On again-off again: COP9 signalosome turns the key on protein degradation

A plant cytorhabdovirus modulates locomotor activity of insect vectors to enhance virus transmission

Transmission of many plant viruses relies on phloem-feeding insect vectors. However, how plant viruses directly modulate insect behavior is largely unknown. Barley yellow striate mosaic virus (BYSMV) is transmitted by the small brown planthopper (SBPH, Laodelphax striatellus). Here, we show that BYSMV infects the central nervous system (CNS) of SBPHs, induces insect hyperactivity, and prolongs phloem feeding duration. The BYSMV accessory protein P6 interacts with the COP9 signalosome subunit 5 (LsCSN5) of SBPHs and suppresses LsCSN5-regulated de-neddylation from the Cullin 1 (CUL1), hereby inhibiting CUL1-based E3 ligases-mediated degradation of the circadian clock protein Timeless (TIM). Thus, virus infection or knockdown of LsCSN5 compromises TIM oscillation and induces high insect locomotor activity for transmission. Additionally, expression of BYSMV P6 in the CNS of transgenic Drosophila melanogaster disturbs circadian rhythm and induces high locomotor activity. Together, our results suggest the molecular mechanisms whereby BYSMV modulates locomotor activity of insect vectors for transmission.

Neddylation Alleviates Methicillin-Resistant Staphylococcus aureus Infection by Inducing Macrophage Reactive Oxygen Species Production

Neddylation, a posttranslational modification in which NEDD8 is covalently attached to target proteins, has emerged as an endogenous regulator of innate immunity. However, the role of neddylation in methicillin-resistant Staphylococcus aureus (MRSA) infection remains unknown. In this study, we found that neddylation was activated after MRSA infection in vivo and in vitro. Inhibition of neddylation with MLN4924 promoted injury of liver and kidneys in C57BL/6 mice with MRSA bloodstream infection and increased mortality. Blockade of neddylation, either pharmacologically (MLN4924, DI591) or through the use of Uba3 small interfering RNA, inhibited Cullin3 neddylation and promoted Nrf2 accumulation, thus reducing reactive oxygen species (ROS) induction and bacterial killing ability in mouse peritoneal macrophages. In summary, our findings suggest that activation of neddylation in macrophages plays a critical protective role against MRSA infection by increasing ROS production, partially by signaling through the NEDD8-Cullin3-Nrf2-ROS axis. Furthermore, our results may provide a new non-antibiotic treatment strategy for MRSA infection through targeting of neddylation.

G Protein Pathway Suppressor 1 Promotes Influenza Virus Polymerase Activity by Activating the NF-κB Signaling Pathway

In the present study, we identified G protein pathway suppressor 1 (GPS1) to be a host cellular protein that is important for influenza virus replication. We also found that GPS1 plays a role in viral genome transcription through the NF-κB signaling pathway. Moreover, downregulation of GPS1 also affected the growth of vesicular stomatitis virus. Therefore, GPS1 may be a host target for antiviral drugs against influenza virus and possibly other viruses.

Influenza virus relies heavily on cellular machinery to replicate in host cells. Therefore, to better understand the influenza virus life cycle, it is important to identify which host proteins are involved and how they function in virus replication. Previously, we identified G protein pathway suppressor 1 (GPS1) to be a matrix protein 2 (M2)-interacting host protein. GPS1 is a component of the COP9 signalosome, which regulates the NF-κB signaling pathway. Here, we found that the downregulation of GPS1 expression reduced influenza virus replication by more than 2 log units. Although GPS1 was not involved in the early and late stages of virus replication, such as viral entry, uncoating, assembly, or budding, we found that viral polymerase activity was impaired in GPS1-downregulated cells. Moreover, our results suggest that M2 activates the NF-κB signaling pathway in a GPS1-dependent manner and that activation of NF-κB signaling leads to the upregulation of influenza virus polymerase activity. Our findings indicate that GPS1 is involved in the transcription and replication of influenza virus genomic RNA through the activation of the NF-κB signaling pathway.

A Targeted RNAi Screen Reveals Drosophila Female-Sterile Genes That Control the Size of Germline Stem Cell Niche During Development

Adult stem cells maintain tissue homeostasis. This unique capability largely depends on the stem cell niche, a specialized microenvironment, which preserves stem cell identity through physical contacts and secreted factors. In many cancers, latent tumor cell niches are thought to house stem cells and aid tumor initiation. However, in developing tissue and cancer it is unclear how the niche is established. The well-characterized germline stem cells (GSCs) and niches in the Drosophila melanogaster ovary provide an excellent model to address this fundamental issue. As such, we conducted a small-scale RNAi screen of 560 individually expressed UAS-RNAi lines with targets implicated in female fertility. RNAi was expressed in the soma of larval gonads, and screening for reduced egg production and abnormal ovarian morphology was performed in adults. Twenty candidates that affect ovarian development were identified and subsequently knocked down in the soma only during niche formation. Feminization factors (Transformer, Sex lethal, and Virilizer), a histone methyltransferase (Enhancer of Zeste), a transcriptional machinery component (Enhancer of yellow 1), a chromatin remodeling complex member (Enhancer of yellow 3) and a chromosome passenger complex constituent (Incenp) were identified as potentially functioning in the control of niche size. The identification of these molecules highlights specific molecular events that are critical for niche formation and will provide a basis for future studies to fully understand the mechanisms of GSC recruitment and maintenance.

Corrected and Republished from: The COP9 Signalosome Interacts with and Regulates Interferon Regulatory Factor 5 Protein Stability

The transcription factor interferon regulatory factor 5 (IRF5) exerts crucial functions in the regulation of host immunity against extracellular pathogens, DNA damage-induced apoptosis, death receptor signaling, and macrophage polarization. Tight regulation of IRF5 is thus warranted for an efficient response to extracellular stressors and for limiting autoimmune and inflammatory responses. Here we report that the COP9 signalosome (CSN), a general modulator of diverse cellular and developmental processes, associates constitutively with IRF5 and promotes its protein stability. The constitutive CSN/IRF5 interaction was identified using proteomics and confirmed by endogenous immunoprecipitations. The CSN/IRF5 interaction occurred on the carboxyl and amino termini of IRF5; a single internal deletion (Δ455-466) was found to significantly reduce IRF5 protein stability. CSN3 was identified as a direct interacting partner of IRF5, and knockdown of this subunit with small interfering RNAs (siRNAs) resulted in enhanced degradation. Degradation was further augmented by knockdown of CSN1 and CSN3 together. The ubiquitin E1 inhibitor UBEI-41 or the proteasome inhibitor MG132 prevented IRF5 degradation, supporting that its stability is regulated by the ubiquitin-proteasome system. Importantly, activation of IRF5 by the death receptor ligand tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) resulted in enhanced degradation via loss of the CSN/IRF5 interaction. This study defines the CSN as a new interacting partner of IRF5 that controls its stability.

Deneddylase 1 regulates deneddylase activity of the Cop9 signalosome in Drosophila melanogaster

NEDD8 conjugation of Cullin has an important role in ubiquitin-mediated protein degradation. The COP9 signalosome, of which CSN5 is the major catalytic subunit, is a major Cullin deneddylase. Another deneddylase, Deneddylase 1, has also been shown to process the Nedd8 precursor. In Drosophila, the DEN1 mutants do not have increased levels of Cullin neddylation, but instead show a significant decrease in neddylated Cullin. This characteristic decrease in neddylated Cullins in the DEN1^null background can be rescued by UAS-dDEN1^WT overexpression but not by overexpression of mature NEDD8, indicating that this phenotype is distinct from the NEDD8-processing function of DEN1. We examined the role of DEN1-CSN interaction in regulating Cullin neddylation. Overexpression of DEN1 in a CSN5^hypo background slightly reduced unneddylated Cullin levels. The CSN5, DEN1 double mutation partially rescues the premature lethality associated with the CSN5 single mutation. These results suggest that DEN1 regulates Cullin neddylation by suppressing CSN deneddylase activity.

COP9-Signalosome deneddylase activity is enhanced by simultaneous neddylation: insights into the regulation of an enzymatic protein complex

Background

Cullin-RING ubiquitin ligases (CRLs) are regulated by neddylation, which is a post translation modification of the Cullin family proteins. Neddylation of Cul1 activates the ligase through some means of biochemical mechanisms. The rate of neddylation and its extent are regulated by 2 opposing enzymatic processes: neddylation by an enzymatic cascade, and deneddylation by COP9-Signalosome (CSN) complex protein. The mechanism by which COP9-Signalosome catalytic activity is regulated is not well understood.

Methods

We set an in vitro neddylation and deneddylation reaction using as a source for specific COP9/Signalosome deneddylase activity either Hela cells extract or purified Signalosome. Neddylation reaction of either endogenic Cul1 from Hela cells extract or recombinant Cul1 was catalyzed by recombinant neddylation enzymes. Deneddylation rate was tested either simultaneous to neddylation or after termination of neddylation by using an ATP depleting reaction or by directly inhibiting the neddylation activation enzyme named APP-BP1/UBA3 by its specific inhibitor MLN-4924.

Results

We demonstrated that neddylation and deneddylation are catalytically engaged and that inhibition of Cul1 neddylation significantly causes a decline in the rate of COP9-Signalosome deneddylase activity. Since neddylation is an ATP consuming reaction we managed to isolate the 2 opposing processes which surprisingly caused a decline in COP9 activity. Using MLN-4924 we demonstrated that direct inhibition of neddylation negatively influences the rate of deneddylation. The hypothesis that phosphorylation controls deneddylation was ruled out by the fact that no change in the rate of deneddylation was exemplified while converting the use of ATP with AMP-PNP.

Conclusions

We demonstrated that deneddylation of Cul1 is positively regulated through direct simultaneous neddylation and is not dependent upon autophosphorylation. Defining the mechanism that regulates neddylation and deneddylation of Cullin proteins is important due to their effect on highly conserved cellular processes. We showed that minor changes in the degree of Cul1 neddylation linearly control the degree of p27 conjugation to ubiquitin, which emphasizes the hypothetic physiologic significance of our findings.

Electronic supplementary material

The online version of this article (doi:10.1186/s13008-015-0011-0) contains supplementary material, which is available to authorized users.

Mechanisms on boron-induced alleviation of aluminum-toxicity in Citrus grandis seedlings at a transcriptional level revealed by cDNA-AFLP analysis

The physiological and biochemical mechanisms on boron (B)-induced alleviation of aluminum (B)-toxicity in plants have been examined in some details, but our understanding of the molecular mechanisms underlying these processes is very limited. In this study, we first used the cDNA-AFLP to investigate the gene expression patterns in Citrus grandis roots responsive to B and Al interactions, and isolated 100 differentially expressed genes. Results showed that genes related to detoxification of reactive oxygen species (ROS) and aldehydes (i.e., glutathione S-transferase zeta class-like isoform X1, thioredoxin M-type 4, and 2-alkenal reductase (NADP+-dependent)-like), metabolism (i.e., carboxylesterases and lecithin-cholesterol acyltransferase-like 4-like, nicotianamine aminotransferase A-like isoform X3, thiosulfate sulfurtransferase 18-like isoform X1, and FNR, root isozyme 2), cell transport (i.e., non-specific lipid-transfer protein-like protein At2g13820-like and major facilitator superfamily protein), Ca signal and hormone (i.e., calcium-binding protein CML19-like and IAA-amino acid hydrolase ILR1-like 4-like), gene regulation (i.e., Gag-pol polyprotein) and cell wall modification (i.e., glycosyl hydrolase family 10 protein) might play a role in B-induced alleviation of Al-toxicity. Our results are useful not only for our understanding of molecular processes associated with B-induced alleviation of Al-toxicity, but also for obtaining key molecular genes to enhance Al-tolerance of plants in the future.

Regulation of Cop9 signalosome activity by the EF-hand Ca2+-binding protein tescalcin

The Ca(2+)-binding protein tescalcin is known to be involved in hematopoietic cell differentiation; however, this mechanism is poorly understood. Here, we identify CSN4 (subunit 4 of the COP9 signalosome) as a novel binding partner of tescalcin. The COP9 signalosome (CSN) is a multiprotein complex that is essential for development in all eukaryotes. This interaction is selective, Ca(2+)-dependent and involves the PCI domain of CSN4 subunit. We then investigated tescalcin and CSN activity in human erythroleukemia HEL and promyelocytic leukemia K562 cells and find that phorbol 12-myristate 13-acetate (PMA)-induced differentiation, resulting in the upregulation of tescalcin, coincides with reduced deneddylation of cullin-1 (Cul1) and stabilization of p27(Kip1) - molecular events that are associated with CSN activity. The knockdown of tescalcin led to an increase in Cul1 deneddylation, expression of F-box protein Skp2 and the transcription factor c-Jun, whereas the levels of cell cycle regulators p27(Kip1) and p53 decreased. These effects are consistent with the hypothesis that tescalcin might play a role as a negative regulator of CSN activity towards Cul1 in the process of induced cell differentiation.

CSN5 specifically interacts with CDK2 and controls senescence in a cytoplasmic cyclin E-mediated manner

The fifth component (CSN5) of the mammalian COP9 signalosome complex plays an essential role in cell proliferation and senescence, but its molecular mediator remains to be determined. Here, we searched for interactors among various cell cycle regulators, and found that CSN5, but not the CSN holo-complex, bound to CDK2 in vivo and in vitro. Depletion of CSN5 enhanced phosphorylation of CDK2 by Akt, resulting in cytoplasmic accumulation of CDK2 together with cyclin E in a leptomycin B-resistant manner, and impaired phosphorylation of the retinoblastoma protein. Additional knockdown of CDK2, which reduced the expression of cyclin E to the normal level, did not restore cell proliferation, but significantly suppressed senescence in CSN5-depleted cells. Enforced expression of cytoplasmic cyclin E induced premature senescence in immortalized cell lines. These results show that CSN5 functions through CDK2 to control premature senescence in a novel way, depending on cyclin E in the cytoplasm.

The COP9 signalosome interacts with and regulates interferon regulatory factor 5 protein stability

The transcription factor interferon regulatory factor 5 (IRF5) exerts crucial functions in the regulation of host immunity against extracellular pathogens, DNA damage-induced apoptosis, death receptor signaling, and macrophage polarization. Tight regulation of IRF5 is thus warranted for an efficient response toward extracellular stressors and for limiting autoimmune and inflammatory responses. Here we report that the COP9 signalosome (CSN), a general modulator of diverse cellular and developmental processes, associates constitutively with IRF5 and promotes its protein stability. The constitutive CSN/IRF5 interaction was identified using proteomics and confirmed by endogenous immunoprecipitations. The CSN/IRF5 interaction occurred on the carboxyl and amino termini of IRF5; a single internal deletion from amino acids 455 to 466 (Δ455-466) was found to significantly reduce IRF5 protein stability. CSN subunit 3 (CSN3) was identified as a direct interacting partner of IRF5, and knockdown of this subunit with small interfering RNAs resulted in enhanced degradation. Degradation was further augmented by knockdown of CSN1 and CSN3 together. The ubiquitin E1 inhibitor UBEI-41 or the proteasome inhibitor MG132 prevented IRF5 degradation, supporting the idea that its stability is regulated by the ubiquitin-proteasome system. Importantly, activation of IRF5 by the death receptor ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) resulted in enhanced degradation via loss of the CSN/IRF5 interaction. This study defines CSN to be a new interacting partner of IRF5 that controls its stability.

The e3 ubiquitin ligase gene family in plants: regulation by degradation

The regulation of protein expression and activity has been for long time considered only in terms of transcription/translation efficiency. In the last years, the discovery of post-transcriptional and post-translational regulation mechanisms pointed out that the key factor in determining transcript/protein amount is the synthesis/degradation ratio, together with post-translational modifications of proteins. Polyubiquitinaytion marks target proteins directed to degradation mediated by 26S-proteasome. Recent functional genomics studies pointed out that about 5% of Arabidopsis genome codes for proteins of ubiquitination pathway. The most of them (more than one thousand genes) correspond to E3 ubiquitin ligases that specifically recognise target proteins. The huge size of this gene family, whose members are involved in regulation of a number of biological processes including hormonal control of vegetative growth, plant reproduction, light response, biotic and abiotic stress tolerance and DNA repair, indicates a major role for protein degradation in control of plant life.

CSN complex controls the stability of selected synaptic proteins via a torsinA-dependent process

DYT1 dystonia is caused by an autosomal dominant mutation that leads to a glutamic acid deletion in torsinA (TA), a member of the AAA+ ATPase superfamily. In this study, we identified a novel-binding partner of TA, the subunit 4 (CSN4) of CSN signalosome. TA binds CSN4 and the synaptic regulator snapin in neuroblastoma cells and in brain synaptosomes. CSN4 and TA are required for the stability of both snapin and the synaptotagmin-specific endocytic adaptor stonin 2, as downregulation of CSN4 or TA reduces the levels of both proteins. Snapin is phosphorylated by the CSN-associated kinase protein kinase D (PKD) and its expression is decreased upon PKD inhibition. In contrast, the stability of stonin 2 is regulated by neddylation, another CSN-associated activity. Overexpression of the pathological TA mutant (ΔE-TA) reduces stonin 2 expression, causing the accumulation of the calcium sensor synaptotagmin 1 on the cell surface. Retrieval of surface-stranded synaptotagmin 1 is restored by overexpression of stonin 2 in ΔE-TA-expressing cells, suggesting that the DYT1 mutation compromises the role of TA in protein stabilisation and synaptic vesicle recycling.

Tbata modulates thymic stromal cell proliferation and thymus function

Niche availability provided by stromal cells is critical to thymus function. Thymi with diminished function contain fewer stromal cells, whereas thymi with robust function contain proliferating stromal cell populations. Here, we show that the thymus, brain, and testes-associated gene (Tbata; also known as SPATIAL) regulates thymic epithelial cell (TEC) proliferation and thymus size. Tbata is expressed in thymic stromal cells and interacts with the enzyme Uba3, thereby inhibiting the Nedd8 pathway and cell proliferation. Thymi from aged Tbata-deficient mice are larger and contain more dividing TECs than wild-type littermate controls. In addition, thymic reconstitution after bone marrow transplantation occurred more rapidly in Rag2(-/-)Tbata(-/-) mice than in Rag2(-/-)Tbata(+/+) littermate controls. These findings suggest that Tbata modulates thymus function by regulating stromal cell proliferation via the Nedd8 pathway.

F-box-directed CRL complex assembly and regulation by the CSN and CAND1

The COP9 signalosome (CSN) is thought to maintain the stability of cullin-RING ubiquitin ligases (CRL) by limiting the autocatalytic destruction of substrate adapters such as F box proteins (FBPs). CAND1, a protein associated with unneddylated CUL1, was proposed to assist in this role in an as yet unclear fashion. We found that only a subset of Schizosaccharomyces pombe FBPs, which feature a critical F box proline that promotes their interaction with CUL1, required CSN for stability. Unlike the CRL3 adaptor Btb3p, none of the CSN-sensitive FBPs were affected by deletion of ubp12. Contrary to current models, CAND1 does not control adaptor stability but maintains the cellular balance of CRL1 complexes by preventing rare FBPs from being outcompeted for binding to CUL1 by more ample adapters. These findings were integrated into a refined model of CRL control in which substrate availability toggles CRLs between independent CSN and CAND1 cycles.

Symmetrical modularity of the COP9 signalosome complex suggests its multifunctionality

The COP9 signalosome (CSN) is an eight-subunit protein complex that is found in all eukaryotes. Accumulating evidence indicates its diverse biological functions that are often linked to ubiquitin-mediated proteolysis. Here we applied an emerging mass spectrometry approach to gain insight into the structure of the CSN complex. Our results indicate that the catalytically active human complex, reconstituted in vitro, is composed of a single copy of each of the eight subunits. By forming a total of 35 subcomplexes, we are able to build a comprehensive interaction map that shows two symmetrical modules, Csn1/2/3/8 and Csn4/5/6/7, connected by interactions between Csn1-Csn6. Overall the stable modules and multiple subcomplexes observed here are in agreement with the "mini-CSN" complexes reported previously. This suggests that the propensity of the CSN complex to change and adapt its subunit composition might underlie its ability to perform multiple functions in vivo.

Identifying determinants of cullin binding specificity among the three functionally different Drosophila melanogaster Roc proteins via domain swapping

BACKGROUND

Cullin-dependent E3 ubiquitin ligases (CDL) are key regulators of protein destruction that participate in a wide range of cell biological processes. The Roc subunit of CDL contains an evolutionarily conserved RING domain that binds ubiquitin charged E2 and is essential for ubiquitylation. Drosophila melanogaster contains three highly related Roc proteins: Roc1a and Roc2, which are conserved in vertebrates, and Roc1b, which is specific to Drosophila. Our previous genetic data analyzing Roc1a and Roc1b mutants suggested that Roc proteins are functionally distinct, but the molecular basis for this distinction is not known.

METHODOLOGY/PRINCIPAL FINDINGS

Using co-immunoprecipitation studies we show that Drosophila Roc proteins bind specific Cullins: Roc1a binds Cul1-4, Roc1b binds Cul3, and Roc2 binds Cul5. Through domain swapping experiments, we demonstrate that Cullin binding specificity is strongly influenced by the Roc NH(2)-terminal domain, which forms an inter-molecular beta sheet with the Cullin. Substitution of the Roc1a RING domain with that of Roc1b results in a protein with similar Cullin binding properties to Roc1a that is active as an E3 ligase but cannot complement Roc1a mutant lethality, indicating that the identity of the RING domain can be an important determinant of CDL function. In contrast, the converse chimeric protein with a substitution of the Roc1b RING domain with that of Roc1a can rescue the male sterility of Roc1b mutants, but only when expressed from the endogenous Roc1b promoter. We also identified mutations of Roc2 and Cul5 and show that they cause no overt developmental phenotype, consistent with our finding that Roc2 and Cul5 proteins are exclusive binding partners, which others have observed in human cells as well.

CONCLUSIONS

The Drosophila Roc proteins are highly similar, but have diverged during evolution to bind a distinct set of Cullins and to utilize RING domains that have overlapping, but not identical, function in vivo.

COPS3 amplification and clinical outcome in osteosarcoma

BACKGROUND

Amplification of several genes that map to a region of chromosome 17p11.2, including COPS3, was observed in high-grade osteosarcoma. These genes were also shown to be overexpressed and may be involved in osteosarcoma tumorigenesis. COPS3 encodes a subunit of the COP9 signalosome implicated in the ubiquitination and ultimately degradation of the P53 tumor suppressor. To determine the relation between COPS3 amplification, P53 mutation, and patient outcome in osteosarcoma, tumors from a large cohort of patients with high-grade osteosarcoma and long-term clinical follow-up were examined.

METHODS

Quantitative real-time polymerase chain reaction (PCR) was performed to detect copy number changes for COPS3, as well as additional genes (NCOR1, TOM1L2, and PMP22) from the 17p11.2 amplicon, in 155 osteosarcomas from a prospective collection of tumors with corresponding clinical data. Univariate and multivariate analyses were performed to assess differences in survival between groups.

RESULTS

Amplification of COPS3, detected in 31% of the osteosarcomas, was strongly associated with large tumor size (P=.0009), but was not associated with age at diagnosis, site, sex, and tumor necrosis. COPS3 amplification was significantly correlated with a shorter time to metastasis with an estimated hazard ratio (HR) of 1.61 (95% confidence interval [CI], 1.02-2.55) in univariate analysis (log-rank test, P=.042). However, in an a priori multivariate Cox model including the other clinical parameters, the HR for COPS3 amplification decreased to 1.32 (95% CI, 0.82-2.13, P=.25), mainly due to the strong correlation with tumor size. COPS3 amplification and P53 mutation frequently occurred in the same tumors, suggesting that these are not mutually exclusive events in osteosarcoma. Although not statistically significant, patients whose tumors exhibited both molecular alterations tended to be more likely to develop metastasis compared with patients with either COPS3 amplification or P53 mutation alone.

CONCLUSIONS

COPS3 is the likely target of the 17p11.2 amplicon. COPS3 may function as an oncogene in osteosarcoma, and an increased copy number may lead to an unfavorable prognosis.

Protection of cullin-RING E3 ligases by CSN-UBP12

Neddylation, a process that conjugates the ubiquitin-like polypeptide NEDD8 to cullin proteins, activates cullin-RING ubiquitin ligases (CRLs). Deneddylation, in which the COP9 signalosome (CSN) removes NEDD8 from cullins, inactivates CRLs. However, genetic studies of CSN function conclude that deneddylation also promotes CRL activity. It has been proposed that a cyclic transition through neddylation and deneddylation is required for the regulation of CRL activity in vivo. Recent discoveries suggest that an additional level of complexity exists, whereby CRL components are targets for degradation, mediated either by autocatalytic ubiquitination or by unknown mechanisms. Deneddylation by CSN and deubiquitylation by CSN-associated ubiquitin-specific protease 12 protect CRL components from cellular depletion, thus maintaining the physiological CRL activities.

Neddylation and deneddylation regulate Cul1 and Cul3 protein accumulation

Cullin family proteins organize ubiquitin ligase (E3) complexes to target numerous cellular proteins for proteasomal degradation. Neddylation, the process that conjugates the ubiquitin-like polypeptide Nedd8 to the conserved lysines of cullins, is essential for in vivo cullin-organized E3 activities. Deneddylation, which removes the Nedd8 moiety, requires the isopeptidase activity of the COP9 signalosome (CSN). Here we show that in cells deficient for CSN activity, cullin1 (Cul1) and cullin3 (Cul3) proteins are unstable, and that to preserve their normal cellular levels, CSN isopeptidase activity is required. We further show that neddylated Cul1 and Cul3 are unstable - as suggested by the evidence that Nedd8 promotes the instability of both cullins - and that the unneddylatable forms of cullins are stable. The protein stability of Nedd8 is also subject to CSN regulation and this regulation depends on its cullin-conjugating ability, suggesting that Nedd8-conjugated cullins are degraded en bloc. We propose that while Nedd8 promotes cullin activation through neddylation, neddylation also renders cullins unstable. Thus, CSN deneddylation recycles the unstable, neddylated cullins into stable, unneddylated ones, and promotes cullin-organized E3 activity in vivo.

Nuclear hormone receptor co-repressors

Gene silencing is an essential transcriptional regulatory process. Co-repressors mediate gene repression through their recruitment by DNA bound transcriptional silencer proteins. Co-repressors repress gene expression through several mechanisms, mostly investigated on the level of chromatin. Lack or aberrant gene silencing is associated with many defects both on cellular and organismic level. Several human diseases are based on dysregulated co-repressor binding to transcriptional silencers indicating that co-repressor recruitment and the strength of gene silencing must be under strict control. In line with that gene silencing is important for animal development, cellular proliferation and transformation. Co-repressors play also a major role in the treatment of hormone-dependent growing cancers, such as for breast and prostate cancer therapy. The molecular basis of anti-hormone therapy lies in the recruitment of co-repressors to the estrogen or androgen receptors, respectively, which leads to their inactivation and to inhibition of cancer growth. The molecular mechanisms of selected topics are summarized here.