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)

Degradation of the HIV receptor CD4 by the proteasome, mediated by the HIV-1 protein Vpu, is crucial for the release of fully infectious virions. To promote CD4 degradation Vpu has to be phosphorylated on a motif DSGXXS, which is conserved in several signalling proteins known to be degraded by the proteasome upon phosphorylation. Such phosphorylation is required for the interaction of Vpu with the ubiquitin ligase SCF-beta-TrCP that triggers CD4 degradation by the proteasome. In the present work, we used two peptides of 22 amino acids between residues 41 and 62 of Vpu. Vpu41-62 was predicted to form an alpha-helix-flexible-alpha-helix including the phosphorylation motif DS52GNES56 and Vpu_P41-62 was phosphorylated at the two sites Ser52 and Ser56. We analysed the conformational change induced by the phosphorylation of this peptide on the residues Ser52 and Ser56. Homo- and heteronuclear NMR techniques were used to assess the structural influence of phosphorylation. The spectra of the free peptides, Vpu_P41-62 and Vpu41-62, in both H2O (at pH 3.5 and 7.2) and a 1:1 mixture of H2O and trifluoroethanol were completely assigned by a combined application of several two-dimensional proton NMR methods. Analysis of the short- and medium-range NOE connectivities and of the secondary chemical shifts indicated that the peptide segment (42-49) shows a less well-defined helix propensity. The Vpu_P41-62 domain of residues 50-62 forms a loop with the phosphate group pointing away, a short beta-strand and a flexible extended 'tail' of residues 60-62. Residues 50-60 exhibit alpha-proton NMR secondary chemical shift changes from random coil toward more beta-like structure with the combined (temperature, solvent and pH) NMR and molecular calculation experiments. Differences in this molecular region 50-62 suggest that conformational changes of Vpu_P play an important role in Vpu_P-induced degradation of CD4 molecules.
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PMID:HIV-1 encoded virus protein U (Vpu) solution structure of the 41-62 hydrophilic region containing the phosphorylated sites Ser52 and Ser56. 1189 91

Eukaryotic cells possess overlapping mechanisms to ensure that DNA replication is restricted to the S phase of the cell cycle. The levels of hOrc1p, the largest subunit of the human origin recognition complex, vary during the cell division cycle. In rapidly proliferating cells, hOrc1p is expressed and targeted to chromatin as cells exit mitosis and prereplicative complexes are formed. Later, as cyclin A accumulates and cells enter S phase, hOrc1p is ubiquitinated on chromatin and then degraded. hOrc1p destruction occurs through the proteasome and is signaled in part by the SCF(Skp2) ubiquitin-ligase complex. Other hORC subunits are stable throughout the cell cycle. The regulation of hOrc1p may be an important mechanism in maintaining the ploidy in human cells.
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PMID:Human origin recognition complex large subunit is degraded by ubiquitin-mediated proteolysis after initiation of DNA replication. 1193 57

The COP9 signalosome (CSN) is a conserved protein complex with homologies to the lid subcomplex of the 26S proteasome. It promotes cleavage of the Nedd8 conjugate (deneddylation) from the cullin component of SCF ubiquitin ligases. We provide evidence that cullin neddylation and deneddylation is highly dynamic, that its equilibrium can be effectively modulated by CSN, and that neddylation allows Cul1 to form larger protein complexes. CSN2 integrates into the CSN complex via its C-terminal region and its N-terminal half region is necessary for direct interaction with Cul1. The polyclonal antibodies against CSN2 but not other CSN subunits cause accumulation of neddylated Cul1/Cul2 in HeLa cell extract, indicating that CSN2 is essential in cullin deneddylation. Further, CSN inhibits ubiquitination and degradation of the cyclin-dependent kinase inhibitor p27(kip1) in vitro. Microinjection of the CSN complex impeded the G1 cells from entering the S phase. Moreover, anti-CSN2 antibodies negate the CSN-dependent p27 stabilization and the G1/S blockage, suggesting that these functions require the deneddylation activity. We conclude that CSN inhibits SCF ubiquitin ligase activity in targeting p27 proteolysis and negatively regulates cell cycle at the G1 phase by promoting deneddylation of Cul1.
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PMID:The COP9 signalosome inhibits p27(kip1) degradation and impedes G1-S phase progression via deneddylation of SCF Cul1. 1196 55

The multiprotein von Hippel-Lindau (VHL) tumor suppressor (CBC(VHL), Cul2-Elongin BC-VHL) and SCF (Skp1-Cul1/Cdc53-F-box protein) complexes are members of structurally related families of E3 ubiquitin ligases that use a heterodimeric module composed of a member of the Cullin protein family and the RING finger protein Rbx1 (ROC1/Hrt1) to activate ubiquitylation of target proteins by the E2 ubiquitin-conjugating enzymes Ubc5 and Cdc34. VHL and F-box proteins function as the substrate recruitment subunits of CBC(VHL) and SCF complexes, respectively. In cells, many F-box proteins are short lived and are proposed to be ubiquitylated by an autocatalytic mechanism and destroyed by the proteasome following assembly into SCF complexes. In contrast, the VHL protein is stabilized by interaction with the Elongin B and C subunits of CBC(VHL) in cells. In this report, we have presented direct biochemical evidence that unlike the F-box protein Cdc4, which is ubiquitylated in vitro by Cdc34 in the context of the SCF, the VHL protein is protected from Ubc5-catalyzed ubiquitylation following assembly into the CBC(VHL) complex. CBC(VHL) is continuously required for negative regulation of hypoxia-inducible transcription factors in normoxic cells and of SCF complexes, many of which function only transiently during the cell cycle or in response to cellular signals. Our findings provide a molecular basis for the different modes of cellular regulation of VHL and F-box proteins and are consistent with the known roles of CBC(VHL).
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PMID:A molecular basis for stabilization of the von Hippel-Lindau (VHL) tumor suppressor protein by components of the VHL ubiquitin ligase. 1204 97

Degradation of Saccharomyces cerevisiae G(1) cyclins Cln1 and Cln2 is mediated by the ubiquitin-proteasome pathway and involves the SCF E3 ubiquitin-ligase complex containing the F-box protein Grr1 (SCF(Grr1)). Here we identify the domain of Cln2 that confers instability and describe the signals in Cln2 that result in binding to Grr1 and rapid degradation. We demonstrate that mutants of Cln2 that lack a cluster of four Cdc28 consensus phosphorylation sites are highly stabilized and fail to interact with Grr1 in vivo. Since one of the phosphorylation sites lies within the Cln2 PEST motif, a sequence rich in proline, aspartate or glutamate, serine, and threonine residues found in many unstable proteins, we fused various Cln2 C-terminal domains containing combinations of the PEST and the phosphoacceptor motifs to stable reporter proteins. We show that fusion of the Cln2 domain to a stabilized form of the cyclin-dependent kinase inhibitor Sic1 (Delta N-Sic1), a substrate of SCF(Cdc4), results in degradation in a phosphorylation-dependent manner. Fusion of Cln2 degradation domains to Delta N-Sic1 switches degradation of Sic1 from SCF(Cdc4) to SCF(Grr1). Delta N-Sic1 fused with a Cln2 domain containing the PEST motif and four phosphorylation sites binds to Grr1 and is unstable and ubiquitinated in vivo. Interestingly, the phosphoacceptor domain of Cln2 binds to Grr1 but is not ubiquitinated and is stable. In summary, we have identified a small transferable domain in Cln2 that can redirect a stabilized SCF(Cdc4) target for SCF(Grr1)-mediated degradation by the ubiquitin-proteasome pathway.
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PMID:Transferable domain in the G(1) cyclin Cln2 sufficient to switch degradation of Sic1 from the E3 ubiquitin ligase SCF(Cdc4) to SCF(Grr1). 1205 57

In order to investigate activation and internalization of c-kit we created a functional c-kit-EGFP chimera by inserting EYFP (enhanced yellow fluorescent protein) within the extracellular domain of c-kit immediately downstream of the signal sequence, SS-EYFP-kit. This location was chosen because the C-terminal fusion of EGFP to c-kit unexpectedly caused constitutive activation of the c-kit tyrosine kinase. As analysed in fixed cells and by real time imaging in vivo, SCF induced activation led to internalization of the fusion construct and translocation to punctate structures resembling vesicles. Analysis of the internalization process by time lapse imaging revealed high mobility and discontinuous movement of these vesicles and their predominantly radial tracks. Two subsets of vesicles were observed: Traffic of the majority of vesicles was directed from the periphery to the center of the cell and most likely represents the internalization of activated receptor molecules via the endosomal pathway. However, some vesicular structures were observed to move towards the periphery of the cell and probably contain newly synthesized protein to replace internalized receptor molecules. The calculated velocity of moving vesicles ranged from 0.05 to 0.2 microm per se. Vesicle formation upon SCF induced dimerization of the receptor was strictly dependent on kinase activity of c-kit. Treatment of cells with phenylarsine oxide, an agent blocking receptor internalization, prior to SCF stimulation resulted in abrogation of the translocation of the chimera to vesicles whereas accumulation of vesicles was observed when cells were treated with proteasome inhibitors. Cholesterol depletion of the cell membrane by methyl-beta-cyclodextrin resulted in dose dependent reduction of receptor internalization indicating that c-kit may be present in lipid rafts or that intact lipid rafts are required for efficient internalization of the receptor. Using the induction of vesicular structures as a sign of efficient internalization of the receptor analysis of mutant c-kit constructs deficient either in activation of PI3-Kinase or Src revealed that internalization of c-kit is dependent on recruitment of Src but not PI3-Kinase.
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PMID:Analysing c-kit internalization using a functional c-kit-EGFP chimera containing the fluorochrome within the extracellular domain. 1208 29

The tobacco N gene confers resistance to Tobacco mosaic virus (TMV) and encodes a toll-interleukin-1 receptor/nucleotide binding/Leu-rich repeat class protein. Recent evidence indicates that the Nicotiana benthamiana Rar1 gene (NbRar1), which encodes a protein with a zinc finger motif called CHORD (Cys- and His-rich domain), is required for the function of N. To investigate the role of NbRar1 in plant defense, we identified its interaction partners. We show that the NbRar1 protein interacts with NbSGT1, a highly conserved component of the SCF (Skp1/Cullin/F-box protein)-type E3 ubiquitin ligase complex involved in protein degradation. In addition, we show that NbSGT1 interacts with NbSKP1. Suppression of NbSGT1 and NbSKP1 shows that these genes play an important role in the N-mediated resistance response to TMV. Both NbRar1 and NbSGT1 associate with the COP9 signalosome, another multiprotein complex involved in protein degradation via the ubiquitin-proteasome pathway. Silencing of the NbCOP9 signalosome also compromises N-mediated resistance to TMV. Our results reveal new roles for SCF and the COP9 signalosome in plant defense signaling.
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PMID:Role of SCF ubiquitin-ligase and the COP9 signalosome in the N gene-mediated resistance response to Tobacco mosaic virus. 1211 69

Cyclin F, a cyclin that can form SCF complexes and bind to cyclin B, oscillates in the cell cycle with a pattern similar to cyclin A and cyclin B. Ectopic expression of cyclin F arrests the cell cycle in G(2)/M. How the level of cyclin F is regulated during the cell cycle is completely obscure. Here we show that, similar to cyclin A, cyclin F is degraded when the spindle assembly checkpoint is activated and accumulates when the DNA damage checkpoint is activated. Cyclin F is a very unstable protein throughout much of the cell cycle. Unlike other cyclins, degradation of cyclin F is independent of ubiquitination and proteasome-mediated pathways. Interestingly, proteolysis of cyclin F is likely to involve metalloproteases. Rapid destruction of cyclin F does not require the N-terminal F-box motif but requires the COOH-terminal PEST sequences. The PEST region alone is sufficient to interfere with the degradation of cyclin F and confer instability when fused to cyclin A. These data show that although cyclin F is degraded at similar time as the mitotic cyclins, the underlying mechanisms are entirely distinct.
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PMID:Cyclin F is degraded during G2-M by mechanisms fundamentally different from other cyclins. 1212 6

N-glycosylation of proteins in the endoplasmic reticulum (ER) has a central role in protein quality control. Here we report that N-glycan serves as a signal for degradation by the Skp1-Cullin1-Fbx2-Roc1 (SCF(Fbx2)) ubiquitin ligase complex. The F-box protein Fbx2 (ref. 4) binds specifically to proteins attached to N-linked high-mannose oligosaccharides and subsequently contributes to ubiquitination of N-glycosylated proteins. Pre-integrin beta 1 is a target of Fbx2; these two proteins interact in the cytosol after inhibition of the proteasome. In addition, expression of the mutant Fbx2 Delta F, which lacks the F-box domain that is essential for forming the SCF complex, appreciably blocks degradation of typical substrates of the ER-associated degradation pathway. Our results indicate that SCF(Fbx2) ubiquitinates N-glycosylated proteins that are translocated from the ER to the cytosol by the quality control mechanism.
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PMID:E3 ubiquitin ligase that recognizes sugar chains. 1214 May 60

The ubiquitin system has been recently implicated in various aspects of transcriptional regulation, including proteasome-dependent degradation of transcriptional activators. In yeast, the activator Met4 is inhibited by the SCF(Met30) ubiquitin ligase, which recognizes and oligo-ubiquitylates Met4. Here, we demonstrate that in minimal media, Met4 is ubiquitylated and rapidly degraded in response to methionine excess, whereas in rich media, Met4 is oligo-ubiquitylated but remains stable. In the latter growth condition, oligo-ubiquitylated Met4 is not recruited to MET gene promoters, but is recruited to the SAM genes, which are required for production of S-adenosylmethionine, an unstable metabolite that is not present in rich medium. Thus, ubiquitylation not only regulates Met4 by distinct degradation-dependent and -independent mechanisms, but also controls differential recruitment of a single transcription factor to distinct promoters, thereby diversifying transcriptional activator specificity.
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PMID:Dual regulation of the met4 transcription factor by ubiquitin-dependent degradation and inhibition of promoter recruitment. 1215 Sep 8


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