Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P62988 (Ubiquitin)
 4,326 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Growth factors stimulate specific receptor tyrosine kinases, but subsequent receptor endocytosis terminates signaling. The ubiquitin ligase c-Cbl targets epidermal growth factor receptors (EGFRs) to endocytosis by tagging them with multiple ubiquitin molecules. However, the type of ubiquitylation is unknown; whereas polyubiquitin chains signal proteasomal degradation, ubiquitin monomers control other processes. We report that in isolation c-Cbl mediates monoubiquitylation rather than polyubiquitylation of EGFRs. Consistent with the sufficiency of monoubiquitylation, when fused to the tail of EGFR, a single ubiquitin induces receptor endocytosis and degradation in cells. By using receptor and ubiquitin mutants, we infer that c-Cbl attaches a founder monoubiquitin to the kinase domain of EGFR and this is complemented by the conjugation of additional monoubiquitins. Hence, receptor tyrosine kinases are desensitized through conjugation of multiple monoubiquitins, which is distinct from polyubiquitin-dependent proteasomal degradation.
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PMID:Endocytosis of receptor tyrosine kinases is driven by monoubiquitylation, not polyubiquitylation. 1271 35

Ubiquitin-dependent proteolysis by the 26S proteasome plays a pivotal role in cell cycle progression as well as in tumorigenesis. In this pathway, ubiquitin-conjugating enzyme (E2), together with ubiquitin ligase (E3), transfers ubiquitin to the specific substrate protein(s); however, little is known about the potential contribution of E2 to tumorigenesis. In this study, we examined the expression levels of 17 E2 genes in 25 different human normal tissues and 24 human cancerous cell lines by using a quantitative real-time reverse transcription-PCR. Among the E2 gene family, the expression level of UbcH10 was extremely low in many of the normal tissues but prominent in the majority of cancerous cell lines. Intriguingly, UbcH10 was expressed at high levels in primary tumors derived from the lung, stomach, uterus, and bladder as compared with their corresponding normal tissues, suggesting that UbcH10 is involved in tumorigenesis or progression of the tumor. To further investigate a possible contribution of UbcH10 to malignant transformation and tumor cell proliferation, NIH3T3 cells were transfected with the expression plasmid encoding UbcH10, and stable transfectants were subsequently established. UbcH10-overexpressing cells exhibited an increased incorporation of bromodeoxyuridine, an enhanced growth rate, an increase in saturation density, and a promotion of colony formation in soft agar medium as compared with parental NIH3T3 cells and the control transfectants. Collectively, our present results provide the first evidence that UbcH10 is highly expressed in various human primary tumors and that UbcH10 has an ability to promote cell growth and malignant transformation.
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PMID:UbcH10 is the cancer-related E2 ubiquitin-conjugating enzyme. 1287 22

We recently described a novel checkpoint pathway that functions early in mitosis to delay chromosome condensation in response to microtubule poisons. The only gene implicated so far in this checkpoint pathway is chfr, whose protein product contains a RING domain and has ubiquitin ligase activity in vitro. The significance of this activity in vivo is unclear. A recent report suggested that the Chfr protein targets itself for proteasome-dependent degradation in mitotic cells through autoubiquitination. However, we observe that in mitosis Chfr exhibits a phosphorylation-dependent electrophoretic mobility shift with no change in overall protein levels. Further analysis of its ubiquitin ligase activity revealed that Chfr can catalyse the formation of noncanonical Lys63-linked polyubiquitin chains with Ubc13-Mms2 acting as the ubiquitin-conjugating enzyme. Ubc13-Mms2 and Lys63-polyubiquitin chains are not associated with targeting proteins to the proteasome, but rather with signaling cellular stress. We propose that Chfr may have a role in signaling the presence of mitotic stress induced by microtubule poisons.
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PMID:The Chfr mitotic checkpoint protein functions with Ubc13-Mms2 to form Lys63-linked polyubiquitin chains. 1456 38

The breast and ovarian cancer suppressor BRCA1 acquires significant ubiquitin ligase activity when bound to BARD1 as a RING heterodimer. Although the activity may well be important for the role of BRCA1 as a tumor suppressor, the biochemical consequence of the activity is not yet known. Here we report that BRCA1-BARD1 catalyzes Lys-6-linked polyubiquitin chain formation. K6R mutation of ubiquitin dramatically reduces the polyubiquitin products mediated by BRCA1-BARD1 in vitro. BRCA1-BARD1 preferentially utilizes ubiquitin with a single Lys residue at Lys-6 or Lys-29 to mediate autoubiquitination of BRCA1 in vivo. Furthermore, mass spectrometry analysis identified the Lys-6-linked branched ubiquitin fragment from the polyubiquitin chain produced by BRCA1-BARD1 using wild type ubiquitin. The BRCA1-BARD1-mediated Lys-6-linked polyubiquitin chains are deubiquitinated by 26 S proteasome in vitro, whereas autoubiquitinated CUL1 through Lys-48-linked polyubiquitin chains is degraded. Proteasome inhibitors do not alter the steady state level of the autoubiquitinated BRCA1 in vivo. Hence, the results indicate that BRCA1-BARD1 mediates novel polyubiquitin chains that may be distinctly edited by 26 S proteasome from conventional Lys-48-linked polyubiquitin chains.
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PMID:Mass spectrometric and mutational analyses reveal Lys-6-linked polyubiquitin chains catalyzed by BRCA1-BARD1 ubiquitin ligase. 1463 90

Degradation of cellular proteins via the ubiquitin-proteasome system (UPS) involves: (i) generation of a substrate-anchored polyubiquitin degradation signal and (ii) destruction of the tagged protein by the 26S proteasome with release of free and reusable ubiquitin. For most substrates, it is believed that the first ubiquitin moiety is conjugated to a epsilon-NH(2) group of an internal Lys residue. Recent findings indicate that for several proteins, the first ubiquitin moiety is fused, in a linear manner, to the free alpha-NH(2) group of the protein. Here, we demonstrate that the inhibitor of differentiation (or inhibitor of DNA binding) 2, Id2, that downregulates gene expression in undifferentiated and self-renewing cells, is degraded by the UPS following ubiquitination at its N-terminal residue. Lysine-less (LL) Id2 is degraded efficiently by the proteasome following ubiquitination. Fusion of a Myc tag to the N-terminal but not to the C-terminal residue of Id2 stabilizes the protein. Furthermore, deletion of the first 15 N-terminal residues of Id2 stabilizes the protein, suggesting that this domain serves as a recognition element, possibly for the ubiquitin ligase, E3. The mechanisms and structural motives that govern Id2 stability may have important implications to the regulation of the protein during normal differentiation and malignant transformation.
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PMID:Degradation of the Id2 developmental regulator: targeting via N-terminal ubiquitination. 1473 35

Ubiquitin inhibitors act at many levels to enhance apoptosis signaling. For TNF-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis signaling, there are at least five mechanisms by which apoptosis are regulated by the ubiquitin-proteasome pathway. First, proteasome inhibitors can decrease Fas-like inhibitor protein (FLIP) protein levels in tumors, resulting in increased apoptosis signaling due to increased caspase-8 activation. This appears to involve the ubiquitin ligase TNF receptor activation factor-2 (TRAF2) and acts indirectly by causing cell-cycle arrest at a stage where there is high degradation of the FLIP-TRAF2 complex. Second, the regulation of the proapoptotic Bcl-2 family member BAX occurs indirectly. Apoptosis signaling and caspase activation results in a confirmation change in the normally monomeric BAX, which exposes the BH3 domain of BAX, leading to dimerization and resistance to ubiquitin degradation. BAX then translocates into the mitochondria, resulting in the release of proapoptotic mitochondrial factors such as cytochrome c and second mitochondria-derived activator of caspase (SMAC). This results in the activation of caspase-9 and formation of the apoptosome and efficient apoptosis signaling. A third mechanism of the regulation of TRAIL signaling in the ubiquitin-proteasome pathway is mediated by the inhibitor of apoptosis proteins (IAP) E3 ligases. These IAPs can directly bind to caspases but also can act as ubiquitin ligases for caspases, resulting in the degradation of these caspases. IAP binding to caspases can be inhibited by SMAC, which exhibits a caspase-9 homology domain. The fourth mechanism for apoptosis activation by proteasome inhibitors is through the stabilization of the inhibitor of the kappaB (IkappaB)/NF-kappaB complex and prevention of nuclear translocation of the antiapoptosis transcription factor NF-kappaB. During TRAIL-DR4, DR5 signaling, this pathway is activated by interactions of activated Fas-associated death domain with activated receptor-interacting protein (RIP), which in turn activates NF-kappaB-inducing kinase and phosphorylates IkappaB. Therefore, the inhibition of IkappaB degradation blocks this RIP-mediated antiapoptosis signaling event. Last, p53 protein levels, and susceptibility to apoptosis, can be deregulated by the human homolog Hdm2 (Mdm2) E3 ligase. This process is inhibited by p53 phosphorylation and by sequestration of Mdm2 by ARF. Better mechanisms to inhibit the ubiquitin-proteasome pathway targeted at the ubiquitin-proteasome degradation process itself, or more specifically at the E3 ligases known to modulate and downregulate proapoptosis pathways will lead to the enhancement of TRAIL apoptosis signaling and better cancer therapeutic outcomes act through this pathway.
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PMID:Regulation of apoptosis proteins in cancer cells by ubiquitin. 1502 88

In our previous study, we found that a human F-box DNA helicase, named hFBH1, interacted with SKP1 to form an SCF (SKP1-Cul1-F-box protein) complex together with CUL1 and ROC1 in an F-box-dependent manner. The complex immunoprecipitated from crude cell extracts catalyzed polyubiquitin formation in the presence of the ubiquitin-activating and ubiquitin-conjugating enzymes, E1 and E2, respectively. In this report, we characterized the enzymatic properties of the recombinant SCF(hFBH1) complex purified from insect cells expressing hFBH1, SKP1, CUL1 and ROC1. The SCF(hFBH1) complex was isolated as a single tight complex that retained DNA helicase, DNA-dependent ATPase and E3 ubiquitin ligase activities. The helicase and ATPase activities residing in the SCF(hFBH1) complex were indistinguishable from those of the hFBH1 protein alone. Moreover, the ubiquitin ligase activity of the SCF(hFBH1) complex was hardly affected by single-stranded or double-stranded DNA. The multiple activities present in this complex act independently of each other, suggesting that the SCF(hFBH1) complex can catalyze a ubiquitination reaction while acting as a DNA helicase or translocating along DNA. The potential roles of the SCF(hFBH1) complex in DNA metabolism based upon the enzymatic activities associated with this complex are discussed.
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PMID:SCFhFBH1 can act as helicase and E3 ubiquitin ligase. 1511 74

The breast and ovarian tumor suppressor BRCA1 forms a heterodimeric RING-type ubiquitin ligase with BARD1 to catalyze untraditional Lys-6-linked polyubiquitin chains. It is not clear how the BRCA1-BARD1 ligase regulates various cellular processes such as DNA repair, cell-cycle progression, transcriptional regulation, and centrosome duplication. Here we report that BRCA1-BARD1 catalyzes the polyubiquitination of nucleolar phosphoprotein nucleophosmin/B23 (NPM). Two different mass spectrometry screens for protein ubiquitinated by BRCA1-BARD1 both identified NPM. NPM interacts with N-terminal fragments of BRCA1 and BARD1 in a manner dependent upon BRCA1-BARD1 heterodimer formation. NPM colocalizes with BRCA1 and BARD1 in mitotic cells suggesting the possibility of NPM regulation by BRCA1-BARD1 during mitosis. BRCA1-BARD1 catalyzes the ubiquitination of NPM in vitro and in vivo, and BRCA1-BARD1 co-expression in cells causes NPM stabilization rather than degradation. This is consistent with the notion that this ligase catalyzes untraditional polyubiquitin chains. Given the many overlapped functions between NPM and BRCA1, we propose that NPM is a strong candidate as a substrate of the BRCA1-BARD1 ubiquitin ligase.
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PMID:Nucleophosmin/B23 is a candidate substrate for the BRCA1-BARD1 ubiquitin ligase. 1518 79

Ubiquitin-dependent proteolysis makes a major contribution to decreasing the levels of p27. Ubiquitin-dependent proteolysis of p27(kip1) is growth and cell cycle regulated in two ways: first, skp2, a component of the E3-ubiquitin ligase, is growth regulated, and second, a kinase must phosphorylate the threonine-187 position on p27 so that it can be recognized by skp2. In vitro, p27 is phosphorylated by cyclin E- and cyclin A-associated cdk2 as well as by cyclin B1-cdk1. Having analyzed the effect of different cyclin-cyclin-dependent kinase complexes on ubiquitination of p27 in a reconstitution assay system, we now report a noncatalytic requirement for cyclin A-cdk2. Multiparameter flow cytometric analysis also indicates that p27 turnover correlates best with the onset of S phase, once the levels of cyclin A become nearly maximal. Finally, increasing the amount of both cyclin E-cdk2 and skp2 was less efficient at promoting p27 ubiquitination than was increasing the amount of cyclin A-cdk2 alone in extracts prepared from cultures of >93%-purified G(1) cells. Together these lines of evidence suggest that cyclin A-cdk2 plays an ancillary noncatalytic role in the ubiquitination of p27 by the SCF(skp2) complex.
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PMID:Noncatalytic requirement for cyclin A-cdk2 in p27 turnover. 1519 59

The activation of NF-kappaB and IKK requires an upstream kinase complex consisting of TAK1 and adaptor proteins such as TAB1, TAB2, or TAB3. TAK1 is in turn activated by TRAF6, a RING domain ubiquitin ligase that facilitates the synthesis of lysine 63-linked polyubiquitin chains. Here we present evidence that TAB2 and TAB3 are receptors that bind preferentially to lysine 63-linked polyubiquitin chains through a highly conserved zinc finger (ZnF) domain. Mutations of the ZnF domain abolish the ability of TAB2 and TAB3 to bind polyubiquitin chains, as well as their ability to activate TAK1 and IKK. Significantly, replacement of the ZnF domain with a heterologous ubiquitin binding domain restored the ability of TAB2 and TAB3 to activate TAK1 and IKK. We also show that TAB2 binds to polyubiquitinated RIP following TNFalpha stimulation. These results indicate that polyubiquitin binding domains represent a new class of signaling domains that regulate protein kinase activity through a nonproteolytic mechanism.
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PMID:TAB2 and TAB3 activate the NF-kappaB pathway through binding to polyubiquitin chains. 1532 70


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