Dimerization characteristics of the 94-kDa glucose-regulated protein

Activation of autophagy depends on Atg1/Ulk1-mediated phosphorylation and inhibition of the Hsp90 chaperone machinery

Cellular homeostasis relies on both the chaperoning of proteins and the intracellular degradation system that delivers cytoplasmic constituents to the lysosome, a process known as autophagy. The crosstalk between these processes and their underlying regulatory mechanisms is poorly understood. Here, we show that the molecular chaperone heat shock protein 90 (Hsp90) forms a complex with the autophagy-initiating kinase Atg1 (yeast)/Ulk1 (mammalian), which suppresses its kinase activity. Conversely, environmental cues lead to Atg1/Ulk1-mediated phosphorylation of a conserved serine in the amino domain of Hsp90, inhibiting its ATPase activity and altering the chaperone dynamics. These events impact a conformotypic peptide adjacent to the activation and catalytic loop of Atg1/Ulk1. Finally, Atg1/Ulk1-mediated phosphorylation of Hsp90 leads to dissociation of the Hsp90:Atg1/Ulk1 complex and activation of Atg1/Ulk1, which is essential for initiation of autophagy. Our work indicates a reciprocal regulatory mechanism between the chaperone Hsp90 and the autophagy kinase Atg1/Ulk1 and consequent maintenance of cellular proteostasis.

Heterologous aggregates promote de novo prion appearance via more than one mechanism

Prions are self-perpetuating conformational variants of particular proteins. In yeast, prions cause heritable phenotypic traits. Most known yeast prions contain a glutamine (Q)/asparagine (N)-rich region in their prion domains. [PSI⁺], the prion form of Sup35, appears de novo at dramatically enhanced rates following transient overproduction of Sup35 in the presence of [PIN⁺], the prion form of Rnq1. Here, we establish the temporal de novo appearance of Sup35 aggregates during such overexpression in relation to other cellular proteins. Fluorescently-labeled Sup35 initially forms one or a few dots when overexpressed in [PIN⁺] cells. One of the dots is perivacuolar, colocalizes with the aggregated Rnq1 dot and grows into peripheral rings/lines, some of which also colocalize with Rnq1. Sup35 dots that are not near the vacuole do not always colocalize with Rnq1 and disappear by the time rings start to grow. Bimolecular fluorescence complementation failed to detect any interaction between Sup35-VN and Rnq1-VC in [PSI⁺][PIN⁺] cells. In contrast, all Sup35 aggregates, whether newly induced or in established [PSI⁺], completely colocalize with the molecular chaperones Hsp104, Sis1, Ssa1 and eukaryotic release factor Sup45. In the absence of [PIN⁺], overexpressed aggregating proteins such as the Q/N-rich Pin4C or the non-Q/N-rich Mod5 can also promote the de novo appearance of [PSI⁺]. Similar to Rnq1, overexpressed Pin4C transiently colocalizes with newly appearing Sup35 aggregates. However, no interaction was detected between Mod5 and Sup35 during [PSI⁺] induction in the absence of [PIN⁺]. While the colocalization of Sup35 and aggregates of Rnq1 or Pin4C are consistent with the model that the heterologous aggregates cross-seed the de novo appearance of [PSI⁺], the lack of interaction between Mod5 and Sup35 leaves open the possibility of other mechanisms. We also show that Hsp104 is required in the de novo appearance of [PSI⁺] aggregates in a [PIN⁺]-independent pathway.

Certain proteins can misfold into β-sheet-rich, self-seeding aggregates. Such proteins appear to be associated with neurodegenerative diseases such as prion, Alzheimer's and Parkinson's. Yeast prions also misfold into self-seeding aggregates and provide a good model to study how these rogue polymers first appear. De novo prion appearance can be made very frequent in yeast by transient overexpression of the prion protein in the presence of heterologous prions or prion-like aggregates. Here, we show that the aggregates of one such newly induced prion are initially formed in a dot-like structure near the vacuole. These dots then grow into rings at the periphery of the cell prior to becoming smaller rings surrounding the vacuole and maturing into the characteristic heritable prion tiny dots found throughout the cytoplasm. We found considerable colocalization of two heterologous prion/prion-like aggregates with the newly appearing prion protein aggregates, which is consistent with the prevalent model that existing prion aggregates can cross-seed the de novo aggregation of a heterologous prion protein. However, we failed to find any physical interaction between another heterologous aggregating protein and the newly appearing prion aggregates it stimulated to appear, which is inconsistent with cross-seeding.

GRP94 in ER quality control and stress responses

A system of endoplasmic reticulum (ER) chaperones has evolved to optimize the output of properly folded secretory and membrane proteins. An important player in this network is Glucose Regulated Protein 94 (GRP94). Over the last decade, new structural and functional data have begun to delineate the unique characteristics of GRP94 and have solidified its importance in ER quality control pathways. This review describes our current understanding of GRP94 and the four ways in which it contributes to the ER quality control: (1) chaperoning the folding of proteins; (2) interacting with other components of the ER protein folding machinery; (3) storing calcium; and (4) assisting in the targeting of malfolded proteins to ER-associated degradation (ERAD).

Functional characterization of orchardgrass endoplasmic reticulum-resident Hsp90 (DgHsp90) as a chaperone and an ATPase

Hsp90 proteins are essential molecular chaperones regulating multiple cellular processes in distinct subcellular organelles. In this study, we report the functional characterization of a cDNA encoding endoplasmic reticulum (ER)-resident Hsp90 from orchardgrass (DgHsp90). DgHsp90 is a 2742bp cDNA with an open reading frame predicted to encode an 808 amino acid protein. DgHsp90 has a well conserved N-terminal ATPase domain and a C-terminal Hsp90 domain and ER-retention motif. Expression of DgHsp90 increased during heat stress at 35 degrees C or H(2)O(2) treatment. DgHsp90 also functions as a chaperone protein by preventing thermal aggregation of malate dehydrogenase (EC 1.1.1.37) and citrate synthase (EC 2.3.3.1). The intrinsic ATPase activity of DgHsp90 was inhibited by geldanamycin, an Hsp90 inhibitor, and the inhibition reduced the chaperone activity of DgHsp90. Yeast cells overexpressing DgHsp90 exhibited enhanced thermotolerance.

Substitution of only two residues of human Hsp90alpha causes impeded dimerization of Hsp90beta

Two isoforms of the 90-kDa heat-shock protein (Hsp90), i.e., Hsp90alpha and Hsp90beta, are expressed in the cytosol of mammalian cells. Although Hsp90 predominantly exists as a dimer, the dimer-forming potential of the beta isoform of human and mouse Hsp90 is less than that of the alpha isoform. The 16 amino acid substitutions located in the 561-685 amino acid region of the C-terminal dimerization domain should be responsible for this impeded dimerization of Hsp90beta (Nemoto T, Ohara-Nemoto Y, Ota M, Takagi T, Yokoyama K. Eur J Biochem 233: 1-8, 1995). The present study was performed to define the amino acid substitutions that cause the impeded dimerization of Hsp90beta. Bacterial two-hybrid analysis revealed that among the 16 amino acids, the conversion from Ala(558) of Hsp90beta to Thr(566) of Hsp90alpha and that from Met(621) of Hsp90beta to Ala(629) of Hsp90alpha most efficiently reversed the dimeric interaction, and that the inverse changes from those of Hsp90alpha to Hsp90beta primarily explained the impeded dimerization of Hsp90beta We conclude that taken together, the conversion of Thr(566) and Ala(629) of Hsp90alpha to Ala(558) and Met(621) is primarily responsible for impeded dimerization of Hsp90beta.

Identification of novel quaternary domain interactions in the Hsp90 chaperone, GRP94

The structural basis for the coupling of ATP binding and hydrolysis to chaperone activity remains a central question in Hsp90 biology. By analogy to MutL, ATP binding to Hsp90 is thought to promote intramolecular N-terminal dimerization, yielding a molecular clamp functioning in substrate protein activation. Though observed in studies with recombinant domains, whether such quaternary states are present in native Hsp90s is unknown. In this study, native subunit interactions in GRP94, the endoplasmic reticulum Hsp90, were analyzed using chemical cross-linking in conjunction with tandem mass spectrometry. We report the identification of two distinct intermolecular interaction sites. Consistent with previous studies, one site comprises the C-terminal dimerization domain. The remaining site represents a novel intermolecular contact between the N-terminal and middle (M) domains of opposing subunits. This N+M domain interaction was present in the nucleotide-empty, ADP-, ATP-, or geldanamycin-bound states and could be selectively disrupted upon addition of synthetic geldanamycin dimers. These results identify a compact, intertwined quaternary conformation of native GRP94 and suggest that intersubunit N+M interactions are integral to the structural biology of Hsp90.

Conformational lability of two molecular chaperones Hsc70 and gp96: effects of pH and temperature

Hsc70 and gp96 are two heat shock proteins with molecular chaperone and immune-related activities. The dynamic conformational properties of heat shock proteins appear to play a critical role in their biological activities. In this study, we investigated the effects of pH and temperature on the conformational states of Hsc70 and gp96. The quaternary, tertiary, and secondary structures of both proteins are evaluated by a variety of spectroscopic techniques, including far-UV circular dichroism, Trp fluorescence, ANS fluorescence, and derivative UV absorption spectroscopy. The results are summarized and compared employing an empirical phase diagram approach. Very similar behaviors are seen for both proteins despite their differences in sequence and tertiary structure. Both proteins show substantial conformational lability in responses to the pH and temperature changes of their environment. This study suggests a natural selection for related functional properties through common conformational dynamics rather than immediate structural homology.

Changes in spatial and temporal localization of Dictyostelium homologues of TRAP1 and GRP94 revealed by immunoelectron microscopy

TRAP1 (tumor necrosis factor receptor-associated protein 1) is a member of the molecular chaperone HSP90 (90-kDa heat shock protein) family. In this study, we mainly examined the behavior of Dictyostelium TRAP1 homologue, Dd-TRAP1, during Dictyostelium development by immunoelectron microscopy. In vegetatively growing D. discoideum Ax-2 cells, Dd-TRAP1 locates in nucleolus and vesicles in addition to the cell cortex including cell membrane. Many of Dd-TRAP1 molecules moved to the mitochondrial matrix in response to differentiation, although Dd-TRAP1 on the cell membrane seems to be retained. Some Dd-TRAP1 was also found to be secreted to locate outside the cell membrane in Ax-2 cells starved for 6 h. At the multicellular slug stage, Dd-TRAP1 was primarily located in mitochondria and cell membrane in both prestalk and prespore cells. More importantly, in differentiating prespore cells, a significant number of Dd-TRAP1 locates in the PSV (prespore-specific vacuole) that is a sole cell type-specific organelle and essential for spore wall formation, whereas some Dd-TRAP1 in the cell cortical region of prestalk cells. These findings strongly suggest the importance of Dd-TRAP1 regulated temporally and spatially during Dictyostelium development. Incidentally, we also have certified that the glucose-regulated protein 94 (Dd-GRP94) is predominantly located in Golgi vesicles and cisternae, followed by its colocalization with Dd-TRAP1 in the PSV.

Identification of the pentapeptide constituting a dominant epitope common to all eukaryotic heat shock protein 90 molecular chaperones

We previously reported that, in human heat shock protein (Hsp) 90 (hHsp90), there are 4 highly immunogenic sites, designated sites Ia, Ib, Ic, and II. This study was performed to further characterize their epitopes and to identify the epitope that is potentially common to all members of the Hsp90 family. Panning of a bacterial library carrying randomized dodecapeptides revealed that Glu251-Ser-X-Asp254 constituted site Ia and Pro295-Ile-Trp-Thr-Arg299, site Ic. Site II (Asp701-Pro717) was composed of several epitopes. When 19 anti-hHsp90 monoclonal antibodies (mAbs) were subjected to immunoblotting against recombinant forms of 7 Hsp90-family members, 2 mAbs (K41110 and K41116C) that recognized site Ic bound to yeast Hsp90 with affinity identical to that for hHsp90, and 1 mAb (K3729) that recognized Glu222-Ala23, of hHsp90beta could bind to human 94-kDa glucose-regulated protein (Grp94), an endoplasmic reticulum paralog of Hsp90. Among the 5 amino acids constituting site Ic, Trp297 and Pro295 were essential for recognition by all anti-site-Ic mAbs, and Arg299 was important for most of them. The necessity of Ile296, Thr298, and Arg299, which are replaced by Leu, Met/Leu, and Lys, respectively, in some eukaryotic Hsp90, was dependent on the mAbs, and K41110 and K41116C could react with Hsp90s carrying these substitutions. From these data taken together, we propose that the pentapeptide Pro295-Ile-Trp-Thr-Arg299 of hHsp90 functions as an immunodominant epitope common to all eukaryotic Hsp90.

The region adjacent to the highly immunogenic site and shielded by the middle domain is responsible for self-oligomerization/client binding of the HSP90 molecular chaperone

We here investigated the mechanism of self-oligomerization of the 90-kDa heat shock protein (HSP90) molecular chaperone, because it is known that this oligomerization reflects the client-binding activity. The transition temperatures for the self-oligomerization of the full-length forms of human HSP90alpha and HtpG (bacterial HSP90), i.e., 45 and 60 degrees C, respectively, were identical to those for the dissociation of the recombinant N domain (residues 1-400 of human HSP90alpha and residues 1-336 of HtpG in our definition) from the remainder of the molecule. The N domain of human HSP90alpha expressed in Escherichia coli was oligomeric, and the oligomerization activity was localized within residues 311-350, i.e., C-terminally adjacent to the highly immunogenic site (residues 291-304). Particularly, residues 341-350 were critical on oligomerization. On the other hand, residues 289-389 were indispensable for the interaction with the M domain (residues 401-618) of the molecule. Oligomer formation of the N domain was efficiently suppressed by its extension until Lys546, i.e., residues 401-546, which is required for the interaction with the N domain. Among highly conserved amino acids at residues 289-400, Trp297, Pro379, and Phe384 were essential for the interaction with the M domain. With these observations taken together, we propose as the activation mechanism of HSP90 molecular chaperone that heat stress induces the liberation of the oligomerization/client-binding site of residues 311-350 by disrupting the intramolecular interaction between residues 289-389 and 401-546.

The crystal structure of the carboxy-terminal dimerization domain of htpG, the Escherichia coli Hsp90, reveals a potential substrate binding site

Hsp90 is a ubiquitous, well-conserved molecular chaperone involved in the folding and stabilization of diverse proteins. Beyond its capacity for general protein folding, Hsp90 influences a wide array of cellular signaling pathways that underlie key biological and disease processes. It has been proposed that Hsp90 functions as a molecular clamp, dimerizing through its carboxy-terminal domain and utilizing ATP binding and hydrolysis to drive large conformational changes including transient dimerization of the amino-terminal and middle domains. We have determined the 2.6 A X-ray crystal structure of the carboxy-terminal domain of htpG, the Escherichia coli Hsp90. This structure reveals a novel fold and that dimerization is dependent upon the formation of a four-helix bundle. Remarkably, proximal to the helical dimerization motif, each monomer projects a short helix into solvent. The location, flexibility, and amphipathic character of this helix suggests that it may play a role in substrate binding and hence chaperone activity.

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.

A hydrophobic segment within the C-terminal domain is essential for both client-binding and dimer formation of the HSP90-family molecular chaperone

The alpha isoform of human 90-kDa heat shock protein (HSP90alpha) is composed of three domains: the N-terminal (residues 1-400); middle (residues 401-615) and C-terminal (residues 621-732). The middle domain is simultaneously associated with the N- and C-terminal domains, and the interaction with the latter mediates the dimeric configuration of HSP90. Besides one in the N-terminal domain, an additional client-binding site exists in the C-terminal domain of HSP90. The aim of the present study is to elucidate the regions within the C-terminal domain responsible for the bindings to the middle domain and to a client protein, and to define the relationship between the two functions. A bacterial two-hybrid system revealed that residues 650-697 of HSP90alpha were essential for the binding to the middle domain. An almost identical region (residues 657-720) was required for the suppression of heat-induced aggregation of citrate synthase, a model client protein. Replacement of either Leu665-Leu666 or Leu671-Leu672 to Ser-Ser within the hydrophobic segment (residues 662-678) of the C-terminal domain caused the loss of bindings to both the middle domain and the client protein. The interaction between the middle and C-terminal domains was also found in human 94-kDa glucose-regulated protein. Moreover, Escherichia coli HtpG, a bacterial HSP90 homologue, formed heterodimeric complexes with HSP90alpha and the 94-kDa glucose-regulated protein through their middle-C-terminal domains. Taken together, it is concluded that the identical region including the hydrophobic segment of the C-terminal domain is essential for both the client binding and dimer formation of the HSP90-family molecular chaperone and that the dimeric configuration appears to be similar in the HSP90-family proteins.

Interaction between the N-terminal and middle regions is essential for the in vivo function of HSP90 molecular chaperone

At the primary structure level, the 90-kDa heat shock protein (HSP90) is composed of three regions: the N-terminal (Met(1)-Arg(400)), middle (Glu(401)-Lys(615)), and C-terminal (Asp(621)-Asp(732)) regions. In the present study, we investigated potential subregion structures of these three regions and their roles. Limited proteolysis revealed that the N-terminal region could be split into two fragments carrying residues Met(1) to Lys(281) (or Lys(283)) and Glu(282) (or Tyr(284)) to Arg(400). The former is known to carry the ATP-binding domain. The fragments carrying the N-terminal two-thirds (Glu(401)-Lys(546)) and C-terminal one-third of the middle region were sufficient for the interactions with the N- and C-terminal regions, respectively. Yeast HSC82 that carried point mutations in the middle region causing deficient binding to the N-terminal region could not support the growth of HSP82-depleted cells at an elevated temperature. Taken together, our data show that the N-terminal and middle regions of the HSP90 family protein are structurally divided into two respective subregions. Moreover, the interaction between the N-terminal and middle regions is essential for the in vivo function of HSP90 in yeast.

Domain-domain interactions of HtpG, an Escherichia coli homologue of eukaryotic HSP90 molecular chaperone

In the present study, we investigated the domain structure and domain-domain interactions of HtpG, an Escherichia coli homologue of eukaryotic HSP90. Limited proteolysis of recombinant HtpG, revealed three major tryptic sites, i.e. Arg7-Gly8, Arg336-Glu337 and Lys552-Leu553, of which the latter two were located at the positions equivalent to the major cleavage sites of human HSP90alpha. A similar pattern was obtained by papain treatment under nondenaturing conditions but not under denaturing conditions. Thus, HtpG consists of three domains, i.e. Domain A, Met1-Arg336; domain B, Glu337-Lys552; and domain C, Leu553-Ser624, as does HSP90. The domains of HtpG were expressed and their interactions were estimated on polyacrylamide gel electrophoresis under nondenaturing conditions. As a result, two kinds of domain-domain interactions were revealed: domain B interaction with domain A of the same polypeptide and domain C of one partner with domain B of the other in the dimer. Domain B could be structurally and functionally divided into two subdomains, the N-terminal two-thirds (subdomain BI) that interacted with domain A and the C-terminal one-third (subdomain BII) that interacted with domain C. The C-terminal two-thirds of domain A, i.e. Asp116-Arg336, were sufficient for the binding to domain B. We finally propose the domain organization of an HtpG dimer.

Liberation of the intramolecular interaction as the mechanism of heat-induced activation of HSP90 molecular chaperone

The molecular chaperone function of HSP90 is activated under heat-stress conditions. In the present study, we investigated the role of the interactions in the heat-induced activation of HSP90 molecular chaperone. The preceding paper demonstrated two domain-domain interactions of HtpG, an Escherichia coli homologue of mammalian HSP90, i.e. an intra-molecular interaction between the N-terminal and middle domains and an intermolecular one between the middle and C-terminal domains. A bacterial two-hybrid system revealed that the two interactions also existed in human HSP90alpha. Partners of the interaction between the N-terminal and middle domains of human HSP90alpha could, but those between the middle and C-terminal domains could not, be replaced by the domains of HtpG. Thus, the interface between the N-terminal and middle domains is essentially unvaried from bacterial to human members of the HSP90-family proteins. The citrate synthase-binding activity of HtpG at an elevated temperature was solely localized in the N-terminal domain, but HSP90alpha possessed two sites in the N-terminal and other domains. The citrate-synthase-binding activity of the N-terminal domain was suppressed by the association of the middle domain. The complex between the N-terminal and middle domains is labile at elevated temperatures, but the other is stable even at 70 degrees C. Taken together, we propose the liberation of the N-terminal client-binding domain from the middle suppressor domain is involved in the temperature-dependent activation mechanism of HSP90 molecular chaperone.

Substrate-binding characteristics of proteins in the 90 kDa heat shock protein family

In the present study we investigated the substrate-binding characteristics of three members of the 90 kDa heat shock protein (HSP90) family, namely the alpha isoform of human HSP90 (HSP90alpha), human GRP94 (94 kDa glucose-regulated protein, a form of HSP90 from endoplasmic reticulum), and HtpG (the Escherichia coli homologue of HSP90) and the domain responsible for these characteristics. The recombinant forms of HSP90alpha, GRP94 and HtpG existed as dimers and became oligomerized at higher temperatures. Among the three family members, HtpG required the highest temperature (65 degrees C) for its transition to oligomeric forms. The precipitation of the substrate protein, glutathione S-transferase, which occurred at 55 degrees C, was efficiently prevented by the simultaneous presence of a sufficient amount of HSP90alpha or GRP94, but not by HtpG, which was still present as a dimer at that temperature. However, precipitation was stopped completely at 65-70 degrees C, at which temperature HtpG was oligomerized. Thus the transition of HSP90-family proteins to a state with self-oligomerization ability is essential for preventing the precipitation of substrate proteins. We then investigated the domain responsible for the substrate binding of HtpG on the basis of the three domain structures. The self-oligomerizing and substrate-binding activities towards glutathione S-transferase and citrate synthase were both located in a single domain, the N-terminal domain (residues 1-336) of HtpG. We therefore propose that the primary peptide-binding site is located in the N-terminal domain of HSP90-family proteins.

The C-terminal domain of human grp94 protects the catalytic subunit of protein kinase CK2 (CK2alpha) against thermal aggregation. Role of disulfide bonds

The C-terminal domain (residues 518-803) of the 94 kDa glucose regulated protein (grp94) was expressed in Escherichia coli as a fusion protein with a His6-N-terminal tag (grp94-CT). This truncated form of grp94 formed dimers and oligomers that could be dissociated into monomers by treatment with dithiothreitol. Grp94-CT conferred protection against aggregation on the catalytic subunit of protein kinase CK2 (CK2alpha), although it did not protect against thermal inactivation. This anti-aggregation effect of grp94-CT was concentration dependent, with full protection achieved at grp94-CT/CK2alpha molar ratios of 4 : 1. The presence of dithiothreitol markedly reduced the anti-aggregation effects of grp94-CT on CK2alpha without altering the solubility of the chaperone. It is concluded that the chaperone activity of the C-terminal domain of human grp94 requires the maintenance of its quaternary structure (dimers and oligomers), which seems to be stabilised by disulphide bonds.

Ligand interactions in the adenosine nucleotide-binding domain of the Hsp90 chaperone, GRP94. I. Evidence for allosteric regulation of ligand binding

X-ray crystallographic studies of the N-terminal domain of Hsp90 have identified an unconventional ATP binding fold, thereby inferring a role for ATP in the regulation of the Hsp90 activity. In this report, N-ethylcarboxamidoadenosine (NECA) was used to investigate the nucleotide binding properties of GRP94, the endoplasmic reticulum paralog of Hsp90. Whereas Hsp90 did not bind NECA, GRP94 bound NECA in a saturable manner with a K(d) of 200 nm. NECA binding to GRP94 was efficiently blocked by geldanamycin and radicicol. Analysis of ligand binding stoichiometries by radioligand and calorimetric techniques indicated that GRP94 bound 1 mol of NECA/mol of GRP94 dimer. In contrast, GRP94 bound radicicol at a stoichiometry of 2 mol of radicicol/mol of GRP94 dimer. In [(3)H]NECA displacement assays, GRP94 displayed binding interactions with ATP, dATP, ADP, AMP, cAMP, and adenosine, but not GTP, CTP, or UTP. To accommodate the 0.5 mol of NECA:mol of GRP94 binding stoichiometry observed for the native GRP94 dimer, a model for allosteric regulation (negative cooperativity) of ligand binding is proposed. A hypothesis on the regulation of GRP94 conformation and activity by adenosine-based ligand(s) other than ATP and ADP is presented.

Ligand interactions in the adenosine nucleotide-binding domain of the Hsp90 chaperone, GRP94. II. Ligand-mediated activation of GRP94 molecular chaperone and peptide binding activity

The N-terminal domain of eukaryotic Hsp90 proteins contains a conserved adenosine nucleotide binding pocket that also serves as the binding site for the Hsp90 inhibitors geldanamycin and radicicol. Although this domain is essential for Hsp90 function, the molecular basis for adenosine nucleotide-dependent regulation of GRP94, the endoplasmic reticulum paralog of Hsp90, remains to be established. We report that bis-ANS (1,1'-bis(4-anilino-5-napthalenesulfonic acid), an environment sensitive fluorophore known to interact with nucleotide-binding domains, binds to the adenosine nucleotide-binding domain of GRP94 and thereby activates its molecular chaperone and peptide binding activities. bis-ANS was observed to elicit a tertiary conformational change in GRP94 similar to that occurring upon heat shock, which also activates GRP94 function. bis-ANS activation of GRP94 function was efficiently blocked by radicicol, an established inhibitory ligand for the adenosine nucleotide binding pocket. Confirmation of the N-terminal nucleotide binding pocket as the bis-ANS-binding site was obtained following covalent incorporation of bis-ANS into GRP94, trypsinolysis, and sequencing of bis-ANS-labeled limit digestion products. These data identify a ligand dependent regulation of GRP94 function and suggest a model whereby GRP94 function is regulated through a ligand-dependent conversion of GRP94 from an inactive to an active conformation.

The affinity binding sites of pancreatic bile salt-dependent lipase in pancreatic and intestinal tissues

In previous studies, we have shown that the bile salt-dependent lipase (BSDL) associates with the Grp94 molecular chaperone, an association that appears to play essential roles in the folding of BSDL. More recently, combined biochemical and immunocytochemical investigations were carried out to show that the transport of BSDL occurs via an association with the Grp94 all along the pancreatic secretory route (ER-Golgi-granules). The Grp94-BSDL complex is secreted with the pancreatic juice into the acinar lumen and reaches the duodenal lumen, where it is internalized by enterocytes. The dissociation of the complex could take place within the endosomal compartment because BSDL continues further on its way to the basolateral membrane of the enterocyte. To localize the affinity binding sites of pancreatic BSDL in pancreatic and duodenal tissues, we have used an affinity-gold ultrastructural technique. BSDL coupled to gold particles appears to interact with specific sites in tissue sections. This was confirmed by another indirect morphological approach using biotin-labeled BSDL and streptavidin-gold complexes on tissue sections. We have shown that BSDL associates with sites in the pancreatic secretory pathway compartments and in the microvilli, the endosomal compartment, and the basolateral membrane of enterocytes. By biochemical approaches, biotin-labeled BSDL displayed affinities with proteins of 180-190 kD in both pancreatic and duodenal tissues. We have also shown that the Grp94-BSDL complexes, which are insensitive to denaturing conditions, are present in pancreatic homogenate but not in duodenal lysate. Thus, BSDL is able to bind protein complexes formed by either BSDL-Grp94 or Grp94 dimers. (J Histochem Cytochem 48:267-276, 2000)

Xanthurenic acid provokes formation of unfolded proteins in endoplasmic reticulum of the lens epithelial cells

The role of xanthurenic acid in a cell is unknown, but it is suspected to provoke several diseases. This study shows that accumulation of xanthurenic acid in the lens epithelial cells leads to an overexpression of endoplasmic reticulum (ER) resident stress chaperones proteins, glucose-regulated protein (Grp94), and calreticulin. Both chaperones proteins are overexpressed in the presence of unfolded proteins. A formation of the unfolded protein in the presence of xanthurenic acid may take place due to covalent binding of xanthurenic acid to protein. Grp94 is responsible for scavenging of the unfolded proteins. The results suggest that Grp94 scavenged xanthurenic acid-modified proteins, and for this reason become preferentially yellow-stained in the presence of yellow xanthurenic acid. Such a modified Grp94 is weakly recognized by anti-Grp94 antibody. An end point of the xanthurenic acid accumulation in the cell is the cell death. In conclusion xanthurenic acid can lead to cell pathology.

GRP94, an ER chaperone with protein and peptide binding properties

GRP94 is the ER representative of the HSP90 family of stress-induced proteins. It binds to a limited number of proteins in the secretory pathway, apparently by recognizing advanced folding intermediates or incompletely assembled proteins, GRP94 also binds peptides and can act as a tumor vaccine, delivering the peptides for presentation to T lymphocytes. Here, we review the current data about GRP94 and propose a structural model that integrates the biochemical data and known functions of the protein.

The 90-kDa molecular chaperone family: structure, function, and clinical applications. A comprehensive review

The 90-kDa molecular chaperone family (which comprises, among other proteins, the 90-kDa heat-shock protein, hsp90 and the 94-kDa glucose-regulated protein, grp94, major molecular chaperones of the cytosol and of the endoplasmic reticulum, respectively) has become an increasingly active subject of research in the past couple of years. These ubiquitous, well-conserved proteins account for 1-2% of all cellular proteins in most cells. However, their precise function is still far from being elucidated. Their involvement in the aetiology of several autoimmune diseases, in various infections, in recognition of malignant cells, and in antigen-presentation already demonstrates the essential role they likely will play in clinical practice of the next decade. The present review summarizes our current knowledge about the cellular functions, expression, and clinical implications of the 90-kDa molecular chaperone family and some approaches for future research.

Oligomeric forms of the 90-kDa heat shock protein

Two isoforms of the 90-kDa heat shock protein, HSP90alpha and HSP90beta, are present in the cytosol of mammalian cells. Analysis by polyacrylamide gel electrophoresis under nondenaturing conditions (native PAGE) revealed that HSP90alpha predominantly exists as a homodimer and that HSP90beta is present mainly as a monomer [Minami, Kawasaki, Miyata, Suzuki and Yahara (1991) J. Biol. Chem. 266, 10099-10103]. However, only the dimeric form has been observed under other analytical conditions such as gradient centrifugation. In this study, therefore, we investigated native forms of HSP90 by use of immunochemical techniques with isoform-specific monoclonal antibodies recently developed in our laboratory. Glycerol gradient centrifugation at the physiological salt concentration as well as native PAGE analysis of rat liver cytosol revealed oligomeric forms of HSP90alpha sedimenting at 8-10S as predominant ones. On the other hand, the glycerol gradient centrifugation revealed multiple forms of HSP90beta oligomers sedimenting at 6-12S. All of the HSP90beta oligomers, however, migrated at 100-kDa monomer and 190-kDa dimer positions on native PAGE. A novel two-dimensional double native PAGE revealed that the entity was converted from the HSP90beta dimer to monomers during the electrophoresis. The same PAGE further revealed that the HSP90alpha oligomer also dissociated into dimers during the electrophoresis. Full-length form of bacterially-expressed human HSP90alpha migrated as dimers, but a considerable amount did not penetrate into the gel under native PAGE conditions, indicating the existence of oligomeric forms. Electrophoretic studies of deletion mutants of HSP90 demonstrated that the C-terminal 200 amino acids were capable of forming oligomers. Taken together, we conclude that both of the HSP90 isoforms predominantly exist as oligomeric forms in the cytosol even under unstressed conditions but that they artificially dissociate into smaller forms when subjected to native PAGE.

Domain structures and immunogenic regions of the 90-kDa heat-shock protein (HSP90). Probing with a library of anti-HSP90 monoclonal antibodies and limited proteolysis

Domain structures of the 90-kDa heat-shock protein (HSP90) have been investigated with a library of anti-HSP90 monoclonal antibodies (mAbs) and by limited proteolysis with trypsin and chymotrypsin. Thirty-three mAbs were obtained by immunization with bacterially expressed human HSP90alpha and HSP90beta isoforms. Among them, ten and three mAbs reacted specifically with HSP90alpha and HSP90beta, respectively. Immunoblotting and enzyme-linked immunosorbent analyses revealed that major immunogenic domains were located at two restricted regions of HSP90alpha, i.e. amino acids 227-310 (designated Region I) and 702-716 (Region II), corresponding to a highly charged region and a region near the C terminus, respectively. Taken together with the characteristics of the amino acid sequences, these two immunogenic regions appeared to be exposed at the outer surface of HSP90. We further investigated the domain structures of HSP90 by limited proteolysis in combination with N-terminal sequencing and immunoblotting analyses. Tryptic cleavages of HSP90alpha at low concentrations revealed the existence of major susceptible sites at Arg400-Glu401, Lys615-Ala616, and Arg620-Asp621. Proteolysis at higher trypsin concentrations caused successive cleavages only toward the N-terminal direction from these sites, and Region I was included in the region selectively deleted during this process, thereby further suggesting its surface location. From these results, we propose three domain structures of HSP90 consisting of amino acids 1-400, 401-615, and 621-732. Differences in the protease sensitivity and immunogenicity further suggest that every domain is composed of two subdomains. This is the first study describing the domain structures and the immunogenic regions of HSP90.