The peptidyl-prolyl isomerase domain of the CyP-40 cyclophilin homolog Cpr7 is not required to support growth or glucocorticoid receptor activity in Saccharomyces cerevisiae

Insights into Hsp90 mechanism and in vivo functions learned from studies in the yeast, Saccharomyces cerevisiae

The molecular chaperone Hsp90 (Heat shock protein, 90 kDa) is an abundant and essential cytosolic protein required for the stability and/or folding of hundreds of client proteins. Hsp90, along with helper cochaperone proteins, assists client protein folding in an ATP-dependent pathway. The laboratory of Susan Lindquist, in collaboration with other researchers, was the first to establish the yeast Saccharomyces cerevisiae as a model organism to study the functional interaction between Hsp90 and clients. Important insights from studies in her lab were that Hsp90 is essential, and that Hsp90 functions and cochaperone interactions are highly conserved between yeast and mammalian cells. Here, we describe key mechanistic insights into the Hsp90 folding cycle that were obtained using the yeast system. We highlight the early contributions of the laboratory of Susan Lindquist and extend our analysis into the broader use of the yeast system to analyze the understanding of the conformational cycle of Hsp90 and the impact of altered Hsp90 function on the proteome.

Characterization and regulation of salt upregulated cyclophilin from a halotolerant strain of Penicillium oxalicum

Penicillium species are an industrially important group of fungi. Cyclophilins are ubiquitous proteins and several members of this family exhibit peptidyl-prolyl cis-trans isomerase (PPIase) activity. We had earlier demonstrated that the salt-induced PPIase activity in a halotolerant strain of P. oxalicum was associated with enhanced expression of a cyclophilin gene, PoxCYP18. Cloning and characterization of PoxCYP18 revealed that its cDNA consists of 522 bp encoding a protein of 173 amino acid residues, with predicted molecular mass and pI values of 18.91 kDa and 8.87, respectively. The recombinant PoxCYP18 can catalyze cis-trans isomerization of peptidyl-prolyl bond with a catalytic efficiency of 1.46 × 107 M-1 s-1 and is inhibited specifically only by cyclosporin A, with an inhibition constant of 5.04 ± 1.13 nM. PoxCYP18 consists of two cysteine residues at positions - 45 and - 170, and loses its activity under oxidizing conditions. Substitution of these residues alone or together by site-directed mutagenesis revealed that the PPIase activity of PoxCYP18 is regulated through a redox mechanism involving the formation of disulfide linkages. Heterologous expression of PoxCYP18 conferred enhanced tolerance to salt stress in transgenic E. coli cells, implying that this protein imparts protection to cellular processes against salt-induced damage.

Structure, Function, and Regulation of the Hsp90 Machinery

Heat shock protein 90 (Hsp90) is a molecular chaperone involved in the maturation of a plethora of substrates ("clients"), including protein kinases, transcription factors, and E3 ubiquitin ligases, positioning Hsp90 as a central regulator of cellular proteostasis. Hsp90 undergoes large conformational changes during its ATPase cycle. The processing of clients by cytosolic Hsp90 is assisted by a cohort of cochaperones that affect client recruitment, Hsp90 ATPase function or conformational rearrangements in Hsp90. Because of the importance of Hsp90 in regulating central cellular pathways, strategies for the pharmacological inhibition of the Hsp90 machinery in diseases such as cancer and neurodegeneration are being developed. In this review, we summarize recent structural and mechanistic progress in defining the function of organelle-specific and cytosolic Hsp90, including the impact of individual cochaperones on the maturation of specific clients and complexes with clients as well as ways of exploiting Hsp90 as a drug target.

Nucleotide-Dependent Dimer Association and Dissociation of the Chaperone Hsp90

Hsp90 is an essential molecular chaperone, which has to be in a dimeric form for its correct function. While the affinity of the dimer has previously been measured, little is known about how it associates and dissociates and the factors that influence this. We perform an in-depth single molecule characterization of the C-terminal association and dissociation of Hsp90. We find more than one dissociation rate, indicating that the dimer has a stable and an unstable state. Furthermore, we find that the stability of the C-terminal association is dependent on the presence of ATP, despite the C-terminal dimerization interface being distal to the catalytic site.

Evidence for Hsp90 Co-chaperones in Regulating Hsp90 Function and Promoting Client Protein Folding

Molecular chaperones are a diverse group of highly conserved proteins that transiently interact with partially folded polypeptide chains during normal cellular processes such as protein translation, translocation, and disassembly of protein complexes. Prior to folding or after denaturation, hydrophobic residues that are normally sequestered within a folded protein are exposed to the aqueous environment and are prone to aggregation or misfolding. Multiple classes of molecular chaperones, such as Hsp70s and Hsp40s, recognize and transiently bind polypeptides with exposed hydrophobic stretches in order to prevent misfolding. Other types of chaperones, such as Hsp90, have more specialized functions in that they appear to interact with only a subset of cellular proteins. This chapter focuses on the role of Hsp90 and partner co-chaperones in promoting the folding and activation of a diverse group of proteins with critical roles in cellular signaling and function.

The FKBP51 Glucocorticoid Receptor Co-Chaperone: Regulation, Function, and Implications in Health and Disease

Among the chaperones and co-chaperones regulating the glucocorticoid receptor (GR), FK506 binding protein (FKBP) 51 is the most intensely investigated across different disciplines. This review provides an update on the role of the different co-chaperones of Hsp70 and Hsp90 in the regulation of GR function. The development leading to the focus on FKBP51 is outlined. Further, a survey of the vast literature on the mechanism and function of FKBP51 is provided. This includes its structure and biochemical function, its regulation on different levels—transcription, post-transcription, and post-translation—and its function in signaling pathways. The evidence portraying FKBP51 as a scaffolding protein organizing protein complexes rather than a chaperone contributing to the folding of individual proteins is collated. Finally, FKBP51’s involvement in physiology and disease is outlined, and the promising efforts in developing drugs targeting FKBP51 are discussed.

The crystal structure of the Hsp90 co-chaperone Cpr7 from Saccharomyces cerevisiae

The versatility of Hsp90 can be attributed to the variety of co-chaperone proteins that modulate the role of Hsp90 in many cellular processes. As a co-chaperone of Hsp90, Cpr7 is essential for accelerating the cell growth in an Hsp90-containing trimeric complex. Here, we report the crystal structure of Cpr7 at a resolution of 1.8Å. It consists of an N-terminal PPI domain and a C-terminal TPR domain, and exhibits a U-shape conformation. Our studies revealed the aggregation state of Cpr7 in solution and the interaction properties between Cpr7 and the MEEVD sequence from the C-terminus of Hsp90. In addition, the structure and sequence analysis between Cpr7 and homologues revealed the structure basis both for the function differences between Cpr6 and Cpr7 and the functional complements between Cns1 and Cpr7. Our studies facilitate the understanding of Cpr7 and provide decent insights into the molecular mechanisms of the Hsp90 co-chaperone pathway.

Hsp90-Associated Immunophilin Homolog Cpr7 Is Required for the Mitotic Stability of [URE3] Prion in Saccharomyces cerevisiae

The role of Hsp70 chaperones in yeast prion propagation is well established. Highly conserved Hsp90 chaperones participate in a number of cellular processes, such as client protein maturation, protein degradation, cellular signalling and apoptosis, but little is known about their role in propagation of infectious prion like aggregates. Here, we examine the influence of Hsp90 in the maintenance of yeast prion [URE3] which is a prion form of native protein Ure2, and reveal a previously unknown role of Hsp90 as an important regulator of [URE3] stability. We show that the C-terminal MEEVD pentapeptide motif, but not the client maturation activity of Hsp90, is essential for [URE3] prion stability. In testing deletions of various Hsp90 co-chaperones known to bind this motif, we find the immunophilin homolog Cpr7 is essential for [URE3] propagation. We show that Cpr7 interacts with Ure2 and enhances its fibrillation. The requirement of Cpr7 is specific for [URE3] as its deletion does not antagonize both strong and weak variant of another yeast prion [PSI+], suggesting a distinct role of the Hsp90 co-chaperone with different yeast prions. Our data show that, similar to the Hsp70 family, the Hsp90 chaperones also influence yeast prion maintenance, and that immunophilins could regulate protein multimerization independently of their activity as peptidyl-prolyl isomerases.

Interaction of p53 with prolyl isomerases: Healthy and unhealthy relationships

BACKGROUND

The p53 protein family, comprising p53, p63 and p73, is primarily involved in preserving genome integrity and preventing tumor onset, and also affects a range of physiological processes. Signal-dependent modifications of its members and of other pathway components provide cells with a sophisticated code to transduce a variety of stress signaling into appropriate responses. TP53 mutations are highly frequent in cancer and lead to the expression of mutant p53 proteins that are endowed with oncogenic activities and sensitive to stress signaling.

SCOPE OF REVIEW

p53 family proteins have unique structural and functional plasticity, and here we discuss the relevance of prolyl-isomerization to actively shape these features.

MAJOR CONCLUSIONS

The anti-proliferative functions of the p53 family are carefully activated upon severe stress and this involves the interaction with prolyl-isomerases. In particular, stress-induced stabilization of p53, activation of its transcriptional control over arrest- and cell death-related target genes and of its mitochondrial apoptotic function, as well as certain p63 and p73 functions, all require phosphorylation of specific S/T-P motifs and their subsequent isomerization by the prolyl-isomerase Pin1. While these functions of p53 counteract tumorigenesis, under some circumstances their activation by prolyl-isomerases may have negative repercussions (e.g. tissue damage induced by anticancer therapies and ischemia-reperfusion, neurodegeneration). Moreover, elevated Pin1 levels in tumor cells may transduce deregulated phosphorylation signaling into activation of mutant p53 oncogenic functions.

GENERAL SIGNIFICANCE

The complex repertoire of biological outcomes induced by p53 finds mechanistic explanations, at least in part, in the association between prolyl-isomerases and the p53 pathway. This article is part of a Special Issue entitled Proline-directed foldases: Cell signaling catalysts and drug targets.

The Hsp90 cochaperones Cpr6, Cpr7, and Cns1 interact with the intact ribosome

The abundant molecular chaperone Hsp90 is essential for the folding and stabilization of hundreds of distinct client proteins. Hsp90 is assisted by multiple cochaperones that modulate Hsp90's ATPase activity and/or promote client interaction, but the in vivo functions of many of these cochaperones are largely unknown. We found that Cpr6, Cpr7, and Cns1 interact with the intact ribosome and that Saccharomyces cerevisiae lacking CPR7 or containing mutations in CNS1 exhibited sensitivity to the translation inhibitor hygromycin. Cpr6 contains a peptidyl-prolyl isomerase (PPIase) domain and a tetratricopeptide repeat (TPR) domain flanked by charged regions. Truncation or alteration of basic residues near the carboxy terminus of Cpr6 disrupted ribosome interaction. Cns1 contains an amino-terminal TPR domain and a poorly characterized carboxy-terminal domain. The isolated carboxy-terminal domain was able to interact with the ribosome. Although loss of CPR6 does not cause noticeable growth defects, overexpression of CPR6 results in enhanced growth defects in cells expressing the temperature-sensitive cns1-G90D mutation (the G-to-D change at position 90 encoded by cns1). Cpr6 mutants that exhibit reduced ribosome interaction failed to cause growth defects, indicating that ribosome interaction is required for in vivo functions of Cpr6. Together, these results represent a novel link between the Hsp90 molecular-chaperone machine and protein synthesis.

Molecular cochaperones: tumor growth and cancer treatment

Molecular chaperones play important roles in all cellular organisms by maintaining the proteome in an optimally folded state. They appear to be at a premium in cancer cells whose evolution along the malignant pathways requires the fostering of cohorts of mutant proteins that are employed to overcome tumor suppressive regulation. To function at significant rates in cells, HSPs interact with cochaperones, proteins that assist in catalyzing individual steps in molecular chaperoning as well as in posttranslational modification and intracellular localization. We review current knowledge regarding the roles of chaperones such as heat shock protein 90 (Hsp90) and Hsp70 and their cochaperones in cancer. Cochaperones are potential targets for cancer therapy in themselves and can be used to assess the likely prognosis of individual malignancies. Hsp70 cochaperones Bag1, Bag3, and Hop play significant roles in the etiology of some cancers as do Hsp90 cochaperones Aha1, p23, Cdc37, and FKBP1. Others such as the J domain protein family, HspBP1, TTC4, and FKBPL appear to be associated with more benign tumor phenotypes. The key importance of cochaperones for many pathways of protein folding in cancer suggests high promise for the future development of novel pharmaceutical agents.

Chaperoning the chaperone: a role for the co-chaperone Cpr7 in modulating Hsp90 function in Saccharomyces cerevisiae

Heat-shock protein 90 (Hsp90) of Saccharomyces cerevisiae is an abundant essential eukaryotic molecular chaperone involved in the activation and stabilization of client proteins, including several transcription factors and oncogenic kinases. Hsp90 undergoes a complex series of conformational changes and interacts with partner co-chaperones such as Sba1, Cpr6, Cpr7, and Cns1 as it binds and hydrolyzes ATP. In the absence of nucleotide, Hsp90 is dimerized only at the carboxy-terminus. In the presence of ATP, Hsp90 also dimerizes at the amino-terminus, creating a binding site for Sba1. Truncation of a charged linker region of yeast Hsp90 (Hsp82Δlinker) was known to disrupt the ability of Hsp82 to undergo amino-terminal dimerization and bind Sba1. We found that yeast expressing Hsp82Δlinker constructs exhibited a specific synthetic lethal phenotype in cells lacking CPR7. The isolated tetratricopeptide repeat domain of Cpr7 was both necessary and sufficient for growth in those strains. Cpr6 and Cpr7 stably bound the carboxy-terminus of wild-type Hsp82 only in the presence of nonhydrolyzable ATP and formed an Hsp82-Cpr6-Cpr7 ternary complex. However, in cells expressing Hsp82Δlinker or lacking CPR7, Cpr6 was able to bind Hsp82 in the presence or absence of nucleotide. Overexpression of CNS1, but not of other co-chaperones, in cpr7 cells restored nucleotide-dependent Hsp82-Cpr6 interaction. Together, our results suggest that the in vivo functions of Cpr7 include modulating Hsp90 conformational changes, mediating proper signaling of the nucleotide-bound state to the carboxy-terminus of Hsp82, or regulating Hsp82-Cpr6 interaction.

The TPR domain in the host Cyp40-like cyclophilin binds to the viral replication protein and inhibits the assembly of the tombusviral replicase

Replication of plus-stranded RNA viruses is greatly affected by numerous host-coded proteins acting either as susceptibility or resistance factors. Previous genome-wide screens and global proteomics approaches with Tomato bushy stunt tombusvirus (TBSV) in a yeast model host revealed the involvement of cyclophilins, which are a large family of host prolyl isomerases, in TBSV replication. In this paper, we identified those members of the large cyclophilin family that interacted with the viral replication proteins and inhibited TBSV replication. Further characterization of the most effective cyclophilin, the Cyp40-like Cpr7p, revealed that it strongly inhibits many steps during TBSV replication in a cell-free replication assay. These steps include viral RNA recruitment inhibited via binding of Cpr7p to the RNA-binding region of the viral replication protein; the assembly of the viral replicase complex and viral RNA synthesis. Since the TPR (tetratricopeptide repeats) domain, but not the catalytic domain of Cpr7p is needed for the inhibitory effect on TBSV replication, it seems that the chaperone activity of Cpr7p provides the negative regulatory function. We also show that three Cyp40-like proteins from plants can inhibit TBSV replication in vitro and Cpr7p is also effective against Nodamura virus, an insect pathogen. Overall, the current work revealed a role for Cyp40-like proteins and their TPR domains as regulators of RNA virus replication.

Cyclophilin 40 facilitates HSP90-mediated RISC assembly in plants

Posttranscriptional gene silencing is mediated by RNA-induced silencing complexes (RISCs) that contain AGO proteins and single-stranded small RNAs. The assembly of plant AGO1-containing RISCs depends on the molecular chaperone HSP90. Here, we demonstrate that cyclophilin 40 (CYP40), protein phosphatase 5 (PP5), and several other proteins with the tetratricopeptide repeat (TPR) domain associates with AGO1 in an HSP90-dependent manner in extracts of evacuolated tobacco protoplasts (BYL). Intriguingly, CYP40, but not the other TPR proteins, could form a complex with small RNA duplex-bound AGO1. Moreover, CYP40 that was synthesized by in-vitro translation using BYL uniquely facilitated binding of small RNA duplexes to AGO1, and as a result, increased the amount of mature RISCs that could cleave target RNAs. CYP40 was not contained in mature RISCs, indicating that the association is transient. Addition of PP5 or cyclophilin-binding drug cyclosporine A prevented the association of endogenous CYP40 with HSP90-AGO1 complex and inhibited RISC assembly. These results suggest that a complex of AGO1, HSP90, CYP40, and a small RNA duplex is a key intermediate of RISC assembly in plants.

Binding of the cyclophilin 40 ortholog SQUINT to Hsp90 protein is required for SQUINT function in Arabidopsis

SQN (SQUINT) is the Arabidopsis ortholog of the immunophilin CyP40 (cyclophilin 40) and promotes microRNA activity by promoting the activity of AGO1. In animals and Saccharomyces cerevisiae, CyP40 promotes protein activity in association with the protein chaperone Hsp90. To determine whether CyP40 also acts in association with Hsp90 in plants, we examined the interaction between SQN and Hsp90 in vitro and tested the importance of this interaction for the function of SQN in planta. We found that SQN interacts with cytoplasmic Hsp90 proteins but not with Hsp90 proteins localized to chloroplasts, mitochondria, or the endoplasmic reticulum. The interaction between SQN and Hsp90 in vitro requires the MEEVD domain of Hsp90, as well as several conserved amino acids within the tetratricopeptide repeat domain of SQN. Amino acid substitutions that disrupt the interaction between SQN and Hsp90 in vitro also impair the activity of SQN in planta. Our results indicate that the interaction between CyP40 and Hsp90 is conserved in plants and that this interaction is essential for the function of CyP40.

The role of p23, Hop, immunophilins, and other co-chaperones in regulating Hsp90 function

Molecular chaperones are a diverse group of highly conserved proteins that transiently interact with partially folded polypeptide chains during normal cellular processes, such as protein translation, translocation, and disassembly of protein complexes (1). Prior to folding or after denaturation, hydrophobic residues that are normally sequestered within a folded protein are exposed to the aqueous environment and are prone to aggregation or misfolding. Multiple classes of molecular chaperones, such as Hsp70s and Hsp40s, recognize and transiently bind polypeptides with exposed hydrophobic stretches in order to prevent misfolding. Other types of chaperones, such as Hsp90, have more specialized functions in that they appear to interact with only a subset of cellular proteins. This chapter focuses on the role of Hsp90 and partner co-chaperones in promoting the folding and activation of a diverse group of proteins with critical roles in cellular signaling and function.

Cyclophilin-40 has a cellular role in the aryl hydrocarbon receptor signaling

Cyclophilin-40 (CyP40) promotes the formation of the gel shift complex that contains the aryl hydrocarbon receptor (AhR), AhR nuclear translocator (Arnt) and dioxin response element (DRE) using baculovirus expressed proteins. Here we reported that CyP40 plays a role in the AhR signaling. When the CyP40 content in MCF-7 cells is reduced, up-regulation of cyp1a1 and cyp1b1 by 3-methylchloranthrene (3MC) is also reduced, suggesting that CyP40 is essential for maximal AhR function. The CyP40 region containing amino acids 186-215, but not the peptidyl-prolyl cis-trans isomerase and tetratricopeptide repeat domains, is essential for forming the AhR/Arnt/DRE complex. CyP40 is found in the cell nucleus after 3MC treatment and appears to promote the DRE binding form of the AhR/Arnt heterodimer.

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 chaperone function of cyclophilin 40 maps to a cleft between the prolyl isomerase and tetratricopeptide repeat domains

Cyclophilin 40 (CyP40), an immunophilin cochaperone present in steroid receptor-Hsp90 complexes, contains an N-terminal peptidylprolyl isomerase (PPIase) domain separated from a C-terminal Hsp90-binding tetratricopeptide repeat (TPR) domain by a 30-residue linker. To map CyP40 chaperone function, CyP40 deletion mutants were prepared and analysed for chaperone activity. CyP40 fragments containing the PPIase domain plus linker or the linker region and the adjoining TPR domain retained chaperone activity, whilst individually, the catalytic and TPR domains were devoid of chaperoning ability. CyP40 chaperone function then, is localized within the linker that forms a binding cleft with potential to accommodate non-native substrates.

Functional interactions between Hsp90 and the co-chaperones Cns1 and Cpr7 in Saccharomyces cerevisiae

Hsp90 complexes contain a class of co-chaperones characterized by a tetratricopeptide repeat (TPR) domain, which mediates binding to a carboxyl-terminal EEVD region in Hsp90. Among Hsp90 TPR co-chaperones in Saccharomyces cerevisiae, only Cns1 is essential. The amino terminus of Cns1, which harbors the TPR domain, is sufficient for viability when overexpressed. In a screen for temperature-sensitive alleles of CNS1, we identified mutations resulting in substitutions of conserved residues in the TPR domain. Mutations in CNS1 disrupt in vitro and in vivo interaction with Hsp90 and reduce Hsp90 function, indicating that Cns1 is a bona fide co-chaperone. Genetic interactions between CNS1 and another Hsp90 co-chaperone, CPR7, suggest that the two co-chaperones share an essential role in the cell. Although both the TPR and the isomerase domains of the cyclophilin Cpr7 are required for viability of cns1 mutant cells, this requirement does not depend on the catalytic function of the isomerase domain. Instead, hydrophilic residues on the surface of this domain appear to be important for the common Cns1.Cpr7 function. Although both co-chaperones interact with Hsp90 primarily through the carboxyl terminus (EEVD), Cns1 and Cpr7 are mostly found in complexes distinct from Hsp90. EEVD is required for normal growth in cns1 mutant cells, demonstrating for the first time in vivo requirement for this conserved region of Hsp90. Overall, our findings reveal a considerable degree of complexity in the interactions not only between Hsp90 and its co-chaperones, but also among the co-chaperones themselves.

A Single-domain Cyclophilin from Leishmania donovaniReactivates Soluble Aggregates of Adenosine Kinase by Isomerase-independent Chaperone Function

Disaggregation and reactivation of aggregated proteins by chaperones is well established. However, little is known regarding such kind of function of single-domain small cyclophilins (CyPs). Here we demonstrate that, with increasing concentrations, fully active adenosine kinase (AdK) of Leishmania donovani tends to form soluble aggregates, resulting in inactivation. Using this inactive enzyme as the substrate, it is shown that a CyP from L. donovani (LdCyP) alone can cause complete disaggregation, leading to reactivation of the enzyme. The reactivating ability of LdCyP remains unaffected even in the presence of cyclosporin A and macromolecular crowding agents. The reactivation occurs noncatalytically and is reversible. A truncated LdCyP, devoid of 88 amino acids from the N terminus, is found to be required in near stoichiometric proportion to reactivate AdK, suggesting essentiality of the C-terminal region. Gel filtration and light-scattering experiments together with protein cross-linking studies revealed that both full-length LdCyP and the truncated form directly interact with AdK and convert oligomeric forms of the enzyme to monomeric state. Homology modeling studies suggest that the exposed hydrophobic residues of LdCyP, by interacting with solvent-accessible hydrophobic surface of AdK, pull apart its aggregated inactive oligomers to functional monomers. Clearly, the results are consistent with the interpretation that the higher efficiency of the truncated LdCyP is most likely due to increased exposure of the hydrophobic residues on its surface. These observations, besides establishing L. donovani AdK as one of the model enzymes to study aggregation-disaggregation of proteins, raise the possibility that single-domain small CyPs, under physiological conditions, may regulate the activity of aggregation-prone proteins by ensuring their disaggregation.

Cloning and characterization of Giardia intestinalis cyclophilin

The cyclophilins (Cyps) are family members of proteins that exhibit peptidylprolyl cis-trans isomerase (PPIase, EC 5.2.1.8) activity and bind the immunosuppressive agent cyclosprin A (CsA) in varying degrees. During the process of random sequencing of a cDNA library made from Giardia intestinalis WB strain, the cyclophilin gene (gicyp 1) was isolated. An open reading frame of gicyp 1 gene was 576 nucleotides, which corresponded to a translation product of 176 amino acids (Gicyp 1). The identity with other Cyps was about 58-71%. The 13 residues that constituted the CsA binding site of human cyclophilin were also detected in the amino acid sequence of Gicyp 1, including tryptophan residue essential for the drug binding. The single copy of the gicyp 1 gene was detected in the G. intestinalis chromosome by southern hybridization analysis. Recombinant Gicyp 1 protein clearly accelerated the rate of cis-->trans isomerization of the peptide substrate and the catalysis was completely inhibited by the addition of 0.5 microM CsA.

The involvement of mammalian and plant FK506-binding proteins (FKBPs) in development

The FK506-binding proteins (FKBPs) are peptidyl prolyl cis/trans isomerases and the information gathered in the last 10 years reveals their involvement in diverse biological systems affecting the function and structure of target proteins. Members of the FKBP family were shown to be growth-regulated and participate in signal transduction. In this review we have chosen to focus on a few examples of the mammalian and plant systems in which members of the FKBP family have been demonstrated to affect the function of proteins or development. The technologies that enable production of knockout mice, Arabidopsis mutants and overexpression in transgenic organisms have revealed the contribution of FKBP to development in higher eukaryotes. It appears that members of the FKBP family have conserved some of their basic functions in the animal and plant kingdom, whereas other functions became unique. Studies that will take advantage of the full genome sequence available for Arabidopsis and the human genome, DNA chip technologies and the use of transgenic complementation system will contribute to the elucidation of the molecular mechanism and biological function of FKBPs.

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.

Regulation of vegetative phase change in Arabidopsis thaliana by cyclophilin 40

During its development, a plant shoot progresses from a juvenile to an adult phase of vegetative growth and from a reproductively incompetent to a reproductively competent state. In Arabidopsis, loss-of-function mutations in SQUINT (SQN) reduced the number of juvenile leaves and had subtle effects on inflorescence morphology but had no effect on flowering time or on reproductive competence. SQN encodes the Arabidopsis homolog of cyclophilin 40 (CyP40), a protein found in association with the Hsp90 chaperone complex in yeast, mammals, and plants. Thus, in Arabidopsis, CyP40 is specifically required for the vegetative but not the reproductive maturation of the shoot.

Cpr6 and Cpr7, two closely related Hsp90-associated immunophilins from Saccharomyces cerevisiae, differ in their functional properties

Hsp90 is an abundant cytosolic molecular chaperone. It controls the folding of target proteins including steroid hormone receptors and kinases in complex with several partner proteins. Prominent members of this protein family are large peptidyl prolyl cis/trans isomerases (PPIases), which catalyze the cis/trans isomerization of prolyl peptide bonds in proteins and possess chaperone activity. In Saccharomyces cerevisiae, two closely related large Hsp90-associated PPIases, Cpr6 and Cpr7, exist. We show here that these homologous proteins bind with comparable affinity to Hsp90 but exhibit significant structural and functional differences. Cpr6 is more stable than Cpr7 against thermal denaturation and displays an up to 100-fold higher PPIase activity. In contrast, the chaperone activity of Cpr6 is much lower than that of Cpr7. Based on these results we suggest that the two immunophilins perform overlapping but not identical tasks in the Hsp90 chaperone cycle.

Overlapping sites of tetratricopeptide repeat protein binding and chaperone activity in heat shock protein 90

The sequential binding of different tetratricopeptide repeat (TPR) proteins to heat shock protein 90 (hsp90) is essential to its chaperone function in vivo. We have previously shown that three basic residues in the TPR domain of PP5 are required for binding to the acidic C-terminal domain of hsp90. We have now tested which acidic residues in this C-terminal domain are required for binding to three different TPR proteins as follows: PP5, FKBP52, and Hop. Mutation of Glu-729, Glu-730, and Asp-732 at the C terminus of hsp90 interfered with binding of all three TPR proteins. Mutation of Glu-720, Asp-722, Asp-723, and Asp-724 inhibited binding of FKBP52 and PP5 but not of Hop. Mutation of Glu-651 and Asp-653 did not affect binding of FKBP52 or PP5 but inhibited both Hop binding and hsp90 chaperone activity. We also found that a conserved Lys residue required for PP5 binding to hsp90 was critical for the binding of FKBP52 but not for the binding of Hop to hsp90. These results suggest distinct but overlapping binding sites on hsp90 for different TPR proteins and indicate that the binding site for Hop, which is associated with hsp90 in intermediate stages of protein folding, overlaps with a site of chaperone activity.

Characterization of the cyclophilin of Trichophyton mentagrophytes

A genetic approach to cyclophilins in a dermatophyte, Trichophyton mentagrophytes, was carried out. The nucleotide and deduced amino acid sequences of the cyclophilin of T. mentagrophytes shared about 70% sequence similarity with those of Schizosaccharomyces pombe, Saccharomyces cerevisiae and Candida albicans. However, the first 21 amino acid and the C-terminal amino acid regions of 188 to 226 of the T. mentagrophytes cyclophilin were distinct from those of the other fungal cyclophilins. The recombinant glutathione S-transferase (GST)-T. mentagrophytes cyclophilin fusion protein produced by Escherichia coli was purified. The protease digest of the fusion protein had a molecular weight of about 13 kDa and peptidyl-prolyl cis-trans isomerase (PPI) activity. This digest protein from T. mentagrophytes was confirmed to be cyclophilin by proving PPI activity.

Structure and in vivo function of Hsp90

Until recently, Hsp90 was one of the least well understood of the molecular chaperones, but considerable progress is now being made in unravelling its biochemistry. Hsp90 has now been shown to possess an inherent ATPase that is essential for the activation of authentic 'client' proteins in vivo and in vitro. The molecular detail of Hsp90's interactions with co-chaperones is also becoming clearer and the identification of key roles in assembling regulatory and signalling pathways has made it a target for anticancer drug development. Despite this, a clear understanding of how Hsp90 contributes to the folding and/or activation of its client proteins remains some way off.

Identification of conserved residues required for the binding of a tetratricopeptide repeat domain to heat shock protein 90

The sequential binding of heat shock protein 90 (hsp90) to a series of tetratricopeptide repeat (TPR) proteins is critical to its function as a molecular chaperone. We have used site-directed mutagenesis to clarify the structural basis for the binding of hsp90 to the TPR domain of phosphoprotein phosphatase 5 (PP5). This TPR domain was chosen for study because its three-dimensional structure is known. We examined co-immunoprecipitation of hsp90 with wild type and mutant TPR constructs from transfected cells. Only mutations located on one face of the TPR domain affected hsp90 binding. This allowed the identification of a binding groove. Three basic residues that are highly conserved in hsp90-binding TPR proteins extend prominently into this groove. Lys-97 and Arg-101 were absolutely required for hsp90 binding, while mutation of Arg-74 diminished, but did not abrogate, hsp90 binding. Mutation of Lys-32, another conserved basic residue in the binding groove, also blocked hsp90 binding. The TPR domain of PP5 bound specifically to a 12-kDa C-terminal fragment of hsp90. This binding was reduced by mutation of acidic residues in the hsp90 fragment. These data suggest conservation, among hsp90-binding TPR proteins, of a binding groove containing basic residues that interact with acidic residues near the C terminus of hsp90.

Hsp90's secrets unfold: new insights from structural and functional studies

Hsp90 is a molecular chaperone associated with the folding of signal-transducing proteins, such as steroid hormone receptors and protein kinases. Results from recent studies have shed light on the structure of Hsp90 and have demonstrated that it can bind to and hydrolyse ATP. Hsp90 forms several discrete subcomplexes, each containing distinct groups of co-chaperones that function in folding pathways. Although Hsp90 is not generally involved in the folding of nascent polypeptide chains, there is a growing list of proteins whose activity depends on its function, including heat-shock factor. This review addresses recent developments in our understanding of the structure and function of Hsp90.

Characterization of the Ah receptor-associated protein, ARA9

The unliganded aryl hydrocarbon receptor (AHR) is found in a complex with other proteins including the 90-kDa heat shock protein (Hsp90) and a 37-kDa protein we refer to as ARA9. We found that the three tetratricopeptide repeats found in the COOH terminus of ARA9 are necessary and sufficient for interaction with the AHR complex. Conversely, the AHR's "repressor"/Hsp90 binding domain is required for interaction with ARA9. Because ARA9 closely resembles the 52-kDa FK506-binding protein (FKBP52), found in the unliganded glucocorticoid receptor (GR) complex, we compared the binding specificities of ARA9 and FKBP52 for AHR and GR. In co-immunoprecipitation experiments, ARA9 specifically associated with AHR-Hsp90 complex but not with GR-Hsp90 complexes. In addition, ARA9 showed a greater capacity than FKBP52 to associate with AHR-Hsp90 complexes. The biological importance of this interaction was suggested by the observation that in a yeast expression system ARA9 expression enhanced the response of AHR to the agonist beta-napthoflavone, decreasing the EC50 by greater than 5-fold and increasing the maximal response 2.5-fold. In contrast, co-expression of FKBP52 had no effect on AHR signaling. In addition, although ARA9 contains a domain similar to that found in other FK506-binding proteins, ARA9 binding to 3H-FK506 could not be detected. Finally, we have characterized the developmental and expression pattern of ARA9 in the developing mouse embryo and mapped the ARA9 locus to human chromosome 11q13.3.

CNS1 encodes an essential p60/Sti1 homolog in Saccharomyces cerevisiae that suppresses cyclophilin 40 mutations and interacts with Hsp90

Cyclophilins are cis-trans-peptidyl-prolyl isomerases that bind to and are inhibited by the immunosuppressant cyclosporin A (CsA). The toxic effects of CsA are mediated by the 18-kDa cyclophilin A protein. A larger cyclophilin of 40 kDa, cyclophilin 40, is a component of Hsp90-steroid receptor complexes and contains two domains, an amino-terminal prolyl isomerase domain and a carboxy-terminal tetratricopeptide repeat (TPR) domain. There are two cyclophilin 40 homologs in the yeast Saccharomyces cerevisiae, encoded by the CPR6 and CPR7 genes. Yeast strains lacking the Cpr7 enzyme are viable but exhibit a slow-growth phenotype. In addition, we show here that cpr7 mutant strains are hypersensitive to the Hsp90 inhibitor geldanamycin. When overexpressed, the TPR domain of Cpr7 alone complements both cpr7 mutant phenotypes, while overexpression of the cyclophilin domain of Cpr7, full-length Cpr6, or human cyclophilin 40 does not. The open reading frame YBR155w, which has moderate identity to the yeast p60 homolog STI1, was isolated as a high-copy-number suppressor of the cpr7 slow-growth phenotype. We show that this Sti1 homolog Cns1 (cyclophilin seven suppressor) is constitutively expressed, essential, and found in protein complexes with both yeast Hsp90 and Cpr7 but not with Cpr6. Cyclosporin A inhibited Cpr7 interactions with Cns1 but not with Hsp90. In summary, our findings identify a novel component of the Hsp90 chaperone complex that shares function with cyclophilin 40 and provide evidence that there are functional differences between two conserved sets of Hsp90 binding proteins in yeast.

Cns1 is an essential protein associated with the hsp90 chaperone complex in Saccharomyces cerevisiae that can restore cyclophilin 40-dependent functions in cpr7Delta cells

Saccharomyces cerevisiae harbors two cyclophilin 40-type enzymes, Cpr6 and Cpr7, which are components of the Hsp90 molecular chaperone machinery. Cpr7 is required for normal growth and is required for maximal activity of heterologous Hsp90-dependent substrates, including glucocorticoid receptor (GR) and the oncogenic tyrosine kinase pp60(v-src). In addition, it has recently been shown that Cpr7 plays a major role in negative regulation of the S. cerevisiae heat shock transcription factor (HSF). To better understand functions associated with Cpr7, a search was undertaken for multicopy suppressors of the cpr7Delta slow-growth phenotype. The screen identified a single gene, designated CNS1 (for cyclophilin seven suppressor), capable of suppressing the cpr7Delta growth defect. Overexpression of CNS1 in cpr7Delta cells also largely restored GR activity and negative regulation of HSF. In vitro protein retention experiments in which Hsp90 heterocomplexes were precipitated resulted in coprecipitation of Cns1. Interaction between Cns1 and the carboxy terminus of Hsp90 was also shown by two-hybrid analysis. The functional consequences of CNS1 overexpression and its physical association with the Hsp90 machinery indicate that Cns1 is a previously unidentified component of molecular chaperone complexes. Thus far, Cns1 is the only tetratricopeptide repeat-containing component of Hsp90 heterocomplexes found to be essential for cell viability under all conditions tested.