Crystal structure of an Hsp90-nucleotide-p23/Sba1 closed chaperone complex

Hsp90-dependent activation of protein kinases is regulated by chaperone-targeted dephosphorylation of Cdc37

Activation of protein kinase clients by the Hsp90 system is mediated by the cochaperone protein Cdc37. Cdc37 requires phosphorylation at Ser13, but little is known about the regulation of this essential posttranslational modification. We show that Ser13 of uncomplexed Cdc37 is phosphorylated in vivo, as well as in binary complex with a kinase (C-K), or in ternary complex with Hsp90 and kinase (H-C-K). Whereas pSer13-Cdc37 in the H-C-K complex is resistant to nonspecific phosphatases, it is efficiently dephosphorylated by the chaperone-targeted protein phosphatase 5 (PP5/Ppt1), which does not affect isolated Cdc37. We show that Cdc37 and PP5/Ppt1 associate in Hsp90 complexes in yeast and in human tumor cells, and that PP5/Ppt1 regulates phosphorylation of Ser13-Cdc37 in vivo, directly affecting activation of protein kinase clients by Hsp90-Cdc37. These data reveal a cyclic regulatory mechanism for Cdc37, in which its constitutive phosphorylation is reversed by targeted dephosphorylation in Hsp90 complexes.

Cisplatin differently affects amino terminal and carboxyl terminal domains of HSP90

The 90-kDa heat shock protein (HSP90) is a molecular chaperone that assists in the folding and assembly of proteins in the cytosol. We previously demonstrated that the antineoplastic reagent, cisplatin, inhibits the aggregation prevention activity of mammalian HSP90. We now show that cisplatin binds both the amino terminal and carboxyl terminal domains of the human HSP90 and differently affects these two domains. Cisplatin blocks the aggregation prevention activity of HSP90C, but not HSP90N. In contrast, cisplatin induces a conformational change in HSP90N, but not HSP90C. These results indicate that cisplatin modulates the HSP90 activities through two different mechanisms using the two distinct binding sites of the HSP90 molecule.

Structural and functional coupling of Hsp90- and Sgt1-centred multi-protein complexes

Sgt1 is an adaptor protein implicated in a variety of processes, including formation of the kinetochore complex in yeast, and regulation of innate immunity systems in plants and animals. Sgt1 has been found to associate with SCF E3 ubiquitin ligases, the CBF3 kinetochore complex, plant R proteins and related animal Nod-like receptors, and with the Hsp90 molecular chaperone. We have determined the crystal structure of the core Hsp90-Sgt1 complex, revealing a distinct site of interaction on the Hsp90 N-terminal domain. Using the structure, we developed mutations in Sgt1 interfacial residues, which specifically abrogate interaction with Hsp90, and disrupt Sgt1-dependent functions in vivo, in plants and yeast. We show that Sgt1 bridges the Hsp90 molecular chaperone system to the substrate-specific arm of SCF ubiquitin ligase complexes, suggesting a role in SCF assembly and regulation, and providing multiple complementary routes for ubiquitination of Hsp90 client proteins.

Structural and functional analysis of SGT1-HSP90 core complex required for innate immunity in plants

SGT1 (Suppressor of G2 allele of skp1), a co-chaperone of HSP90 (Heat-shock protein 90), is required for innate immunity in plants and animals. Unveiling the cross talks between SGT1 and other co-chaperones such as p23, AHA1 (Activator of HSP90 ATPase 1) or RAR1 (Required for Mla12 resistance) is an important step towards understanding the HSP90 machinery. Nuclear magnetic resonance spectroscopy and mutational analyses of HSP90 revealed the nature of its binding with the CS domain of SGT1. Although CS is structurally similar to p23, these domains were found to non-competitively bind to various regions of HSP90; yet, unexpectedly, full-length SGT1 could displace p23 from HSP90. RAR1 partly shares the same binding site with HSP90 as the CS domain, whereas AHA1 does not. This analysis allowed us to build a structural model of the HSP90-SGT1 complex and to obtain a compensatory mutant pair between both partners that is able to restore virus resistance in vivo through Rx (Resistance to potato virus X) immune sensor stabilization.

Amyloid oligomer conformation in a group of natively folded proteins

Recent in vitro and in vivo studies suggest that destabilized proteins with defective folding induce aggregation and toxicity in protein-misfolding diseases. One such unstable protein state is called amyloid oligomer, a precursor of fully aggregated forms of amyloid. Detection of various amyloid oligomers with A11, an anti-amyloid oligomer conformation-specific antibody, revealed that the amyloid oligomer represents a generic conformation and suggested that toxic beta-aggregation processes possess a common mechanism. By using A11 antibody as a probe in combination with mass spectrometric analysis, we identified GroEL in bacterial lysates as a protein that may potentially have an amyloid oligomer conformation. Surprisingly, A11 reacted not only with purified GroEL but also with several purified heat shock proteins, including human Hsp27, 40, 70, 90; yeast Hsp104; and bovine Hsc70. The native folds of A11-reactive proteins in purified samples were characterized by their anti-beta-aggregation activity in terms of both functionality and in contrast to the beta-aggregation promoting activity of misfolded pathogenic amyloid oligomers. The conformation-dependent binding of A11 with natively folded Hsp27 was supported by the concurrent loss of A11 reactivity and anti-beta-aggregation activity of heat-treated Hsp27 samples. Moreover, we observed consistent anti-beta-aggregation activity not only by chaperones containing an amyloid oligomer conformation but also by several A11-immunoreactive non-chaperone proteins. From these results, we suggest that the amyloid oligomer conformation is present in a group of natively folded proteins. The inhibitory effects of A11 antibody on both GroEL/ES-assisted luciferase refolding and Hsp70-mediated decelerated nucleation of Abeta aggregation suggested that the A11-binding sites on these chaperones might be functionally important. Finally, we employed a computational approach to uncover possible A11-binding sites on these targets. Since the beta-sheet edge was a common structural motif having the most similar physicochemical properties in the A11-reactive proteins we analyzed, we propose that the beta-sheet edge in some natively folded amyloid oligomers is designed positively to prevent beta aggregation.

Role of acetylation and extracellular location of heat shock protein 90alpha in tumor cell invasion

Heat shock protein (hsp) 90 is an ATP-dependent molecular chaperone that maintains the active conformation of client oncoproteins in cancer cells. An isoform, hsp90alpha, promotes extracellular maturation of matrix metalloproteinase (MMP)-2, involved in tumor invasion and metastasis. Knockdown of histone deacetylase (HDAC) 6, which deacetylates lysine residues in hsp90, induces reversible hyperacetylation and attenuates ATP binding and chaperone function of hsp90. Here, using mass spectrometry, we identified seven lysine residues in hsp90alpha that are hyperacetylated after treatment of eukaryotic cells with a pan-HDAC inhibitor that also inhibits HDAC6. Depending on the specific lysine residue in the middle domain involved, although acetylation affects ATP, cochaperone, and client protein binding to hsp90alpha, acetylation of all seven lysines increased the binding of hsp90alpha to 17-allyl-amino-demethoxy geldanamycin. Notably, after treatment with the pan-HDAC inhibitor panobinostat (LBH589), the extracellular hsp90alpha was hyperacetylated and it bound to MMP-2, which was associated with increased in vitro tumor cell invasiveness. Treatment with antiacetylated hsp90alpha antibody inhibited in vitro invasion by tumor cells. Thus, reversible hyperacetylation modulates the intracellular and extracellular chaperone function of hsp90, and targeting extracellular hyperacetylated hsp90alpha may undermine tumor invasion and metastasis.

Multiple conformations of E. coli Hsp90 in solution: insights into the conformational dynamics of Hsp90

Hsp90, an essential eukaryotic chaperone, depends upon its intrinsic ATPase activity for function. Crystal structures of the bacterial Hsp90 homolog, HtpG, and the yeast Hsp90 reveal large domain rearrangements between the nucleotide-free and the nucleotide-bound forms. We used small-angle X-ray scattering and recently developed molecular modeling methods to characterize the solution structure of HtpG and demonstrate how it differs from known Hsp90 conformations. In addition to this HtpG conformation, we demonstrate that under physiologically relevant conditions, multiple conformations coexist in equilibrium. In solution, nucleotide-free HtpG adopts a more extended conformation than observed in the crystal, and upon the addition of AMPPNP, HtpG is in equilibrium between this open state and a closed state that is in good agreement with the yeast AMPPNP crystal structure. These studies provide a unique view of Hsp90 conformational dynamics and provide a model for the role of nucleotide in effecting conformational change.

Apo-Hsp90 coexists in two open conformational states in solution

BACKGROUND INFORMATION

Hsp90 (90 kDa heat-shock protein) plays a key role in the folding and activation of many client proteins involved in signal transduction and cell cycle control. The cycle of Hsp90 has been intimately associated with large conformational rearrangements, which are nucleotide-binding-dependent. However, up to now, our understanding of Hsp90 conformational changes derives from structural information, which refers to the crystal states of either recombinant Hsp90 constructs or the prokaryotic homologue HtpG (Hsp90 prokaryotic homologue).

RESULTS AND DISCUSSION

Here, we present the first nucleotide-free structures of the entire eukaryotic Hsp90 (apo-Hsp90) obtained by small-angle X-ray scattering and single-particle cryo-EM (cryo-electron microscopy). We show that, in solution, apo-Hsp90 is in a conformational equilibrium between two open states that have never been described previously. By comparing our cryo-EM maps with HtpG and known Hsp90 structures, we establish that the structural changes involved in switching between the two Hsp90 apo-forms require large movements of the NTD (N-terminal domain) and MD (middle domain) around two flexible hinge regions.

CONCLUSIONS

The present study shows, for the first time, the structure of the entire eukaryotic apo-Hsp90, along with its intrinsic flexibility. Although large structural rearrangements, leading to partial closure of the Hsp90 dimer, were previously attributed to the binding of nucleotides, our results reveal that they are in fact mainly due to the intrinsic flexibility of Hsp90 dimer. Taking into account the preponderant role of the dynamic nature of the structure of Hsp90, we reconsider the Hsp90 ATPase cycle.

Structural frameworks for considering microbial protein- and nucleic acid-dependent motor ATPases

Many fundamental cellular processes depend on enzymes that utilize chemical energy to catalyse unfavourable reactions. Certain classes of ATPases provide a particularly vivid example of the process of energy conversion, employing cycles of nucleotide turnover to move and/or rearrange biological polymers such as proteins and nucleic acids. Four well-characterized classes of ATP-dependent protein/nucleic acid translocases and remodelling factors are found in all three domains of life (bacteria, archaea and eukarya): additional strand catalytic 'E' (ASCE) P-loop NTPases, GHL proteins, actin-fold enzymes and chaperonins. These unrelated protein superfamilies have each evolved the ability to couple ATP binding and hydrolysis to the generation of motion and force along or within their substrates. The past several years have witnessed the emergence of a wealth of structural data that help explain how such molecular engines link nucleotide turnover to conformational change. In this review, we highlight several recent advances to illustrate some of the mechanisms by which each family of ATP-dependent motors facilitates the rearrangement and movement of proteins, protein complexes and nucleic acids.

HDAC6 inhibition enhances 17-AAG--mediated abrogation of hsp90 chaperone function in human leukemia cells

Histone deacetylase 6 (HDAC6) is a heat shock protein 90 (hsp90) deacetylase. Treatment with pan-HDAC inhibitors or depletion of HDAC6 by siRNA induces hyperacetylation and inhibits ATP binding and chaperone function of hsp90. Treatment with 17-allylamino-demothoxy geldanamycin (17-AAG) also inhibits ATP binding and chaperone function of hsp90, resulting in polyubiquitylation and proteasomal degradation of hsp90 client proteins. In this study, we determined the effect of hsp90 hyperacetylation on the anti-hsp90 and antileukemia activity of 17-AAG. Hyperacetylation of hsp90 increased its binding to 17-AAG, as well as enhanced 17-AAG-mediated attenuation of ATP and the cochaperone p23 binding to hsp90. Notably, treatment with 17-AAG alone also reduced HDAC6 binding to hsp90 and induced hyperacetylation of hsp90. This promoted the proteasomal degradation of HDAC6. Cotreatment with 17-AAG and siRNA to HDAC6 induced more inhibition of hsp90 chaperone function and depletion of BCR-ABL and c-Raf than treatment with either agent alone. In addition, cotreatment with 17-AAG and tubacin augmented the loss of survival of K562 cells and viability of primary acute myeloid leukemia (AML) and chronic myeloid leukemia (CML) samples. These findings demonstrate that HDAC6 is an hsp90 client protein and hyperacetylation of hsp90 augments the anti-hsp90 and antileukemia effects of 17-AAG.

Understanding ligand-based modulation of the Hsp90 molecular chaperone dynamics at atomic resolution

Molecular switching and ligand-based modulation of the 90-kDa heat-shock protein (Hsp90) chaperone activity may ultimately facilitate conformational coupling to the ATPase cycle along with activation and recruitment of the broad range of client proteins. We present an atomic resolution analysis of the Hsp90 N-terminal domain (NTD) binding energy landscape by simulating protein dynamics with a range of binding partners. We show that the activity of the molecular chaperone may be linked to (i) local folding-unfolding transitions and conformational switching of the "active site lid" upon binding and (ii) differences in the underlying protein dynamics as a function of the binding partner. This study suggests that structural plasticity of the Hsp90 NTD can be exploited by the molecular chaperone machinery to modulate enhanced structural rigidity during ATP binding and increased protein flexibility as a consequence of the inhibitor binding. The present study agrees with the experimental structural data and provides a plausible molecular model for understanding mechanisms of modulation of molecular chaperone activities by binding partners.

Intra- and intermonomer interactions are required to synergistically facilitate ATP hydrolysis in Hsp90

Nucleotide-dependent conformational changes of the constitutively dimeric molecular chaperone Hsp90 are integral to its molecular mechanism. Recent full-length crystal structures (Protein Data Bank codes 2IOQ, 2CG9, AND 2IOP) of Hsp90 homologs reveal large scale quaternary domain rearrangements upon the addition of nucleotides. Although previous work has shown the importance of C-terminal domain dimerization for efficient ATP hydrolysis, which should imply cooperativity, other studies suggest that the two ATPases function independently. Using the crystal structures as a guide, we examined the role of intra- and intermonomer interactions in stabilizing the ATPase activity of a single active site within an intact dimer. This was accomplished by creating heterodimers that allow us to differentially mutate each monomer, probing the context in which particular residues are important for ATP hydrolysis. Although the ATPase activity of each monomer can function independently, we found that the activity of one monomer could be inhibited by the mutation of hydrophobic residues on the trans N-terminal domain (opposite monomer). Furthermore, these trans interactions are synergistically mediated by a loop on the cis middle domain. This loop contains hydrophobic residues as well as a critical arginine that provides a direct linkage to the γ-phosphate of bound ATP. Small angle x-ray scattering demonstrates that deleterious mutations block domain closure in the presence of AMPPNP (5′-adenylyl-β,γ-imidodiphosphate), providing a direct linkage between structural changes and functional consequences. Together, these data indicate that both the cis monomer and the trans monomer and the intradomain and interdomain interactions cooperatively stabilize the active conformation of each active site and help explain the importance of dimer formation.

Conserved conformational changes in the ATPase cycle of human Hsp90

The dimeric molecular chaperone Hsp90 is required for the activation and stabilization of hundreds of substrate proteins, many of which participate in signal transduction pathways. The activation process depends on the hydrolysis of ATP by Hsp90. Hsp90 consists of a C-terminal dimerization domain, a middle domain, which may interact with substrate protein, and an N-terminal ATP-binding domain. A complex cycle of conformational changes has been proposed for the ATPase cycle of yeast Hsp90, where a critical step during the reaction requires the transient N-terminal dimerization of the two protomers. The ATPase cycle of human Hsp90 is less well understood, and significant differences have been proposed regarding key mechanistic aspects. ATP hydrolysis by human Hsp90alpha and Hsp90beta is 10-fold slower than that of yeast Hsp90. Despite these differences, our experiments suggest that the underlying enzymatic mechanisms are highly similar. In both cases, a concerted conformational rearrangement involving the N-terminal domains of both subunits is controlling the rate of ATP turnover, and N-terminal cross-talk determines the rate-limiting steps. Furthermore, similar to yeast Hsp90, the slow ATP hydrolysis by human Hsp90s can be stimulated up to over 100-fold by the addition of the co-chaperone Aha1 from either human or yeast origin. Together, our results show that the basic principles of the Hsp90 ATPase reaction are conserved between yeast and humans, including the dimerization of the N-terminal domains and its regulation by the repositioning of the ATP lid from its original position to a catalytically competent one.

A novel Hsp90 inhibitor to disrupt Hsp90/Cdc37 complex against pancreatic cancer cells

Pancreatic cancer is an aggressive disease with multiple biochemical and genetic alterations. Thus, a single agent to hit one molecular target may not be sufficient to treat this disease. The purpose of this study is to identify a novel Hsp90 inhibitor to disrupt protein-protein interactions of Hsp90 and its cochaperones for down-regulating many oncogenes simultaneously against pancreatic cancer cells. Here, we reported that celastrol disrupted Hsp90-Cdc37 interaction in the superchaperone complex to exhibit antitumor activity in vitro and in vivo. Molecular docking and molecular dynamic simulations showed that celastrol blocked the critical interaction of Glu33 (Hsp90) and Arg167 (Cdc37). Immunoprecipitation confirmed that celastrol (10 micromol/L) disrupted the Hsp90-Cdc37 interaction in the pancreatic cancer cell line Panc-1. In contrast to classic Hsp90 inhibitor (geldanamycin), celastrol (0.1-100 micromol/L) did not interfere with ATP binding to Hsp90. However, celastrol (1-5 micromol/L) induced Hsp90 client protein degradation (Cdk4 and Akt) by 70% to 80% and increased Hsp70 expression by 12-fold. Celastrol induced apoptosis in vitro and significantly inhibited tumor growth in Panc-1 xenografts. Moreover, celastrol (3 mg/kg) effectively suppressed tumor metastasis by more than 80% in RIP1-Tag2 transgenic mouse model with pancreatic islet cell carcinogenesis. The data suggest that celastrol is a novel Hsp90 inhibitor to disrupt Hsp90-Cdc37 interaction against pancreatic cancer cells.

The Hsp90 molecular chaperone: an open and shut case for treatment

The molecular chaperone Hsp90 (90 kDa heat-shock protein) is a remarkably versatile protein involved in the stress response and in normal homoeostatic control mechanisms. It interacts with 'client proteins', including protein kinases, transcription factors and others, and either facilitates their stabilization and activation or directs them for proteasomal degradation. By this means, Hsp90 displays a multifaceted ability to influence signal transduction, chromatin remodelling and epigenetic regulation, development and morphological evolution. Hsp90 operates as a dimer in a conformational cycle driven by ATP binding and hydrolysis at the N-terminus. The cycle is also regulated by a group of co-chaperones and accessory proteins. Here we review the biology of the Hsp90 molecular chaperone, emphasizing recent progress in our understanding of structure-function relationships and the identification of new client proteins. In addition we describe the exciting progress that has been made in the development of Hsp90 inhibitors, which are now showing promise in the clinic for cancer treatment. We also identify the gaps in our current understanding and highlight important topics for future research.

Molecular chaperones and photoreceptor function

Molecular chaperones facilitate and regulate protein conformational change within cells. This encompasses many fundamental cellular processes: including the correct folding of nascent chains; protein transport and translocation; signal transduction and protein quality control. Chaperones are, therefore, important in several forms of human disease, including neurodegeneration. Within the retina, the highly specialized photoreceptor cell presents a fascinating paradigm to investigate the specialization of molecular chaperone function and reveals unique chaperone requirements essential to photoreceptor function. Mutations in several photoreceptor proteins lead to protein misfolding mediated neurodegeneration. The best characterized of these are mutations in the molecular light sensor, rhodopsin, which cause autosomal dominant retinitis pigmentosa. Rhodopsin biogenesis is likely to require chaperones, while rhodopsin misfolding involves molecular chaperones in quality control and the cellular response to protein aggregation. Furthermore, the specialization of components of the chaperone machinery to photoreceptor specific roles has been revealed by the identification of mutations in molecular chaperones that cause inherited retinal dysfunction and degeneration. These chaperones are involved in several important cellular pathways and further illuminate the essential and diverse roles of molecular chaperones.

Structural models and binding site prediction of the C-terminal domain of human Hsp90: a new target for anticancer drugs

Heat shock protein 90 is a valuable target for anticancer drugs because of its role in the activation and stabilization of multiple oncogenic signalling proteins. While several compounds inhibit heat shock protein 90 by binding the N-terminal domain, recent studies have proved that the C-terminal domain is important for dimerization of the chaperone and contains an additional binding site for inhibitors. Heat shock protein 90 inhibition achieved with molecules binding to the C-terminal domain provides an additional and novel opportunity to design and develop drugs. Therefore, for the first time, we have investigated the structure and the dynamic behaviour of the C-terminal domain of human heat shock protein 90 with and without the small-middle domain, using homology modelling and molecular dynamics simulations. In addition, secondary structure predictions and peptide folding simulations proved useful to investigate a putative additional alpha-helix located between H18 and beta20 of the C-terminal domain. Finally, we used the structural information to infer the location of the binding site located in the C-terminal domain by using a number of computational tools. The predicted pocket is formed by two grooves located between helix H18, the loop downstream of H18 and the loop connecting helices H20 and H21 of each monomer of the C-terminal domain, with only two amino acids contributing from each middle domain.

p23/Sba1p protects against Hsp90 inhibitors independently of its intrinsic chaperone activity

The molecular chaperone Hsp90 assists a subset of cellular proteins and is essential in eukaryotes. A cohort of cochaperones contributes to and regulates the multicomponent Hsp90 machine. Unlike the biochemical activities of the cochaperone p23, its in vivo functions and the structure-function relationship remain poorly understood, even in the genetically tractable model organism Saccharomyces cerevisiae. The SBA1 gene that encodes the p23 ortholog in this species is not an essential gene. We found that in the absence of p23/Sba1p, yeast and mammalian cells are hypersensitive to Hsp90 inhibitors. This protective function of Sba1p depends on its abilities to bind Hsp90 and to block the Hsp90 ATPase and inhibitor binding. In contrast, the protective function of Sba1p does not require the Hsp90-independent molecular chaperone activity of Sba1p. The structure-function analysis suggests that Sba1p undergoes considerable structural rearrangements upon binding Hsp90 and that the large size of the p23/Sba1p-Hsp90 interaction surface facilitates maintenance of high affinity despite sequence divergence during evolution. The large interface may also contribute to preserving a protective function in an environment in which Hsp90 inhibitory compounds can be produced by various microorganisms.

Direct visualization of asymmetric adenine-nucleotide-induced conformational changes in MutL alpha

MutL alpha, the heterodimeric eukaryotic MutL homolog, is required for DNA mismatch repair (MMR) in vivo. It has been suggested that conformational changes, modulated by adenine nucleotides, mediate the interactions of MutL alpha with other proteins in the MMR pathway, coordinating the recognition of DNA mismatches by MutS alpha and the activation of MutL alpha with the downstream events that lead to repair. Thus far, the only evidence for these conformational changes has come from X-ray crystallography of isolated domains, indirect biochemical analyses, and comparison to other members of the GHL ATPase family to which MutL alpha belongs. Using atomic force microscopy (AFM), coupled with biochemical techniques, we demonstrate that adenine nucleotides induce large asymmetric conformational changes in full-length yeast and human MutL alpha and that these changes are associated with significant increases in secondary structure. These data reveal an ATPase cycle in which sequential nucleotide binding, hydrolysis, and release modulate the conformational states of MutL alpha.

The evolution of fungal drug resistance: modulating the trajectory from genotype to phenotype

The emergence of drug resistance in pathogenic microorganisms provides an excellent example of microbial evolution that has had profound consequences for human health. The widespread use of antimicrobial agents in medicine and agriculture exerts strong selection for the evolution of drug resistance. Selection acts on the phenotypic consequences of resistance mutations, which are influenced by the genetic variation in particular genomes. Recent studies have revealed a mechanism by which the molecular chaperone heat shock protein 90 (Hsp90) can alter the relationship between genotype and phenotype in an environmentally contingent manner, thereby 'sculpting' the course of evolution. Harnessing Hsp90 holds great promise for treating life-threatening infectious diseases.

Transforming growth factor alpha (TGFalpha)-stimulated secretion of HSP90alpha: using the receptor LRP-1/CD91 to promote human skin cell migration against a TGFbeta-rich environment during wound healing

Jump-starting and subsequently maintaining epidermal and dermal cell migration are essential processes for skin wound healing. These events are often disrupted in nonhealing wounds, causing patient morbidity and even fatality. Currently available treatments are unsatisfactory. To identify novel wound-healing targets, we investigated secreted molecules from transforming growth factor alpha (TGFalpha)-stimulated human keratinoytes, which contained strong motogenic, but not mitogenic, activity. Protein purification allowed us to identify the heat shock protein 90alpha (hsp90alpha) as the factor fully responsible for the motogenic activity in keratinocyte secretion. TGFalpha causes rapid membrane translocation and subsequent secretion of hsp90alpha via the unconventional exosome pathway in the cells. Secreted hsp90alpha promotes both epidermal and dermal cell migration through the surface receptor LRP-1 (LDL receptor-related protein 1)/CD91. The promotility activity resides in the middle domain plus the charged sequence of hsp90alpha but is independent of the ATPase activity. Neutralizing the extracellular function of hsp90alpha blocks TGFalpha-induced keratinicyte migration. Most intriguingly, unlike the effects of canonical growth factors, the hsp90alpha signaling overrides the inhibition of TGFbeta, an abundant inhibitor of dermal cell migration in skin wounds. This finding provides a long-sought answer to the question of how dermal cells migrate into the wound environment to build new connective tissues and blood vessels. Thus, secreted hsp90alpha is potentially a new agent for wound healing.

Membrane-associated stress proteins: more than simply chaperones

The protein- and/or lipid-mediated association of chaperone proteins to membranes is a widespread phenomenon and implicated in a number of physiological and pathological events that were earlier partially or completely overlooked. A temporary association of certain HSPs with membranes can re-establish the fluidity and bilayer stability and thereby restore the membrane functionality during stress conditions. The fluidity and microdomain organization of membranes are decisive factors in the perception and transduction of stresses into signals that trigger the activation of specific HS genes. Conversely, the membrane association of HSPs may result in the inactivation of membrane-perturbing signals, thereby switch off the heat shock response. Interactions between certain HSPs and specific lipid microdomains ("rafts") might be a previously unrecognized means for the compartmentalization of HSPs to specific signaling platforms, where key signaling proteins are known to be concentrated. Any modulations of the membranes, especially the raft-lipid composition of the cells can alter the extracellular release and thus the immuno-stimulatory activity of certain HSPs. Reliable techniques, allowing mapping of the composition and dynamics of lipid microdomains and simultaneously the spatio-temporal localization of HSPs in and near the plasma membrane can provide suitable means with which to address fundamental questions, such as how HSPs are transported to and translocated through the plasma membrane. The possession of such information is critical if we are to target the membrane association principles of HSPs for successful drug development in most various diseases.

Photophysical properties of popular fluorescent adenosine nucleotide analogs used in enzyme mechanism probing

Fluorescent nucleotide analogs are widely used in mechanistic studies of nucleotide binding and utilizing proteins. We describe here an overview of the photophysical parameters of the most popular nucleotide analogs that have a fluorescent N-methylanthraniloyl-group attached at various positions of the nucleotide. Steady state absorption and fluorescence spectra of free chromophores depend on the type of modification (ribose, base or phosphate moiety) and the addition of proteins suggests that the labeled nucleotides also vary in sensitivity depending upon their local protein environment. Fluorescence lifetime measurements imply two to three lifetimes for each nucleotide with complex changes in dependence on solvent but more importantly also on the protein. The measured quantum yields quantify the increase in fluorescence for (C8)-MABA-ADP, MANT-ATP and (Pgamma)-MABA-ATP as 153%, 93% and 14% when bound to DnaK, ClpB and Trap1, respectively, compared to free in buffer solution.

The ATPase cycle of the mitochondrial Hsp90 analog Trap1

Hsp90 is an ATP-dependent molecular chaperone whose mechanism is not yet understood in detail. Here, we present the first ATPase cycle for the mitochondrial member of the Hsp90 family called Trap1 (tumor necrosis factor receptor-associated protein 1). Using biochemical, thermodynamic, and rapid kinetic methods we dissected the kinetics of the nucleotide-regulated rearrangements between the open and the closed conformations. Surprisingly, upon ATP binding, Trap1 shifts predominantly to the closed conformation (70%), but, unlike cytosolic Hsp90 from yeast, this process is rather slow at 0.076 s(-1). Because reopening (0.034 s(-1)) is about ten times faster than hydrolysis (k(hyd) = 0.0039 s(-1)), which is the rate-limiting step, Trap1 is not able to commit ATP to hydrolysis. The proposed ATPase cycle was further scrutinized by a global fitting procedure that utilizes all relevant experimental data simultaneously. This analysis corroborates our model of a two-step binding mechanism of ATP followed by irreversible ATP hydrolysis and a one-step product (ADP) release.

Structural studies on the co-chaperone Hop and its complexes with Hsp90

The tetratricopeptide repeat domain (TPR)-containing co-chaperone Hsp-organising protein (Hop) plays a critical role in mediating interactions between Heat Shock Protein (Hsp)70 and Hsp90 as part of the cellular assembly machine. It also modulates the ATPase activity of both Hsp70 and Hsp90, thus facilitating client protein transfer between the two. Despite structural work on the individual domains of Hop, no structure for the full-length protein exists, nor is it clear exactly how Hop interacts with Hsp90, although it is known that its primary binding site is the C-terminal MEEVD motif. Here, we have undertaken a biophysical analysis of the structure and binding of Hop to Hsp90 using a variety of truncation mutants of both Hop and Hsp90, in addition to mutants of Hsp90 that are thought to modulate the conformation, in particular the N-terminal dimerisation of the chaperone. The results establish that whilst the primary binding site of Hop is the C-terminal MEEVD peptide of Hsp90, binding also occurs at additional sites in the C-terminal and middle domain. In contrast, we show that another TPR-containing co-chaperone, CyP40, binds solely to the C-terminus of Hsp90. Truncation mutants of Hop were generated and used to investigate the dimerisation interface of the protein. In good agreement with recently published data, we find that the TPR2a domain that contains the Hsp90-binding site is also the primary site for dimerisation. However, our results suggest that residues within the TPR2b may play a role. Together, these data along with shape reconstruction analysis from small-angle X-ray scattering measurements are used to generate a solution structure for full-length Hop, which we show has an overall butterfly-like quaternary structure. Studies on the nucleotide dependence of Hop binding to Hsp90 establish that Hop binds to the nucleotide-free, 'open' state of Hsp90. However, the Hsp90-Hop complex is weakened by the conformational changes that occur in Hsp90 upon ATP binding. Together, the data are used to propose a detailed model of how Hop may help present the client protein to Hsp90 by aligning the bound client on Hsp70 with the middle domain of Hsp90. It is likely that Hop binds to both monomers of Hsp90 in the form of a clamp, interacting with residues in the middle domain of Hsp90, thus preventing ATP hydrolysis, possibly by the prevention of association of N-terminal and middle domains in individual Hsp90 monomers.

A novel HSP90 chaperone complex regulates intracellular vesicle transport

Heat shock protein 90 (HSP90) is considered a specialized molecular chaperone that controls the folding of cell-regulatory proteins such as steroid receptors and kinases. However, its high abundance is suggestive of a more general function in other fundamental processes. Here, we show that HSP90 is required for vesicular protein transport in the cell. We have identified a novel chaperone complex comprising HSP90 and TPR1 that is recruited to the membrane protein VAP-33. Depletion of the TPR1 protein in mammalian cells inhibits transport of vesicular stomatitis virus glycoprotein (VSVG) and leads to accumulation of this cargo protein in the Golgi apparatus. Furthermore, trafficking of VSVG between Golgi stacks is dependent on the ATPase function of HSP90 and can be inhibited by drugs specific for HSP90. Our results identify a new role for HSP90 in protein sorting, pointing to a central role for this molecular chaperone in the cell.

Structures of GRP94-nucleotide complexes reveal mechanistic differences between the hsp90 chaperones

GRP94, an essential endoplasmic reticulum chaperone, is required for the conformational maturation of proteins destined for cell-surface display or export. The extent to which GRP94 and its cytosolic paralog, Hsp90, share a common mechanism remains controversial. GRP94 has not been shown conclusively to hydrolyze ATP or bind cochaperones, and both activities, by contrast, result in conformational changes and N-terminal dimerization in Hsp90 that are critical for its function. Here, we report the 2.4 A crystal structure of mammalian GRP94 in complex with AMPPNP and ADP. The chaperone is conformationally insensitive to the identity of the bound nucleotide, adopting a "twisted V" conformation that precludes N-terminal domain dimerization. We also present conclusive evidence that GRP94 possesses ATPase activity. Our observations provide a structural explanation for GRP94's observed rate of ATP hydrolysis and suggest a model for the role of ATP binding and hydrolysis in the GRP94 chaperone cycle.

The ATPase-dependent chaperoning activity of Hsp90a regulates thick filament formation and integration during skeletal muscle myofibrillogenesis

The mechanisms that regulate sarcomere assembly during myofibril formation are poorly understood. In this study, we characterise the zebrafish sloth(u45) mutant, in which the initial steps in sarcomere assembly take place, but thick filaments are absent and filamentous I-Z-I brushes fail to align or adopt correct spacing. The mutation only affects skeletal muscle and mutant embryos show no other obvious phenotypes. Surprisingly, we find that the phenotype is due to mutation in one copy of a tandemly duplicated hsp90a gene. The mutation disrupts the chaperoning function of Hsp90a through interference with ATPase activity. Despite being located only 2 kb from hsp90a, hsp90a2 has no obvious role in sarcomere assembly. Loss of Hsp90a function leads to the downregulation of genes encoding sarcomeric proteins and upregulation of hsp90a and several other genes encoding proteins that may act with Hsp90a during sarcomere assembly. Our studies reveal a surprisingly specific developmental role for a single Hsp90 gene in a regulatory pathway controlling late steps in sarcomere assembly.

Chaperone machines in action

How do chaperones operate in cells? For some major chaperones it is clear what they do, though mostly not how they do it. Hsp60, 70 and 100 families carry out folding, unfolding or disaggregation of proteins. Regarding mechanisms of action, we have the clearest picture of the ATP-driven mechanism of the bacterial Hsp60s, and structures of full-length Hsp70 and 90 family members are beginning to give insights into their allosteric mechanisms. Recent advances are giving an improved understanding of the nature of chaperone interactions with their non-native substrate proteins. There have also been significant advances in understanding the engagement of chaperones in preventing the formation of toxic aggregates in degenerative disease and the relationship of protein quality control to complex biological processes such as ageing.

Discovery and design of novel HSP90 inhibitors using multiple fragment-based design strategies

The molecular chaperone HSP90 has been shown to facilitate cancer cell survival by stabilizing key proteins responsible for a malignant phenotype. We report here the results of parallel fragment-based drug design approaches in the design of novel HSP90 inhibitors. Initial aminopyrimidine leads were elaborated using high-throughput organic synthesis to yield nanomolar inhibitors of the enzyme. Second site leads were also identified which bound to HSP90 in two distinct conformations, an 'open' and 'closed' form. Intriguingly, linked fragment approaches targeting both of these conformations were successful in producing novel, micromolar inhibitors. Overall, this study shows that, with only a few fragment hits, multiple lead series can be generated for HSP90 due to the inherent flexibility of the active site. Thus, ample opportunities exist to use these lead series in the development of clinically useful HSP90 inhibitors for the treatment of cancers.

A Grp on the Hsp90 mechanism

In a recent issue of Molecular Cell, Dollins et al. (2007) present the crystal structure of Grp94, which highlights the similarity between Grp94 and Hsp90 and provides insight into the resting state of Grp94 and potentially other Hsp90 family members.

Novobiocin and additional inhibitors of the Hsp90 C-terminal nucleotide-binding pocket

The 90 kDa heat shock proteins (Hsp90), which are integrally involved in cell signaling, proliferation, and survival, are ubiquitously expressed in cells. Many proteins in tumor cells are dependent upon the Hsp90 protein folding machinery for their stability, refolding, and maturation. Inhibition of Hsp90 uniquely targets client proteins associated with all six hallmarks of cancer. Thus, Hsp90 has emerged as a promising target for the treatment of cancer. Hsp90 exists as a homodimer, which contains three domains. The N-terminal domain contains an ATP-binding site that binds the natural products geldanamycin and radicicol. The middle domain is highly charged and has high affinity for co-chaperones and client proteins. Initial studies by Csermely and co-workers suggested a second ATP-binding site in the C-terminus of Hsp90. This C-terminal nucleotide binding pocket has been shown to not only bind ATP, but cisplatin, novobiocin, epilgallocatechin-3-gallate (EGCG) and taxol. The coumarin antibiotics novobiocin, clorobiocin, and coumermycin A1 were isolated from several streptomyces strains and exhibit potent activity against Gram-positive bacteria. These compounds bind type II topoisomerases, including DNA gyrase, and inhibit the enzyme-catalyzed hydrolysis of ATP. As a result, novobiocin analogues have garnered the attention of numerous researchers as an attractive agent for the treatment of bacterial infection. Novobiocin was reported to bind weakly to the newly discovered Hsp90 C-terminal ATP binding site ( approximately 700 M in SkBr3 cells) and induce degradation of Hsp90 client proteins. Structural modification of this compound has led to an increase of 1000-fold in activity in anti-proliferative assays. Recent studies of structure-activity relationship (SAR) by Renoir and co-workers highlighted the crucial role of the C-4 and/or C-7 positions of the coumarin and removal of the noviose moiety, which appeared to be essential for degradation of Hsp90 client proteins. Unlike the N-terminal ATP binding site, there is no reported co-crystal structure of Hsp90 C-terminus bound to any inhibitor. The Hsp90 C-terminal domain, however, is known to contain a conserved pentapeptide sequence (MEEVD) which is recognized by co-chaperones. Cisplatin is a platinum-containing chemotherapeutic used to treat various types of cancers, including testicular, ovarian, bladder, and small cell lung cancer. Most notably, cisplatin coordinates to DNA bases, resulting in cross-linked DNA, which prohibits rapidly dividing cells from duplicating DNA for mitosis. Itoh and co-workers reported that cisplatin decreases the chaperone activity of Hsp90. This group applied bovine brain cytosol to a cisplatin affinity column, eluted with cisplatin and detected Hsp90 in the eluent. Subsequent experiments indicated that cisplatin exhibits high affinity for Hsp90. Moreover Csermely and co-workers determined that the cisplatin binding site is located proximal to the C-terminal ATP binding site. EGCG is one of the active ingredients found in green tea. EGCG is known to inhibit the activity of many Hsp90-dependent client proteins, including telomerase, several kinases, and the aryl hydrocarbon receptor (AhR). Recently Gasiewicz and co-workers reported that EGCG manifests its antagonistic activity against AhR through binding Hsp90. Similar to novobiocin, EGCG was shown to bind the C-terminus of Hsp90. Unlike previously identified N-terminal Hsp90 inhibitors, EGCG does not appear to prevent Hsp90 from forming multiprotein complexes. Studies are currently underway to determine whether EGCG competes with novobiocin or cisplatin binding. Taxol, a well-known drug for the treatment of cancer, is responsible for the stabilization of microtubules and the inhibition of mitosis. Previous studies have shown that taxol induces the activation of kinases and transcription factors, and mimics the effect of bacterial lipopolysaccharide (LPS), an attribute unrelated to its tubulin-binding properties. Rosen and co-workers prepared a biotinylated taxol derivative and performed affinity chromatography experiments with lysates from both mouse brain and macrophage cell lines. These studies led to identification of two chaperones, Hsp70 and Hsp90, by mass spectrometry. In contrast to typical Hsp90-binding drugs, taxol exhibits a stimulatory response. Recently it was reported that the geldanamycin derivative 17-AAG behaves synergistically with taxol-induced apoptosis. This review describes the different C-terminal inhibitors of Hsp90, with specific emphasis on structure-activity relationship studies of novobiocin and their effects on anti-proliferative activity.

Three-dimensional structure of heat shock protein 90 from Plasmodium falciparum: molecular modelling approach to rational drug design against malaria

We have recently implicated heat shock protein 90 from Plasmodium falciparum (PfHsp90) as a potential drug target against malaria. Using inhibitors specific to the nucleotide binding domain of Hsp90, we have shown potent growth inhibitory effects on development of malarial parasite in human erythrocytes. To gain better understanding of the vital role played by PfHsp90 in parasite growth,we have modeled its three dimensional structure using recently described full length structure of yeast Hsp90.S equence similarity found between PfHsp90 and yeast Hsp90 allowed us to model the core structure with high confidence. The superimposition of the predicted structure with that of the template yeast Hsp90 structure reveals an RMSD of 3.31 Angstrom. The N-terminal and middle domains showed the least RMSD (1.76 Angstrom) while the more divergent C-terminus showed a greater RMSD (2.84 Angstrom) with respect to the template. The structure shows overall conservation of domains involved in nucleotide binding, ATPase activity, co-chaperone binding as well as inter-subunit interactions. Important co-chaperones known to modulate Hsp90 function in other eukaryotes are conserved in malarial parasite as well. An acidic stretch of amino acids found in the linker region, which is uniquely extended in PfHsp90 could not be modeled in this structure suggesting a flexible conformation. Our results provide a basis to compare the overall structure and functional pathways dependent on PfHsp90 in malarial parasite. Further analysis of differences found between human and parasite Hsp90 may make it possible to design inhibitors targeted specifically against malaria.

Structural and functional analysis of SGT1 reveals that its interaction with HSP90 is required for the accumulation of Rx, an R protein involved in plant immunity

SGT1 (for suppressor of G2 allele of skp1) and RAR1 (for required for Mla12 resistance) are highly conserved eukaryotic proteins that interact with the molecular chaperone HSP90 (for heat shock protein90). In plants, SGT1, RAR1, and HSP90 are essential for disease resistance triggered by a number of resistance (R) proteins. Here, we present structural and functional characterization of plant SGT1 proteins. Random mutagenesis of Arabidopsis thaliana SGT1b revealed that its CS (for CHORD-SGT1) and SGS (for SGT1 specific) domains are essential for disease resistance. NMR-based interaction surface mapping and mutational analyses of the CS domain showed that the CHORD II domain of RAR1 and the N-terminal domain of HSP90 interact with opposite sides of the CS domain. Functional analysis of the CS mutations indicated that the interaction between SGT1 and HSP90 is required for the accumulation of Rx, a potato (Solanum tuberosum) R protein. Biochemical reconstitution experiments suggest that RAR1 may function to enhance the SGT1-HSP90 interaction by promoting ternary complex formation.

The ATPase cycle of the endoplasmic chaperone Grp94

Grp94, the Hsp90 paralog of the endoplasmic reticulum, plays a crucial role in protein secretion. Like cytoplasmic Hsp90, Grp94 is regulated by nucleotide binding to its N-terminal domain. However, the question of whether Grp94 hydrolyzes ATP was controversial. This sets Grp94 apart from other members of the Hsp90 family where a slow but specific turnover of ATP has been unambiguously established. In this study we aimed at analyzing the nucleotide binding properties and the potential ATPase activity of Grp94. We show here that Grp94 has an ATPase activity comparable with that of yeast Hsp90 with a k(cat) of 0.36 min(-1) at 25 degrees C. Kinetic and equilibrium constants of the partial reactions of the ATPase cycle were determined using transient kinetic methods. Nucleotide binding appears to be tighter compared with other Hsp90s investigated, with dissociation constants (K(D)) of approximately 4 microm for ADP, ATP, and AMP-PCP. Interestingly, all nucleotides and inhibitors (radicicol, 5'-N-ethylcarboxamidoadenosine) studied here bind with similar rate constants for association (0.2-0.3 x 10(6) M(-1) s(-1)). Furthermore, there is a marked difference from cytosolic Hsp90s in that after binding, the ATP molecule does not seem to become trapped by conformational changes in Grp94. Grp94 stays predominantly in the open state concerning the nucleotide-binding pocket as evidenced by kinetic analyses. Thus, Grp94 shows mechanistically important differences in the interaction with adenosine nucleotides, but the basic hydrolysis reaction seems to be conserved between cytosolic and endoplasmic members of the Hsp90 family.

Dimerization of Hsp90 is required for in vivo function. Design and analysis of monomers and dimers

Heat shock protein 90 (Hsp90) plays a central role in signal transduction and has emerged as a promising target for anti-cancer therapeutics, but its molecular mechanism is poorly understood. At physiological concentration, Hsp90 predominantly forms dimers, but the function of full-length monomers in cells is not clear. Hsp90 contains three domains: the N-terminal and middle domains contribute directly to ATP binding and hydrolysis and the C domain mediates dimerization. To study the function of Hsp90 monomers, we used a single-chain strategy that duplicated the C-terminal dimerization domain. This novel monomerization strategy had the dual effect of stabilizing the C domain to denaturation and hindering intermolecular association of the ATPase domain. The resulting construct was predominantly monomeric at physiological concentration and did not function to support yeast viability as the sole Hsp90. The monomeric construct was also defective at ATP hydrolysis and the activation of a kinase and steroid receptor substrate in yeast cells. The ability to support yeast growth was rescued by the addition of a coiled-coil dimerization domain, indicating that the parental single-chain construct is functionally defective because it is monomeric.

Chaperone activation of the hepadnaviral reverse transcriptase for template RNA binding is established by the Hsp70 and stimulated by the Hsp90 system

Hepadnaviruses are DNA viruses that replicate by protein-primed reverse transcription, employing a specialized reverse transcriptase (RT), P protein. DNA synthesis from the pregenomic RNA is initiated by binding of P to the ε signal. Using ε as template and a Tyr-residue for initiation, the RT synthesizes a DNA oligo (priming) as primer for full-length DNA. Priming strictly requires prior RT activation by chaperones. Active P–ε complexes have been reconstituted in vitro, but whether in addition to the heat-shock protein 70 (Hsp70) system the Hsp90 system is essential has been controversial. Here we quantitatively compared Hsp70 versus Hsp70 plus Hsp90 RT activation, and corroborated that the Hsp70 system alone is sufficient; however, Hsp90 as well the Hsp70 nucleotide exchange factor Bag-1 markedly stimulated activation by increasing the steady-state concentration of the activated metastable RT form P*, though by different mechanisms. Hsp90 inhibition in intact cells by geldanamycin analogs blocked hepadnavirus replication, however not completely and only at severely cytotoxic inhibitor concentrations. While compatible with a basal level of Hsp90 independent in vivo replication, unambiguous statements are precluded by the simultaneous massive upregulation of Hsp70 and Hsp90.

The DNA maturation domain of gpA, the DNA packaging motor protein of bacteriophage lambda, contains an ATPase site associated with endonuclease activity

Terminase enzymes are common to double-stranded DNA (dsDNA) viruses and are responsible for packaging viral DNA into the confines of an empty capsid shell. In bacteriophage lambda the catalytic terminase subunit is gpA, which is responsible for maturation of the genome end prior to packaging and subsequent translocation of the matured DNA into the capsid. DNA packaging requires an ATPase catalytic site situated in the N terminus of the protein. A second ATPase catalytic site associated with the DNA maturation activities of the protein has been proposed; however, direct demonstration of this putative second site is lacking. Here we describe biochemical studies that define protease-resistant peptides of gpA and expression of these putative domains in Escherichia coli. Biochemical characterization of gpA-DeltaN179, a construct in which the N-terminal 179 residues of gpA have been deleted, indicates that this protein encompasses the DNA maturation domain of gpA. The construct is folded, soluble and possesses an ATP-dependent nuclease activity. Moreover, the construct binds and hydrolyzes ATP despite the fact that the DNA packaging ATPase site in the N terminus of gpA has been deleted. Mutation of lysine 497, which alters the conserved lysine in a predicted Walker A "P-loop" sequence, does not affect ATP binding but severely impairs ATP hydrolysis. Further, this mutation abrogates the ATP-dependent nuclease activity of the protein. These studies provide direct evidence for the elusive nucleotide-binding site in gpA that is directly associated with the DNA maturation activity of the protein. The implications of these results with respect to the two roles of the terminase holoenzyme, DNA maturation and DNA packaging, are discussed.

Potentiation of chemotherapeutics by the Hsp90 antagonist geldanamycin requires a steady serum condition

Inhibition of Hsp90 potentiates diverse chemotherapeutics, but it is not clear if this applies only to specific agents, tumor types or conditions. The aim of this report is to determine the effect of serum starvation (SS) on potentiation. SUIT2 cells were cultured with and without the presence of serum and 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assays were carried out at time intervals. Cytotoxic agents were added individually or in combination. Immunohistochemistry of tumor samples and immunofluorescence of cultured cells were used to examine Hsp90 localization. In the presence of serum an at least additive effect of combining the Hsp90 inhibitor geldanamycin (GA) with 5-fluorouracil (5FU) was demonstrated. Following pretreatment with GA, 5FU and GA were synergistic. However, during SS GA was protective against 5FU. Geldanamycin also protected cells from 12-O-tetradecanoylphorbol-13-acetate (TPA) during SS. Protection of cells is transitory, as after 24 h of SS GA again has an at least additive negative effect on vitality with 5FU or TPA. Serum starvation of pancreatic cancer cell lines causes normally largely cytoplasmic Hsp90 to become predominantly nuclear localized. Hsp90 nuclear localization was observed in pancreatic and prostate tumors. Hsp90 binding to a pro-apoptotic client could explain the transitory protection of cells by Hsp90 inhibition during SS. Although potentiation of chemotherapeutics by Hsp90 inhibition is probably a general phenomenon, design of clinical trials should take into account that continuous co-administration may be ineffective because of a balance of synergy of the drugs in some cells and mutual inhibition of the two drug activities in other cells.

Activation segment exchange: a common mechanism of kinase autophosphorylation?

The crystal structure of the kinase domain from human checkpoint kinase 2 (Chk2) has shown, for the first time, the reciprocal exchange of activation segments between two adjacent molecules and provides the molecular basis for understanding the observed mode of Chk2 kinase activation via trans-autophosphorylation. With further examples of activation segment exchanged kinase domains now publicly available (i.e. Ste20-like kinase, Ser/Thr kinase 10 and Death-associated protein kinase 3), we suggest that this phenomenon represents a common mechanism of activation amongst a particular subset of protein kinases, that is, those that are dimeric (either transiently or constitutively), that undergo activation by autophosphorylation and that have activation segment amino acid sequences that do not resemble those of their substrate consensus sequence.

Multiple 40-kDa heat-shock protein chaperones function in Tom70-dependent mitochondrial import

Mitochondrial preproteins that are imported via the translocase of the mitochondrial outer membrane (Tom)70 receptor are complexed with cytosolic chaperones before targeting to the mitochondrial outer membrane. The adenine nucleotide transporter (ANT) follows this pathway, and its purified mature form is identical to the preprotein. Purified ANT was reconstituted with chaperones in reticulocyte lysate, and bound proteins were identified by mass spectrometry. In addition to 70-kDa heat-shock cognate protein (Hsc70) and 90-kDa heat-shock protein (Hsp90), a specific subset of cochaperones were found, but no mitochondria-specific targeting factors were found. Interestingly, three different Hsp40-related J-domain proteins were identified: DJA1, DJA2, and DJA4. The DJAs bound preproteins to different extents through their C-terminal regions. DJA dominant-negative mutants lacking the N-terminal J-domains impaired mitochondrial import. The mutants blocked the binding of Hsc70 to preprotein, but with varying efficiency. The DJAs also showed significant differences in activation of the Hsc70 ATPase and Hsc70-dependent protein refolding. In HeLa cells, the DJAs increased new protein folding and mitochondrial import, although to different extents. No single DJA was superior to the others in all aspects, but each had a profile of partial specialization. The Hsp90 cochaperones p23 and Aha1 also regulated Hsp90-preprotein interactions. We suggest that multiple cochaperones with similar yet partially specialized properties cooperate in optimal chaperone-preprotein complexes.

HSP90 cross-links branched actin filaments induced by N-WASP and the Arp2/3 complex

N-WASP induces filopodial actin cytoskeleton through activation of the Arp2/3 complex. Here, we show that heat shock protein 90 (HSP90) regulates the structure of actin filaments induced by N-WASP and the Arp2/3 complex. HSP90 binds to N-WASP and to F-actin and bundles actin filaments. Bundling activity of HSP90 does not affect actin filament nucleation induced by N-WASP and the Arp2/3 complex. HSP90 is co-localized with N-WASP at branching points of actin filaments produced by the Arp2/3 complex and thereby bundles branched filaments; this bundled actin structure is inhibited by blocking direct binding between HSP90 and N-WASP. Furthermore, HSP90 converts branched actin filaments on N-WASP-coated beads to filopodia-like star-shaped bundles. These findings indicate that HSP90 promotes the formation of N-WASP/Arp2/3 complex-induced unbranched filopodial actin structures.