Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:3.4.25.1 (proteasome)
28,817 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

3-hydroxy-3-methylglutaryl-CoA reductase (HMG-R), a key enzyme of sterol synthesis, is an integral membrane protein of the endoplasmic reticulum (ER). In both humans and yeast, HMG-R is degraded at or in the ER. The degradation of HMG-R is regulated as part of feedback control of the mevalonate pathway. Neither the mechanism of degradation nor the nature of the signals that couple the degradation of HMG-R to the mevalonate pathway is known. We have launched a genetic analysis of the degradation of HMG-R in Saccharomyces cerevisiae using a selection for mutants that are deficient in the degradation of Hmg2p, an HMG-R isozyme. The underlying genes are called HRD (pronounced "herd"), for HMG-CoA reductase degradation. So far we have discovered mutants in three genes: HRD1, HRD2, and HRD3. The sequence of the HRD2 gene is homologous to the p97 activator of the 26S proteasome. This p97 protein, also called TRAP-2, has been proposed to be a component of the mature 26S proteasome. The hrd2-1 mutant had numerous pleiotropic phenotypes expected for cells with a compromised proteasome, and these phenotypes were complemented by the human TRAP-2/p97 coding region. In contrast, HRD1 and HRD3 genes encoded previously unknown proteins predicted to be membrane bound. The Hrd3p protein was homologous to the Caenorhabditis elegans sel-1 protein, a negative regulator of at least two different membrane proteins, and contained an HRD3 motif shared with several other proteins. Hrd1p had no full-length homologues, but contained an H2 ring finger motif. These data suggested a model of ER protein degradation in which the Hrd1p and Hrd3p proteins conspire to deliver HMG-R to the 26S proteasome. Moreover, our results lend in vivo support to the proposed role of the p97/TRAP-2/Hrd2p protein as a functionally important component of the 26S proteasome. Because the HRD genes were required for the degradation of both regulated and unregulated substrates of ER degradation, the HRD genes are the agents of HMG-R degradation but not the regulators of that degradation.
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PMID:Role of 26S proteasome and HRD genes in the degradation of 3-hydroxy-3-methylglutaryl-CoA reductase, an integral endoplasmic reticulum membrane protein. 897 Jan 63

Polypeptide import into the yeast endoplasmic reticulum (ER) requires two hsp70s, Ssa1p in the cytosol and BiP (Kar2p) in the ER lumen. After import, aberrant polypeptides may be exported to the cytoplasm for degradation by the proteasome, and defects in the ER chaperone calnexin (Cne1p) compromise their degradation. Both import and export require BiP and the Sec61p translocation complex, suggesting that import and export may be mechanistically related. We now show that the cne1Delta and two kar2 mutant alleles exhibit a synthetic interaction and that the export and degradation of pro-alpha factor is defective in kar2 mutant microsomes. Pulse-chase analysis indicates that A1PiZ, another substrate for degradation, is stabilized in the kar2 strains at the restrictive temperature. Because two of the kar2 mutants examined are proficient for polypeptide import, the roles of BiP during ER protein export and import differ, indicating that these processes must be mechanistically distinct. To examine whether Ssa1p drives polypeptides from the ER and is also required for degradation, we assembled reactions using strains either containing a mutation in SSA1 or in which the level of Ssa1p could be regulated. We found that pro-alpha factor and A1PiZ were degraded normally, indicating further that import and export are distinct and that other cytosolic factors may pull polypeptides from the ER.
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PMID:The requirement for molecular chaperones during endoplasmic reticulum-associated protein degradation demonstrates that protein export and import are mechanistically distinct. 992 Aug 90

In eukaryotic cells, the ubiquitin-proteasome pathway is the major mechanism for the targeted degradation of proteins with short half-lives. The covalent attachment of ubiquitin to lysine residues of targeted proteins is a signal for the recognition and rapid degradation by the proteasome, a large multi-subunit protease. In this report, we demonstrate that the human estrogen receptor (ER) protein is rapidly degraded in mammalian cells in an estradiol-dependent manner. The treatment of mammalian cells with the proteasome inhibitor MG132 inhibits activity of the proteasome and blocks ER degradation, suggesting that ER protein is turned over through the ubiquitin-proteasome pathway. In addition, we show that in vitro ER degradation depends on ubiquitin-activating E1 enzyme (UBA) and ubiquitin-conjugating E2 enzymes (UBCs), and the proteasome inhibitors MG132 and lactacystin block ER protein degradation in vitro. Furthermore, the UBA/UBCs and proteasome inhibitors promote the accumulation of higher molecular weight forms of ER. The UBA and UBCs, which promote ER degradation in vitro, have no significant effect on human progesterone receptor and human thyroid hormone receptor beta proteins.
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PMID:Proteasome-dependent degradation of the human estrogen receptor. 1005 59

Eukaryotic cells have a complex degradation machinery that eliminates misfolded or unassembled secretory proteins from the endoplasmic reticulum (ER). The proteins are retained in an ER/pre-Golgi compartment and then hydrolysed by the cytosolic ubiquitin-proteasome system. This requires retrograde translocation of proteins from the ER back to the cytoplasm, which is mediated by Sec61, the central component of the ER protein-import channel. This proteolytic pathway prevents a potentially lethal aggregation of secretory proteins; however, several viruses misuse it to escape detection, and bacterial and plant toxins might also exploit it. Underactive or overactive ER degradation machinery contributes to the pathogenesis of several severe human diseases.
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PMID:Retrograde protein translocation: ERADication of secretory proteins in health and disease. 1039 Jun 15

Regulation of estrogen receptor (ER) concentration is a key component in limiting estrogen responsiveness in target cells. Yet the mechanisms governing ER concentration in the lactotrope cells of the anterior pituitary, a major site of estrogen action, are undetermined. In this study, we used a lactotrope cell line, PR1, to explore regulation of ER protein by estrogen. Estrogen treatment resulted in an approximate 60% decrease in ER steady state protein levels. Suprisingly, the decline in ER protein was apparent within 1 h of estrogen treatment and occurred in the absence of protein synthesis and transcription. Direct regulation of ER protein was further confirmed by pulse chase analysis, which showed that ER protein half-life was shortened from greater than 3 h to 1 h in the presence of estrogen. The estrogen-induced degradation of ER protein could be prevented by pretreatment with peptide aldehyde inhibitors of proteasome protease whereas inhibitors of calpain and lysosomal proteases were ineffective. Inhibition of proteasome activity maintained ER protein at a level equivalent to control cells not stimulated with estrogen but increased estrogen-binding activity by 1.75-fold. Proteolytic regulation of ER by the proteasome is not limited to pituitary lactotrope cells but is also operational in MCF-7 breast cancer cells, suggesting that this may be a common regulatory pathway used by estrogen. These studies describe a nongenomic action of estrogen that involves nuclear ER: rapid proteolysis of ER protein via a proteasome-mediated pathway.
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PMID:Proteasome-mediated proteolysis of estrogen receptor: a novel component in autologous down-regulation. 1047 43

The translocation of secretory polypeptides into and across the membrane of the endoplasmic reticulum (ER) occurs at the translocon, a pore-forming structure that orchestrates the transport and maturation of polypeptides at the ER membrane. Recent data also suggest that misfolded or unassembled polypeptides exit the ER via the translocon for degradation by the cytosolic ubiquitin/proteasome pathway. Sec61p is a highly conserved multispanning membrane protein that constitutes a core component of the translocon. We have found that the essential function of the Saccharomyces cerevisiae Sec61p is retained upon deletion of either of two internal regions that include transmembrane domains 2 and 3, respectively. However, a deletion mutation encompassing both of these domains was found to be nonfunctional. Characterization of yeast mutants expressing the viable deletion alleles of Sec61p has revealed defects in post-translational translocation. In addition, the transmembrane domain 3 deletion mutant is induced for the unfolded protein response and is defective in the dislocation of a misfolded ER protein. These data demonstrate that the various activities of Sec61p can be functionally dissected. In particular, the transmembrane domain 2 region plays a role in post-translational translocation that is required neither for cotranslational translocation nor for protein dislocation.
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PMID:Distinct domains within yeast Sec61p involved in post-translational translocation and protein dislocation. 1061 47

Cytochrome P450, CYP3A4, is the dominant human liver endoplasmic reticulum (ER) hemoprotein enzyme, responsible for the metabolism of over 60% of clinically relevant drugs. We have previously shown that mechanism-based suicide inactivation of CYP3A4 and its rat liver ER orthologs, CYPs 3A, via heme-modification of their protein moieties, results in their ubiquitin (Ub)-dependent 26S proteasomal degradation (Korsmeyer et al. (1999) Arch. Biochem. Biophys. 365, 31; Wang et al. (1999) Arch. Biochem. Biophys. 365, 45). This is not surprising given that the heme-modified CYP3A proteins are structurally damaged. To determine whether the turnover of the native enzyme similarly recruited this pathway, we heterologously expressed this protein in wild-type Saccharomyces cerevisiae and mutant strains (hrd1Delta, hrd2-1, and hrd3Delta) previously shown to be deficient in the Ub-dependent 26S proteasomal degradation of the polytopic ER protein 3-hydroxy-3-methylglutaryl-CoA reductase (isoform Hmg2p), the rate-limiting enzyme in sterol biosynthesis, as well as in strains deficient in ER-associated Ub-conjugating enzymes, Ubc6p and/or Ubc7p (Hampton et al. (1996) Mol. Biol. Cell 7, 2029; Hampton and Bhakta (1997) Proc. Natl. Acad. Sci. USA 94, 12,944). Our findings reveal that in common with the degradation of Hmg2p, that of native CYP3A4 also requires Hrd2p (a subunit of the 19S cap complex of the 26S proteasome) and Ubc7p, and to a much lesser extent Hrd3p, a component of the ER-associated Ub-ligase complex. In contrast to Hmg2p-degradation, that of native CYP3A4 does not appear to absolutely require Hrd1p, another component of the ER-associated Ub-ligase complex. Furthermore, studies in a S. cerevisiae pep4Delta strain proven to be deficient in the vacuolar degradation of carboxypeptidase Y indicated that CYP3A4 degradation is also largely independent of vacuolar (lysosomal) proteolytic function. The degradation of two other native ER proteins, Sec61p and Sec63p, normal components of the ER translocon, were also examined in parallel and found to be stabilized to some extent in HRD2- and UBC7-deficient strains. Together these findings attest to the remarkable mechanistic diversity in the normal degradation of ER proteins.
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PMID:Ubiquitin-dependent 26S proteasomal pathway: a role in the degradation of native human liver CYP3A4 expressed in Saccharomyces cerevisiae? 1151 67

Proteins destined for the secretory pathway must first fold and assemble in the lumen of endoplasmic reticulum (ER). The pathway maintains a quality control mechanism to assure that aberrantly processed proteins are not delivered to their sites of function. As part of this mechanism, misfolded proteins are returned to the cytosol via the ER protein translocation pore where they are ubiquitinated and degraded by the 26S proteasome. Previously, little was known regarding the recognition and targeting of proteins before degradation. By tracking the fate of several mutant proteins subject to quality control, we demonstrate the existence of two distinct sorting mechanisms. In the ER, substrates are either sorted for retention in the ER or are transported to the Golgi apparatus via COPII-coated vesicles. Proteins transported to the Golgi are retrieved to the ER via the retrograde transport system. Ultimately, both retained and retrieved proteins converge at a common machinery at the ER for degradation. Furthermore, we report the identification of a gene playing a novel role specific to the retrieval pathway. The gene, BST1, is required for the transport of misfolded proteins to the Golgi, although dispensable for the transport of many normal cargo proteins.
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PMID:Distinct retrieval and retention mechanisms are required for the quality control of endoplasmic reticulum protein folding. 1167 77

Synthetic estrogen receptor ligands such as tamoxifen and raloxifene produce biologic responses which can be either estrogenic or anti-estrogenic, depending upon the tissue in which their action is examined. To reflect the fact that they are not 'pure' antagonists, such ligands have been more accurately termed selective estrogen receptor modulators (SERMs). Recent progress in our understanding of the molecular biology of estrogen receptor (ER) action has provided a great deal of evidence which promises to increase our understanding of the mechanism through which SERMs elicit their tissue-specific effects. The identification of numerous coactivators and corepressors which modulate receptor function and the realization of two subtypes of ER attest to the potential complexity through which SERMs produce diverse tissue-specific responses. Evidence from co-crystal structures of ER ligand-binding domains complexed with SERMs provides additional information as to how this class of ligands can elicit diverse biologic responses. SERMs also influence the stability of the ER protein, and recent information on the determinants of receptor stability and the role of proteasome-mediated protein degradation in ER-driven transcription also promises to give a fuller understanding of SERM biology. These aspects of the molecular biology of estrogen receptor action may help clarify the mechanism(s) of SERM biologic action and will be addressed in further detail in this review.
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PMID:Molecular perspectives on selective estrogen receptor modulators (SERMs): progress in understanding their tissue-specific agonist and antagonist actions. 1172 17

In eukaryotic cells, incorrectly folded proteins in the endoplasmic reticulum (ER) are exported into the cytosol and degraded by the proteasome. This pathway is co-opted by some viruses. For example, the US11 protein of the human cytomegalovirus targets the major histocompatibility complex class I heavy chain for cytosolic degradation. How proteins are extracted from the ER membrane is unknown. In bacteria and mitochondria, members of the AAA ATPase family are involved in extracting and degrading membrane proteins. Here we demonstrate that another member of this family, Cdc48 in yeast and p97 in mammals, is required for the export of ER proteins into the cytosol. Whereas Cdc48/p97 was previously known to function in a complex with the cofactor p47 (ref. 5) in membrane fusion, we demonstrate that its role in ER protein export requires the interacting partners Ufd1 and Npl4. The AAA ATPase interacts with substrates at the ER membrane and is needed to release them as polyubiquitinated species into the cytosol. We propose that the Cdc48/p97-Ufd1-Npl4 complex extracts proteins from the ER membrane for cytosolic degradation.
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PMID:The AAA ATPase Cdc48/p97 and its partners transport proteins from the ER into the cytosol. 1174 May 63


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