Binding of aryl hydrocarbon receptor (AhR) to AhR-interacting protein. The role of hsp90

The aryl hydrocarbon receptor (AhR) has been shown to interact with an immunophilin-like molecule known as AhR-interacting protein (AIP) and to enhance AhR function. We show here that AIP associates with AhR homologues from mouse and fish, which can bind ligands such as dioxin, but nonligand binding homologues from Caenorhabditis elegans or Drosophila do not bind to AIP. However, a minimal ligand-binding domain of the AhR is incapable of binding AIP. The binding of AIP to AhR in reticulocyte lysate shows several of the characteristics of an hsp90-dependent process, including sensitivity to geldanamycin and temperature and a requirement for ATP or nonhydrolyzable analogues. Purified AIP binds to the C terminus of hsp90, and mutation of a conserved basic residue in the tetratricopeptide repeats of AIP (K266A, analogous to K97A in protein phosphatase 5) abolishes binding to hsp90. Mutation of K266A in AIP reduces binding to AhR by 75-80%; the geldanamycin sensitivity of this complex shows that AhR stabilizes the AIP-hsp90-AhR complex. The alpha-helical C terminus of AIP, which is outside the tetratricopeptide repeat domain, is absolutely required for binding to AhR as shown by deletions of the C-terminal 5 amino acids or alanine-scanning mutagenesis, but it is not required for binding of AIP to hsp90. The data support a model where 1) AIP binds to both hsp90 and AhR; 2) hsp90 is required for AhR-AIP binding; and 3) the binding of AhR to AIP stabilizes the AIP-hsp90-AhR complex.     

Heat shock protein 90beta1 is essential for polyunsaturated fatty acid-induced mitochondrial Ca2+ efflux

Nonesterified fatty acids may influence mitochondrial function by alterations in gene expression, metabolism, and/or mitochondrial Ca(2+) ([Ca(2+)](m)) homeostasis. We have previously reported that polyunsaturated fatty acids induce Ca(2+) efflux from mitochondria, an action that may deplete [Ca(2+)](m) and thus contribute to nonesterified fatty acid-responsive mitochondrial dysfunction. Here we show that the chaperone protein heat shock protein 90 beta1 (hsp90beta1) is required for polyunsaturated fatty acid-induced mitochondrial Ca(2+) efflux (PIMCE). Retinoic acid induced differentiation of human teratocarcinoma NT2 cells in association with attenuation of PIMCE. Proteomic analysis of mitochondrial proteins revealed that hsp90beta1, among other proteins, was reduced in retinoic acid-differentiated cells. Blockade of PIMCE in NT2 cells by 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin, a known inhibitor of the chaperone activity of hsp90, and hsp90beta1 RNA interference demonstrated that hsp90beta1 is essential for PIMCE. We also show localization of hsp90beta1 in mitochondria by Western blot and immunofluorescence. Distinctive effects of inhibitors binding to the N or C terminus of hsp90 on PIMCE in isolated mitochondria suggested that the C terminus of hsp90beta1 plays a critical role in PIMCE.     

Regulation of the atrial natriuretic peptide receptor by heat shock protein 90 complexes

Heat shock protein 90 (hsp90) is a chaperone required for the proper folding and trafficking of many proteins involved in signal transduction. We tested whether hsp90 plays a role as a chaperone for GC-A, the membrane guanylate cyclase that acts as a receptor for atrial natriuretic peptide (ANP). When cultured cells expressing recombinant GC-A were treated with geldanamycin, an inhibitor of hsp90 function, the ANP-stimulated production of cyclic GMP was inhibited. This suggested that hsp90 was required for GC-A processing and/or stability. A physical association between hsp90 and GC-A was demonstrated in coimmunoprecipitation experiments. Treatment with geldanamycin disrupted this association and led to the accumulation of complexes containing GC-A and heat shock protein 70 (hsp70). Protein folding pathways involving hsp70 and hsp90 include several pathway-specific co-chaperones. Complexes between GC-A and hsp90 contained the co-chaperone p50(cdc37), typically found associated with protein kinase.hsp90 heterocomplexes. GC-A immunoprecipitates did not contain detectable amounts of Hop, FKBP51, FKBP52, PP5, or p23, all co-chaperones found in hsp90 complexes with other signaling proteins. The association of hsp90 and p50(cdc37) with GC-A was dependent on the kinase homology domain of this receptor but not on its ANP-binding, transmembrane, or guanylate cyclase domains. The data suggest that GC-A is regulated by hsp90 complexes similar to those involved in the maturation of protein kinases.     

Hsp90 chaperone activity requires the full-length protein and interaction among its multiple domains

Hsp90 is an abundant and ubiquitous protein involved in a diverse array of cellular processes. Mechanistically we understand little of the apparently complex interactions of this molecular chaperone. Recently, progress has been made in assigning some of the known functions of hsp90, such as nucleotide binding and peptide binding, to particular domains within the protein. We used fragments of hsp90 and chimeric proteins containing functional domains from hsp90 or its mitochondrial homolog, TRAP1, to study the requirements for this protein in the folding of firefly luciferase as well as in the prevention of citrate synthase aggregation. In agreement with others who have found peptide binding and limited chaperone ability in fragments of hsp90, we see that multiple fragments from hsp90 can prevent the aggregation of thermally denatured citrate synthase, a measure of passive chaperoning activity. However, in contrast to these results, the luciferase folding assay was found to be much more demanding. Here, folding is mediated by hsp70 and hsp40, requires ATP, and thus is a measure of active chaperoning. Hsp90 and the co-chaperone, Hop, enhance this process. This hsp90 activity was only observed using full-length hsp90 indicating that the cooperation of multiple functional domains is essential for active, chaperone-mediated folding.     

Inhibition of MDM2 by hsp90 contributes to mutant p53 stabilization

Stabilization and overexpression are hallmarks of mutant p53 found in nearly 50% of human tumors. Mutations in the conformation-sensitive core domain of p53 often lead to association with molecular chaperones such as hsp70 and hsp90. Inhibition of hsp90 function accelerates mutant p53 degradation. We recently found that expression of p53 core domain mutants inhibits MDM2 degradation, suggesting that mutant p53 can modulate MDM2 functions. In this report, we show that mutant p53 mediates formation of MDM2-p53-hsp90 complexes. Release of MDM2 from the p53-hsp90 complex after DNA damage restores MDM2 but not p53 turnover, whereas dissociation of hsp90 by geldanamycin increases the degradation of both MDM2 and mutant p53. Mutant p53 degradation after hsp90 inhibition requires MDM2 expression. The interaction between MDM2 and hsp90 is disrupted by the 2A10 antibody, which recognizes a site on MDM2 important for binding to alternative reading frame (ARF). Expression of mutant p53 prevents MDM2 from binding ARF and accumulating in the nucleolus in an hsp90-dependent fashion. These results suggest that hsp90 recruited by mutant p53 conceals the ARF-binding site on MDM2 and inhibits its ubiquitin-protein isopeptide ligase function, resulting in the stabilization of both mutant p53 and MDM2.     

The biochemical role of the heat shock protein 90 chaperone complex in establishing human telomerase activity

Telomerase is a ribonucleoprotein complex that synthesizes the G-rich DNA found at the 3'-ends of linear chromosomes. Human telomerase consists minimally of a catalytic protein (hTERT) and a template-containing RNA (hTR), although other proteins are involved in regulating telomerase activity in vivo. Several chaperone proteins, including hsp90 and p23, have demonstrable roles in establishing telomerase activity both in vitro and in vivo, and previous reports indicate that hsp90 and p23 are required for the reconstitution of telomerase activity from recombinant hTERT and hTR. Here we report that hTERT and hTR associate in the absence of a functional hsp90-p23 heterocomplex. We also report that hsp90 inhibitors geldanamycin and novobiocin inhibit recombinant telomerase even after telomerase is assembled. Inhibition by geldanamycin could be overcome by allowing telomerase to first bind its primer, suggesting a role for hsp90 in loading telomerase onto the telomere. Inhibition by novobiocin could not similarly be overcome but instead resulted in destabilization of the hTERT polypeptide. We propose that the hsp90-p23 complex fine tunes and stabilizes a functional telomerase structure, allowing primer loading and extension.     

The importance of ATP binding and hydrolysis by hsp90 in formation and function of protein heterocomplexes.

The chaperone hsp90 is capable of binding and hydrolyzing ATP. Using information on a related ATPase, DNA gyrase B, we selected three conserved residues in hsp90's ATP-binding domain for mutation. Two of these mutations eliminate nucleotide binding, while the third retains nucleotide binding but is apparently deficient in ATP hydrolysis. We first analyzed how these mutations affect hsp90's binding to the co-chaperones p23 and Hop, and to the hydrophobic resin, phenyl-Sepharose. These experiments showed that ATP's effects, specifically, increased affinity for p23 and decreased affinity for Hop and phenyl-Sepharose, are brought on by ATP binding alone. We also tested the ability of hsp90 mutants to assist hsp70, hsp40, and Hop in the refolding of denatured firefly luciferase. While hsp90 is capable of participating in this process in a nucleotide-independent manner, the ability to hydrolyze ATP markedly potentiates hsp90's effect. Finally, we assembled progesterone receptor heterocomplexes with hsp70, hsp40, Hop, p23, and wild type or mutant hsp90. While neither ATP binding nor hydrolysis was necessary to bind hsp90 to the receptor, mature complexes containing p23 and capable of hormone binding were only obtained with wild type hsp90.     

Progesterone receptor structure and function altered by geldanamycin, an hsp90-binding agent.

The assembly of progesterone receptor (PR) heterocomplexes in vitro involves at least eight components of the molecular chaperone machinery, and as earlier reports have shown, these proteins exhibit complex, dynamic, but ordered, interactions with one another and PR. Using the selective hsp90 binding agent geldanamycin (GA), we have found that PR assembly in vitro can be arrested at a previously observed intermediate assembly step. Like mature PR complexes, the intermediate complexes contain hsp90, but they differ from mature complexes by the presence of hsp70, p60, and p48 and the absence of immunophilins and p23. Arrest of PR assembly is likely due to GA's ability to directly block binding of p23 to hsp90. An important functional consequence of GA-mediated assembly arrest in vitro is the inability of the resulting PR complexes to bind progesterone, despite the presence of hsp90 in the receptor complexes. The biological significance of the in vitro observations is demonstrated by GA's ability to (i) rapidly block PR's hormone binding capacity in intact cells and (ii) alter the composition of COS cell PR complexes in a manner similar to that observed during in vitro reconstitutions. An updated model for the cyclic assembly pathway of PR complexes that incorporates the present findings with earlier results is presented.     

A heat shock protein complex isolated from rabbit reticulocyte lysate can reconstitute a functional glucocorticoid receptor-Hsp90 complex.

When unliganded glucocorticoid receptor that has been stripped free of associated proteins is incubated with rabbit reticulocyte lysate, the receptor becomes associated with the 70- and 90-kDa heat shock proteins (hsp70 and hsp90), and the untransformed state of the receptor is functionally reconstituted [Scherrer, L. C., Dalman, F. C., Massa, E., Meshinchi, S., & Pratt, W. B. (1990) J. Biol. Chem. 265, 21397-21400]. Recently, an hsp70-containing protein complex (200-250 kDa) purified from rabbit reticulocyte lysate was shown to maintain a fusion protein bearing the mitochondrial matrix-targeting signal in a state that is competent for mitochondrial import [Sheffield, W. P., Shore, G. C., & Randall, S. K. (1990) J. Biol. Chem. 265, 11069-11076]. In this work, we show that this partially purified mitochondrial import-competent fraction contains both hsp90 and hsp70. When the purified fraction is immunoadsorbed with a monoclonal antibody specific for hsp90, a significant portion of the hsp70 is co-immunoadsorbed, suggesting that hsp90 and hsp70 are present together as a complex. The partially purified fraction maintains a hybrid precursor protein containing the mitochondrial matrix-targeting signal of rat pre-ornithine carbamyl transferase in an import-competent state. Incubation of immunopurified glucocorticoid receptor with this fraction of reticulocyte lysate results in ATP-dependent association of the receptor with both hsp70 and hsp90, and the resulting complexes are functional as assessed by return of the receptor to the high-affinity steroid binding conformation. The glucocorticoid receptor hetero-complex reconstituting activity of the lysate fraction is low relative to its mitochondrial import activity. Importantly, however, this is the first demonstration of the functional and structural reconstitution of the untransformed state of any steroid receptor utilizing a partially purified system.     

Heat shock protein 90 mediates protein-protein interactions between human aminoacyl-tRNA synthetases

Heat shock protein 90 (hsp90) is a molecular chaperone responsible for protein folding and maturation in vivo. Interaction of hsp90 with human glutamyl-prolyl-tRNA synthetase (EPRS) was found by genetic screening, co-immunoprecipitation, and in vitro binding experiments. This interaction was sensitive to the hsp90 inhibitor, geldanamycin, and also ATP, suggesting that the chaperone activity of hsp90 is required for interaction with EPRS. Interaction of EPRS with hsp90 was targeted to the region of three tandem repeats linking the two catalytic domains of EPRS that is also responsible for the interaction with isoleucyl-tRNA synthetase (IRS). Interaction of EPRS and IRS also depended on the activity of hsp90, implying that their association was mediated by hsp90. EPRS and IRS form a macromolecular protein complex with at least six other tRNA synthetases and three cofactors. hsp90 preferentially binds to most of the complex-forming enzymes rather than those that are not found in the complex. In addition, inactivation of hsp90 interfered with the in vivo incorporation of the nascent aminoacyl-tRNA synthetases into the multi-ARS complex. Thus, hsp90 appears to mediate protein-protein interactions of mammalian tRNA synthetases.     

The receptor for heat shock protein 60 on macrophages is saturable, specific, and distinct from receptors for other heat shock proteins.

Previous studies have shown that human heat shock protein (hsp) 60 elicits a strong proinflammatory response in cells of the innate immune system with CD14, Toll-like receptor (TLR) 2, and TLR4 as mediators of signaling, but probably not of binding. In the present study, we directly demonstrate binding of hsp60 to the macrophage surface and find the binding receptor for hsp60 different from the previously described common receptor for several other heat shock proteins, including hsp70, hsp90, and gp96. Fluorescence-labeled human hsp60 bound to cell surfaces of the murine macrophage lines J774 A.1 and RAW264.7 and to mouse bone marrow-derived macrophages. By flow cytometry, we could demonstrate for the first time that hsp60 binding to macrophages occurred at submicromolar concentrations, is saturable, and can be competed by unlabeled hsp60, but not by unrelated proteins, thus confirming the classic characteristics of specific ligand-receptor interactions. Binding of hsp60 at 4 degrees C was followed by endocytosis at 37 degrees C. Hsp60 binding to macrophages could not be competed by excess hsp70, hsp90, or gp96, all of which share the alpha(2)-macroglobulin receptor as binding site. Hsp60 binding occurred in the absence of surface TLR4. However, no cytokine response was induced by hsp60 in TLR4-deficient macrophages. We conclude that hsp60 binds to a stereo-specific receptor on macrophages, and that different surface molecules are engaged in binding and signal transduction. Furthermore, the binding site for hsp60 is separate from the common receptor for hsp70, hsp90, and gp96, which suggests an independent role of hsp60 as danger Ag and in immunoregulation.     

Molybdate inhibits hsp90, induces structural changes in its C-terminal domain, and alters its interactions with substrates

To examine the biochemical mechanism by which hsp90 exerts its essential positive function on certain signal transduction proteins, we characterized the effects of molybdate and geldanamycin on hsp90 function and structure. Molybdate inhibited hsp90-mediated p56lck biogenesis and luciferase renaturation while enforcing salt-stable interactions with these substrates. Molybdate also reduced the amount of free hsp90 present in cell lysates, inhibited hsp90's ability to bind geldanamycin, and induced resistance to proteolysis at a specific region within the C-terminal domain of hsp90. In contrast, the hsp90 inhibitor geldanamycin prevented hsp90 from assuming natural or molybdate-induced conformations that allow salt-stable interactions with substrates. When these compounds were applied sequentially, the order of addition determined the effects observed, indicating that these agents had opposing effects on hsp90. We conclude that a specific region within the C-terminal domain of hsp90 (near residue 600) determines the mode by which hsp90 interacts with substrates and that the ability of hsp90 to cycle between alternative modes of interaction is obligatory for hsp90 function.     

Visualization and mechanism of assembly of a glucocorticoid receptor.Hsp70 complex that is primed for subsequent Hsp90-dependent opening of the steroid binding cleft

A minimal system of five proteins, hsp90, hsp70, Hop, hsp40, and p23, assembles glucocorticoid receptor (GR).hsp90 heterocomplexes and causes the simultaneous opening of the steroid binding cleft to access by steroid. The first step in assembly is the ATP-dependent and hsp40 (YDJ-1)-dependent formation of a GR.hsp70 complex that primes the receptor for subsequent ATP-dependent activation by hsp90, Hop, and p23. This study focuses on three aspects of the GR priming reaction with hsp70. First, we have visualized the primed GR.hsp70 complexes by atomic force microscopy, and we find the most common stoichiometry to be 1:1, with some complexes of a size approximately 1:2 and a few complexes of larger size. Second, in a recent study of progesterone receptor priming, it was shown that hsp40 binds first, leading to the notion that it targets hsp70 to the receptor. We show here that hsp40 does not perform such a targeting function in priming the GR. Third, we focus on a short amino-terminal segment of the ligand binding domain that is required for GR.hsp90 heterocomplex assembly. By using two glutathione S-transferase (GST)/ligand binding domain fusions with (GST/520C) and without (GST/554C) hsp90 binding and steroid binding activity, we show that the priming step with hsp70 occurs with GST/554C, and it is the subsequent assembly step with hsp90 that is defective.     

Identification of a 60-kilodalton stress-related protein, p60, which interacts with hsp90 and hsp70.

Immunoaffinity purification of hsp90 from chick oviduct cytosol reveals two major proteins, hsp70 and a 60-kDa protein (p60), copurifying with hsp90. A similar result is obtained when hsp90 is immunoaffinity purified from chick liver and brain cytosols, avian fibroblasts, and rabbit reticulocyte lysate. This p60 is the same protein previously identified in certain assembly complexes of chick progesterone receptor generated in a cell-free reconstitution system. Tryptic and cyanogen bromide peptide fragments were generated from gel-purified p60, and partial N-terminal sequences were determined from eight peptides. The sequences show a striking similarity to the sequence of a 63-kDa human protein (IEF SSP 3521) whose abundance is increased in MRC-5 fibroblasts following simian virus 40 transformation. A monoclonal antibody was prepared against avian p60; Western immunoblot analysis showed that p60 was present in each of eight chick tissues examined and in each of the human, rat, rabbit, and Xenopus tissues tested. Immunoaffinity purifications from both chick oviduct cytosol and rabbit reticulocyte lysate using anti-p60 and anti-hsp70 monoclonal antibodies confirm that there is a relatively abundant complex in these extracts containing hsp90, hsp70, and p60. This complex appears to comprise an important functional unit in the assembly of progesterone receptor complexes. However, judging from the abundance and widespread occurrence of this multiprotein complex, hsp90, hsp70, and p60 probably function interactively in other systems as well.     

Regulation of heat shock protein 90 ATPase activity by sequences in the carboxyl terminus.

Hsp90, in addition to being an abundant and pivotal cytoplasmic chaperone protein, has been shown to be a weak ATPase. In an effort to characterize the ATPase activity of hsp90, we have observed marked differences in activities among various species of hsp90. Chicken or human hsp90 hydrolyzed ATP with a k(cat) of 0.02 min(-1) and a K(m) greater than 300 microm. In contrast, yeast hsp90 and TRAP1, an hsp90 homologue found in mitochondria, were 10-100-fold more active as ATPases. Sedimentation studies confirmed that all are dimeric proteins. Chicken hsp90 mutants were then analyzed to identify regions within the protein that influence ATPase activity. A truncation mutant of chicken hsp90, N1-573, was found to be monomeric, and yet the catalytic efficiency (k(cat)/K(m)) was greater than 100 times that of the full-length protein (k(cat) of 0.24 min(-1) and K(m) of 60 microm). In contrast, an internal deletion mutant, Delta661-677, was also monomeric but failed to hydrolyze ATP. Finally, deletion of the last 30 amino acids resulted in a dimeric protein with an ATPase activity very similar to full-length hsp90. These data indicate that sequences within the last one-fourth of hsp90 regulate ATP hydrolysis.