development. We show that the production of each mutant protein results in lethality over a range of developmental stages, depending on the levels of protein produced and which tissues are targeted. We demonstrate that the functions of both Hsc3p and Hsc4p are required for proper tissue establishment and maintenance. Production of mutant Hsc4p, but not Hsc3p, results in induction of the stress-inducible Hsp70 at normal temperatures. Evidence is presented that lethality is caused by tissue-specific defects that result from a global accumulation of misfolded protein caused by lack of functional Hsc70. We show that both mutant Hsc3ps are defective in ATP-induced substrate release, although Hsc3p(D231S) does undergo an ATP-induced conformational change. We believe that the amino acid substitutions in Hsc3p interfere with the structural coupling of ATP binding to substrate release, and this defect is the basis for the mutant proteins' dominant negative effects in vivo.

Mutations in the substrate binding domain of the Escherichia coli 70 kDa molecular chaperone, DnaK, which alter substrate affinity or interdomain coupling

In Escherichia coli, DnaK is essential for the replication of bacteriophage lambda DNA; this in vivo activity provides the basis of a screen for mutations affecting DnaK function. Mn PCR was used to introduce mutations into residues 405-468 of the C-terminal polypeptide-binding domain of DnaK. These mutant proteins were screened for the ability to propagate bacteriophage lambda in the background of a dnaK deficient cell line, BB1553. This initial screen identified several proteins which were mutant at multiple positions. The multiple mutants were further dissected into single mutants which remained negative for lambda propagation. Four of these single-site mutants were purified and assayed for biochemical functionality. Two single-site mutations, F426S and S427P, are localized in the peptide binding site and display weakened peptide binding affinity. This indicates that the crystallographically determined peptide binding site is also critical for in vivo lambda replication. Two other mutations, K414I and N451K, are located at the edge of the beta-sandwich domain near alpha-helix A. The K414I mutant binds peptide moderately well, yet displays defects in allosteric functions, including peptide-stimulated ATPase activity, ATP-induced changes in tryptophan fluorescence, ATP-induced peptide release, and elevated ATPase activity. The K414 position is close in tertiary structure to the linker region to the ATPase domain and reflects a specific area of the peptide-binding domain which is necessary for interdomain coupling. The mutant N451K displays defects in both peptide binding and allosteric interaction.