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

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.

Hsp104 interacts with Hsp90 cochaperones in respiring yeast

The highly abundant molecular chaperone Hsp90 functions with assistance from auxiliary factors, collectively referred to as Hsp90 cochaperones, and the Hsp70 system. Hsp104, a molecular chaperone required for stress tolerance and for maintenance of [psi(+)] prions in the budding yeast Saccharomyces cerevisiae, appears to collaborate only with the Hsp70 system. We now report that several cochaperones previously thought to be dedicated to Hsp90 are shared with Hsp104. We show that the Hsp90 cochaperones Sti1, Cpr7, and Cns1, which utilize tetratricopeptide repeat (TPR) domains to interact with a common surface on Hsp90, form complexes with Hsp104 in vivo and that Sti1 and Cpr7 interact with Hsp104 directly in vitro. The interaction is Hsp90 independent, as further emphasized by the fact that two distinct TPR domains of Sti1 are required for binding Hsp90 and Hsp104. In a striking parallel to the sequence requirements of Hsp90 for binding TPR proteins, binding of Sti1 to Hsp104 requires a related acidic sequence at the C-terminal tail of Hsp104. While Hsp90 efficiently sequesters the cochaperones during fermentative growth, respiratory conditions induce the interaction of a fraction of Hsp90 cochaperones with Hsp104. This suggests that cochaperone sharing may favor adaptation to altered metabolic conditions.

A novel tetratricopeptide repeat-containing J-protein localized in a plasma membrane-bound protein complex of the phytopathogenic oomycete Phytophthora megasperma

Phytoalexins originating from plant tissues may cause within cells of fungi or oomycetes a change in the localization of actin, tubulin and chaperones. To test the hypothesis in a filamentously growing oomycete, we compared the distribution of cellular markers in the presence and absence of hydroxystilbene phytoalexins. Using cDNA from the phytopathogenic organism Phytophthora megasperma, the causal agent of root rot on soybean and many other plants, and including probes for Hsp70 and Hsp40, we cloned a DnaJ-protein (Jcp) with the capacity of interacting with both a particular Hsp70 isoform via its J-domain and with other proteins via its tetratricopeptide repeat (TPR) domain. Antisera raised against the bacterially expressed protein Jcp allowed the analysis of its intracellular localization during hyphal growth. Following the subfractionation of cell homogenates, we detected virtually all immunoreactive Jcp in the plasma membrane-enriched fraction and as constituent of a membrane-associated protein complex. In agreement with the biochemical findings, immunocytochemical stains of hyphae showed Jcp as part of cortical patches positioned along the plasma membrane similar to the distribution of actin patches. Confocal microscopy, however, revealed that the Jcp-containing patches did not generally co-localize with the patches visualized by the actin stain. The 59-kDa Jcp, characterized by a large 8-fold TPR domain at the N-terminal region and a J-domain close to the C-terminus, is a good candidate for bridging the gap between Hsp70 and Hsp90 by protein-protein interactions. By administration of plant-derived phytoalexins it was shown that the presence of resveratrol or piceatannol significantly reduces the amount of the Jcp-containing patches, but does not lead to a relocalization of intracellular Jcp.

Hsp90: a specialized but essential protein-folding tool

Hsp90 is unique among molecular chaperones. The majority of its known substrates are signal transduction proteins, and recent work indicates that it uses a novel protein-folding strategy.

Functional analysis of the Hsp90-associated human peptidyl prolyl cis/trans isomerases FKBP51, FKBP52 and Cyp40

Large peptidyl-prolyl cis/trans isomerases (PPIases) are important components of the Hsp90 chaperone complex. In mammalian cells, either Cyp40, FKBP51 or FKBP52 is incorporated into these complexes. It has been suggested that members of this protein family exhibit both prolyl isomerase and chaperone activity. Here we define the structural and functional properties of the three mammalian large PPIases. We find that in all cases two PPIase monomers bind to an Hsp90 dimer. However, the affinities of the PPIases are different with FKBP52 exhibiting the strongest interaction and Cyp40 the weakest. Furthermore, in the mammalian system, in contrast to the yeast system, the catalytic activity of prolyl isomerization corresponds well to that of the respective small PPIases. Interestingly, Cyp40 and FKBP51 are the more potent chaperones. Thus, it seems that both the affinity for Hsp90 and the differences in their chaperone properties, which may reflect their interaction with the non-native protein in the Hsp90 complex, are critical for the selective incorporation of a specific large PPIase.

The Drosophila Dpit47 protein is a nuclear Hsp90 co-chaperone that interacts with DNA polymerase α

Hsp90 is gaining increasing importance as a protein involved in controlling the normal functioning of the cell. To do this it apparently interacts with a battery of co-chaperone proteins that are involved in both substrate recognition and the progression of the Hsp90 catalytic pathway. In this report we have identified the Drosophila Dpit47 protein (DNA polymerase interacting tpr containing protein of 47 kDa) through its interaction with the DNA polymerase α. This protein is a predominantly nuclear protein, which forms a tight and stoichiometric interaction with Hsp90 and shows interaction with Hsp70. It also has substantial homology to other known Hsp90 co-chaperones, e.g. CNS1 and hop1, making it likely that this protein also functions as an Hsp90 co-chaperone.

The interaction with the DNA polymerase α is not related to the special situation in early embryos where there are large amounts of maternal protein stockpiles of the polymerase, as it occurs to the same level in early and late embryos and also in proliferating cell culture. However, it does not occur in quiescent cells, making it likely that the protein is related to proliferation. This is also consistent with Dpit47 expression being higher in proliferating cells. The interaction between the Dpit47 and the polymerase takes place predominantly in the nucleoplasm, and seems to involve several subunits of the polymerase in comparable amounts, making it unlikely that it is solely required for the assembly of the polymerase complex. The polymerase can also be seen to interact with Hsp90, and the interaction between Dpit47 and the polymerase is increased by the specific Hsp90 inhibitor geldanamycin. This suggests that a complex of the Dpit47, Hsp90 and DNA polymerase exists in the cell. The interaction between DNA polymerase α and Dpit47 completely inhibits the activity of the polymerase.

These results suggest that Hsp90 acts as a chaperone for DNA polymerase α and that this interaction is mediated through the novel co-chaperone Dpit47. This provides the first suggestion of a role for chaperones in DNA replication in higher eukaryotes.

Two structures of cyclophilin 40: folding and fidelity in the TPR domains

BACKGROUND

The "large immunophilin" family consists of domains of cyclophilin or FK506 binding protein linked to a tetratricopeptide (TPR) domain. They are intimately associated with steroid receptor complexes and bind to the C-terminal domain of Hsp90 via the TPR domain. The competitive binding of specific large immunophilins and other TPR-Hsp90 proteins provides a regulatory mechanism for Hsp90 chaperone activity.

RESULTS

We have solved the X-ray structures of monoclinic and tetragonal forms of Cyp40. In the monoclinic form, the TPR domain consists of seven helices of variable length incorporating three TPR motifs, which provide a convincing binding surface for the Hsp90 C-terminal MEEVD sequence. The C-terminal residues of Cyp40 protrude out beyond the body of the TPR domain to form a charged helix-the putative calmodulin binding site. However, in the tetragonal form, two of the TPR helices have straightened out to form one extended helix, providing a dramatically different conformation of the molecule.

CONCLUSIONS

The X-ray structures are consistent with the role of Cyclophilin 40 as a multifunctional signaling protein involved in a variety of protein-protein interactions. The intermolecular helix-helix interactions in the tetragonal form mimic the intramolecular interactions found in the fully folded monoclinic form. These conserved intra- and intermolecular TPR-TPR interactions are illustrative of a high-fidelity recognition mechanism. The two structures also open up the possibility that partially folded forms of TPR may be important in domain swapping and protein recognition.

Hsp90 chaperone complexes are required for the activity and stability of yeast protein kinases Mik1, Wee1 and Swe1

The Wee1 protein kinase negatively regulates entry into mitosis by mediating the inhibitory tyrosine phosphorylation of Cdc2-cyclin B kinase. The stability and activity of Wee1 from the fission yeast Schizosaccharomyces pombe is critically dependent on functional Hsp90 chaperones. Here we identify two related tyrosine protein kinases, Mik1 from fission yeast and its Saccharomyces cerevisiae homolog Swe1, as Hsp90 substrates and show that the kinase domain is sufficient to mediate this interaction. Morphological and biochemical defects arising from overexpression of the kinases in fission yeast are suppressed in the conditional Hsp90 mutant swo1-26. A subset of all three kinases is associated with the Hsp90 cochaperones cyclophilin 40 and p23. Under conditions of impaired chaperone function or treatment with the Hsp90 inhibitory drug geldanamycin, intracellular levels of the kinases are reduced and the proteins become rapidly degraded by the proteasome machinery, indicating that Wee1, Mik1 and Swe1 require Hsp90 heterocomplexes for their stability and maintenance of function.

Maturation of steroid receptors: an example of functional cooperation among molecular chaperones and their associated proteins

The selective modulation of transcription exerted by steroids depends upon recognition of signalling molecules by properly folded cytoplasmic receptors and their subsequent translocation into the nucleus. These events require a sequential and dynamic series of protein-protein interactions in order to fashion receptors that bind stably to steroids. Central to receptor maturation, therefore, are several molecular chaperones and their accessory proteins; Hsp70, Hsp40, and hip modulate the 3-dimensional conformation of steroid receptors, permitting reaction via hop with Hsp90, arguably the central protein in the process. Binding to Hsp90 leads to dissociation of some proteins from the receptor complex while others are recruited. Notably, p23 stabilizes receptors in a steroid binding state, and the immunophilins, principally CyP40 and Hsp56, arrive late in receptor complex assembly. In this review, the functions of molecular chaperones during steroid receptor maturation are explored, leading to a general mechanistic model indicative of chaperone cooperation in protein folding.

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.

Cyclophilin A and Ess1 interact with and regulate silencing by the Sin3-Rpd3 histone deacetylase

Three families of prolyl isomerases have been identified: cyclophilins, FK506-binding proteins (FKBPs) and parvulins. All 12 cyclophilins and FKBPs are dispensable for growth in yeast, whereas the one parvulin homolog, Ess1, is essential. We report here that cyclophilin A becomes essential when Ess1 function is compromised. We also show that overexpression of cyclophilin A suppresses ess1 conditional and null mutations, and that cyclophilin A enzymatic activity is required for suppression. These results indicate that cyclophilin A and Ess1 function in parallel pathways and act on common targets by a mechanism that requires prolyl isomerization. Using genetic and biochemical approaches, we found that one of these targets is the Sin3-Rpd3 histone deacetylase complex, and that cyclophilin A increases and Ess1 decreases disruption of gene silencing by this complex. We show that conditions that favor acetylation over deacetylation suppress ess1 mutations. Our findings support a model in which Ess1 and cyclophilin A modulate the activity of the Sin3-Rpd3 complex, and excess histone deacetylation causes mitotic arrest in ess1 mutants.

Structure of TPR domain-peptide complexes: critical elements in the assembly of the Hsp70-Hsp90 multichaperone machine

The adaptor protein Hop mediates the association of the molecular chaperones Hsp70 and Hsp90. The TPR1 domain of Hop specifically recognizes the C-terminal heptapeptide of Hsp70 while the TPR2A domain binds the C-terminal pentapeptide of Hsp90. Both sequences end with the motif EEVD. The crystal structures of the TPR-peptide complexes show the peptides in an extended conformation, spanning a groove in the TPR domains. Peptide binding is mediated by electrostatic interactions with the EEVD motif, with the C-terminal aspartate acting as a two-carboxylate anchor, and by hydrophobic interactions with residues upstream of EEVD. The hydrophobic contacts with the peptide are critical for specificity. These results explain how TPR domains participate in the ordered assembly of Hsp70-Hsp90 multichaperone complexes.

Maturation of steroid receptors: an example of functional cooperation among molecular chaperones and their associated proteins

The selective modulation of transcription exerted by steroids depends upon recognition of signalling molecules by properly folded cytoplasmic receptors and their subsequent translocation into the nucleus. These events require a sequential and dynamic series of protein-protein interactions in order to fashion receptors that bind stably to steroids. Central to receptor maturation, therefore, are several molecular chaperones and their accessory proteins; Hsp70, Hsp40, and hip modulate the 3-dimensional conformation of steroid receptors, permitting reaction via hop with Hsp90, arguably the central protein in the process. Binding to Hsp90 leads to dissociation of some proteins from the receptor complex while others are recruited. Notably, p23 stabilizes receptors in a steroid binding state, and the immunophilins, principally CyP40 and Hsp56, arrive late in receptor complex assembly. In this review, the functions of molecular chaperones during steroid receptor maturation are explored, leading to a general mechanistic model indicative of chaperone cooperation in protein folding.

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.

Antifungal activities of antineoplastic agents: Saccharomyces cerevisiae as a model system to study drug action

Recent evolutionary studies reveal that microorganisms including yeasts and fungi are more closely related to mammals than was previously appreciated. Possibly as a consequence, many natural-product toxins that have antimicrobial activity are also toxic to mammalian cells. While this makes it difficult to discover antifungal agents without toxic side effects, it also has enabled detailed studies of drug action in simple genetic model systems. We review here studies on the antifungal actions of antineoplasmic agents. Topics covered include the mechanisms of action of inhibitors of topoisomerases I and II; the immunosuppressants rapamycin, cyclosporin A, and FK506; the phosphatidylinositol 3-kinase inhibitor wortmannin; the angiogenesis inhibitors fumagillin and ovalicin; the HSP90 inhibitor geldanamycin; and agents that inhibit sphingolipid metabolism. In general, these natural products inhibit target proteins conserved from microorganisms to humans. These studies highlight the potential of microorganisms as screening tools to elucidate the mechanisms of action of novel pharmacological agents with unique effects against specific mammalian cell types, including neoplastic cells. In addition, this analysis suggests that antineoplastic agents and derivatives might find novel indications in the treatment of fungal infections, for which few agents are presently available, toxicity remains a serious concern, and drug resistance is emerging.

Genetic approaches to the study of protein-protein interactions

This article describes genetic approaches to the study of heterologous protein-protein interactions, focusing on the yeast Saccharomyces cerevisiae as a useful eukaryotic model system. Several methods are described that can be used to search for new interactions, including extragenic suppression, multicopy suppression, synthetic lethality, and transdominant inhibition. Strategies for screening, genetic characterization, and clone identification are described, along with recent examples from the literature. In addition, genetic methods are discussed that can be used to further characterize a newly discovered protein-protein interaction. These include the creation of mutant libraries of a given protein by chemical mutagenesis or polymerase chain reaction, the production of dominant-negative mutants, and strategies for introducing these mutant alleles back into yeast for analysis. Although these genetic methods are quite powerful, they are often just a starting point for further biochemical or cell biological experiments.

The yeast Hsp110 family member, Sse1, is an Hsp90 cochaperone

In eukaryotes, production of the diverse repertoire of molecular chaperones during normal growth and in response to stress is governed by the heat shock transcription factor HSF. The HSC82 and HSP82 genes, encoding isoforms of the yeast Hsp90 molecular chaperone, were recently identified as targets of the HSF carboxyl-terminal activation domain (CTA), whose expression is required for cell cycle progression during prolonged heat stress conditions. In the present study, we have identified additional target genes of the HSF CTA, which include nearly all of the heat shock-inducible members of the Hsp90 chaperone complex, demonstrating coordinate regulation of these components by HSF. Heat shock induction of SSE1, encoding a member of the Hsp110 family of heat shock proteins, was also dependent on the HSF CTA. Disruption of SSE1 along with STI1, encoding an established subunit of the Hsp90 chaperone complex, resulted in a severe synthetic growth phenotype. Sse1 associated with partially purified Hsp90 complexes and deletion of the SSE1 gene rendered cells susceptible to the Hsp90 inhibitors macbecin and geldanamycin, suggesting functional interaction between Sse1 and Hsp90. Sse1 is required for function of the glucocorticoid receptor, a model substrate of the Hsp90 chaperone machinery, and Hsp90-based repression of HSF under nonstress conditions. Taken together, these data establish Sse1 as an integral new component of the Hsp90 chaperone complex in yeast.

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.

Identification of SSF1, CNS1, and HCH1 as multicopy suppressors of a Saccharomyces cerevisiae Hsp90 loss-of-function mutation

Hsp90 functions in a multicomponent chaperone system to promote the maturation and maintenance of a diverse, but specific, set of target proteins that play key roles in the regulation of cell growth and development. To identify additional components of the Hsp90 chaperone system and its targets, we searched for multicopy suppressors of various temperature-sensitive mutations in the yeast Hsp90 gene, HSP82. Three suppressors were isolated for one Hsp90 mutant (glutamate --> lysine at amino acid 381). Each exhibited a unique, allele-specific pattern of suppression with other Hsp90 mutants and had unique structural and biological properties. SSF1 is a member of an essential gene family and functions in the response to mating pheromones. CNS1 is an essential gene that encodes a component of the Hsp90 chaperone machinery. The role of HCH1 is unknown; its sequence has no strong homology to any protein of known function. SSF1 and CNS1 were weak suppressors, whereas HCH1 restored wild-type growth rates at all temperatures tested to cells expressing the E381K mutant. Overexpression of CNS1 or HCH1, but not SSF1, enhanced the maturation of a heterologous Hsp90 target protein, p60(v-src). These results suggest that like Cns1p, Hch1p is a general modulator of Hsp90 chaperone functions, whereas Ssf1p likely is either an Hsp90 target protein or functions in the same pathway as an Hsp90 target protein.

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.