Component Of Ubiquitin Ligase Research Articles

Phosphorylation of ETS1 by Src Family Kinases Prevents Its Recognition by the COP1 Tumor Suppressor

Oncoproteins and tumor suppressors antagonistically converge on critical nodes governing neoplastic growth, invasion, and metastasis. We discovered that phosphorylation of the ETS1 and ETS2 transcriptional oncoproteins at specific serine or threonine residues creates binding sites for the COP1 tumor suppressor protein, which is an ubiquitin ligase component, leading to their destruction. In the case of ETS1, however, phosphorylation of a neighboring tyrosine residue by Src family kinases disrupts COP1 binding, thereby stabilizing ETS1. Src-dependent accumulation of ETS1 in breast cancer cells promotes anchorage-independent growth invitro and tumor growth invivo. These findings expand the list of potential COP1 substrates to include proteins whose COP1-binding sites are subject to regulatory phosphorylation and provide insights into transformation by Src family kinases.

Time-of-day- and light-dependent expression of ubiquitin protein ligase E3 component N-recognin 4 (UBR4) in the suprachiasmatic nucleus circadian clock.

Circadian rhythms of behavior and physiology are driven by the biological clock that operates endogenously but can also be entrained to the light-dark cycle of the environment. In mammals, the master circadian pacemaker is located in the suprachiasmatic nucleus (SCN), which is composed of individual cellular oscillators that are driven by a set of core clock genes interacting in transcriptional/translational feedback loops. Light signals can trigger molecular events in the SCN that ultimately impact on the phase of expression of core clock genes to reset the master pacemaker. While transcriptional regulation has received much attention in the field of circadian biology in the past, other mechanisms including targeted protein degradation likely contribute to the clock timing and entrainment process. In the present study, proteome-wide screens of the murine SCN led to the identification of ubiquitin protein ligase E3 component N-recognin 4 (UBR4), a novel E3 ubiquitin ligase component of the N-end rule pathway, as a time-of-day-dependent and light-inducible protein. The spatial and temporal expression pattern of UBR4 in the SCN was subsequently characterized by immunofluorescence microscopy. UBR4 is expressed across the entire rostrocaudal extent of the SCN in a time-of-day-dependent fashion. UBR4 is localized exclusively to arginine vasopressin (AVP)-expressing neurons of the SCN shell. Upon photic stimulation in the early subjective night, the number of UBR4-expressing cells within the SCN increases. This study is the first to identify a novel E3 ubiquitin ligase component, UBR4, in the murine SCN and to implicate the N-end rule degradation pathway as a potential player in regulating core clock mechanisms and photic entrainment.

Novel Cul3 binding proteins function to remodel E3 ligase complexes.

BackgroundCullins belong to a family of scaffold proteins that assemble multi-subunit ubiquitin ligase complexes to recruit protein substrates for ubiquitination via unique sets of substrate adaptor, such as Skp1 or Elongin B, and a substrate-binding protein with a conserved protein-protein interacting domain, such as leucine-rich repeats (LRR), a WD40 domain, or a zinc-finger domain. In the case of the Cullin3 (Cul3), it forms a BTB-Cul3-Rbx1 (BCR) ubiquitin ligase complex where it is believed that a BTB domain-containing protein performs dual functions where it serves as both the substrate adaptor and the substrate recognition protein.ResultsTandem affinity purification and LC/MS-MS analysis of the BCR complex led to the identification of 10,225 peptides. After the SEQUEST algorithm and CDART program were used for protein identification and domain prediction, we discovered a group of Cul3-bound proteins that contain either the LRR or WD40 domain (CLWs). Further biochemical analysis revealed that the LRR domain-containing CLWs could bind both Cul3 and BTB domain-containing proteins. The dual binding role for the LRR domain-containing CLWs results in causing the BTB-domain protein to become a substrate instead of an adaptor.To further distinguish potential substrates from other components that are part of the BCR ubiquitin ligase complex, we altered the parameters in the SEQUEST algorithm to select for peptide fragments with a modified lysine residue. This method not only identifies the potential substrates of the BCR ubiquitin ligase complex, but it also pinpoints the lysine residue in which the post-translational modification occurs. Interestingly, none of the CLWs were identified by this method, supporting our hypothesis that CLWs were not potential substrates but rather additional components of the BCR ubiquitin ligase complex.ConclusionOur study identified a new set of Cul3-binding proteins known as CLWs via tandem affinity purification and LC/MS-MS analysis. Subsequently, our biochemical analysis revealed that some CLWs modify binding of BTB domain-containing proteins to the complex, causing degradation of the BTB domain-containing protein. As these CLWs were excluded from our list of substrates, we propose that CLWs serve as unique Cul3 binding proteins that provide an alternative regulatory mechanism for the complex.

Ubiquitin-mediated response to microsporidia and virus infection in C. elegans.

Microsporidia comprise a phylum of over 1400 species of obligate intracellular pathogens that can infect almost all animals, but little is known about the host response to these parasites. Here we use the whole-animal host C. elegans to show an in vivo role for ubiquitin-mediated response to the microsporidian species Nematocida parisii, as well to the Orsay virus, another natural intracellular pathogen of C. elegans. We analyze gene expression of C. elegans in response to N. parisii, and find that it is similar to response to viral infection. Notably, we find an upregulation of SCF ubiquitin ligase components, such as the cullin ortholog cul-6, which we show is important for ubiquitin targeting of N. parisii cells in the intestine. We show that ubiquitylation components, the proteasome, and the autophagy pathway are all important for defense against N. parisii infection. We also find that SCF ligase components like cul-6 promote defense against viral infection, where they have a more robust role than against N. parisii infection. This difference may be due to suppression of the host ubiquitylation system by N. parisii: when N. parisii is crippled by anti-microsporidia drugs, the host can more effectively target pathogen cells for ubiquitylation. Intriguingly, inhibition of the ubiquitin-proteasome system (UPS) increases expression of infection-upregulated SCF ligase components, indicating that a trigger for transcriptional response to intracellular infection by N. parisii and virus may be perturbation of the UPS. Altogether, our results demonstrate an in vivo role for ubiquitin-mediated defense against microsporidian and viral infections in C. elegans.

Development of a multipurpose GATEWAY-based lentiviral tetracycline-regulated conditional RNAi system (GLTR).

RNA interference (RNAi) has become an essential technology for functional gene analysis. Its success, however, depends on the effective expression of RNAi-inducing small double-stranded interfering RNA molecules (siRNAs) in target cells. In many cell types, RNAi can be achieved by transfection of chemically synthesised siRNAs, which results in transient knockdown of protein expression. Expression of double-stranded short hairpin RNA (shRNA) provides another means to induce RNAi in cells that are hard to transfect. To facilitate the generation of stable, conditional RNAi cell lines, we have developed novel one- and two-component vector GATEWAY-compatible lentiviral tetracycline-regulated RNAi (GLTR) systems. The combination of a modified RNA-polymerase-III-dependent H1 RNA promoter (designated ‘THT’) for conditional shRNA expression with different lentiviral delivery vectors allows (1) the use of fluorescent proteins for colour-coded combinatorial RNAi or for monitoring RNAi induction (pGLTR-FP), (2) selection of transduced cells (pGLTR-S), and (3) the generation of conditional cell lines using a one vector system (pGLTR-X). All three systems were found to be suitable for the analysis of essential genes, such as CDC27, a component of the mitotic ubiquitin ligase APC/C, in cell lines and primary human cells.

Regulation of plant immunity through ubiquitin‐mediated modulation of Ca2+–calmodulin–AtSR1/CAMTA3 signaling

Transient changes in intracellular Ca(2+) concentration are essential signals for activation of plant immunity. It has also been reported that Ca(2+) signals suppress salicylic acid-mediated plant defense through AtSR1/CAMTA3, a member of the Ca(2+) /calmodulin-regulated transcription factor family that is conserved in multicellular eukaryotes. How plants overcome this negative regulation to mount an effective defense response during a stage of intracellular Ca(2+) surge is unclear. Here we report the identification and functional characterization of an important component of ubiquitin ligase, and the associated AtSR1 turnover. The AtSR1 interaction protein1 (SR1IP1) was identified by CytoTrap two-hybrid screening. The loss-of-function mutant of SR1IP1 is more susceptible to bacterial pathogens, and over-expression of SR1IP1 confers enhanced resistance, indicating that SR1IP1 acts as a positive regulator of plant defense. SR1IP1 and AtSR1 act in the same signaling pathway to regulate plant immunity. SR1IP1 contains the structural features of a substrate adaptor in cullin3-based E3 ubiquitin ligase, and was shown to serve as a substrate adaptor that recruits AtSR1 for ubiquitination and degradation when plants are challenged with pathogens. Hence, SR1IP1 positively regulates plant immunity by removing the defense suppressor AtSR1. These findings provide a mechanistic insight into how Ca(2+) -mediated actions are coordinated to achieve effective plant immunity.

SPOP Promotes Tumorigenesis by Acting as a Key Regulatory Hub in Kidney Cancer

Hypoxic stress and hypoxia-inducible factors (HIFs) play important roles in a wide range of tumors. We demonstrate that SPOP, which encodes an E3 ubiquitin ligase component, is a direct transcriptional target of HIFs in clear cell renal cell carcinoma (ccRCC). Furthermore, hypoxia results in cytoplasmic accumulation of SPOP, which is sufficient to induce tumorigenesis. This tumorigenic activity occurs through the ubiquitination and degradation of multiple regulators of cellular proliferation and apoptosis, including the tumor suppressor PTEN, ERK phosphatases, the proapoptotic molecule Daxx, and the Hedgehog pathway transcription factor Gli2. Knockdown of SPOP specifically kills ccRCC cells, indicating that it may be a promising therapeutic target. Collectively, our results indicate that SPOP serves as a regulatory hub to promote ccRCC tumorigenesis.

Truncation of the E3 ubiquitin ligase component FBXO31 causes non-syndromic autosomal recessive intellectual disability in a Pakistani family

In this study, we have performed autozygosity mapping on a large consanguineous Pakistani family segregating with intellectual disability. We identified two large regions of homozygosity-by-descent (HBD) on 16q12.2-q21 and 16q24.1-q24.3. Whole exome sequencing (WES) was performed on an affected individual from the family, but initially, no obvious mutation was detected. However, three genes within the HBD regions that were not fully captured during the WES were Sanger sequenced and we identified a five base pair deletion (actually six base pairs deleted plus one base pair inserted) in exon 7 of the gene FBXO31. The variant segregated completely in the family, in recessive fashion giving a LOD score of 3.95. This variant leads to a frameshift and a premature stop codon and truncation of the FBXO31 protein, p.(Cys283Asnfs*81). Quantification of mRNA and protein expression suggests that nonsense-mediated mRNA decay also contributes to the loss of FBXO31 protein in affected individuals. FBXO31 functions as a centrosomal E3 ubiquitin ligase, in association with SKP1 and Cullin-1, involved in ubiquitination of proteins targeted for degradation. The FBXO31/SKP1/Cullin1 complex is important for neuronal morphogenesis and axonal identity. FBXO31 also plays a role in dendrite growth and neuronal migration in developing cerebellar cortex. Our finding adds further evidence of the involvement of disruption of the protein ubiquitination pathway in intellectual disability.

Quantitative proteomics identifies the membrane-associated peroxidase GPx8 as a cellular substrate of the hepatitis C virus NS3-4A protease

The hepatitis C virus (HCV) NS3-4A protease is not only an essential component of the viral replication complex and a prime target for antiviral intervention but also a key player in the persistence and pathogenesis of HCV. It cleaves and thereby inactivates two crucial adaptor proteins in viral RNA sensing and innate immunity, mitochondrial antiviral signaling protein (MAVS) and TRIF, a phosphatase involved in growth factor signaling, T-cell protein tyrosine phosphatase (TC-PTP), and the E3 ubiquitin ligase component UV-damaged DNA-binding protein 1 (DDB1). Here we explored quantitative proteomics to identify novel cellular substrates of the NS3-4A protease. Cell lines inducibly expressing the NS3-4A protease were analyzed by stable isotopic labeling using amino acids in cell culture (SILAC) coupled with protein separation and mass spectrometry. This approach identified the membrane-associated peroxidase GPx8 as a bona fide cellular substrate of the HCV NS3-4A protease. Cleavage by NS3-4A occurs at Cys 11, removing the cytosolic tip of GPx8, and was observed in different experimental systems as well as in liver biopsies from patients with chronic HCV. Overexpression and RNA silencing studies revealed that GPx8 is involved in viral particle production but not in HCV entry or RNA replication. We provide proof-of-concept for the use of quantitative proteomics to identify cellular substrates of a viral protease and describe GPx8 as a novel proviral host factor targeted by the HCV NS3-4A protease.

Prognostic impact of RING box protein-1 (RBX1) expression in gastric cancer

RING box protein-1 (RBX1) is an essential component of the E3 ubiquitin ligase Skp1/Cullin/RBX1/F-box protein complex. Although an altered expression of RBX1 has been reported in several human cancers, the role of RBX1 in gastric cancer remains unknown. We investigated the RBX1 expression in primary gastric cancer tissues from 145 patients by immunohistochemistry, and explored its clinical relevance and prognostic value. Furthermore, the effect of RBX1 expression on cancer cell proliferation was analyzed in vitro using a siRNA silencing technique. The RBX1 expression was abundant in gastric cancer tissues. There was a significant difference in the expression level of RBX1 in terms of the tumor depth (P = 0.008), presence of distant metastasis (P = 0.016) and venous invasion (P = 0.005). The postoperative overall (P < 0.001) and relapse-free survival (P < 0.001) rates were significantly poorer in patients with RBX1-high tumors than in patients with RBX1-low tumors. There was a significant correlation of the RBX1 status with postoperative hematogenous recurrence (P = 0.013). Importantly, the RBX1 status was identified as an independent prognostic factor for gastric cancer (P = 0.002). Furthermore, RBX1 gene silencing significantly inhibited the proliferation of gastric cancer cells in vitro. The RBX1 expression has a significant prognostic value in gastric cancer. RBX1 might play an important role in regulating the proliferation of gastric cancer cells and promoting the development of postoperative recurrence. Our data provide a rationale for developing a novel therapy targeting RBX1 for gastric cancer.

ATM regulates Cdt1 stability during the unperturbed S phase to prevent re-replication

Ataxia-telangiectasia mutated (ATM) plays crucial roles in DNA damage responses, especially with regard to DNA double-strand breaks (DSBs). However, it appears that ATM can be activated not only by DSB, but also by some changes in chromatin architecture, suggesting potential ATM function in cell cycle control. Here, we found that ATM is involved in timely degradation of Cdt1, a critical replication licensing factor, during the unperturbed S phase. At least in certain cell types, degradation of p27Kip1 was also impaired by ATM inhibition. The novel ATM function for Cdt1 regulation was dependent on its kinase activity and NBS1. Indeed, we found that ATM is moderately phosphorylated at Ser1981 during the S phase. ATM silencing induced partial reduction in levels of Skp2, a component of SCFSkp2 ubiquitin ligase that controls Cdt1 degradation. Furthermore, Skp2 silencing resulted in Cdt1 stabilization like ATM inhibition. In addition, as reported previously, ATM silencing partially prevented Akt phosphorylation at Ser473, indicative of its activation, and Akt inhibition led to modest stabilization of Cdt1. Therefore, the ATM-Akt-SCFSkp2 pathway may partly contribute to the novel ATM function. Finally, ATM inhibition rendered cells hypersensitive to induction of re-replication, indicating importance for maintenance of genome stability.

Flavanoids induce expression of the suppressor of cytokine signalling 3 (SOCS3) gene and suppress IL-6-activated signal transducer and activator of transcription 3 (STAT3) activation in vascular endothelial cells

The atherogenic cytokine IL-6 (interleukin-6) induces pro-inflammatory gene expression in VECs (vascular endothelial cells) by activating the JAK (Janus kinase)/STAT3 (signal transducer and activator of transcription 3) signalling pathway, which is normally down-regulated by the STAT3-dependent induction of the E3 ubiquitin ligase component SOCS3 (suppressor of cytokine signalling 3). Novel treatments based on the regulation of SOCS3 protein levels could therefore have value in the treatment of diseases with an inflammatory component, such as atherosclerosis. To this end we carried out a screen of 1031 existing medicinal compounds to identify inducers of SOCS3 gene expression and identified the flavanoids naringenin and flavone as effective inducers of SOCS3 protein, mRNA and promoter activity. This was in contrast with the action of traditional JAK/STAT3 inhibitors and the polyphenol resveratrol, which effectively suppress SOCS3 gene expression. Both naringenin and flavone also effectively suppressed IL-6-stimulated phosphorylation of STAT3 (Tyr705) which led to suppression of IL-6-induction of the atherogenic STAT3 target gene MCP1 (monocyte chemotactic protein-1), suggesting that their ability to induce SOCS3 gene expression is STAT3-independent. Supporting this idea was the observation that the general kinase inhibitor compound C inhibits flavone- and cAMP-dependent, but not JAK-dependent, SOCS3 induction in VECs. Indeed, the ability of flavanoids to induce SOCS3 expression requires activation of the ERK (extracellular-signal-regulated kinase)-dependent transcription factor SP3, and not STAT3. In the present paper we therefore describe novel molecular actions of flavanoids, which control SOCS3 gene induction and suppression of STAT3 signalling in VECs. These mechanisms could potentially be exploited to develop novel anti-atherogenic therapies.

Fbw7 Repression by Hes5 Creates a Feedback Loop That Modulates Notch-Mediated Intestinal and Neural Stem Cell Fate Decisions

Author SummaryThe Notch signalling pathway is a highly conserved system that controls cell differentiation decisions in a wide range of animal species and cell types, and at different steps during cell lineage progression. An important function of the Notch pathway is in lateral inhibition—an interaction between equal adjacent cells that drives them towards different final states. The basic principle of lateral inhibition is that activation of the Notch cell surface receptor represses production of the Notch ligand (also borne on the cell surface). Consequently, cells expressing less Notch produce more Notch ligand that can activate the Notch pathway in neighboring cells and thereby amplify the differences between these cells. However, the additional regulatory circuits required to fine-tune this delicate process have so far remained elusive. Here we describe the identification of a novel intracellular positive feedback loop that connects Fbw7 (the ubiquitin ligase responsible for targeting Notch for degradation) and Notch itself. We show that Fbw7 reduces the stability of Notch intracellular domain (NICD) protein, as previously established, but also that the fbw7 gene is itself transcriptionally downregulated by the Notch effector Hes5. Thus we conclude that increased Notch activity causes NICD stabilisation. Further, we demonstrate that perturbation of this regulatory loop is responsible for the Fbw7 haploinsufficiency observed for Notch-dependent functions in intestine and brain stem cells.

14-3-3 Regulates 1-Aminocyclopropane-1-Carboxylate Synthase Protein Turnover in Arabidopsis

14-3-3 proteins are a family of conserved phospho-specific binding proteins involved in diverse physiological processes. Plants have large 14-3-3 gene families, and many binding partners have been identified, though relatively few functions have been defined. Here, we demonstrate that 14-3-3 proteins interact with multiple 1-aminocyclopropane-1-carboxylate synthase (ACS) isoforms in Arabidopsis thaliana. ACS catalyzes the generally rate-limiting step in the biosynthesis of the phytohormone ethylene. This interaction increases the stability of the ACS proteins. 14-3-3s also interact with the ETHYLENE-OVERPRODUCER1 (ETO1)/ETO1-LIKE (EOLs), a group of three functionally redundant proteins that are components of a CULLIN-3 E3 ubiquitin ligase that target a subset of the ACS proteins for rapid degradation by the 26S proteasome. In contrast with ACS, the interaction with 14-3-3 destabilizes the ETO1/EOLs. The level of the ETO1/EOLs in vivo plays a role in mediating ACS protein turnover, with increased levels leading to a decrease in ACS protein levels. These studies demonstrate that regulation of ethylene biosynthesis occurs by a mechanism in which 14-3-3 proteins act through a direct interaction and stabilization of ACS and through decreasing the abundance of the ubiquitin ligases that target a subset of ACS proteins for degradation.

Differences in degradation lead to asynchronous expression of cyclin E1 and cyclin E2 in cancer cells

Cyclin E1 is expressed at the G₁/S phase transition of the cell cycle to drive the initiation of DNA replication and is degraded during S/G₂M. Deregulation of its periodic degradation is observed in cancer and is associated with increased proliferation and genomic instability. We identify that in cancer cells, unlike normal cells, the closely related protein cyclin E2 is expressed predominantly in S phase, concurrent with DNA replication. This occurs at least in part because the ubiquitin ligase component that is responsible for cyclin E1 downregulation in S phase, Fbw7, fails to effectively target cyclin E2 for proteosomal degradation. The distinct cell cycle expression of the two E-type cyclins in cancer cells has implications for their roles in genomic instability and proliferation and may explain their associations with different signatures of disease.

Germline-specific MATH-BTB substrate adaptor MAB1 regulates spindle length and nuclei identity in maize.

Germline and early embryo development constitute ideal model systems to study the establishment of polarity, cell identity, and asymmetric cell divisions (ACDs) in plants. We describe here the function of the MATH-BTB domain protein MAB1 that is exclusively expressed in the germ lineages and the zygote of maize (Zea mays). mab1 (RNA interference [RNAi]) mutant plants display chromosome segregation defects and short spindles during meiosis that cause insufficient separation and migration of nuclei. After the meiosis-to-mitosis transition, two attached nuclei of similar identity are formed in mab1 (RNAi) mutants leading to an arrest of further germline development. Transient expression studies of MAB1 in tobacco (Nicotiana tabacum) Bright Yellow-2 cells revealed a cell cycle-dependent nuclear localization pattern but no direct colocalization with the spindle apparatus. MAB1 is able to form homodimers and interacts with the E3 ubiquitin ligase component Cullin 3a (CUL3a) in the cytoplasm, likely as a substrate-specific adapter protein. The microtubule-severing subunit p60 of katanin was identified as a candidate substrate for MAB1, suggesting that MAB1 resembles the animal key ACD regulator Maternal Effect Lethal 26 (MEL-26). In summary, our findings provide further evidence for the importance of posttranslational regulation for asymmetric divisions and germline progression in plants and identified an unstable key protein that seems to be involved in regulating the stability of a spindle apparatus regulator(s).

The Hect Domain E3 Ligase Tom1 and the F-box Protein Dia2 Control Cdc6 Degradation in G1 Phase

The accurate replication of genetic information is critical to maintaining chromosomal integrity. Cdc6 functions in the assembly of pre-replicative complexes and is specifically required to load the Mcm2-7 replicative helicase complex at replication origins. Cdc6 is targeted for protein degradation by multiple mechanisms in Saccharomyces cerevisiae, although only a single pathway and E3 ubiquitin ligase for Cdc6 has been identified, the SCF(Cdc4) (Skp1/Cdc53/F-box protein) complex. Notably, Cdc6 is unstable during the G(1) phase of the cell cycle, but the ubiquitination pathway has not been previously identified. Using a genetic approach, we identified two additional E3 ubiquitin ligase components required for Cdc6 degradation, the F-box protein Dia2 and the Hect domain E3 Tom1. Both Dia2 and Tom1 control Cdc6 turnover during G(1) phase of the cell cycle and act separately from SCF(Cdc4). Ubiquitination of Cdc6 is significantly reduced in dia2Δ and tom1Δ cells. Tom1 and Dia2 each independently immunoprecipitate Cdc6, binding to a C-terminal region of the protein. Tom1 and Dia2 cannot compensate for each other in Cdc6 degradation. Cdc6 and Mcm4 chromatin association is aberrant in tom1Δ and dia2Δ cells in G(1) phase. Together, these results present evidence for a novel degradation pathway that controls Cdc6 turnover in G(1) that may regulate pre-replicative complex assembly.

SEL-10/Fbw7-dependent negative feedback regulation of LIN-45/Braf signaling in C. elegans via a conserved phosphodegron

The conserved E3 ubiquitin ligase component named SEL-10 in Caenorhabditis elegans and Fbw7 in mammals targets substrates for ubiquitin-mediated degradation through a high-affinity binding site called a Cdc4 phosphodegron (CPD). As many known substrates of Fbw7 are oncoproteins, the identification of new substrates may offer insight into cancer biology as well as aspects of proteome regulation. Here, we evaluated whether the presence of an evolutionarily conserved CPD would be a feasible complement to proteomics-based approaches for identifying new potential substrates. For functional assessments, we focused on LIN-45, a component of the signal transduction pathway underlying vulval induction and the ortholog of human Braf, an effector of Ras in numerous cancers. Our analysis demonstrates that LIN-45 behaves as a bona fide substrate of SEL-10, with mutation of the CPD or loss of sel-10 resulting in increased activity and protein stability in vivo. Furthermore, during vulval induction, the downstream kinase MPK-1/ERK is also required for LIN-45 protein degradation in a negative feedback loop, resulting in degradation of LIN-45 where ERK is highly active. As the CPD consensus sequence is conserved in human Braf, we propose that Fbw7 may also regulate Braf stability in some cell contexts. We discuss the implications of our findings for vulval development in C. elegans, the potential applicability to human Braf, and the value of a CPD-based predictive approach for human Fbw7 substrates.

FBXW7 is involved in Aurora B degradation

FBXW7, a component of E3 ubiquitin ligase, plays an important role in mitotic checkpoint, but its role remains unclear. Aurora B is a mitotic checkpoint kinase that plays a pivotal role in mitosis by ensuring correct chromosome segregation and normal progression through mitosis. Whether Aurora B and FBXW7 are coordinately regulated during mitosis is not known. Here, we show that FBXW7 is a negative regulator for Aurora B. Ectopic expression of FBXW7 can suppress the expression of Aurora B. Accordingly, FBXW7 deficiency leads to Aurora B elevation. Mechanistic studies show that all FBXW7 isoforms are negative regulators of Aurora B expression through ubiquitination-mediated protein degradation. Aurora B interacts with R465 and R505 residues of WD 40 domain of FBXW7. Significantly, inverse correlation between FBXW7 and Aurora B elevation is translated into the deregulation of mitosis. FBWX7 expression mitigates Aurora B-mediated cell growth and mitotic deregulation. In addition, FBXW7 reduces the percentage of multinucleated cells caused by Aurora B overexpression. These data suggest that FBXW7 is an important negative regulator of Aurora B, and that the loss or mutation of FBXW7 as seen in many types of cancer could lead to an abnormal elevation of Aurora B and result in deregulated mitosis, which accelerates cancer cell growth.

Cullin-3 protein expression levels correlate with breast cancer progression

Cullin-3 is a component of the Cullin-Ring ubiquitin ligase (CRL) family that plays an important role in mediating protein degradation. Deregulation of Cullin-3 expression has been observed in human cancers; however, a role for Cullin-3 in tumor progression has not been previously recognized. Using the MCF10DCIS.com human breast cancer xenograft model, we show that Cullin-3 is increasingly expressed during progression from comedo ductal carcinoma in situ (DCIS) to invasive carcinomas. Cullin-3 protein is not detected in early lesions but is noticeably increased in DCIS tumors and significantly overexpressed in invasive cancers. In experimental metastasis assays, high expression of Cullin-3 was observed in the lung site. Importantly, Cullin-3 staining is detected in human breast cancer tissues, not in normal breast tissues and its expression level positively correlates with tumor stage. These data suggest that Cullin-3 may play an important role in tumor progression from DCIS to invasive cancer and may serve as a biomarker for the diagnosis of aggressive breast cancer.