p50(cdc37) acting in concert with Hsp90 is required for Raf-1 function

Meaningful relationships: the regulation of the Ras/Raf/MEK/ERK pathway by protein interactions

The Ras/Raf/MEK (mitogen-activated protein kinase/ERK kinase)/ERK (extracellular-signal-regulated kinase) pathway is at the heart of signalling networks that govern proliferation, differentiation and cell survival. Although the basic regulatory steps have been elucidated, many features of this pathway are only beginning to emerge. This review focuses on the role of protein-protein interactions in the regulation of this pathway, and how they contribute to co-ordinate activation steps, subcellular redistribution, substrate phosphorylation and cross-talk with other signalling pathways.

Hsp90 regulates p50(cdc37) function during the biogenesis of the activeconformation of the heme-regulated eIF2 alpha kinase

Recent studies indicate that p50(cdc37) facilitates Hsp90-mediated biogenesis of certain protein kinases. In this report, we examined whether p50(cdc37) is required for the biogenesis of the heme-regulated eIF2 alpha kinase (HRI) in reticulocyte lysate. p50(cdc37) interacted with nascent HRI co-translationally and this interaction persisted during the maturation and activation of HRI. p50(cdc37) stimulated HRI's activation in response to heme deficiency, but did not activate HRI per se. p50(cdc37) function was specific to immature and inactive forms of the kinase. Analysis of mutant Cdc37 gene products indicated that the N-terminal portion of p50(cdc37) interacted with immature HRI, but not with Hsp90, while the C-terminal portion of p50(cdc37) interacted with Hsp90. The Hsp90-specific inhibitor geldanamycin disrupted the ability of both Hsp90 and p50(cdc37) to bind HRI and promote its activation, but did not disrupt the native association of p50(cdc37) with Hsp90. A C-terminal truncated mutant of p50(cdc37) inhibited HRI's activation, prevented the interaction of Hsp90 with HRI, and bound to HRI irrespective of geldanamycin treatment. Additionally, native complexes of HRI with p50(cdc37) were detected in cultured K562 erythroleukemia cells. These results suggest that p50(cdc37) provides an activity essential to HRI biogenesis via a process regulated by nucleotide-mediated conformational switching of its partner Hsp90.

In vitro reconstitution of a functional duck hepatitis B virus reverse transcriptase: posttranslational activation by Hsp90

Reverse transcription in hepatitis B viruses is initiated through a unique protein priming mechanism whereby the viral reverse transcriptase (RT) first assembles into a ribonucleoprotein (RNP) complex with its RNA template and then initiates DNA synthesis de novo using the RT itself as a protein primer. RNP formation and protein priming require the assistance of host cell factors, including the molecular chaperone heat shock protein 90 (Hsp90). To better understand the mechanism of RT activation by Hsp90, we have now mapped the minimal RT sequences of the duck hepatitis B virus that are required for chaperone binding, RNP formation, and protein priming. Furthermore, we have reconstituted in vitro both RNP formation and protein priming using purified RT proteins and host factors. Our results show that (i) Hsp90 recognizes two independent domains of the RT, both of which are necessary for RNP formation and protein priming; (ii) Hsp90 function is required not only to establish, but also to maintain, the RT in a state competent for RNA binding; and (iii) Hsp90 is not required during RT synthesis and can activate the RT posttranslationally. Based on these findings, we propose a model for Hsp90 function whereby the chaperone acts as an active interdomain bridge to bring the two RT domains into a poised but labile conformation competent for RNP formation. It is anticipated that the reconstitution system established here will facilitate the isolation of additional host factors required for RT functions and further elucidation of the mechanisms of RT activation.

Genomic organization and the promoter region of the round-spotted pufferfish (Tetraodon fluviatilis) CDC37 gene

The CDC37 gene was isolated from a round-spotted pufferfish genomic library and characterized. This gene is composed of nine exons spanning 3.5 kb. Exon 1 contains the 5'-untranslated region and exon 2 contains the putative translation initiation site. By 5'-RACE (rapid amplication of cDNA ends) and sequence analysis, we deduced the promoter region for the CDC37 gene and found that it does not contain typical TATA or CCAAT box. The 1.8 kb DNA fragment upstream of the putative transcription initiation site contains numerous potential binding sites for transcription factors including CREB, E2A, Ets-1, GATA, NF-IL6 and PEA3. When this DNA fragment was placed upstream of the chloramphenicol acetyltransferase (CAT) reporter gene and transfected into a carp CF cell line, it could drive the synthesis of CAT enzyme four times more efficiently than the promoterless pCAT-Basic did. In addition, the CDC37 gene is linked to the TYK2 gene in a tail-to-head manner with a small intergenic region of 292 bp.

Heat shock protein 80 of Neurospora crassa: sequence analysis of the gene and expression during the asexual phase

Heat shock protein 80 (Hsp80) of Neurospora crassa, a member of the stress-90 protein family, is a cytosolic molecular chaperone that interacts directly with Hsp70 to form a hetero-oligomeric complex. The complete nucleotide sequence of the gene encoding this protein, along with the 5'- and 3'-flanking DNA, is reported. The coding sequence is interrupted by two introns, 61 and 30 nucleotides, respectively, in length. The deduced amino acid sequence corresponds to a 695-residue polypeptide with a calculated molecular mass of 78,894 Da and an average pI of 4.94. Primer extension experiments demonstrated two transcription start sites, a major and a minor one. No sequence motifs resembling the standard eukaryotic heat shock elements were evident in the putative promoter region. Immunoblot analysis showed Hsp80 protein to be present in the mature, dormant conidia, while the hsp80 transcripts were not detected. Both the transcripts and the protein were present in the germinating conidia in the absence of externally applied stress.

Activation of B-Raf kinase requires phosphorylation of the conserved residues Thr598 and Ser601

The Raf kinase family serves as a central intermediate to relay signals from Ras to ERK. The precise molecular mechanism for Raf activation is still not fully understood. Here we report that phosphorylation of Thr598 and Ser601, which lie between kinase subdomains VII and VIII, is essential for B-Raf activation by Ras. Substitution of these residues by alanine (B-RafAA) abolished Ras-induced B-Raf activation without altering the association of B-Raf with other signaling proteins. Phosphopeptide mapping and immunoblotting with phospho-specific antibodies confirmed that Thr598 and Ser601 are in vivo phosphorylation sites induced by Ras. Furthermore, replacement of these two sites by acidic residues (B-RafED) renders B-Raf constitutively active. Con sistent with these data, B-RafAA and B-RafED exhibited diminished and enhanced ability, respectively, to stimulate ERK activation and Elk-dependent transcription. Moreover, functional studies revealed that B-RafED was able to promote NIH 3T3 cell transformation and PC12 cell differentiation. Since Thr598 and Ser601 are conserved in all Raf family members from Caenorhabditis elegans to mammals, we propose that phosphorylation of these two residues may be a general mechanism for Raf activation.

Meaningful relationships: the regulation of the Ras/Raf/MEK/ERK pathway by protein interactions

The Ras/Raf/MEK (mitogen-activated protein kinase/ERK kinase)/ERK (extracellular-signal-regulated kinase) pathway is at the heart of signalling networks that govern proliferation, differentiation and cell survival. Although the basic regulatory steps have been elucidated, many features of this pathway are only beginning to emerge. This review focuses on the role of protein-protein interactions in the regulation of this pathway, and how they contribute to co-ordinate activation steps, subcellular redistribution, substrate phosphorylation and cross-talk with other signalling pathways.

p50(Cdc37) can buffer the temperature-sensitive properties of a mutant of Hck

Genetic studies have previously revealed that Cdc37p is required for the catalytic competence of v-Src in yeast. We have reasoned that temperature-sensitive mutants of Src family kinases might be more sensitive to the cellular level of p50(Cdc37), the mammalian homolog of Cdc37p, than their wild-type counterpart, thus potentially providing a unique opportunity to elucidate the involvement of p50(Cdc37) in the folding and stabilization of Src family kinases. A temperature-sensitive mutant of a constitutively active form of Hck (i.e., tsHck499F) was created by mutating two amino acids within the kinase domain of Hck499F. Significantly, overexpression of p50(Cdc37) rescues the catalytic activity of tsHck499F at 33 degrees C, while partially buffering it against inactivation at higher temperatures (e.g., 37 and 39 degrees C). Hsp90 function is required for tsHck499F activity and its stabilization by p50(Cdc37), but overexpression of Hsp90 is not sufficient to stabilize tsHck499F. Overexpression of p50(Cdc37) promotes the association of tsHck499F with Hsp90, suggesting that the cellular level of p50(Cdc37) might be the rate-limiting step in the association of tsHck499F with Hsp90. A truncation mutant of p50(Cdc37) that cannot bind Hsp90 still has a limited capacity to rescue the catalytic activity of tsHck499F and promote its association with Hsp90. This is a particularly important observation, since it argues that rather than solely acting as a passive adapter protein to tether tsHck499F to Hsp90, p50(Cdc37) may also act allosterically to enhance the association of tsHck499F with Hsp90. The findings presented here might also have implications for our understanding of the evolution of protein kinases and tumor development.

Geldanamycin induces ErbB-2 degradation by proteolytic fragmentation

Exposure of carcinoma cell lines to the antibiotic geldanamycin induces the degradation of ErbB-2, a co-receptor tyrosine kinase that is frequently overexpressed in certain tumors. Using ErbB-2 mutants expressed as chimeric receptors or green fluorescent protein fusion proteins, we report that the kinase domain of ErbB-2 is essential for geldanamycin-induced degradation. The kinase domain of the related epidermal growth factor receptor was not sensitive to this drug. The data further indicate mechanistic aspects of ErbB-2 degradation by geldanamycin. The data show that exposure to the drug induces at least one cleavage within the cytoplasmic domain of ErbB-2 producing a 135-kDa fragment and a 23-kDa fragment. The latter represents the carboxyl-terminal domain of ErbB-2, whereas the former represents the ectodomain and part of the cytoplasmic domain. Degradation of the carboxyl-terminal fragment is prevented by proteasome inhibitors, whereas degradation of the membrane-anchored 135-kDa ErbB-2 fragment is blocked by inhibitors of the endocytosis-dependent degradation pathway. Confocal microscopy studies confirm a geldanamycin-induced localization of ErbB-2 on intracellular vesicles.

AKT/PKB and other D3 phosphoinositide-regulated kinases: kinase activation by phosphoinositide-dependent phosphorylation

The protein kinase Akt/PKB is activated via a multistep process by a variety of signals. In the early steps of this process, PI-3 kinase-generated D3-phosphorylated phosphoinositides bind the Akt PH domain and induce the translocation of the kinase to the plasma membrane where it co-localizes with phosphoinositide-dependent kinase-1. By binding to the PH domains of both Akt and phosphoinositide-dependent kinase-1, D3-phosphorylated phosphoinositides appear to also induce conformational changes that permit phosphoinositide-dependent kinase-1 to phosphorylate the activation loop of Akt. The paradigm of Akt activation via phosphoinositide-dependent phosphorylation provided a framework for research into the mechanism of activation of other members of the AGC kinase group (p70S6K, PKC, and PKA) and members of the Tec tyrosine kinase family (TecI, TecII, Btk/Atk, Itk/Tsk/Emt, Txk/Rlk, and Bm/Etk). The result was the discovery that these kinases and Akt are activated by overlapping pathways. In this review, we present our current understanding of the regulation and function of the Akt kinase and we discuss the common and unique features of the activation processes of Akt and the AGC and Tec kinase families. In addition, we present an overview of the biosynthesis of phosphoinositides that contribute to the regulation of these kinases.

p50(cdc37) is a nonexclusive Hsp90 cohort which participates intimately in Hsp90-mediated folding of immature kinase molecules

Hsp90 and p50(cdc37) provide a poorly understood biochemical function essential to certain protein kinases, and recent models describe p50(cdc37) as an exclusive hsp90 cohort which links hsp90 machinery to client kinases. We describe here the recovery of p50(cdc37) in immunoadsorptions directed against the hsp90 cohorts FKBP52, cyp40, p60HOP, hsp70, and p23. Additionally, monoclonal antibodies against FKBP52 coadsorb maturation intermediates of the hsp90-dependent kinases p56(lck) and HRI, and the presence of these maturation intermediates significantly increases the representation of p50(cdc37) and hsp90 on FKPB52 machinery. Although the native heterocomplex between hsp90 and p50(cdc37) is salt-labile, their dynamic interactions with kinase substrates produce kinase-chaperone heterocomplexes which are highly salt-resistant. The hsp90 inhibitor geldanamycin does not directly disrupt the native association of hsp90 with p50(cdc37) per se, but does result in the formation of salt-labile hsp90-kinase heterocomplexes which lack the p50(cdc37) cohort. We conclude that p50(cdc37) does not simply serve as a passive structural bridge between hsp90 and its kinase substrates; instead, p50(cdc37) is a nonexclusive hsp90 cohort which responds to hsp90's nucleotide-regulated conformational switching during the generation of high-affinity interactions within the hsp90-kinase-p50(cdc37) heterocomplex.

The oncoprotein kinase chaperone CDC37 functions as an oncogene in mice and collaborates with both c-myc and cyclin D1 in transformation of multiple tissues

CDC37 encodes a 50-kDa protein that targets intrinsically unstable oncoprotein kinases including Cdk4, Raf-1, and v-src to the molecular chaperone Hsp90, an interaction that is thought to be important for the establishment of signaling pathways. CDC37 is required for proliferation in budding yeast and is coexpressed with cyclin D1 in proliferative zones during mouse development, a finding consistent with a positive role in cell proliferation. CDC37 expression may not only be required to support proliferation in cells that are developmentally programmed to proliferate but may also be required in cells that are inappropriately induced to initiate proliferation by oncogenes. Here we report that mouse mammary tumor virus (MMTV)-CDC37 transgenic mice develop mammary gland tumors at a rate comparable to that observed previously in MMTV-cyclin D1 mice. Moreover, CDC37 was found to collaborate with MMTV-c-myc in the transformation of multiple tissues, including mammary and salivary glands in females and testis in males, and also collaborates with cyclin D1 to transform the female mammary gland. These data indicate that CDC37 can function as an oncogene in mice and suggests that the establishment of protein kinase pathways mediated by Cdc37-Hsp90 can be a rate-limiting event in epithelial cell transformation.

Induction of human Cdc37 in prostate cancer correlates with the ability of targeted Cdc37 expression to promote prostatic hyperplasia

The Cdc37 gene encodes a 50 kDa protein which targets intrinsically unstable oncoprotein kinases such as Cdk4, Raf-1, and src to the molecular chaperone Hsp90. This activity is thought to play an important role in the establishment of signaling pathways controlling cell proliferation. The budding yeast Cdc37 homolog is required for cell division and mammalian Cdc37 is expressed in proliferative zones during embryonic development and in adult tissues, consistent with a positive role in proliferation. Here we report that human prostatic tumors, neoplasias and certain pre-malignant lesions display increased Cdc37 expression, suggesting an important and early role for Cdc37 in prostatic transformation. To test the consequences of increased Cdc37 levels, transgenic mice expressing Cdc37 in the prostate were generated. These mice displayed a wide range of growth-related abnormalities including prostatic epithelial cell hyperplasia and dysplasia. These data suggest that the expression of Cdc37 may promote inappropriate proliferation and may be an important early step in the development of human prostate cancer.

The molecular chaperone Cdc37 is required for Ste11 function and pheromone-induced cell cycle arrest

The molecular chaperone Cdc37 is thought to act in part as a targeting subunit of the heat-shock protein 90 (Hsp90) chaperone complex. We demonstrate here that Cdc37 is required for activity of the kinase Ste11 in budding yeast. A cdc37 mutant strain is defective in Ste11-mediated pheromone signaling and in accumulation and functional maturation of the constitutively active Ste11 version Ste11DeltaN. Moreover, Cdc37, Ste11DeltaN and Hsp90 coprecipitate pairwise. Thus, Hsp90 and Cdc37 may transiently associate with Ste11 to promote proper folding and/or association with additional regulatory factors. Our results establish Ste11 as the first endogenous Cdc37 client protein in yeast.

Cdc37 promotes the stability of protein kinases Cdc28 and Cak1

In the budding yeast Saccharomyces cerevisiae, Cdc37 is required for the productive formation of Cdc28-cyclin complexes. The cdc37-1 mutant arrests at Start with low levels of Cdc28 protein, which is predominantly unphosphorylated at Thr169, fails to bind cyclin, and has little protein kinase activity. We show here that Cdc28 and not cyclin is specifically defective in the cdc37-1 mutant and that Cdc37 likely does not act as an assembly factor for Cdc28-cyclin complex formation. We have also found that the levels and activity of the protein kinase Cak1 are significantly reduced in the cdc37-1 mutant. Pulse-chase analysis indicates that Cdc28 and Cak1 proteins are both destabilized when Cdc37 function is absent during but not after translation. In addition, Cdc37 promotes the production of Cak1, but not that of Cdc28, when coexpressed in insect cells. We conclude that budding yeast Cdc37, like its higher eukaryotic homologs, promotes the physical integrity of multiple protein kinases, perhaps by virtue of a cotranslational role 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.

Extracellular signal-regulated kinase binds to TFII-I and regulates its activation of the c-fos promoter

We have previously shown that TFII-I enhances transcriptional activation of the c-fos promoter through interactions with upstream elements in a signal-dependent manner. Here we demonstrate that activated Ras and RhoA synergize with TFII-I for c-fos promoter activation, whereas dominant-negative Ras and RhoA inhibit these effects of TFII-I. The Mek1 inhibitor, PD98059 abrogates the enhancement of the c-fos promoter by TFII-I, indicating that TFII-I function is dependent on an active mitogen-activated protein (MAP) kinase pathway. Analysis of the TFII-I protein sequence revealed that TFII-I contains a consensus MAP kinase interaction domain (D box). Consistent with this, we have found that TFII-I forms an in vivo complex with extracellular signal-related kinase (ERK). Point mutations within the consensus MAP kinase binding motif of TFII-I inhibit its ability to bind ERK and its ability to enhance the c-fos promoter. Therefore, the D box of TFII-I is required for its activity on the c-fos promoter. Moreover, the interaction between TFII-I and ERK can be regulated. Serum stimulation enhances complex formation between TFII-I and ERK, and dominant-negative Ras abrogates this interaction. In addition, TFII-I can be phosphorylated in vitro by ERK and mutation of consensus MAP kinase substrate sites at serines 627 and 633 impairs the phosphorylation of TFII-I by ERK and its activity on the c-fos promoter. These results suggest that ERK regulates the activity of TFII-I by direct phosphorylation.

Regulation of c-myc expression by IFN-gamma through Stat1-dependent and -independent pathways

Interferons (IFNs) inhibit cell growth in a Stat1-dependent fashion that involves regulation of c-myc expression. IFN-gamma suppresses c-myc in wild-type mouse embryo fibroblasts, but not in Stat1-null cells, where IFNs induce c-myc mRNA rapidly and transiently, thus revealing a novel signaling pathway. Both tyrosine and serine phosphorylation of Stat1 are required for suppression. Induced expression of c-myc is likely to contribute to the proliferation of Stat1-null cells in response to IFNs. IFNs also suppress platelet-derived growth factor (PDGF)-induced c-myc expression in wild-type but not in Stat1-null cells. A gamma-activated sequence element in the promoter is necessary but not sufficient to suppress c-myc expression in wild-type cells. In PKR-null cells, the phosphorylation of Stat1 on Ser727 and transactivation are both defective, and c-myc mRNA is induced, not suppressed, in response to IFN-gamma. A role for Raf-1 in the Stat1-independent pathway is revealed by studies with geldanamycin, an HSP90-specific inhibitor, and by expression of a mutant of p50(cdc37) that is unable to recruit HSP90 to the Raf-1 complex. Both agents abrogated the IFN-gamma-dependent induction of c-myc expression in Stat1-null cells.

Requirement for a kinase-specific chaperone pathway in the production of a Cdk9/cyclin T1 heterodimer responsible for P-TEFb-mediated tat stimulation of HIV-1 transcription

Tat activation of HIV-1 transcription is mediated by human transcription elongation factor P-TEFb, which interacts with Tat and phosphorylates the C-terminal domain of RNA polymerase II. The catalytic subunit of the P-TEFb complex, Cdk9, has been shown to interact with cyclin T and several other proteins of unknown identity. Consequently, the exact subunit composition of active P-TEFb has not been determined. Here we report the affinity purification and identification of the Cdk9-associated proteins. In addition to forming a heterodimer with cyclin T1, Cdk9 interacted with the molecular chaperone Hsp70 or a kinase-specific chaperone complex, Hsp90/Cdc37, to form two separate chaperone-Cdk9 complexes. Although the Cdk9/cyclin T1 dimer was exceptionally stable and produced slowly in the cell, free and unprotected Cdk9 appeared to be degraded rapidly. Several lines of evidence indicate the heterodimer of Cdk9/cyclin T1 to be the mature, active form of P-TEFb responsible for phosphorylation of the C-terminal domain of RNA polymerase II interaction with the Tat activation domain, and mediation of Tat activation of HIV-1 transcription. Pharmacological inactivation of Hsp90/Cdc37 function by geldanamycin revealed an essential role for the chaperone-Cdk9 complexes in generation of Cdk9/cyclin T1. Our data suggest a previously unrecognized chaperone-dependent pathway involving the sequential actions of Hsp70 and Hsp90/Cdc37 in the stabilization/folding of Cdk9 as well as the assembly of an active Cdk9/cyclin T1 complex responsible for P-TEFb-mediated Tat transactivation.

Temperature-sensitive ZAP70 mutants degrading through a proteasome-independent pathway. Restoration of a kinase domain mutant by Cdc37

CD8 deficiency is an autosomal recessive form of severe combined immunodeficiency diseases characterized by the absence of CD8(+) T lymphocytes and impaired T cell functions. We identified two novel mis-sense mutations in the zap70 genes of a CD8-deficiency patient. One mutation (P80Q) affects a residue in an SH2 domain and another (M572L) in the kinase subdomain XI. Both mutations cause a degradation of ZAP70 protein in a temperature-sensitive manner through an ATP-dependent and proteasome-independent pathway. We further demonstrated that Cdc37, a protein kinase-specific chaperone, bound to M572L but not P80Q mutant and restored the expression of the M572L mutant when overexpressed. The restoration of M572L mutant by Cdc37 required the function of HSP90. These results indicate that Cdc37 in conjunction with HSP90 functions as a molecular chaperone for a temperature-sensitive kinase domain mutant of ZAP70.

The ankyrin repeat-containing adaptor protein Tvl-1 is a novel substrate and regulator of Raf-1

Tvl-1 is a 269-amino acid ankyrin repeat protein expressed primarily in thymus, lung, and testes that was identified by screening a murine T-cell two-hybrid cDNA library for proteins that associate with the serine-threonine kinase Raf-1. The interaction of Tvl-1 with Raf-1 was confirmed by co-immunoprecipitation of the two proteins from COS-1 cells transiently transfected with Tvl-1 and Raf-1 expression constructs as well as by co-immunoprecipitation of the endogenous proteins from CV-1 and NB2 cells. Tvl-1 interacts with Raf-1 via its carboxyl-terminal ankyrin repeat domain. The same domain also mediates Tvl-1 homodimerization. Tvl-1 was detected by immunofluorescence in both the cytoplasm and the nucleus suggesting that in addition to Raf-1 it may also interact with nuclear proteins. Activated Raf-1 phosphorylates Tvl-1 both in vitro and in vivo. In baculovirus-infected Sf9 insect cells, Tvl-1 potentiates the activation of Raf-1 by Src and Ras while in COS-1 cells it potentiates the activation of Raf-1 by EGF. These data suggest that Tvl-1 is both a target as well as a regulator of Raf-1. The human homologue of Tvl-1 maps to chromosome 19p12, upstream of MEF2B with the two genes in a head to head arrangement.