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Query: UNIPROT:P62988 (Ubiquitin)
 4,326 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The distribution of protein gene product 9.5 (PGP) and ubiquitin in the spermatozoa and epithelial cells in the different regions of the rat ductus epididymidis (proximal caput, distal caput, corpus and cauda) was studied by Western blotting analyses and electron microscopical immunogold labelling. Western blotting analyses showed that the PGP immunoreactive band was very intense in the caput and cauda epididymidis and almost irrelevant in the corpus, while the ubiquitin immunoreactive band was intense in the distal caput and cauda. No ubiquitin immunoreactive band was observed in the proximal caput and only a very weak band was seen in the corpus. The results of electron microscopical immunogold labelling varied from one epididymal region to another. The proximal caput epididymidis presented immunoreaction to PGP in the rough endoplasmic reticulum, cytosol, mitochondria and microvilli of most principal cells, and in the cytosol, rough endoplasmic reticulum and mitochondria of most basal cells. No ubiquitin immunoreaction was observed in this epididymal region. In the distal caput epididymidis, PGP immunoreactivity was detected in some principal and basal cells in the same intracellular locations as described in the proximal caput. In this region, ubiquitin immunoreactivity appears in the apical cytosol and mitochondria of principal cells. The corpus epididymidis showed no immunoreaction to PGP or ubiquitin. In the cauda epididymidis, immunostaining to PGP was observed in most clear cells and in isolated principal cells. The intracellular location of PGP in both cell types was the cytosol, mitochondria and microvilli. Ubiquitin immunoreactivity was detected in the perinuclear cytosol and mitochondria-but not in the digestive vacuoles-of some clear cells.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Ultrastructural localization of PGP 9.5 and ubiquitin immunoreactivities in rat ductus epididymidis epithelium. 755 92

Ubiquitin-conjugating enzymes function in selective proteolysis pathways and catalyse the covalent attachment of ubiquitin to proteolytic substrates. Here we report the identification of an integral membrane ubiquitin-conjugating enzyme. This enzyme, UBC6, localizes to the endoplasmic reticulum (ER), with the catalytic domain facing the cytosol. ubc6 loss-of-function mutants suppress the protein translocation defect caused by a mutation in SEC61, which encodes a key component of a multisubunit protein translocation apparatus of the ER. The expression of the sec61 mutant phenotype requires both the activity of UBC6 and its localization at the ER membrane. This suggests that UBC6 may mediate selective degradation of ER membrane proteins and that the protein translocation defect of sec61 may be caused by proteolysis of components of a structurally distorted mutant translocation apparatus.
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PMID:A protein translocation defect linked to ubiquitin conjugation at the endoplasmic reticulum. 839 28

The accumulation of misfolded proteins in the cytosol leads to increased expression of heat-shock proteins, while accumulation of such proteins in the endoplasmic reticulum (ER) stimulates the expression of many ER resident proteins, most of which function as molecular chaperones. Recently, inhibitors of the proteasome have been identified that can block the rapid degradation of abnormal cytosolic and ER-associated proteins. We therefore tested whether these agents, by causing the accumulation of abnormal proteins, might stimulate the expression of cytosolic heat-shock proteins and/or ER molecular chaperones and thereby induce thermotolerance. Exposure of Madin-Darby canine kidney cells to various proteasome inhibitors, including the peptide aldehydes (MG132, MG115, N-acetyl-leucyl-leucyl-norleucinal) and lactacystin, inhibited the degradation of short-lived proteins and increased markedly the levels of mRNAs encoding cytosolic heat-shock proteins (Hsp70, polyubiquitin) and ER chaperones (BiP, Grp94, ERp72), as shown by Northern blot analysis. However, inhibitors of cysteine proteases (E64), serine proteases (leupeptin), or metalloproteases (1, 10-phenanthroline) had no effect on the levels of these mRNAs. The relative efficacies of the peptide aldehyde inhibitors in inducing these mRNAs correlated with their potencies against the proteasome. Furthermore, reduction of the aldehyde group of MG132 decreased its inhibitory effect on proteolysis and largely prevented the induction of these mRNAs. Although treatment with the proteasome inhibitors caused rapid increases in mRNA levels (as early as 2 h after treatment with MG132), the inhibitors did not detectably affect total protein synthesis, total protein secretion, ER morphology, or the retention of ER-lumenal proteins, even after 18 h of treatment. Together, the findings suggest that inhibition of proteasome function induces heat-shock proteins and ER chaperones due to the accumulation of sufficient amounts of abnormal proteins and/or the inhibition of degradation of a key regulatory factor (e.g. heat-shock factor). Since expression of heat-shock proteins can protect cells from thermal injury, we tested whether the proteasome inhibitors might also confer thermotolerance. Treatment of cells with MG132 for as little as 2 h, markedly increased the survival of cells subjected to high temperatures (up to 46 degrees C). Thus, these agents may have applications in protecting against cell injury.
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PMID:Proteasome inhibition leads to a heat-shock response, induction of endoplasmic reticulum chaperones, and thermotolerance. 908 35

The modification of cytosolic proteins with polyubiquitin chains targets them for recognition and degradation by the multisubunit proteolytic particle, the 26S proteasome. Membrane proteins are also substrates for ubiquitination. Integral membrane proteins of the endoplasmic reticulum are ubiquitinated and destroyed by the proteasome. However, it has been shown recently that the ubiquitination of Saccharomyces cerevisiae plasma membrane proteins signals their degradation by the proteolytic system in the lysosome-like vacuole. Ubiquitination of several different classes of cell surface proteins serves as a signal for their entry into the endocytic pathway; this leads to their transport to the vacuole, where they are permanently inactivated by degradation. In yeast, ubiquitin has been implicated as an internalization signal for most, if not all, endogenous plasma membrane proteins that are known to be endocytosed. Ubiquitin-dependent internalization has been best characterized for two proteins: the mating pheromone alpha-factor receptor and the uracil permease. Some mammalian cell surface receptors are also ubiquitinated at the plasma membrane. Ubiquitination machinery is required for ligand-induced endocytosis of the growth hormone receptor, suggesting that ubiquitin-dependent endocytosis and sorting is also an important regulatory process in mammalian cells. Mammalian receptors may also be down-regulated through the degradation of their cytosolic domains by a proteasome-dependent pathway.
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PMID:Ubiquitin-dependent internalization and down-regulation of plasma membrane proteins. 940 40

The human immunodeficiency virus type 1 (HIV-1) vpu gene encodes a type I anchored integral membrane phosphoprotein with two independent functions. First, it regulates virus release from a post-endoplasmic reticulum (ER) compartment by an ion channel activity mediated by its transmembrane anchor. Second, it induces the selective down regulation of host cell receptor proteins (CD4 and major histocompatibility complex class I molecules) in a process involving its phosphorylated cytoplasmic tail. In the present work, we show that the Vpu-induced proteolysis of nascent CD4 can be completely blocked by peptide aldehydes that act as competitive inhibitors of proteasome function and also by lactacystin, which blocks proteasome activity by covalently binding to the catalytic beta subunits of proteasomes. The sensitivity of Vpu-induced CD4 degradation to proteasome inhibitors paralleled the inhibition of proteasome degradation of a model ubiquitinated substrate. Characterization of CD4-associated oligosaccharides indicated that CD4 rescued from Vpu-induced degradation by proteasome inhibitors is exported from the ER to the Golgi complex. This finding suggests that retranslocation of CD4 from the ER to the cytosol may be coupled to its proteasomal degradation. CD4 degradation mediated by Vpu does not require the ER chaperone calnexin and is dependent on an intact ubiquitin-conjugating system. This was demonstrated by inhibition of CD4 degradation (i) in cells expressing a thermally inactivated form of the ubiquitin-activating enzyme E1 or (ii) following expression of a mutant form of ubiquitin (Lys48 mutated to Arg48) known to compromise ubiquitin targeting by interfering with the formation of polyubiquitin complexes. CD4 degradation was also prevented by altering the four Lys residues in its cytosolic domain to Arg, suggesting a role for ubiquitination of one or more of these residues in the process of degradation. The results clearly demonstrate a role for the cytosolic ubiquitin-proteasome pathway in the process of Vpu-induced CD4 degradation. In contrast to other viral proteins (human cytomegalovirus US2 and US11), however, whose translocation of host ER molecules into the cytosol occurs in the presence of proteasome inhibitors, Vpu-targeted CD4 remains in the ER in a transport-competent form when proteasome activity is blocked.
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PMID:CD4 glycoprotein degradation induced by human immunodeficiency virus type 1 Vpu protein requires the function of proteasomes and the ubiquitin-conjugating pathway. 949 87

Several secretory proteins, including apolipoprotein B, have been shown to undergo degradation by proteasomes. We found that the rapid degradation of nascent apolipoprotein B in HepG2 cells was diminished but not abolished by the addition of any of three different inhibitors of proteasomes. Ubiquitin is conjugated to apolipoprotein B that is not assembled with sufficient lipids either during or soon after synthesis. In addition, we found that apolipoprotein B that has entered the endoplasmic reticulum sufficiently to become glycosylated can be degraded by proteasomes. Furthermore, we detected ubiquitin-apolipoprotein B that is associated with the Sec61 complex, the major constituent of the translocational channel. Treatment of cells with monomethylethanolamine or dithiothreitol decreased the translocation of apolipoprotein B and increased the proportion of ubiquitin-conjugated molecules associated with Sec61. Conversely, treatment of cells with oleic acid, which increased the proportion of translocated apolipoprotein B, decreased the amount of ubiquitin-apolipoprotein B in the Sec61 complex. Finally, we found that inhibition of the interaction between calnexin and apolipoprotein B decreases the translocation of apolipoprotein B, increases the ubiquitin-apolipoprotein B in the Sec61 complex, and increases the proteasomal degradation of glycosylated apolipoprotein B. Thus, ubiquitin can be attached to unassembled apolipoprotein B in the Sec61 complex, and this process is affected by factors including calnexin that alter the translocation of apolipoprotein B.
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PMID:Calnexin and other factors that alter translocation affect the rapid binding of ubiquitin to apoB in the Sec61 complex. 956 15

To identify factors involved in the expression of ligand-gated ion channels, we expressed nicotinic acetylcholine receptors in HEK cells to characterize roles for oligosaccharide trimming, calnexin association, and targeting to the proteasome. The homologous subunits of the acetylcholine receptor traverse the membrane four times, contain at least one oligosaccharide, and are retained in the endoplasmic reticulum until completely assembled into the circular arrangement of subunits of delta-alpha-gamma-alpha-beta to enclose the ion channel. We previously demonstrated that calnexin is associated with unassembled subunits of the receptor, but appears to dissociate when subunits are assembled in various combinations. We used the glucosidase inhibitor castanospermine to block oligosaccharide processing, and thereby inhibit calnexin's interaction with the oligosaccharides in the receptor subunits. Castanospermine treatment reduces the association of calnexin with the alpha-subunit of the receptor, and diminishes the intracellular accumulation of unassembled receptor subunit protein. However, treatment with castanospermine does not appear to alter subunit folding or assembly. In contrast, co-treatment with proteasome inhibitors and castanospermine enhances the accumulation of polyubiquitin-conjugated alpha-subunits, and generally reverses the castanospermine induced loss of alpha-subunit protein. Co-transfection of cDNAs encoding the alpha- and delta-subunits, which leads to the expression of assembled alpha- and delta- subunits, also inhibits the loss of alpha-subunits expressed in the presence of castanospermine. Taken together, these observations indicate that calnexin association reduces the degradation of unassembled receptor subunits in the ubiquitin-proteasome pathway.
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PMID:Inhibition of glucose trimming with castanospermine reduces calnexin association and promotes proteasome degradation of the alpha-subunit of the nicotinic acetylcholine receptor. 964 71

In this study, we explored how sterol metabolism altered by the expression of cholesterol-7alpha-hydroxylase NADPH:oxygen oxidoreductase (7alpha-hydroxylase) affects the ubiquitin-dependent proteasome degradation of translocation-arrested apoB53 in Chinese hamster ovary cells. Stable expression of two different plasmids that encode either rat or human 7alpha-hydroxylase inhibited the ubiquitin conjugation of apoB and its subsequent degradation by the proteasome. Oxysterols (25-hydroxycholesterol and 7-ketocholesterol) reversed the inhibition of apoB degradation caused by 7alpha-hydroxylase. The combined results suggest that the normally rapid proteasome degradation of translocation-arrested apoB can be regulated by a sterol-sensitive polyubiquitin conjugation step in the endoplasmic reticulum. Blocked ubiquitin-dependent proteasome degradation caused translocation-arrested apoB to become sequestered in segregated membrane domains. Our results described for the first time a novel mechanism through which the "quality control" proteasome endoplasmic reticulum degradative pathway of translocation-arrested apoB is linked to sterol metabolism. Sterol-sensitive blocked ubiquitin conjugation appears to selectively inhibit the proteasome degradation of apoB, but not 7alpha-hydroxylase protein, with no impairment of cell vitality or function. Our findings may help to explain why the hepatic production of lipoproteins is increased when familial hypertriglyceridemic patients are treated with drugs that activate 7alpha-hydroxylase (e.g. bile acid-binding resins).
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PMID:Translocation-arrested apolipoprotein B evades proteasome degradation via a sterol-sensitive block in ubiquitin conjugation. 988 May 70

Ubiquitin-conjugating enzymes (UBCs) selectively target proteins for proteasomal degradation by the covalent attachment of ubiquitin moieties. Yeast UBC6 is unusual in having an active site distinct from all other UBCs and in possessing a transmembrane domain that anchors it to the cytoplasmic surface of the endoplasmic reticulum. During a differential display analysis on chick growth plate chondrocytes we isolated a cDNA encoding a noncanonical ubiquitin-conjugating enzyme (NCUBE1) structurally similar to yeast UBC6. Chick NCUBE1 transcripts were detected in all tissues examined and decreased threefold during chondrocyte terminal differentiation. Database searches identified other related proteins; the human and mouse orthologues of NCUBE1, a second human homologue of yeast UBC6 (NCUBE2), and related proteins from S. pombe, C. elegans, and P. mariana. Together with yeast UBC6 these proteins constitute a distinct family of UBCs sharing a conserved noncanonical active site sequence and a C-terminal transmembrane domain. By analogy with yeast UBC6 they are likely to be localised to the endoplasmic reticulum where they may be involved in targeting retrotranslocated, ER-associated proteins for proteasomal degradation.
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PMID:Identification of a family of noncanonical ubiquitin-conjugating enzymes structurally related to yeast UBC6. 1070 78

In eukaryotes, endoplasmic reticulum-associated degradation (ERAD) functions in cellular quality control and regulation of normal ER-resident proteins. ERAD proceeds by the ubiquitin-proteasome pathway, in which the covalent attachment of ubiquitin to proteins targets them for proteasomal degradation. Ubiquitin-protein ligases (E3s) play a crucial role in this process by recognizing target proteins and initiating their ubiquitination. Here we show that Hrd1p, which is identical to Der3p, is an E3 for ERAD. Hrd1p is required for the degradation and ubiquitination of several ERAD substrates and physically associates with relevant ubiquitin-conjugating enzymes (E2s). A soluble Hrd1 fusion protein shows E3 activity in vitro - catalysing the ubiquitination of itself and test proteins. In this capacity, Hrd1p has an apparent preference for misfolded proteins. We also show that Hrd1p functions as an E3 in vivo, using only Ubc7p or Ubc1p to specifically program the ubiquitination of ERAD substrates.
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PMID:Hrd1p/Der3p is a membrane-anchored ubiquitin ligase required for ER-associated degradation. 1114 22


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