SBA1 encodes a yeast hsp90 cochaperone that is homologous to vertebrate p23 proteins

Contribution of Yap1 towards Saccharomyces cerevisiae adaptation to arsenic-mediated oxidative stress

In the budding yeast Saccharomyces cerevisiae, arsenic detoxification involves the activation of Yap8, a member of the Yap (yeast AP-1-like) family of transcription factors, which in turn regulates ACR2 and ACR3, genes encoding an arsenate reductase and a plasma-membrane arsenite-efflux protein respectively. In addition, Yap1 is involved in the arsenic adaptation process through regulation of the expression of the vacuolar pump encoded by YCF1 (yeast cadmium factor 1 gene) and also contributing to the regulation of ACR genes. Here we show that Yap1 is also involved in the removal of ROS (reactive oxygen species) generated by arsenic compounds. Data on lipid peroxidation and intracellular oxidation indicate that deletion of YAP1 and YAP8 triggers cellular oxidation mediated by inorganic arsenic. In spite of the increased amounts of As(III) absorbed by the yap8 mutant, the enhanced transcriptional activation of the antioxidant genes such as GSH1 (γ- glutamylcysteine synthetase gene), SOD1 (superoxide dismutase 1 gene) and TRX2 (thioredoxin 2 gene) may prevent protein oxidation. In contrast, the yap1 mutant exhibits high contents of protein carbonyl groups and the GSSG/GSH ratio is severely disturbed on exposure to arsenic compounds in these cells. These results point to an additional level of Yap1 contribution to arsenic stress responses by preventing oxidative damage in cells exposed to these compounds. Transcriptional profiling revealed that genes of the functional categories related to sulphur and methionine metabolism and to the maintenance of cell redox homoeostasis are activated to mediate adaptation of the wild-type strain to 2 mM arsenate treatment.

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.

Functioning of the Hsp90 machine in chaperoning checkpoint kinase I (Chk1) and the progesterone receptor (PR)

Hsp90 is an abundant and highly conserved chaperone that functions at later stages of protein folding to maintain and regulate the activity of client proteins. Using a recently described in vitro system to fold a functional model kinase Chk1, we performed a side-by-side comparison of the Hsp90-dependent chaperoning of Chk1 to that of the progesterone receptor (PR) and show that these distinct types of clients have different chaperoning requirements. The less stable PR required more total chaperone protein(s) and p23, whereas Chk1 folding was critically dependent on Cdc37. When the 2 clients were reconstituted under identical conditions, each client folding was dose dependent for Hsp90 protein levels and was inhibited by geldanamycin. Using this tractable system, we found that Chk1 kinase folding was more effective if we used a type II Hsp40 cochaperone, whereas PR is chaperoned equally well with a type I or type II Hsp40. Additional dissection of Chk1-chaperone complexes and the resulting kinase activity suggests that kinase folding, like that previously shown for PR, is a dynamic, multistep process. Importantly, the cochaperones Hop and Cdc37 cooperate as the kinase transitions from immature Hsp70- to mature Hsp90-predominant complexes.

The Hsp110 molecular chaperone stabilizes apolipoprotein B from endoplasmic reticulum-associated degradation (ERAD)

Apolipoprotein B (apoB) is the most abundant protein in low density lipoproteins and plays key roles in cholesterol homeostasis. The co-translational degradation of apoB is controlled by fatty acid levels in the endoplasmic reticulum (ER) and is mediated by the proteasome. To define the mechanism of apoB degradation, we employed a cell-free system in which proteasome-dependent degradation is recapitulated with yeast cytosol, and we developed an apoB yeast expression system. We discovered that a yeast Hsp110, Sse1p, associates with and stabilizes apoB, which contrasts with data indicating that select Hsp70s and Hsp90s facilitate apoB degradation. However, the Ssb Hsp70 chaperones have no effect on apoB turnover. To determine whether our results are relevant in mammalian cells, Hsp110 was overexpressed in hepatocytes, and enhanced apoB secretion was observed. This study indicates that chaperones within distinct complexes can play unique roles during ER-associated degradation (ERAD), establishes a role for Sse1/Hsp110 in ERAD, and identifies Hsp110 as a target to lower cholesterol.

Temporal dissection of T-bet functions

T-bet is a transcription factor of the T-box family that regulates the expression of numerous immune system-associated genes. T-bet directs the acquisition of the Th1-associated genetic program in differentiating CD4(+) lymphocytes. It also influences the development of NK and NKT cells through its regulation of the IL-2/IL-15Rbeta-chain (CD122) and the trafficking of these lymphocytes through CxCR3. The temporal requirements of T-bet activity for the production of IFN-gamma and the regulation of CD122 and CxCR3 expression remain undefined. We produced an ectopically controllable form of T-bet by fusing its C-terminal domain with a mutated ligand-binding domain of human estrogen receptor alpha. By temporally controlling the expression of T-bet-estrogen receptor alpha by the addition or removal of 4-hydroxytamoxifen (4-HT), we show that IFN-gamma, CD122, and CxCR3 are direct gene targets of T-bet whose expression are acutely regulated by T-bet activity.

Structure and mechanism of the Hsp90 molecular chaperone machinery

Heat shock protein 90 (Hsp90) is a molecular chaperone essential for activating many signaling proteins in the eukaryotic cell. Biochemical and structural analysis of Hsp90 has revealed a complex mechanism of ATPase-coupled conformational changes and interactions with cochaperone proteins, which facilitate activation of Hsp90's diverse "clientele." Despite recent progress, key aspects of the ATPase-coupled mechanism of Hsp90 remain controversial, and the nature of the changes, engendered by Hsp90 in client proteins, is largely unknown. Here, we discuss present knowledge of Hsp90 structure and function gleaned from crystallographic studies of individual domains and recent progress in obtaining a structure for the ATP-bound conformation of the intact dimeric chaperone. Additionally, we describe the roles of the plethora of cochaperones with which Hsp90 cooperates and growing insights into their biochemical mechanisms, which come from crystal structures of Hsp90 cochaperone complexes.

The Hsp40 molecular chaperone Ydj1p, along with the protein kinase C pathway, affects cell-wall integrity in the yeast Saccharomyces cerevisiae

Molecular chaperones, such as Hsp40, regulate cellular processes by aiding in the folding, localization, and activation of multi-protein machines. To identify new targets of chaperone action, we performed a multi-copy suppressor screen for genes that improved the slow-growth defect of yeast lacking the YDJ1 chromosomal locus and expressing a defective Hsp40 chimera. Among the genes identified were MID2, which regulates cell-wall integrity, and PKC1, which encodes protein kinase C and is linked to cell-wall biogenesis. We found that ydj1delta yeast exhibit phenotypes consistent with cell-wall defects and that these phenotypes were improved by Mid2p or Pkc1p overexpression or by overexpression of activated downstream components in the PKC pathway. Yeast containing a thermosensitive allele in the gene encoding Hsp90 also exhibited cell-wall defects, and Mid2p or Pkc1p overexpression improved the growth of these cells at elevated temperatures. To determine the physiological basis for suppression of the ydj1delta growth defect, wild-type and ydj1delta yeast were examined by electron microscopy and we found that Mid2p overexpression thickened the mutant's cell wall. Together, these data provide the first direct link between cytoplasmic chaperone function and cell-wall integrity and suggest that chaperones orchestrate the complex biogenesis of this structure.

Nucleotide-dependent interaction of Saccharomyces cerevisiae Hsp90 with the cochaperone proteins Sti1, Cpr6, and Sba1

The ATP-dependent molecular chaperone Hsp90 and partner cochaperone proteins are required for the folding and activity of diverse cellular client proteins, including steroid hormone receptors and multiple oncogenic kinases. Hsp90 undergoes nucleotide-dependent conformational changes, but little is known about how these changes are coupled to client protein activation. In order to clarify how nucleotides affect Hsp90 interactions with cochaperone proteins, we monitored assembly of wild-type and mutant Hsp90 with Sti1, Sba1, and Cpr6 in Saccharomyces cerevisiae cell extracts. Wild-type Hsp90 bound Sti1 in a nucleotide-independent manner, while Sba1 and Cpr6 specifically and independently interacted with Hsp90 in the presence of the nonhydrolyzable analog of ATP, AMP-PNP. Alterations in Hsp90 residues that contribute to ATP binding or hydrolysis prevented or altered Sba1 and Cpr6 interaction; additional alterations affected the specificity of Cpr6 interaction. Some mutant forms of Hsp90 also displayed reduced Sti1 interaction in the presence of a nucleotide. These studies indicate that cycling of Hsp90 between the nucleotide-free, open conformation and the ATP-bound, closed conformation is influenced by residues both within and outside the N-terminal ATPase domain and that these conformational changes have dramatic effects on interaction with cochaperone proteins.

Identification of regions within the F domain of the human estrogen receptor alpha that are important for modulating transactivation and protein-protein interactions

The estrogen receptor (ER)alpha is a biologically and clinically important ligand-modulated transcription factor. The F domain of the ERalpha modulates its functions in a ligand-, promoter-, and cell-specific manner. To identify the region(s) responsible for these functions, we characterized the effects of serial truncations within the F domain. We found that truncating the last 16 residues of the F domain altered the activity of the human ERalpha (hERalpha) on an estrogen response element-driven promoter in response to estradiol or 4-hydroxytamoxifen (4-OHT), its sensitivity to overexpression of the coactivator steroid receptor coactivator-1 in mammalian cells, and its interaction with a receptor-interacting domain of the coactivator steroid receptor coactivator-1 or engineered proteins ("monobodies") that specifically bind to ERalpha/ligand complexes in a yeast two-hybrid system. Most importantly, the ability of the ER to induce pS2 was reduced in MDA-MB-231 cells stably expressing this truncated ER vs. the wild-type ER. The region includes a distinctive segment (residues 579-584; LQKYYIT) having a high content of bulky and/or hydrophobic amino acids that was previously predicted to adopt a beta-strand-like structure. As previously reported, removal of the entire F domain was necessary to eliminate the agonist activity of 4-OHT. In addition, mutation of the vicinal glycine residues between the ligand-binding domain and F domains specifically reduced the 4-OHT-dependent interactions of the hERalpha ligand-binding domain and F domains with monobodies. These results show that regions within the F domain of the hERalpha selectively modulate its activity and its interactions with other proteins.

The middle domain of Hsp90 acts as a discriminator between different types of client proteins

The mechanism of client protein activation by Hsp90 is enigmatic, and it is uncertain whether Hsp90 employs a common route for all proteins. Using a mutational analysis approach, we investigated the activation of two types of client proteins, glucocorticoid receptor (GR) and the kinase v-Src by the middle domain of Hsp90 (Hsp90M) in vivo. Remarkably, the overall cellular activity of v-Src was highly elevated in a W300A mutant yeast strain due to a 10-fold increase in cellular protein levels of the kinase. In contrast, the cellular activity of GR remained almost unaffected by the W300A mutation but was dramatically sensitive to S485Y and T525I exchanges. In addition, we show that mutations S485Y and T525I in Hsp90M reduce the ATP hydrolysis rate, suggesting that Hsp90 ATPase is more tightly regulated than assumed previously. Therefore, the activation of GR and v-Src has various demands on Hsp90 biochemistry and is dependent on separate functional regions of Hsp90M. Thus, Hsp90M seems to discriminate between different substrate types and to adjust the molecular chaperone for proper substrate activation.

Sgt1p is a unique co-chaperone that acts as a client adaptor to link Hsp90 to Skp1p

Sgt1p is a conserved, essential protein required for kinetochore assembly in both yeast and animal cells. Sgt1p has homology to both TPR and p23 domains, sequences often found in proteins that interact with and regulate the molecular chaperone, Hsp90. The presence of these domains and the recent findings that Sgt1p interacts with Hsp90 has led to the speculation that Sgt1p and Hsp90 form a co-chaperone complex. To test this possibility, we have used purified recombinant proteins to characterize the in vitro interactions between yeast Sgt1p and Hsp82p (an Hsp90 homologue in yeast). We show that Sgt1p interacts directly with Hsp82p via its p23 homology region in a nucleotide-dependent manner. However, Sgt1p binding does not alter the enzymatic activity of Hsp82p, suggesting that it is distinct from other co-chaperones. We find that Sgt1p can form a ternary chaperone complex with Hsp82p and Sti1p, a well characterized Hsp90 co-chaperone. Sgt1p interacts with its binding partner Skp1p through its TPR domains and links Skp1p to the core Hsp82p-Sti1p co-chaperone complex. The multidomain nature of Sgt1p and its ability to bridge the interaction between Skp1p and Hsp82p argue that Sgt1p acts as a "client adaptor" recruiting specific clients to Hsp82p co-chaperone complexes.

Reconstitution of human hypoxia inducible factor HIF-1 in yeast: A simple in vivo system to identify and characterize HIF-1α effectors

Hypoxia inducible factor 1 (HIF-1), the master regulator of hypoxia-activated genes, is involved in many diseases and is a valid drug target. In order to develop a simple and genetically tractable in vivo system for HIF-1 analysis, we tested the inducible expression of both human HIF-1 subunits (HIF-1alpha and ARNT) in the yeast Saccharomyces cerevisiae and showed the formation of transcriptionally active HIF-1. The use of this system for the identification and characterization of HIF-1 effectors was first validated by showing that two chemical Hsp90 inhibitors, geldanamycin and radicicol, impaired the activity of HIF-1 in yeast. By applying this system in mutant yeast strains, we then identified Hsp90 co-chaperones, which were required for HIF-1 activity. Furthermore, using yeast strains co-expressing truncated forms of HIF-1alpha with ARNT or both HIF-1alpha and ARNT, we characterized fragments of HIF-1alpha that acted as dominant negative mutants and suppressed HIF-1 activity.

Characterization and expression of p23 gene in the amphioxus Branchiostoma belcheri

The cDNA AmphiP23, encoding an amphioxus p23, was identified from the gut cDNA library of amphioxus Branchiostoma belcheri. It contains a 513 bp open reading frame corresponding to a deduced protein of 170 amino acids. Phylogenetic analysis shows that vertebrate and invertebrate p23/p23-like proteins are each grouped together, with AmphiP23 falling at the base of vertebrate p23/p23-like clade, suggesting that the divergence of vertebrate and invertebrate p23 genes probably occurs prior to the split of invertebrate/vertebrate from a common ancestor around 550 million years ago. Northern blotting reveals a ubiquitous expression pattern of AmphiP23 in all adult tissues examined, while whole mount in situ hybridization demonstrates a tissue- and stage-specific expression pattern of AmphiP23 in developing embryos and larvae. Presumably, the ubiquitous expression pattern of AmphiP23 in adult amphioxus represents the ancestral type of p23 gene prior to its split to human paralogs p23 and tsp23, while the tissue- and stage-specific expression pattern during early embryonic development implicates a role of AmphiP23 in anterior/posterior patterning.

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.

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

Hsp90 (heat shock protein of 90 kDa) is a ubiquitous molecular chaperone responsible for the assembly and regulation of many eukaryotic signalling systems and is an emerging target for rational chemotherapy of many cancers. Although the structures of isolated domains of Hsp90 have been determined, the arrangement and ATP-dependent dynamics of these in the full Hsp90 dimer have been elusive and contentious. Here we present the crystal structure of full-length yeast Hsp90 in complex with an ATP analogue and the co-chaperone p23/Sba1. The structure reveals the complex architecture of the 'closed' state of the Hsp90 chaperone, the extensive interactions between domains and between protein chains, the detailed conformational changes in the amino-terminal domain that accompany ATP binding, and the structural basis for stabilization of the closed state by p23/Sba1. Contrary to expectations, the closed Hsp90 would not enclose its client proteins but provides a bipartite binding surface whose formation and disruption are coupled to the chaperone ATPase cycle.

Aha1 competes with Hop, p50 and p23 for binding to the molecular chaperone Hsp90 and contributes to kinase and hormone receptor activation

The ATP-dependent molecular chaperone Hsp90 (heat-shock protein 90) is essential for the maturation of hormone receptors and protein kinases. During the process of client protein activation, Hsp90 co-operates with cofactors/co-chaperones of unique sequence, e.g. Aha1 (activator of Hsp90 ATPase 1), p23 or p50, and with cofactors containing TPR (tetratricopeptide repeat) domains, e.g. Hop, immunophilins or cyclophilins. Although the binding sites for these different types of cofactors are distributed along the three domains of Hsp90, sterical overlap and competition for binding sites restrict the combinations of cofactors that can bind to Hsp90 at the same time. The recently discovered cofactor Aha1 associates with the middle domain of Hsp90, but its relationship to other cofactors of the molecular chaperone is poorly understood. Therefore we analysed whether complexes of Aha1, p23, p50, Hop and a cyclophilin with Hsp90 are disrupted by the other four cofactors by gel permeation chromatography using purified proteins. It turned out that Aha1 competes with the early cofactors Hop and p50, but can bind to Hsp90 in the presence of cyclophilins, suggesting that Aha1 acts as a late cofactor of Hsp90. In contrast with p50, which can bind to Hop, Aha1 does not interact directly with any of the other four cofactors. In vivo studies in yeast and in mammalian cells revealed that Aha1 is not specific for kinase activation, but also contributes to maturation of hormone receptors, proposing a general role for this cofactor in the activation of Hsp90-dependent client proteins.

The co-chaperone p23 arrests the Hsp90 ATPase cycle to trap client proteins

The action of the molecular chaperone Hsp90 is essential for the activation and assembly of an increasing number of client proteins. This function of Hsp90 has been proposed to be governed by conformational changes driven by ATP binding and hydrolysis. Association of co-chaperones and client proteins regulate the ATPase activity of Hsp90. Here, we have examined the inhibition of the ATPase activity of human Hsp90beta by one such co-chaperone, human p23. We demonstrate that human p23 interacts with Hsp90 in both the absence and presence of nucleotide with a higher affinity in the presence of the ATP analogue AMP-PNP. This is consistent with an analysis of the effect of p23 on the steady-state kinetics that revealed a mixed mechanism of inhibition. Mass spectrometry of the intact Hsp90.p23 complex determined the stoichiometry of binding to be one p23 to each subunit of the Hsp90 dimer. p23 was also shown to interact with a monomeric, truncated fragment of Hsp90, lacking the C-terminal homodimerisation domain, indicating dimerisation of Hsp90 is not a prerequisite for association with p23. Complex formation between Hsp90 and p23 increased the apparent affinity of Hsp90 for AMP-PNP and completely inhibited the ATPase activity. We propose a model where the role of p23 is to lock individual subunits of Hsp90 in an ATP-dependent conformational state that has a high affinity for client proteins.

Chaperoning checkpoint kinase 1 (Chk1), an Hsp90 client, with purified chaperones

Checkpoint kinase 1 (Chk1), a serine/threonine kinase that regulates DNA damage checkpoints, is destabilized when heat shock protein 90 (Hsp90) is inhibited, suggesting that Chk1 is an Hsp90 client. In the present work we examined the interplay between Chk1 and Hsp90 in intact cells, identified a source of unchaperoned Chk1, and report the in vitro chaperoning of Chk1 in reticulocyte lysates and with purified chaperones and co-chaperones. We find that bacterially expressed Chk1 is post-translationally chaperoned to an active kinase. This reaction minimally requires Hsp90, Hsp70, Hsp40, Cdc37, and the protein kinase CK2. The co-chaperone Hop, although not essential for the activation of Chk1 in vitro, enhanced the chaperoning process, whereas the co-chaperone p23 did not stimulate the chaperoning reaction. Additionally, we found that the C-terminal regulatory domain of Chk1 affects the association of Chk1 with Hsp90. Collectively these results provide new insights into Hsp90-dependent chaperoning of a client kinase and identify a novel, biochemically tractable model system that will be useful to further dissect the Hsp90-dependent chaperoning of this important and ubiquitous class of Hsp90 clients.

Novel interaction of the Hsp90 chaperone machine with Ssl2, an essential DNA helicase in Saccharomyces cerevisiae

Hsp90 is an essential molecular chaperone that is critical for the activity of diverse cellular proteins. Hsp90 functions with a number of co-chaperone proteins, including Sti1/Hop. We conducted a genetic screen in Saccharomyces cerevisiae to isolate mutations that exhibit enhanced growth defects in the absence of STI1. We obtained mutations in genes encoding components of the Hsp90 chaperone machine, HSC82, CPR7 and YDJ1, and two essential genes, SSL2 and UTP21, not previously linked to Hsp90. Ssl2, the yeast homologue of XPB, is an ATP-dependent DNA helicase that is a component of the TFIIH multiprotein complex and has dual functions in transcription and DNA repair. In order to determine whether Ssl2 function is dependent on Hsp90, we further examined the interaction between Ssl2 and Hsp90. Multiple mutant alleles of SSL2 exhibited a pronounced growth defect when co-expressed with a mutant allele of Hsp90. In addition, isolation of Ssl2 protein resulted in the co-purification of Hsp90 and Sti1, suggesting that Ssl2 and Hsp90 are in the same protein complexes in vivo. These results suggest a novel role for Hsp90 in the essential cellular functions of transcription and DNA repair.

Role of hsp90 and the hsp90-binding immunophilins in signalling protein movement

The ubiquitous protein chaperone hsp90 has been shown to regulate more than 100 proteins involved in cellular signalling. These proteins are called 'client proteins' for hsp90, and a multiprotein hsp90/hsp70-based chaperone machinery forms client protein.hsp90 heterocomplexes in the cytoplasm and the nucleus. In the case of signalling proteins that act as transcription factors, the client protein.hsp90 complexes also contain one of several TPR domain immunophilins or immunophilin homologs that bind to a TPR domain binding site on hsp90. Using several intracellular receptors and the tumor suppressor p53 as examples, we review evidence that dynamic assembly of heterocomplexes with hsp90 is required for rapid movement through the cytoplasm to the nucleus along microtubular tracks. The role of the immunophilin in this system is to connect the client protein.hsp90 complex to cytoplasmic dynein, the motor protein for retrograde movement toward the nucleus. Upon arrival at the nuclear pores, the receptor.hsp90.immunophilin complexes are transferred to the nuclear interior by importin-dependent facilitated diffusion. The unliganded receptors then distribute within the nucleus to diffuse patches from which they proceed in a ligand-dependent manner to discrete nuclear foci where chromatin binding occurs. We review evidence that dynamic assembly of heterocomplexes with hsp90 is required for movement to these foci and for the dynamic exchange of transcription factors between chromatin and the nucleoplasm.

Investigating the protein-protein interactions of the yeast Hsp90 chaperone system by two-hybrid analysis: potential uses and limitations of this approach

The Hsp90 chaperone cycle involves sequential assembly of different Hsp90-containing multiprotein complexes, the accessory proteins ("cochaperones") that are associated with these complexes being exchanged as the cycle proceeds from its early to its late stages. To gain insight as to whether the 2-hybrid system could be used to probe the interactions of this Hsp90 system, yeast transformants were constructed that express the Gal4p deoxyribonucleic acid-binding domain (BD) fused to the 2 Hsp90 isoforms and the various Hsp90 system cochaperones of yeast. These "bait" fusions were then introduced by mating into other transformants expressing nearly all the 6000 proteins of yeast expressed as fusions to the Gal4p activation domain (AD). High throughput 2-hybrid screening revealed the ability of Hsp90 and Hsp90 system cochaperones to engage in stable interactions in vivo, both with each other and with the various other proteins of the yeast proteome. Consistent with the transience of most chaperone associations, interactions to Hsp90 itself were invariably weak and generally influenced by stress. Mutations within a Hsp90-BD bait fusion and an AD-Cdc37 "prey" fusion were used to provide in vivo confirmation of the in vitro data that shows that Cdc37p is interacting with the "relaxed" conformation of Hsp90 and also to provide indications that Cdc37p needs to be phosphorylated at its N-terminus for any appreciable interaction with Hsp90. A number of potentially novel cochaperone interactions were also identified, providing a framework for these to be analyzed further using other techniques.

Cooperation of heat shock protein 90 and p23 in aryl hydrocarbon receptor signaling

Aryl hydrocarbon receptor (AhR) is a transcription factor that is activated by the binding of xenobiotic and endogenous ligands. AhR interacts with heat shock protein (Hsp) 90 complexes and can be used as a functional substrate to detect chaperone-dependent processes. Yeast Hsp90 (hsp82) mutants that variably affected AhR signaling were identified using reporter gene assays. Some mutated alleles resided in the p23/adenosine triphosphate (ATP)-binding pocket of Hsp90, so the relationship between the cochaperone Sba1 (yeast p23) and adenosine triphosphatase (ATPase) activity was investigated. Deletion of the p23 gene in the hsp82G170D mutant background had a greater effect on AhR signaling than the individual mutations, suggesting that these 2 mutations have separate actions on AhR signaling. In contrast, p23 overexpression suppressed temperature sensitivity and AhR signaling defects in the hsp82G170D mutant strain, suggesting that there is a relationship between these 2 proteins. The mutated hsp82G170D protein lacked detectable ATPase activity and p23 binding in vitro, which may relate to the weakened AhR signaling observed in mutant cells. Sba1 (p23) suppressed Hsp82 ATPase activity in vitro. These studies implicate the p23 protein and the G170 region of Hsp90 as being important, but not essential, for AhR signaling. Our results are consistent with a model in which p23 inhibits Hsp90 ATPase activity, thereby stabilizing ATP-Hsp90-client protein complexes.

Evolutionary epitopes of Hsp90 and p23: implications for their interaction

The amino-terminal domain (N-domain) of Hsp90 represents the ATP binding site and is important for interaction with its cochaperone, p23. Whereas some evidence suggests that p23 may bind to this domain in an ATP-dependent manner and that this process requires the dimerization of two N-domains, the interaction sites between them and the molecular mechanism of coupling these two events to p23 binding remain unsolved. As a first step toward establishing the interaction mechanism, we used the evolutionary tracing (ET) method [Lichtarge, O., Bourne, H. R., and Cohen, F. E. (1996) J. Mol. Biol. 257, 342-358] to identify the putative functional surfaces of Hsp90 and p23, and combined with protein-protein docking techniques, to predict their binding interface. Both evolutionarily privileged surfaces of Hsp90 and p23 identified by ET appear on this putative interface. An analysis of the complex model produced using the ET results combined with available experimental data highlights a putative conformational pathway in the ATP binding domain of Hsp90, where a series of conformational changes transfer the ATP-induced N-domain dimerization signal for the binding of p23. In this pathway, the closure of "lid" may result in reorientation of the helix alpha1 and the following loop (residues 10-27 in yeast Hsp90), which will expose more hydrophobic surface, and thus triggers the dimerization of N-domain.

The Co-chaperone Sba1 connects the ATPase reaction of Hsp90 to the progression of the chaperone cycle

The molecular chaperone Hsp90 mediates the ATP-dependent activation of a large number of proteins involved in signal transduction. During this process, Hsp90 was found to associate transiently with several accessory factors, such as p23/Sba1, Hop/Sti1, and prolyl isomerases. It has been shown that ATP hydrolysis triggers conformational changes within Hsp90, which in turn are thought to mediate conformational changes in the substrate proteins, thereby causing their activation. The specific role of the partner proteins in this process is unknown. Using proteins from Saccharomyces cerevisiae, we characterized the interaction of Hsp90 with its partner protein p23/Sba1. Our results show that the nucleotide-dependent N-terminal dimerization of Hsp90 is necessary for the binding of Sba1 to Hsp90 with an affinity in the nanomolar range. Two Sba1 molecules were found to bind per Hsp90 dimer. Sba1 binding to Hsp90 resulted in a decreased ATPase activity, presumably by trapping the hydrolysis state of Hsp90ATP. Ternary complexes of Hsp90Sba1 could be formed with the prolyl isomerase Cpr6, but not with Sti1. Based on these findings, we propose a model that correlates the ordered assembly of the Hsp90 co-chaperones with distinct steps of the ATP hydrolysis reaction during the chaperone cycle.

Distinct roles for the Hsp40 and Hsp90 molecular chaperones during cystic fibrosis transmembrane conductance regulator degradation in yeast

Aberrant secreted proteins can be destroyed by ER-associated protein degradation (ERAD), and a prominent, medically relevant ERAD substrate is the cystic fibrosis transmembrane conductance regulator (CFTR). To better define the chaperone requirements during CFTR maturation, the protein was expressed in yeast. Because Hsp70 function impacts CFTR biogenesis in yeast and mammals, we first sought ER-associated Hsp40 cochaperones involved in CFTR maturation. Ydj1p and Hlj1p enhanced Hsp70 ATP hydrolysis but CFTR degradation was slowed only in yeast mutated for both YDJ1 and HLJ1, suggesting functional redundancy. In contrast, CFTR degradation was accelerated in an Hsp90 mutant strain, suggesting that Hsp90 preserves CFTR in a folded state, and consistent with this hypothesis, Hsp90 maintained the solubility of an aggregation-prone domain (NBD1) in CFTR. Soluble ERAD substrate degradation was unaffected in the Hsp90 or the Ydj1p/Hlj1p mutants, and surprisingly CFTR degradation was unaffected in yeast mutated for Hsp90 cochaperones. These results indicate that Hsp90, but not the Hsp90 complex, maintains CFTR structural integrity, whereas Ydj1p/Hlj1p catalyze CFTR degradation.

Cns1 Is an Activator of the Ssa1 ATPase Activity

Hsp90 is a key mediator in the folding process of a growing number of client proteins. The molecular chaperone cooperates with many co-chaperones and partner proteins to fulfill its task. In Saccharomyces cerevisiae, several co-chaperones of Hsp90 interact with Hsp90 via a tetratricopeptide repeat (TPR) domain. Here we show that one of these proteins, Cns1, binds both to Hsp90 and to the yeast Hsp70 protein Ssa1 with comparable affinities. This is reminiscent of Sti1, another TPR-containing co-chaperone. Unlike Sti1, Cns1 exhibits no influence on the ATPase of Hsp90. However, it activates the ATPase of Ssa1 up to 30-fold by accelerating the rate-limiting ATP hydrolysis step. This stimulating effect is mediated by the N-terminal TPR-containing part of Cns1, whereas the C-terminal part showed no effect. Competition experiments allow the conclusion that Hsp90 and Ssa1 compete for binding to the single TPR domain of Cns1. Taken together, Cns1 is a potent cochaperone of Ssa1. Our findings highlight the importance of the regulation of Hsp70 function in the context of the Hsp90 chaperone cycle.

Inhibition of GR-mediated transcription by p23 requires interaction with Hsp90

p23 is a regulatory co-chaperone of heat shock protein (Hsp) 90, but can also act as a general molecular chaperone by itself. Using novel point mutations of p23 that disrupt its interaction with Hsp90 we found its co-chaperone function to be required for its inhibitory effect on glucocorticoid receptor (GR). The C-terminal region of p23, which is required for its chaperone activity, is dispensable for inhibition of GR. Importantly, similar results were obtained with a constitutively active GR. Thus, the action of p23 on the nuclear stage of GR regulation requires its Hsp90 co-chaperone function, but not its chaperone activity.

p23, a simple protein with complex activities

p23 is a small but important cochaperone for the Hsp90 chaperoning pathway. It appears to facilitate the adenosine triphosphate-driven cycle of Hsp90 binding to client proteins. It enters at a late stage of the cycle and enhances the maturation of client proteins. Although this role of p23 is fairly well established, recent studies suggest that it may have additional functions in the cell that merit further exploration.

Sti1 and Cdc37 can stabilize Hsp90 in chaperone complexes with a protein kinase

Hsp90 functions in association with several cochaperones for folding of protein kinases and transcription factors, although the relative contribution of each to the overall reaction is unknown. We assayed the role of nine different cochaperones in the activation of Ste11, a Saccharomyces cerevisiae mitogen-activated protein kinase kinase kinase. Studies on signaling via this protein kinase pathway was measured by alpha-factor-stimulated induction of FIG1 or lacZ, and repression of HHF1. Several cochaperone mutants tested had reduced FIG1 induction or HHF1 repression, although to differing extents. The greatest defects were in cpr7Delta, sse1Delta, and ydj1Delta mutants. Assays of Ste11 kinase activity revealed a pattern of defects in the cochaperone mutant strains that were similar to the gene expression studies. Overexpression of CDC37, a chaperone required for protein kinase folding, suppressed defects the sti1Delta mutant back to wild-type levels. CDC37 overexpression also restored stable Hsp90 binding to the Ste11 protein kinase domain in the sti1Delta mutant strain. These data suggest that Cdc37 and Sti1 have functional overlap in stabilizing Hsp90:client complexes. Finally, we show that Cns1 functions in MAP kinase signaling in association with Cpr7.

Hop: more than an Hsp70/Hsp90 adaptor protein

Molecular chaperones facilitate the correct folding of other proteins under physiological and stress conditions. Recently it has become evident that various co-chaperone proteins regulate the cellular functions of these chaperones, particularly Hsp70 and Hsp90. Hop is one of the most extensively studied co-chaperones that is able to directly associate with both Hsp70 and Hsp90. The current dogma proposes that Hop functions primarily as an adaptor that directs Hsp90 to Hsp70-client protein complexes in the cytoplasm. However, recent evidence suggests that Hop can also modulate the chaperone activities of these Hsps, and that it is not dedicated to Hsp70 and Hsp90. While the co-chaperone function of Hop within the cytoplasm has been extensively studied, its association with nuclear complexes and prion proteins remains to be elucidated. This article will review the structural features of Hop, and the evidence that its biological function is considerably broader than previously envisaged.

The Hsp90 Cochaperone p23 Is the Limiting Component of the Multiprotein Hsp90/Hsp70-based Chaperone System in Vivo Where It Acts to Stabilize the Client Protein·Hsp90 Complex

A variety of signaling proteins form heterocomplexes with and are regulated by the heat shock protein chaperone hsp90. These complexes are formed by a multiprotein machinery, including hsp90 and hsp70 as essential and abundant components and Hop, hsp40, and p23 as non-essential cochaperones that are present in much lower abundance in cells. Overexpression of signaling proteins can overwhelm the capacity of this machinery to properly assemble heterocomplexes with hsp90. Here, we show that the limiting component of this assembly machinery in vitro in reticulocyte lysate and in vivo in Sf9 cells is p23. Only a fraction of glucocorticoid receptors (GR) overexpressed in Sf9 cells are in heterocomplex with hsp90 and have steroid binding activity, with the majority of the receptors present as both insoluble and cytosolic GR aggregates. Coexpression of p23 with the GR increases the proportion of cytosolic receptors that are in stable GR.hsp90 heterocomplexes with steroid binding activity, a strictly hsp90-dependent activity for the GR. Coexpression of p23 eliminates the insoluble GR aggregates and shifts the cytosolic receptor from very large aggregates without steroid binding activity to approximately 600-kDa heterocomplexes with steroid binding activity. These data lead us to conclude that p23 acts in vivo to stabilize hsp90 binding to client protein.

Pharmacological and genetic analysis of 90-kDa heat shock isoprotein-aryl hydrocarbon receptor complexes

The 90-kDa heat shock protein (Hsp90) is an abundant chaperone that regulates a diverse set of intracellular signaling proteins. Drugs that inhibit Hsp90 activity have been useful in the identification of novel Hsp90-dependent signaling pathways. One class of inhibitory compounds disrupts Hsp90-dependent processes by binding to the N-terminal ATPase/p23-binding domain of Hsp90, whereas a second inhibitor class binds within the C-terminal domain. We used signaling by aryl hydrocarbon receptor (AhR), an Hsp90-dependent transcription factor, as a functional probe to study the effects of Hsp90 inhibitors in yeast strains with deletion mutations of individual Hsp90 and p23 cochaperone genes. The more abundant and constitutively expressed Hsp90 isoform, Hsc82, functioned best in supporting AhR signaling. Deletion of the more inducible isoform, Hsp82, had no effect on signaling. AhR complexes containing Hsc82 were preferentially sensitive to the effects of low concentrations of the N-terminal inhibitors radicicol and herbimycin A. However, both Hsp90 isoforms were equally sensitive to the AhR-specific effects of novobiocin, which binds to the C terminus. Hsp90 inhibitors had no preferential effects on AhR signaling in strains that lacked p23, suggesting that the inhibitors exert their effects through a p23-independent mechanism. In contrast, overexpression of p23 buffered the effects of radicicol and herbimycin A, but not novobiocin, on AhR signaling. The data collectively suggest preferential use or function of the Hsc82 isoprotein in AhR signaling and provide new insight into the effects of three structurally unrelated Hsp90 inhibitors.

Genetic dissection of p23, an Hsp90 cochaperone, reveals a distinct surface involved in estrogen receptor signaling

p23 is an Hsp90-associated protein that regulates signal transduction by the estrogen receptor alpha (ER); however, the mechanism through which p23 governs ER function remains enigmatic. To obtain a collection of p23 molecules with distinct effects on ER signaling, we screened in yeast a series of random mutations as well as specific sequence alterations based on the p23 crystal structure and further analyzed these mutations for their effect on p23-Hsp90 association in vitro and in vivo. We found that the ability of the p23 mutants to decrease or increase ER signal transduction correlated with their association with Hsp90. We also identified a mutation in the C-terminal tail of p23, which displayed a dominant inhibitory effect on ER transcriptional activation and associates more avidly with Hsp90 relative to the wild type p23. Interestingly, this mutant interacts with Hsp90 in its non-ATP-bound state, whereas the wild type p23 protein interacts exclusively with the ATP-bound form of Hsp90, which may account for its dominant phenotype. In addition, we have uncovered a novel activity of p23 that antagonizes Hsp90 action during times of cell stress. Using molecular modeling and the p23 crystal structure, we found that the p23 mutations affecting ER signaling identified in the screen localized to one face of the molecule, whereas those that had no effect mapped to other parts of the protein. Thus, our structure/function analysis has identified an important regulatory surface on p23 involved in ER signaling and p23 binding to Hsp90.

Use of modular substrates demonstrates mechanistic diversity and reveals differences in chaperone requirement of ERAD

The endoplasmic reticulum (ER) harbors a protein quality control system, which monitors protein folding in the ER. Elimination of malfolded proteins is an important function of this protein quality control. Earlier studies with various soluble and transmembrane ER-associated degradation (ERAD) substrates revealed differences in the ER degradation machinery used. To unravel the nature of these differences we generated two type I membrane ERAD substrates carrying malfolded carboxypeptidase yscY (CPY*) as the ER-luminal ERAD recognition motif. Whereas the first, CT* (CPY*-TM), has no cytoplasmic domain, the second, CTG*, has the green fluorescent protein present in the cytosol. Together with CPY*, these three substrates represent topologically diverse malfolded proteins, degraded via ERAD. Our data show that degradation of all three proteins is dependent on the ubiquitin-proteasome system involving the ubiquitin-protein ligase complex Der3/Hrd1p-Hrd3p, the ubiquitin conjugating enzymes Ubc1p and Ubc7p, as well as the AAA-ATPase complex Cdc48-Ufd1-Npl4 and the 26S proteasome. In contrast to soluble CPY*, degradation of the membrane proteins CT* and CTG* does not require the ER proteins Kar2p (BiP) and Der1p. Instead, CTG* degradation requires cytosolic Hsp70, Hsp40, and Hsp104p chaperones.

The Hsp90 molecular chaperone complex regulates maltose induction and stability of the Saccharomyces MAL gene transcription activator Mal63p

Induction of the Saccharomyces MAL structural genes encoding maltose permease and maltase requires the MAL activator, a DNA-binding transcription activator. Genetic analysis of MAL activator mutations suggested that protein folding and stability play an important role in MAL activator regulation and led us to explore the role of the Hsp90 molecular chaperone complex in the regulation of the MAL activator. Strains carrying mutations in genes encoding components of the Hsp90 chaperone complex, hsc82 Delta hsp82-T101I and hsc82 Delta cpr7 Delta, are defective for maltase induction and exhibit significantly reduced growth rates on media containing a limiting concentration of maltose (0.05%). This growth defect is suppressed by providing maltose in excess. Using epitope-tagged alleles of the MAL63 MAL activator, we showed that Mal63p levels are drastically reduced following depletion of cellular Hsp90. Overexpression ( approximately 3-fold) of Mal63p in the hsc82 Delta hsp82-T101I and hsc82 Delta cpr7 Delta strains suppresses their Mal- growth phenotype, suggesting that Mal63p levels are limiting for maltose utilization in strains with abrogated Hsp90 activity. Consistent with this, the half-life of Mal63p is significantly shorter in the hsc82 Delta cpr7 Delta strain (reduced about 6-fold) and modestly affected in the Hsp90-ts strain (reduced about 2-fold). Most importantly, triple hemagglutinin-tagged Mal63p protein is found in association with Hsp90 as demonstrated by co-immunoprecipitation. Taken together, these results identify the inducible MAL activator as a client protein of the Hsp90 molecular chaperone complex and point to a critical role for chaperone function in alternate carbon source utilization in Saccharomyces cerevisiae.

Aha1 binds to the middle domain of Hsp90, contributes to client protein activation, and stimulates the ATPase activity of the molecular chaperone

The ATP-dependent molecular chaperone Hsp90 is an essential and abundant stress protein in the eukaryotic cytosol that cooperates with a cohort of cofactors/cochaperones to fulfill its cellular tasks. We have identified Aha1 (activator of Hsp90 ATPase) and its relative Hch1 (high copy Hsp90 suppressor) as binding partners of Hsp90 in Saccharomyces cerevisiae. By using genetic and biochemical approaches, the middle domain of Hsp90 (amino acids 272-617) was found to mediate the interaction with Aha1 and Hch1. Data base searches revealed that homologues of Aha1 are conserved from yeast to man, whereas Hch1 was found to be restricted to lower eukaryotes like S. cerevisiae and Candida albicans. In experiments with purified proteins, Aha1 but not Hch1 stimulated the intrinsic ATPase activity of Hsp90 5-fold. To establish their cellular role further, we deleted the genes encoding Aha1 and Hch1 in S. cerevisiae. In vivo experiments demonstrated that Aha1 and Hch1 contributed to efficient activation of the heterologous Hsp90 client protein v-Src. Moreover, Aha1 and Hch1 became crucial for cell viability under non-optimal growth conditions when Hsp90 levels are limiting. Thus, our results identify a novel type of cofactor involved in the regulation of the molecular chaperone Hsp90.

Regulation of signaling protein function and trafficking by the hsp90/hsp70-based chaperone machinery

Nearly 100 proteins are known to be regulated by hsp90. Most of these substrates or "client proteins" are involved in signal transduction, and they are brought into complex with hsp90 by a multiprotein hsp90/hsp70-based chaperone machinery. In addition to binding substrate proteins at the chaperone site(s), hsp90 binds cofactors at other sites that are part of the heterocomplex assembly machinery as well as immunophilins that connect assembled substrate*hsp90 complexes to protein-trafficking systems. In the 5 years since we last reviewed this subject, much has been learned about hsp90 structure, nucleotide-binding, and cochaperone interactions; the most important concept is that ATP hydrolysis by an intrinsic ATPase activity results in a conformational change in hsp90 that is required to induce conformational change in a substrate protein. The conformational change induced in steroid receptors is an opening of the steroid-binding cleft so that it can be accessed by steroid. We have now developed a minimal system of five purified proteins-hsp90, hsp70, Hop, hsp40, and p23- that assembles stable receptor*hsp90 heterocomplexes. An hsp90*Hop*hsp70*hsp40 complex opens the cleft in an ATP-dependent process to produce a receptor*hsp90 heterocomplex with hsp90 in its ATP-bound conformation, and p23 then interacts with the hsp90 to stabilize the complex. Stepwise assembly experiments have shown that hsp70 and hsp40 first interact with the receptor in an ATP-dependent reaction to produce a receptor*hsp70*hsp40 complex that is "primed" to be activated to the steroid-binding state in a second ATP-dependent step with hsp90, Hop, and p23. Successful use of the five-protein system with other substrates indicates that it can assemble signal protein*hsp90 heterocomplexes whether the substrate is a receptor, a protein kinase, or a transcription factor. This purified system should facilitate understanding of how eukaryotic hsp70 and hsp90 work together as essential components of a process that alters the conformations of substrate proteins to states that respond in signal transduction.

The Cdc37 protein kinase-binding domain is sufficient for protein kinase activity and cell viability

Cdc37 is a molecular chaperone required for folding of protein kinases. It functions in association with Hsp90, although little is known of its mechanism of action or where it fits into a folding pathway involving other Hsp90 cochaperones. Using a genetic approach with Saccharomyces cerevisiae, we show that CDC37 overexpression suppressed a defect in v-Src folding in yeast deleted for STI1, which recruits Hsp90 to misfolded clients. Expression of CDC37 truncation mutants that were deleted for the Hsp90-binding site stabilized v-Src and led to some folding in both sti1Delta and hsc82Delta strains. The protein kinase-binding domain of Cdc37 was sufficient for yeast cell viability and permitted efficient signaling through the yeast MAP kinase-signaling pathway. We propose a model in which Cdc37 can function independently of Hsp90, although its ability to do so is restricted by its normally low expression levels. This may be a form of regulation by which cells restrict access to Cdc37 until it has passed through a triage involving other chaperones such as Hsp70 and Hsp90.

Activation of the ATPase activity of hsp90 by the stress-regulated cochaperone aha1

Client protein activation by Hsp90 involves a plethora of cochaperones whose roles are poorly defined. A ubiquitous family of stress-regulated proteins have been identified (Aha1, activator of Hsp90 ATPase) that bind directly to Hsp90 and are required for the in vivo Hsp90-dependent activation of clients such as v-Src, implicating them as cochaperones of the Hsp90 system. In vitro, Aha1 and its shorter homolog, Hch1, stimulate the inherent ATPase activity of yeast and human Hsp90. The identification of these Hsp90 cochaperone activators adds to the complex roles of cochaperones in regulating the ATPase-coupled conformational changes of the Hsp90 chaperone cycle.

p23 and HSP20/alpha-crystallin proteins define a conserved sequence domain present in other eukaryotic protein families

We identified families of proteins characterized by the presence of a domain similar to human p23 protein, which include proteins such as Sgt1, involved in the yeast kinetochore assembly; melusin, involved in specific interactions with the cytoplasmic integrin beta1 domain; Rar1, related to pathogenic resistance in plants, and to development in animals; B5+B5R flavo-hemo cytochrome NAD(P)H oxidoreductase type B in humans and mice; and NudC, involved in nucleus migration during mitosis. We also found that p23 and the HSP20/alpha-crystallin family of heat shock proteins, which share the same three-dimensional folding, show a pattern of conserved residues that points to a common origin in the evolution of both protein domains. The p23 and HSP20/alpha-crystallin phylogenetic relationship and their similar role in chaperone activity suggest a common function, probably involving protein-protein interaction, for those proteins containing p23-like domains.

The Hsp90 co-chaperones Cdc37 and Sti1 interact physically and genetically

Cdc37 associates with the heat-shock protein 90 (Hsp90) molecular chaperone as one of several auxiliary proteins that are collectively referred to as Hsp90 co-chaperones. Cdc37 has been proposed to be a specificity factor for Hsp90, directing it notably towards kinases. It is not known whether Cdc37 is essential for viability in the budding yeast Saccharomyces cerevisiae because of Hsp90-dependent or -independent functions or both. Sti1 and Cpr7 are non-essential Hsp90 co-chaperones that bind to a common surface on Hsp90 through tetratricopeptide repeats (TPR). We have found that Sti1 is specifically retained from yeast extracts by immobilized Cdc37. Similarly, the endogenous proteins are also found in a complex. Moreover, purified recombinant Sti1 and Cdc37 interact in the complete absence of Hsp90. Complexes between Cdc37 and Sti1 are not unique to this TPR protein since endogenous Cdc37 can be co-purified with exogenously expressed Cpr7 fused to glutathione-S-transferase. The heterogeneity of Cdc37 complexes, both with and without Hsp90, may expand the functional diversity of Cdc37. Here we show that the combination of cdc37 and sti1 mutations is synthetically lethal, suggesting that direct contacts between Cdc37 and Sti1 may at least contribute to vital functions in yeast.

Two tetratricopeptide repeat proteins facilitate human aryl hydrocarbon receptor signalling in yeast

A human aryl hydrocarbon (Ah) receptor signalling pathway was constructed in yeast and used to identify regulatory proteins that may be related to those present in mammalian cells. The sequence similarity of human hepatitis B protein X-associated protein 2 (XAP2) protein to yeast Cpr7 and Cns1 proteins suggested that these proteins might be involved in Ah receptor signalling in this model system. Ah receptor signalling from a lacZ reporter gene was reduced by approximately 60% in cells that lacked Cpr7. In vitro interaction experiments indicated that a Cpr7-GST fusion protein and Ah receptor formed a complex. Expression of Cpr7, Cns1 and the isolated tetratricopeptide repeat (TPR) region of Cpr7 from plasmids restored Ah receptor signalling function in the Cpr7-deficient strain. Thus, Cpr7 and Cns1 proteins facilitate the signalling of human Ah receptor expressed in yeast, perhaps in the same manner as the TPR-containing XAP2 protein and related chaperone proteins in mammalian cells.

The p23 co-chaperone facilitates dioxin receptor signaling in a yeast model system

Hsp90, p23 and other chaperosome proteins are critical for the function of several enzymes and steroid hormone receptors. The dioxin receptor (DR) is an Hsp90-regulated transcription factor that binds numerous toxic ligands, including the environmental contaminant 2,3,7,8-tetrachlorodibenzo(p)dioxin. We used a yeast model system that expressed human DR and Arnt proteins to test whether p23 affected DR signaling. Deletion of the SBA1 gene (yeast p23 homolog) in this model system reduced ligand-mediated DR signaling by approximately 40% and shifted the EC(50) of the beta-napthoflavone ligand by five-fold in a reporter gene assay. DR signaling was restored in the sba1 strain by a plasmid-borne SBA1 gene, confirming that the signaling defect was due to SBA1. The human p23 protein substituted for yeast Sba1 protein in this model system. These genetic data show that p23 enhances DR signaling.

Folding of newly translated proteins in vivo: the role of molecular chaperones

Recent years have witnessed dramatic advances in our understanding of how newly translated proteins fold in the cell and the contribution of molecular chaperones to this process. Folding in the cell must be achieved in a highly crowded macromolecular environment, in which release of nonnative polypeptides into the cytosolic solution might lead to formation of potentially toxic aggregates. Here I review the cellular mechanisms that ensure efficient folding of newly translated proteins in vivo. De novo protein folding appears to occur in a protected environment created by a highly processive chaperone machinery that is directly coupled to translation. Genetic and biochemical analysis shows that several distinct chaperone systems, including Hsp70 and the cylindrical chaperonins, assist the folding of proteins upon translation in the cytosol of both prokaryotic and eukaryotic cells. The cellular chaperone machinery is specifically recruited to bind to ribosomes and protects nascent chains and folding intermediates from nonproductive interactions. In addition, initiation of folding during translation appears to be important for efficient folding of multidomain proteins.

Identification and characterization of ART-27, a novel coactivator for the androgen receptor N terminus

The androgen receptor (AR) is a ligand-regulated transcription factor that stimulates cell growth and differentiation in androgen-responsive tissues. The AR N terminus contains two activation functions (AF-1a and AF-1b) that are necessary for maximal transcriptional enhancement by the receptor; however, the mechanisms and components regulating AR transcriptional activation are not fully understood. We sought to identify novel factors that interact with the AR N terminus from an androgen-stimulated human prostate cancer cell library using a yeast two-hybrid approach designed to identify proteins that interact with transcriptional activation domains. A 157-amino acid protein termed ART-27 was cloned and shown to interact predominantly with the AR(153-336), containing AF-1a and a part of AF-1b, localize to the nucleus and increase the transcriptional activity of AR when overexpressed in cultured mammalian cells. ART-27 also enhanced the transcriptional activation by AR(153-336) fused to the LexA DNA-binding domain but not other AR N-terminal subdomains, suggesting that ART-27 exerts its effect via an interaction with a defined region of the AR N terminus. ART-27 interacts with AR in nuclear extracts from LNCaP cells in a ligand-independent manner. Interestingly, velocity gradient sedimentation of HeLa nuclear extracts suggests that native ART-27 is part of a multiprotein complex. ART-27 is expressed in a variety of human tissues, including sites of androgen action such as prostate and skeletal muscle, and is conserved throughout evolution. Thus, ART-27 is a novel cofactor that interacts with the AR N terminus and plays a role in facilitating receptor-induced transcriptional activation.

In vitro reconstitution of functional hepadnavirus reverse transcriptase with cellular chaperone proteins

Initiation of reverse transcription in hepadnaviruses (hepatitis B viruses) depends on the specific binding of an RNA signal (the packaging signal, epsilon) on the pregenomic RNA template by the viral reverse transcriptase (RT) and is primed by the RT itself (protein priming). We have previously shown that the RT-epsilon interaction and protein priming require the cellular heat shock protein, Hsp90. However, additional host factors required for these reactions remained to be identified. We now report that five cellular chaperone proteins, all known cofactors of Hsp90, were sufficient to reconstitute a duck hepatitis B virus RT active in epsilon binding and protein priming in vitro. Four proteins, Hsp90, Hsp70, Hsp40, and Hop, were required for reconstitution of RT activity, and the fifth protein, p23, further enhanced the kinetics of reconstitution. RT activation by the chaperone proteins is a dynamic process dependent on ATP hydrolysis and the Hsp90 ATPase activity. Thus, our results have defined a minimal complement of host factors necessary and sufficient for RT activation. Furthermore, this defined in vitro reconstitution system has now paved the way for future biochemical and structural studies to elucidate the mechanisms of RT activation and chaperone functions.