Cullin-RING E3 Ubiquitin Ligases Research Articles

The NEDD8 E3 ligase DCNL5 is phosphorylated by IKK alpha during Toll-like receptor activation.

The activity of Cullin-RING ubiquitin E3 ligases (CRL) is regulated by NEDD8 modification. DCN-like proteins promote Cullin neddylation as scaffold-like E3s. One DCNL, DCNL5, is highly expressed in immune tissue. Here, we provide evidence that DCNL5 may be involved in innate immunity, as it is a direct substrate of the kinase IKKα during immune signalling. We find that upon activation of Toll-like receptors, DCNL5 gets rapidly and transiently phosphorylated on a specific N-terminal serine residue (S41). This phosphorylation event is specifically mediated by IKKα and not IKKβ. Our data for the first time provides evidence that DCNL proteins are post-translationally modified in an inducible manner. Our findings also provide the first example of a DCNL member as a kinase substrate in a signalling pathway, indicating that the activity of at least some DCNLs may be regulated.

Deletion of DDB1- and CUL4- associated factor-17 (Dcaf17) gene causes spermatogenesis defects and male infertility in mice

DDB1– and CUL4–associated factor 17 (Dcaf17) is a member of DCAF family genes that encode substrate receptor proteins for Cullin-RING E3 ubiquitin ligases, which play critical roles in many cellular processes. To unravel the function of DCAF17, we performed expression profiling of Dcaf17 in different tissues of wild type mouse by qRT-PCR and generated Dcaf17 knockout mice by gene targeting. Expression profiling of Dcaf17 showed highest expression in testis. Analyses of Dcaf17 transcripts during post-natal development of testis at different ages displayed gradual increase in Dcaf17 mRNA levels with the age. Although Dcaf17 disruption did not have any effect on female fertility, Dcaf17 deletion led to male infertility due to abnormal sperm development. The Dcaf17−/− mice produced low number of sperm with abnormal shape and significantly low motility. Histological examination of the Dcaf17−/− testis revealed impaired spermatogenesis with presence of vacuoles and sloughed cells in the seminiferous tubules. Disruption of Dcaf17 caused asymmetric acrosome capping, impaired nuclear compaction and abnormal round spermatid to elongated spermatid transition. For the first time, these data indicate that DCAF17 is essential for spermiogenesis.

The Eukaryotic Proteome Is Shaped by E3 Ubiquitin Ligases Targeting C-Terminal Degrons

Degrons are minimal elements that mediate the interaction of proteins with degradation machineries to promote proteolysis. Despite their central role in proteostasis, the number of known degrons remains small, and a facile technology to characterize them is lacking. Using a strategy combining global protein stability (GPS) profiling with a synthetic human peptidome, we identify thousands of peptides containing degron activity. Employing CRISPR screening, we establish that the stability of many proteins is regulated through degrons located at their C terminus. We characterize eight Cullin-RING E3 ubiquitin ligase (CRL) complex adaptors that regulate C-terminal degrons, including six CRL2 and two CRL4 complexes, and computationally implicate multiple non-CRLs in end recognition. Proteome analysis revealed that the C termini of eukaryotic proteins are depleted for C-terminal degrons, suggesting an E3-ligase-dependent modulation of proteome composition. Thus, we propose that a series of "C-end rules" operate to govern protein stability and shape the eukaryotic proteome.

Role of CRBN in molecular and cellular mechanism of AMPK inhibition.

Cereblon (CRBN), initially identified as a target protein of mild mental retardation in humans, is a substrate receptor of cullin‐ring E3 ubiquitin ligase (CRL) and a primary target of thalidomide‐induced teratogenicity. An earlier study showed that CRBN negatively regulate the functional activity of AMP‐activated protein kinase (AMPK) by binding to the alpha subunit of the AMPK complex, and the ectopic expression of CRBN reduced the content of gamma subunit in AMPK complex. However, the molecular mechanism for the CRBN‐dependent reduction of gamma subunit from AMPK complex was not demonstrated. Thus, we investigated the molecular mechanism of CRBN‐dependent inhibition of AMPK. We noticed that the amount of gamma subunit in AMPK complex was increased in Crbn‐knockout (KO) MEFs and the gamma subunit was degraded more rapidly in wild‐type MEFs than in Crbn‐KO MEFs. After treating MG132, a proteasome inhibitor, the gamma subunit is accumulated more rapidly in wild‐type MEFs compared to Crbn‐KO MEFs. Moreover, exogenous expression of CRBN promoted the degradation of gamma subunit in Crbn‐knockout MEFs only in the presence of alpha subunit. These results strongly suggest that CRBN negatively regulates the AMPK activity by recruiting AMPK complex to CRL and degrading its gamma subunit.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Chromatin-Bound Cullin-Ring Ligases: Regulatory Roles in DNA Replication and Potential Targeting for Cancer Therapy.

Cullin-RING (Really Interesting New Gene) E3 ubiquitin ligases (CRLs), the largest family of E3 ubiquitin ligases, are functional multi-subunit complexes including substrate receptors, adaptors, cullin scaffolds, and RING-box proteins. CRLs are responsible for ubiquitination of ~20% of cellular proteins and are involved in diverse biological processes including cell cycle progression, genome stability, and oncogenesis. Not surprisingly, cullins are deregulated in many diseases and instances of cancer. Recent studies have highlighted the importance of CRL-mediated ubiquitination in the regulation of DNA replication/repair, including specific roles in chromatin assembly and disassembly of the replication machinery. The development of novel therapeutics targeting the CRLs that regulate the replication machinery and chromatin in cancer is now an attractive therapeutic strategy. In this review, we summarize the structure and assembly of CRLs and outline their cellular functions and their diverse roles in cancer, emphasizing the regulatory functions of nuclear CRLs in modulating the DNA replication machinery. Finally, we discuss the current strategies for targeting CRLs against cancer in the clinic.

SIAMESE-RELATED1 Is Regulated Posttranslationally and Participates in Repression of Leaf Growth under Moderate Drought.

The plant cell cycle is tightly regulated by factors that integrate endogenous cues and environmental signals to adapt plant growth to changing conditions. Under drought, cell division in young leaves is blocked by an active mechanism, reducing the evaporative surface and conserving energy resources. The molecular function of cyclin-dependent kinase-inhibitory proteins (CKIs) in regulating the cell cycle has already been well studied, but little is known about their involvement in cell cycle regulation under adverse growth conditions. In this study, we show that the transcript of the CKI gene SIAMESE-RELATED1 (SMR1) is quickly induced under moderate drought in young Arabidopsis (Arabidopsis thaliana) leaves. Functional characterization further revealed that SMR1 inhibits cell division and affects meristem activity, thereby restricting the growth of leaves and roots. Moreover, we demonstrate that SMR1 is a short-lived protein that is degraded by the 26S proteasome after being ubiquitinated by a Cullin-RING E3 ubiquitin ligase. Consequently, overexpression of a more stable variant of the SMR1 protein leads to a much stronger phenotype than overexpression of the native SMR1. Under moderate drought, both the SMR1 transcript and SMR1 protein accumulate. Despite this induction, smr1 mutants do not show overall tolerance to drought stress but do show less growth inhibition of young leaves under drought. Surprisingly, the growth-repressive hormone ethylene promotes SMR1 induction, but the classical drought hormone abscisic acid does not.

A novel mutation in exon 9 of Cullin 3 gene contributes to aberrant splicing in pseudohypoaldosteronism type II.

Pseudohypoaldosteronism type II (PHAII) is a rare renal tubular disease that is inherited in an autosomal dominant manner. Mutations in four genes (WNK1,WNK4,CUL3, and KLHL3) have been identified to be responsible for this disease. Cullin 3 (CUL3) and KLHL3 are subunits of Cullin–RING E3 ubiquitin ligase complexes, and the serine–threonine kinases WNK1 and WNK4 are substrates of this ubiquitin ligase. For CUL3, all mutations associated with PHAII exclusively lead to exon 9 skipping. In this study, we identified a Chinese PHAII kindred caused by a novel synonymous mutation (c.1221A > G p.Glu407Glu) in CUL3, and explored its effects on exon 9 abnormal splicing through an in vitro splicing assay and study of the patients’ RNA. We obtained evidence that this synonymous mutation leads to complete exon 9 skipping, and in silico bioinformatics analysis demonstrated that the CUL3 c.1221A > G mutation might decrease the ratio of exonic splicing enhancers and silencers. This is the first report of PHAII in Chinese patients with a novel CUL3 mutation. Our findings add a novel pathogenic splicing variant to the CUL3 mutational spectrum and provide reference for further research on mechanisms of splicing modulation and development of potential therapeutic reagents for PHAII.

The Neddylation Inhibitor Pevonedistat (MLN4924) Suppresses and Radiosensitizes Head and Neck Squamous Carcinoma Cells and Tumors.

The cullin RING E3 ubiquitin ligase 4 (CRL4) with its substrate receptor CDT2 (CRL4-CDT2) is emerging as a critical regulator of DNA replication through targeting CDT1, SET8, and p21 for ubiquitin-dependent proteolysis. The aberrant increased stability of these proteins in cells with inactivated CRL4-CDT2 results in DNA rereplication, which is deleterious to cells due to the accumulation of replication intermediates and stalled replication forks. Here, we demonstrate that CDT2 is overexpressed in head and neck squamous cell carcinoma (HNSCC), and its depletion by siRNA inhibits the proliferation of human papilloma virus-negative (HPV-ve) HNSCC cells primarily through the induction of rereplication. Treatment of HNSCC with the NEDD8-activating enzyme inhibitor pevonedistat (MLN4924), which inhibits all cullin-based ligases, induces significant rereplication and inhibits HNSCC cell proliferation in culture and HNSCC xenografts in mice. Pevonedistat additionally sensitizes HNSCC cells to ionizing radiation (IR) and enhances IR-induced suppression of xenografts in mice. Induction of rereplication via CDT2 depletion, or via the stabilization or activation of CDT1, also radiosensitizes HNSCC cells. Collectively, these results demonstrate that induction of rereplication represents a novel approach to treating radioresistant HNSCC tumors and suggest that pevonedistat may be considered as an adjuvant for IR-based treatments. Mol Cancer Ther; 17(2); 368-80. ©2017 AACRSee all articles in this MCT Focus section, "Developmental Therapeutics in Radiation Oncology."

Refining Radiation for the Next Century.

![][1] Wilhelm Roentgen's report more than a century ago describing X-ray imaging fostered in the application of ionizing radiation (IR) for cancer therapy. Today, about half of all the patients with cancer are treated with radiotherapy and it remains one of our most effective

Ubiquitinated CD36 sustains insulin-stimulated Akt activation by stabilizing insulin receptor substrate 1 in myotubes

Both the magnitude and duration of insulin signaling are important in executing its cellular functions. Insulin-induced degradation of insulin receptor substrate 1 (IRS1) represents a key negative feedback loop that restricts insulin signaling. Moreover, high concentrations of fatty acids (FAs) and glucose involved in the etiology of obesity-associated insulin resistance also contribute to the regulation of IRS1 degradation. The scavenger receptor CD36 binds many lipid ligands, and its contribution to insulin resistance has been extensively studied, but the exact regulation of insulin sensitivity by CD36 is highly controversial. Herein, we found that CD36 knockdown in C2C12 myotubes accelerated insulin-stimulated Akt activation, but the activated signaling was sustained for a much shorter period of time as compared with WT cells, leading to exacerbated insulin-induced insulin resistance. This was likely due to enhanced insulin-induced IRS1 degradation after CD36 knockdown. Overexpression of WT CD36, but not a ubiquitination-defective CD36 mutant, delayed IRS1 degradation. We also found that CD36 functioned through ubiquitination-dependent binding to IRS1 and inhibiting its interaction with cullin 7, a key component of the multisubunit cullin-RING E3 ubiquitin ligase complex. Moreover, dissociation of the Src family kinase Fyn from CD36 by free FAs or Fyn knockdown/inhibition accelerated insulin-induced IRS1 degradation, likely due to disrupted IRS1 interaction with CD36 and thus enhanced binding to cullin 7. In summary, we identified a CD36-dependent FA-sensing pathway that plays an important role in negative feedback regulation of insulin activation and may open up strategies for preventing or managing type 2 diabetes mellitus.

Characterisation of cellular effects of Burkholderia pseudomallei cycle inhibiting factor (Cif).

ABSTRACTCycle inhibiting factors (Cifs) are type III secretion system effectors produced by some Gram-negative pathogenic bacteria including Burkholderia pseudomallei. Through their deamidase activity, Cifs inhibit the activity of Cullin RING E3 ubiquitin ligases (CRL). CRL inhibition induces the accumulation of cell cycle inhibitors p21 and p27, thereby leading to host cell cycle arrest. However, whether Cif exerts additional effects on host cells that are important in bacterial pathogenesis is currently poorly understood. In this study, we found that Cif exerts a bimodal effect on NF-κB signalling. Cif increases basal NF-κB activity. This effect is dependent on Cif-mediated activation of ERK MAPK. On the other hand, Cif inhibits NF-κB activation by TNFα and Burkholderia thailandensis infection. This inhibitory effect on NF-κB activity is partially mediated by Cif-dependent inhibition of CRLs. We also found that Cif only has a modest effect in stimulating the intracellular replication of the B. pseudomallei surrogate, B. thailandensis. The observed Cif-dependent stimulation of B. thailandensis intracellular replication was not, or was only partially, due to CRL inhibition. Furthermore, the increased B. thailandensis replication induced by Cif was independent of ERK MAPK activation. Our findings suggest that Cif likely exerts additional cellular effects through novel targets.

Targeting of the C-Jun/BCL-XL/P21 Axis Accelerates the Switch from Senescence to Apoptosis Upon ROC1 Knockdown in Gastric Cancer Cells

Regulator of cullins-1 (ROC1) is a pivotal component of cullin-RING E3 ubiquitin ligases, which help to regulate distinct cellular processes by governing the degradation of various substrates. Because the role of ROC1 in gastric cancer is largely uncharacterized, we investigated the relationship between ROC1 expression and the prognosis of gastric cancer patients and explored the biological function of ROC1 and its underlying mechanisms in gastric cancer. Kaplan-Meier and multivariate Cox regression analyses were used to evaluate the correlation between the carcinogenesis of gastric cancer and ROC1. SA-β-gal staining and the senescence-associated secretory phenotype (SASP) were used to assess ROC1 silencing-induced cellular senescence. A xenograft zebrafish model was used to evaluate the effects of BCL-XL and ROC1 co-silencing on tumor formation in vivo. High ROC1 expression was correlated with poor prognosis and a low 5-year survival rate of gastric cancer patients. ROC1 depletion significantly inhibited the growth of gastric cancer cells by sequentially inducing p21-mediated cellular senescence and mitochondrial-dependent apoptosis. Functional studies revealed successive upregulation of c-Jun, BCL-XL, and p21 upon ROC1 knockdown. BCL-XL suppression via RNA interference or a BH3 mimetic (ABT-737 or ABT-263) efficiently enhanced the anti-tumor effects of ROC1 knockdown. Equally as important, BCL-XL silencing strengthened ROC1 knockdown-induced apoptosis by blocking p21-mediated senescence. The c-Jun/BCL-XL/p21 axis promotes senescence to resist ROC1 knockdown-induced apoptosis in gastric cancer cells. Targeted inactivation of BCL-XL could sensitize gastric cancer cells to ROC1 knockdown in clinical practice.

The E3 ubiquitin ligase SCFFBXL14 complex stimulates neuronal differentiation by targeting the Notch signaling factor HES1 for proteolysis

Molecular oscillators are important cellular regulators of, for example, circadian clocks, oscillations of immune regulators, and short-period (ultradian) rhythms during embryonic development. The Notch signaling factor HES1 (hairy and enhancer of split 1) is a well-known repressor of proneural genes, and HES1 ultradian oscillation is essential for keeping cells in an efficiently proliferating progenitor state. HES1 oscillation is driven by both transcriptional self-repression and ubiquitin-dependent proteolysis. However, the E3 ubiquitin ligase targeting HES1 for proteolysis remains unclear. Based on siRNA-mediated gene silencing screening, co-immunoprecipitation, and ubiquitination assays, we discovered that the E3 ubiquitin ligase SCFFBXL14 complex regulates HES1 ubiquitination and proteolysis. siRNA-mediated knockdown of the Cullin-RING E3 ubiquitin ligases RBX1 or CUL1 increased HES1 protein levels, prolonged its half-life, and dampened its oscillation. FBXL14, an F-box protein for SCF ubiquitin ligase, associates with HES1. FBXL14 silencing stabilized HES1, whereas FBXL14 overexpression decreased HES1 protein levels. Of note, the SCFFBXL14 complex promoted the ubiquitination of HES1 in vivo, and a conserved WRPW motif in HES1 was essential for HES1 binding to FBXL14 and for ubiquitin-dependent HES1 degradation. HES1 knockdown promoted neuronal differentiation, but FBXL14 silencing inhibited neuronal differentiation induced by HES1 ablation in mES and F9 cells. Our results suggest that SCFFBXL14 promotes neuronal differentiation by targeting HES1 for ubiquitin-dependent proteolysis and that the C-terminal WRPW motif in HES1 is required for this process.

A potent small-molecule inhibitor of the DCN1-UBC12 interaction that selectively blocks cullin 3 neddylation

The Cullin-RING E3 ubiquitin ligases (CRLs) regulate homeostasis of ~20% of cellular proteins and their activation require neddylation of their cullin subunit. Cullin neddylation is modulated by a scaffolding DCN protein through interactions with both the cullin protein and an E2 enzyme such as UBC12. Here we report the development of DI-591 as a high-affinity, cell-permeable small-molecule inhibitor of the DCN1–UBC12 interaction. DI-591 binds to purified recombinant human DCN1 and DCN2 proteins with Ki values of 10–12 nM, and disrupts the DCN1–UBC12 interaction in cells. Treatment with DI-591 selectively converts cellular cullin 3 into an un-neddylated inactive form with no or minimum effect on other cullin members. Our data firmly establish a previously unrecognized specific role of the DCN1–UBC12 interaction for cellular neddylation of cullin 3. DI-591 is an excellent probe compound to investigate the role of the cullin 3 CRL ligase in biological processes and human diseases.

Characterization and comparative expression analysis of CUL1 genes in rice.

Cullin-RING E3 ubiquitin ligase (CRL) complex is known as the largest family of E3 ligases. The most widely characterized CRL, SCF complex (CRL1), utilizes CUL1 as a scaffold protein to assemble the complex components. To better understand CRL1-mediated cellular processes in rice, three CUL1 genes (OsCUL1s) were isolated in Oryza sativa. Although all OsCUL1 proteins exhibited high levels of amino acid similarities with each other, OsCUL1-3 had a somewhat distinct structure from OsCUL1-1 and OsCUL1-2. Basal expression levels of OsCUL1-3 were much lower than those of OsCUL1-1 and OsCUL1-2 in all selected samples, showing that OsCUL1-1 and OsCUL1-2 play predominant roles relative to OsCUL1-3 in rice. OsCUL1-1 and OsCUL1-2 genes were commonly upregulated in dry seeds and by ABA and salt/drought stresses, implying their involvement in ABA-mediated processes. These genes also showed similar expression patterns in response to various hormones and abiotic stresses, alluding to their functional redundancy. Expression of the OsCUL1-3 gene was also induced in dry seeds and by ABA-related salt and drought stresses, implying their participation in ABA responses. However, its expression pattern in response to hormones and abiotic stresses was somehow different from those of the OsCUL1-1 and OsCUL1-2 genes. Taken together, these findings suggest that the biological role and function of OsCUL1-3 may be distinct from those of OsCUL1-1 and OsCUL1-2. The results of expression analysis of OsCUL1 genes in this study will serve as a useful platform to better understand overlapping and distinct roles of OsCUL1 proteins and CRL1-mediated cellular processes in rice plants.

Structure‐Guided Design of Peptides as Tools to Probe the Protein–Protein Interaction between Cullin‐2 and Elongin BC Substrate Adaptor in Cullin RING E3 Ubiquitin Ligases

Cullin RING E3 ubiquitin ligases (CRLs) are large dynamic multi‐subunit complexes that control the fate of many proteins in cells. CRLs are attractive drug targets for the development of small‐molecule inhibitors and chemical inducers of protein degradation. Herein we describe a structure‐guided biophysical approach to probe the protein–protein interaction (PPI) between the Cullin‐2 scaffold protein and the adaptor subunits Elongin BC within the context of the von Hippel‐Lindau complex (CRL2VHL) using peptides. Two peptides were shown to bind at the targeted binding site on Elongin C, named the “EloC site”, with micromolar dissociation constants, providing a starting point for future optimization. Our results suggest ligandability of the EloC binding site to short linear peptides, unveiling the opportunity and challenges to develop small molecules that have the potential to target selectively the Cul2‐adaptor PPI within CRLs.

Rotavirus NSP1 Requires Casein Kinase II-Mediated Phosphorylation for Hijacking of Cullin-RING Ligases.

ABSTRACTThe rotavirus nonstructural protein NSP1 repurposes cullin-RING E3 ubiquitin ligases (CRLs) to antagonize innate immune responses. By functioning as substrate adaptors of hijacked CRLs, NSP1 causes ubiquitination and proteasomal degradation of host proteins that are essential for expression of interferon (IFN) and IFN-stimulated gene products. The target of most human and porcine rotaviruses is the β-transducin repeat-containing protein (β-TrCP), a regulator of NF-κB activation. β-TrCP recognizes a phosphorylated degron (DSGΦXS) present in the inhibitor of NF-κB (IκB); phosphorylation of the IκB degron is mediated by IκB kinase (IKK). Because NSP1 contains a C-terminal IκB-like degron (ILD; DSGXS) that recruits β-TrCP, we investigated whether the NSP1 ILD is similarly activated by phosphorylation and whether this modification is required to trigger the incorporation of NSP1 into CRLs. Based on mutagenesis and phosphatase treatment studies, we found that both serine residues of the NSP1 ILD are phosphorylated, a pattern mimicking phosphorylation of IκB. A three-pronged approach using small-molecule inhibitors, small interfering RNAs, and mutagenesis demonstrated that NSP1 phosphorylation is mediated by the constitutively active casein kinase II (CKII), rather than IKK. In coimmunoprecipitation assays, we found that this modification was essential for NSP1 recruitment of β-TrCP and induced changes involving the NSP1 N-terminal RING motif that allowed formation of Cul3-NSP1 complexes. Taken together, our results indicate a highly regulated stepwise process in the formation of NSP1-Cul3 CRLs that is initiated by CKII phosphorylation of NSP1, followed by NSP1 recruitment of β-TrCP and ending with incorporation of the NSP1–β-TrCP complex into the CRL via interactions dependent on the highly conserved NSP1 RING motif.

NEDD8-activating enzyme inhibitor, MLN4924 (Pevonedistat) induces NOXA-dependent apoptosis through up-regulation of ATF-4

It has been reported that MLN4924 can inhibit cell growth and metastasis in various kinds of cancer. We have reported that MLN4924 is able to inhibit angiogenesis through the induction of cell apoptosis both in vitro and in vivo models. Moreover, Neddylation inhibition using MLN4924 triggered the accumulation of pro-apoptotic protein NOXA in Human umbilical vein endothelial cells (HUVECs). However, the mechanism of MLN4924-induced NOXA up-regulation has not been addressed in HUVECs yet. In this study, we investigated how MLN4924 induced NOXA expression and cellular apoptosis in HUVECs treated with MLN4924 at indicated concentrations. MLN4924-induced apoptosis was evaluated by Annexin V-FITC/PI analysis and expression of genes associated with apoptosis was assessed by Quantitative RT-PCR and western blotting. As a result, MLN4924 triggered NOXA-dependent apoptosis in a dose-dependent manner in HUVECs. Mechanistically, inactivation of Neddylation pathway caused up-regulation of activating transcription factor 4 (ATF-4), a substrate of Cullin-Ring E3 ubiquitin ligases (CRL). NOXA was subsequently transactivated by ATF-4 and further induced apoptosis. More importantly, knockdown of ATF-4 by siRNA significantly decreased NOXA expression and apoptotic induction in HUVECs. In summary, our study reveals a new mechanism underlying MLN4924-induced NOXA accumulation in HUVECs, which may help extend further study of MLN4924 for angiogenesis inhibition treatment.

The Arabidopsis Cop9 signalosome subunit 4 (CSN4) is involved in adventitious root formation

The COP9 signalosome (CSN) is an evolutionary conserved multiprotein complex that regulates many aspects of plant development by controlling the activity of CULLIN-RING E3 ubiquitin ligases (CRLs). CRLs ubiquitinate and target for proteasomal degradation a vast number of specific substrate proteins involved in many developmental and physiological processes, including light and hormone signaling and cell division. As a consequence of CSN pleiotropic function, complete loss of CSN activity results in seedling lethality. Therefore, a detailed analysis of CSN physiological functions in adult Arabidopsis plants has been hampered by the early seedling lethality of csn null mutants. Here we report the identification and characterization of a viable allele of the Arabidopsis COP9 signalosome subunit 4 (CSN4). The allele, designated csn4-2035, suppresses the adventitious root (AR) phenotype of the Arabidopsis superroot2-1 mutant, potentially by altering its auxin signaling. Furthermore, we show that although the csn4-2035 mutation affects primary and lateral root (LR) formation in the 2035 suppressor mutant, CSN4 and other subunits of the COP9 complex seem to differentially control AR and LR development.

Cockayne syndrome B protein regulates recruitment of the Elongin A ubiquitin ligase to sites of DNA damage

Elongin A performs dual functions as the transcriptionally active subunit of RNA polymerase II (Pol II) elongation factor Elongin and as the substrate recognition subunit of a Cullin-RING E3 ubiquitin ligase that ubiquitylates Pol II in response to DNA damage. Assembly of the Elongin A ubiquitin ligase and its recruitment to sites of DNA damage is a tightly regulated process induced by DNA-damaging agents and α-amanitin, a drug that induces Pol II stalling. In this study, we demonstrate (i) that Elongin A and the ubiquitin ligase subunit CUL5 associate in cells with the Cockayne syndrome B (CSB) protein and (ii) that this interaction is also induced by DNA-damaging agents and α-amanitin. In addition, we present evidence that the CSB protein promotes stable recruitment of the Elongin A ubiquitin ligase to sites of DNA damage. Our findings are consistent with the model that the Elongin A ubiquitin ligase and the CSB protein function together in a common pathway in response to Pol II stalling and DNA damage.