Apoprotein B degradation is promoted by the molecular chaperones hsp90 and hsp70

Apoprotein B (apoB) is the major protein of liver-derived atherogenic lipoproteins. The net production of apoB can be regulated by presecretory degradation mediated by the ubiquitin-proteasome pathway and cytosolic hsp70. To further explore the mechanisms of apoB degradation, we have established a cell-free system in which degradation can be faithfully recapitulated. Human apoB48 synthesized in vitro was translocated into microsomes, glycosylated, and ubiquitinylated. Subsequent incubation with rat hepatic cytosol led to proteasome-mediated degradation. To explore whether hsp90 is required for apoB degradation, geldanamycin (GA) was added during the degradation assay. GA increased the recovery of microsomal apoB48 approximately 3-fold and disrupted the interaction between hsp90 and apoB48. Confirming the hsp90 effect in the cell-free system, we also found that transfection of hsp90 cDNA into rat hepatoma cells enhanced apoB48 degradation. Finally, apoB48 degradation was reconstituted in vitro using cytosol prepared from wild type yeast. Notably, degradation was attenuated when apoB48-containing microsomes were incubated with cytosol supplemented with GA or with cytosol prepared from yeast strains with mutations in the homologues of mammalian hsp70 and hsp90. Overall, our data suggest that hsp90 facilitates the interaction between endoplasmic reticulum-associated apoB and components of the proteasomal pathway, perhaps in cooperation with hsp70.

Functional interaction of human Cdc37 with the androgen receptor but not with the glucocorticoid receptor

Cdc37 is a molecular chaperone closely associated with the folding of protein kinases. Results from studies using a yeast model system showed that it was also important for activation of the human androgen receptor (AR). Based on results from the yeast model system (Fliss, A. E., Fang, Y., Boschelli, F., and Caplan, A. J. (1997) Mol. Biol. Cell 8, 2501-2509), we initiated studies to address whether AR and Cdc37 interact with each other in animal cell systems. Our results show that Cdc37 binds to AR but not to glucocorticoid receptors (GR) synthesized in rabbit reticulocyte lysates. This binding occurs via the ligand-binding domain of the AR in a manner that is partially dependent on Hsp90 and the presence of hormone. Further studies using the yeast system showed that Cdc37 is not interchangeable with Hsp90, suggesting that it functions at a distinct step in the activation pathway. Expression of a dominant negative form of Cdc37 in animal cells down-regulates full-length AR but has very little effect on an AR truncation lacking the ligand-binding domain or full-length GR. These results reveal differences in the mechanisms by which AR and GR become active transcription factors and strengthen the notion that Cdc37 has a wider range of polypeptide clients than was realized previously.

Control of estrogen receptor ligand binding by Hsp90

The molecular chaperone Hsp90 interacts with unliganded steroid hormone receptors and regulates their activity. We have analyzed the function of yeast and mammalian Hsp90 in regulating the ability of the human estrogen receptor (ER) to bind ligands in vivo and in vitro. Using the yeast system, we show that the ER expressed in several different hsp82 mutant strains binds reduced amounts of the synthetic estrogen diethylstilbestrol compared to the wild type. This defect in hormone binding occurs without any significant change in the steady state levels of ER protein. To analyze the role of mammalian Hsp90, we synthesized the human ER in rabbit reticulocyte lysates containing geldanamycin, an Hsp90 inhibitor. At low concentrations of geldanamycin we observed reduced levels of hormone binding by the ER. At higher concentrations, we found reduced synthesis of the receptor. These data indicate that Hsp90 functions to maintain the ER in a high affinity hormone-binding conformation.

Domain requirements of DnaJ-like (Hsp40) molecular chaperones in the activation of a steroid hormone receptor

DnaJ-like proteins function in association with Hsp70 molecular chaperones to facilitate protein folding. We previously demonstrated that a yeast DnaJ-like protein, Ydj1p, was important for activation of heterologously expressed steroid hormone receptors (Caplan, A. J., Langley, E., Wilson, E. M., and Vidal, J. (1995) J. Biol. Chem. 270, 5251-5257). In the present study, we analyzed Ydj1p function by assaying hormone binding to the human androgen receptor (AR) heterologously expressed in yeast. We analyzed hormone binding in strains that were wild type or deleted for the YDJ1 gene. In the deletion mutant, the AR did not bind hormone to the same extent as the wild type. Introduction of mutant forms of Ydj1p to the deletion strain revealed that the J-domain is necessary but not sufficient for Ydj1p action, and that other domains of the protein are also functionally important. Of three human DnaJ-like proteins introduced into the deletion mutant, only Hdj2, which displays full domain conservation with Ydj1p, suppressed the hormone binding defect of the deletion mutant. By comparison of the domains shared by these three human proteins, and with mutants of Ydj1p that were functional, it was deduced that the cysteine-rich zinc binding domain is important for Hdj2/Ydj1p action in hormone receptor function. A model for the mechanism of DnaJ-like protein action is discussed.

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.

Mutations in the cytosolic DnaJ homologue, YDJ1, delay and compromise the efficient translation of heterologous proteins in yeast

The Saccharomyces cerevisiae YDJ1 gene encodes a yeast homologue of DnaJ, an Escherichia coli molecular chaperone and regulator of Hsp70 function. We examined the function of Ydj1p in vivo by analyzing the activity and production of firefly luciferase (FFLux) and green fluorescent protein (GFP) after inducible expression in yeast strains containing a wild type or a mutant YDJ1 gene. Although FFLux and GFP mRNA levels were similar in the wild type and mutant strains, the FFLux protein was translated about half as efficiently in the ydj1-151 mutant compared to the wild type strain; the lower FFLux level was not the result of increased FFLux turnover in the mutant. In contrast, GFP translation was significantly delayed in the ydj1-151 mutant compared to the wild type strain. Surprisingly, we observed that FFLux and GFP mRNA bound efficiently to polysomes in the ydj1-151 mutant. Analysis of polysome profiles also revealed a modest increase in the amount of 60S ribosomal subunits in the ydj1-151 strain, consistent with a translation defect in the mutant, although the Ydj1 protein was not found to be associated with polysomes. To determine whether the inducible expression of an endogenous yeast protein was also less efficient in the ydj1-151 strain, we examined the inducible synthesis of the yeast TATA-binding protein (TBP) but observed no translation defect. Statistical analysis of the FFLux, GFP, and TBP encoding genes suggests that Ydj1p facilitates the expression of proteins that are poorly translated because both FFLux and GFP contain an abundance of codons that are rarely used in yeast.

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

The Saccharomyces cerevisiae SBA1 gene was cloned by PCR amplification from yeast genomic DNA following its identification as encoding an ortholog of human p23, an Hsp90 cochaperone. The SBA1 gene product is constitutively expressed and nonessential, although a disruption mutant grew more slowly than the wild type at both 18 and 37 degreesC. A double deletion of SBA1 and STI1, encoding an Hsp90 cochaperone, displayed synthetic growth defects. Affinity isolation of histidine-tagged Sba1p (Sba1(His6)) after expression in yeast led to coisolation of Hsp90 and the cyclophilin homolog Cpr6. Using an in vitro assembly assay, purified Sba1(His6) bound to Hsp90 only in the presence of adenosine 5'-O-(3-thiotriphosphate) or adenyl-imidodiphosphate. Furthermore, interaction between purified Sba1(His6) and Hsp90 in yeast extracts was inhibited by the benzoquinoid ansamycins geldanamycin and macbecin. The in vitro assay was also used to identify residues in Hsp90 that are important for complex formation with Sba1(His6), and residues in both the N-terminal nucleotide binding domain and C-terminal half were characterized. In vivo analysis of known Hsp90 substrate proteins revealed that Sba1 loss of function had only a mild effect on the activity of the tyrosine kinase v-Src and steroid hormone receptors.

Structure, function and evolution of DnaJ: conservation and adaptation of chaperone function

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Differential in vivo regulation of steroid hormone receptor activation by Cdc37p

The CDC37 gene is essential for the activity of p60(v-src) when expressed in yeast cells. Since the activation pathway for p60(v-src) and steroid hormone receptors is similar, the present study analyzed the hormone-dependent transactivation by androgen receptors and glucocorticoid receptors in yeast cells expressing a mutant version of the CDC37 gene. In this mutant, hormone-dependent transactivation by androgen receptors was defective at both permissive and restrictive temperatures, although transactivation by glucocorticoid receptors was mildly defective only at the restrictive temperature. Cdc37p appears to function via the androgen receptor ligand-binding domain, although it does not influence receptor hormone-binding affinity. Models for Cdc37p regulation of steroid hormone receptors are discussed.

Yeast molecular chaperones and the mechanism of steroid hormone action

The yeast Saccharomyces cerevisiae has become a valuable tool for the analysis of steroid action in vivo. Results from studies using yeast have been productive for understanding the role of molecular chaperones, especially Hsp90 and its cochaperones, in regulating the activation of heterologously expressed steroid hormone receptors. The methodology for these investigations involves assaying receptor function in yeast strains with deletions or mutations in the genes encoding molecular chaperone proteins. The information obtained thus far indicates that the Hsp90 chaperone machine functions in vivo to maintain steroid receptors in a high-affinity hormone-binding conformation and also in downstream events of the activation pathway.

Hsp90 regulates androgen receptor hormone binding affinity in vivo

The regulation of human androgen receptor (AR) by the molecular chaperone Hsp90 was investigated using the yeast Saccharomyces cerevisiae as a model system. These studies were performed in strains expressing a conditional temperature-sensitive mutant allele of the hsp82 gene, which encodes Hsp90 protein. At the restrictive temperature in the mutant, there is a decrease in hormone-dependent transactivation by the AR, although steady state levels of AR protein are unchanged. Quantitative hormone binding studies at the permissive temperature revealed the presence of both high affinity and low affinity hormone binding states. At the restrictive temperature in the hsp82 mutant, the high affinity state was abolished, and only the low affinity state was observed. The change in hormone binding affinity was further investigated by a competition assay with the anti-androgen hydroxyflutamide. Under permissive conditions, hydroxyflutamide competes poorly for the synthetic androgen R1881, but under restrictive conditions in the hsp82 mutant strain, hydroxyflutamide was shown to be a potent competitive inhibitor. Our findings indicate that Hsp90 participates in the activation process by maintaining apoAR in a high affinity ligand binding conformation which is important for efficient response to hormone.

The Ydj1 molecular chaperone facilitates formation of active p60v-src in yeast

Molecular chaperones have been implicated in the formation of active p60v-src tyrosine kinase. In Saccharomyces cerevisiae, expression of p60v-src causes cell death, a phenomenon that requires functional Hsp90. We show here that mutations in a member of a second class of chaperones, the yeast dnaJ homologue YDJ1, suppress the lethality caused by p60v-src. One p60v-src-resistant ydj1 mutant, ydj1-39, which has two point mutations in the highly conserved "J" domain, has reduced levels of v-src mRNA and protein. However, a ydj1 null mutant produces normal quantities of active p60v-src, indicating that Ydj1p facilitates, but is not essential for, the formation of active p60v-src. We also report p60v-src-resistance in a previously identified temperature-sensitive ydj1 mutant, ydj1-151. In this mutant, the level of p60v-src remains unaltered, but the protein is much less active in vivo. In addition, p60v-src immunoprecipitates from the ydj1-151 strain contained Hsp90 and Hsp70 in greater amounts than in wild-type strains. Ydj1 protein was also detected in p60v-src immunoprecipitates from both wild-type and ydj1-151 strains. These results indicate that Ydj1p participates in the formation of active p60v-src via molecular chaperone complexes.

Hormone-dependent transactivation by the human androgen receptor is regulated by a dnaJ protein

Genetic studies were performed to examine the role of eukaryotic dnaJ protein, Ydj1p, in the regulated activation of human androgen receptor (hAR) after heterologous expression in Saccharomyces cerevisiae. Hormone-dependent activation of hAR was measured as a function of lacZ reporter gene expression, which was defective in ydj1-151 and ydj1-2 delta null mutant strains compared to the wild type. This defect was not due to receptor misfolding, since hAR in both wild type and mutant strains had a similar capacity to bind hormone. The target for Ydj1p action was determined to be the hAR hormone binding domain since an N-terminal fragment lacking this region was constitutively active in both wild type and ydj1-151 mutant strains. These data correlate hormone dependence of hAR activation with a requirement for Ydj1p function and are consistent with a role for dnaJ proteins in signal transduction by steroid hormone receptors.

XDJ1, a gene encoding a novel non-essential DnaJ homologue from Saccharomyces cerevisiae

The gene encoding a novel DnaJ-like protein, termed Xdj1, has been identified by amplification of Saccharomyces cerevisiae genomic DNA. An open reading frame of 1380 bp was detected. Disruption of XDJ1 did not yield any detectable new phenotype. A double-deletion strain containing a disruption of both XDJ1 and YDJ1, another gene coding for a DnaJ-like protein, was still viable. Under a variety of growth conditions, no XDJ1 transcripts could be detected by Northern blot analysis and no translation product was found by immunoblotting with antibody against Xdj1 produced in Escherichia coli. Thus, XDJ1 is either expressed only under very specific conditions or represents a silent gene.

YDJ1p facilitates polypeptide translocation across different intracellular membranes by a conserved mechanism

The role of S. cerevisiae YDJ1 protein (YDJ1p) in polypeptide translocation across membranes has been examined. A conditional ydj1 mutant strain (ydj1-151TS) is defective for import of several polypeptides into mitochondria and alpha factor into the endoplasmic reticulum at 37 degrees C. These defects are suppressed by E. coli dnaJ or overexpression of S. cerevisiae SIS1 proteins. A different ydj1 mutant, which cannot be farnesylated (ydj1-S406), displays similar transport defects to the ydj1-151 strain. Furthermore, the ability of purified ydj1-151p to stimulate the ATPase activity of hsp70SSA1 was greatly diminished compared with the wild-type protein. Together, these data suggest that YDJ1p functions in polypeptide translocation in a conserved manner, probably acting at organelle membranes and in association with hsp70 proteins.

Farnesylation of YDJ1p is required for function at elevated growth temperatures in Saccharomyces cerevisiae

The Saccharomyces cerevisiae YDJ1 protein (YDJ1p) contains a C-terminal "CaaX box" motif common to proteins that are modified by prenylation. In the present study we show that YDJ1p is a specific substrate for both yeast and mammalian protein farnesyltransferase enzymes in vitro. A mutant form of YDJ1p, in which the conserved cysteine of the CaaX box is mutated to a serine (ydj1-S406p), cannot be farnesylated in vitro. After expression in S. cerevisiae, ydj1-S406p displays a reduced electrophoretic mobility and an increased cytosolic localization in subcellular fractionation experiments when compared to wild type YDJ1p. Expression of ydj1-S406 in cells lacking YDJ1 results in a temperature-sensitive growth phenotype in S. cerevisiae. These data indicate that farnesylation of YDJ1p is required for its function at elevated temperatures.

Characterization of YDJ1: a yeast homologue of the bacterial dnaJ protein

The YDJ1 (yeast dnaJ) gene was isolated from a yeast expression library using antisera made against a yeast nuclear sub-fraction termed the matrix lamina pore complex. The predicted open reading frame displays a 32% identity with the sequence of the Escherichia coli heat shock protein dnaJ. Localization of YDJ1 protein (YDJ1p) by indirect immunofluorescence reveals it to be concentrated in a perinuclear ring as well as in the cytoplasm. YDJ1p cofractionates with nuclei and also microsomes, suggesting that its perinuclear localization reflects association with the ER. YDJ1p is required for normal growth and disruption of its gene results in very slow growing cells that have pleiotropic morphological defects. Haploid cells carrying the disrupted YDJ1 gene are inviable for growth in liquid media. We further show that a related yeast protein, SIS1, is a multicopy suppressor of YDJ1.

Basal-level expression of the yeast HSP82 gene requires a heat shock regulatory element

Previous studies have shown that heat shock factor is constitutively bound to heat shock elements in Saccharomyces cerevisiae. We demonstrate that mutation of the heat shock element closest to the TATA box of the yeast HSP82 promoter abolishes basal-level transcription without markedly affecting inducibility. The mutated heat shock element no longer bound putative heat shock factor, either in vitro or in vivo, but still resided within a nuclease-hypersensitive site in the chromatin. Thus, constitutive binding of heat shock factor to heat shock elements in S. cerevisiae appears to functionally direct basal-level transcription.