COP9 signalosome subunits 4 and 5 regulate multiple pleiotropic pathways inDrosophila melanogaster

Regulation of COP1 nuclear localization by the COP9 signalosome via direct interaction with CSN1

COP1 and COP9 signalosome (CSN) are key regulators of plant light responses and development. Deficiency in either COP1 or CSN causes a constitutive photomorphogenic phenotype. Through coordinated actions of nuclear- and cytoplasmic-localization signals, COP1 can respond to light signals by differentially partitions between nuclear and cytoplasmic compartments. Previous genetic analysis in Arabidopsis indicated that the nuclear localization of COP1 requires CSN, an eight-subunit heteromeric complex. However the mechanism underlying the functional relationship between COP1 and CSN is unknown. We report here that COP1 weakly associates with CSN in vivo. Furthermore, we report on the direct interaction involving the coiled-coil domain of COP1 and the N-terminal domain of the CSN1 subunit. In onion epidermal cells, expression of CSN1 can stimulate nuclear localization of GUS-COP1, and the N-terminal domain of CSN1 is necessary and sufficient for this function. Moreover, CSN1-induced COP1 nuclear localization requires the nuclear-localization sequences of COP1, as well as its coiled-coil domain, which contains both the cytoplasmic localization sequences and the CSN1 interacting domain. We also provide genetic evidence that the CSN1 N-terminal domain is specifically required for COP1 nuclear localization in Arabidopsis hypocotyl cells. This study advances our understanding of COP1 localization, and the molecular interactions between COP1 and CSN.

Revisiting the COP9 signalosome as a transcriptional regulator

The COP9 signalosome (CSN) is a highly conserved protein complex that was originally described as a repressor of light-dependent growth and transcription in Arabidopsis. The most studied CSN function is the regulation of protein degradation, which occurs primarily through the removal of the ubiquitin-like modifier Nedd8 from cullin-based E3 ubiquitin ligases. This activity can regulate transcription-factor stability and, therefore, transcriptional activity. Recent data suggest that the CSN also regulates transcription on the chromatin by mechanisms that are not yet clearly understood. Furthermore, the CSN subunits CSN5 and CSN2 seem to act as transcriptional coactivators and corepressors, respectively. Here, I re-evaluate the mechanisms by which the CSN acts as a transcriptional regulator, and suggest that they could extend beyond the regulation of protein stability.

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.

Mechanisms of macrophage migration inhibitory factor (MIF)-dependent tumor microenvironmental adaptation

Since its activity was first reported in the mid-1960s, macrophage migration inhibitory factor (MIF) has gone from a cytokine activity modulating monocyte motility to a pleiotropic regulator of a vast array of cellular and biological processes. Studies in recent years suggest that MIF contributes to malignant disease progression on several different levels. Both circulating and intracellular MIF protein levels are elevated in cancer patients and MIF expression reportedly correlates with stage, metastatic spread and disease-free survival. Additionally, MIF expression positively correlates with angiogenic growth factor expression, microvessel density and tumor-associated neovascularization. Not coincidentally, MIF has recently been shown to contribute to tumoral hypoxic adaptation by promoting hypoxia-induced HIF-1alpha stabilization. Intriguingly, hypoxia is a strong regulator of MIF expression and secretion, suggesting that hypoxia-induced MIF acts as an amplifying factor for both hypoxia and normoxia-associated angiogenic growth factor expression in human malignancies. Combined, these findings suggest that MIF overexpression contributes to tumoral hypoxic adaptation and, by extension, therapeutic responsiveness and disease prognosis. This review summarizes recent literature on the contributions of MIF to tumor-associated angiogenic growth factor expression, neovascularization and hypoxic adaptation. We also will review recent efforts aimed at identifying and employing small-molecule antagonists of MIF as a novel approach to cancer therapeutics.

The COP9 signalosome negatively regulates proteasome proteolytic function and is essential to transcription

The COP9 signalosome (CSN) is an evolutionarily conserved protein complex formed by eight subunits (CSN1 through CSN8). Deneddylating cullin family proteins is considered the bona fide function of the CSN. It has been proposed that the CSN regulates the assembly and disassembly of the cullin-based ubiquitin ligases via its deneddylation activity. Here we report that down-regulation of CSN8 by RNA interference destabilized differentially other CSN subunits and reduced the amount of CSN holo-complexes, leading to increases in neddylated cullin proteins and reduction of F-box protein Skp2 in HEK293 cells. Moreover, suppression of CSN8 enhanced the degradation of a proteasome surrogate substrate and cyclin kinase inhibitor p21(cip). Reduced transcript levels of cyclin kinase inhibitor p21(cip) and p27(kip) were also observed upon down-regulation of CSN8. These data suggest that the homeostatic level of CSN8/CSN suppresses proteasome proteolytic function and regulates transcription.

Auxin responses in mutants of the Arabidopsis CONSTITUTIVE PHOTOMORPHOGENIC9 signalosome

The CONSTITUTIVE PHOTOMORPHOGENIC9 (COP9) signalosome (CSN) is an evolutionarily conserved multiprotein complex that interacts with cullin-RING type E3 ubiquitin ligases (CRLs). CSN subunit 5 (CSN5), which, when incorporated into CSN, can deconjugate the NEDD8 modification from the cullin subunit of CRLs, is essential for CSN's role in controlling CRL activity. Whether the CSN5 monomer, which is maintained in csn mutants such as csn3 or csn4, has a functional role, remains to be established. We performed a comparative gene expression-profiling experiment with Arabidopsis (Arabidopsis thaliana) csn3, csn4, and csn5 mutants, and we show here that these mutants cannot be distinguished at the transcriptional level. Furthermore, we show that csn3 csn5 mutants are morphologically indistinguishable from csn3 or csn5 mutants. Taken together, these data suggest that the CSN5 monomer does not have a function that leads to transcriptional or morphological changes in the csn mutants. We further examined auxin responses in csn mutants. Whereas CSN had previously been shown to be required for the auxin response-regulatory E3 complexes, specifically SCF(TIR1), the csn mutant phenotype suggests that CSN is not essential for auxin responses. We present physiological and genetic data that indicate that auxin responses are indeed only partially impaired in csn mutants and that this is not the result of maternally contributed CSN. Finally, we discuss these findings in the context of the current understanding of the role of neddylation and CSN-mediated deneddylation for CRL activity.

The myeloid leukemia factor interacts with COP9 signalosome subunit 3 in Drosophila melanogaster

The human myeloid leukemia factor 1 (hMLF1) gene was first identified as an NPM-hMLF1 fusion gene produced by chromosomal translocation. In Drosophila, dMLF has been identified as a protein homologous to hMLF1 and hMLF2, which interacts with various factors involved in transcriptional regulation. However, the precise cellular function of dMLF remains unclear. To generate further insights, we first examined the behavior of dMLF protein using an antibody specific to dMLF. Immunostaining analyses showed that dMLF localizes in the nucleus in early embryos and cultured cells. Ectopic expression of dMLF in the developing eye imaginal disc using eyeless-GAL4 driver resulted in a small-eye phenotype and co-expression of cyclin E rescued the small-eye phenotype, suggesting the involvement of dMLF in cell-cycle regulation. We therefore analyzed the molecular mechanism of interactions between dMLF and a dMLF-interacting protein, dCSN3, a subunit of the COP9 signalosome, which regulates multiple signaling and cell-cycle pathways. Biochemical and genetic analyses revealed that dMLF interacts with dCSN3 in vivo and glutathione S-transferase pull-down assays revealed that the PCI domain of the dCSN3 protein is sufficient for this to occur, possibly functioning as a structural scaffold for assembly of the COP9 signalosome complex. From these data we propose the possibility that dMLF plays a negative role in assembly of the COP9 signalosome complex.

The proto-oncogene Int6 is essential for neddylation of Cul1 and Cul3 in Drosophila

Int6 is a proto-oncogene implicated in various types of cancer, but the mechanisms underlying its activity are not clear. Int6 encodes a subunit of the eukaryotic translation initiation factor 3, and interacts with two related complexes, the proteasome, whose activity is regulated by Int6 in S. pombe, and the COP9 signalosome. The COP9 signalosome regulates the activity of Cullin-Ring Ubiquitin Ligases via deneddylation of their cullin subunit. We report here the generation and analysis of two Drosophila mutants in Int6. The mutants are lethal demonstrating that Int6 is an essential gene. The mutant larvae accumulate high levels of non-neddylated Cul1, suggesting that Int6 is a positive regulator of cullin neddylation. Overexpression in Int6 in cell culture leads to accumulation of neddylated cullins, further supporting a positive role for Int6 in regulating neddylation. Thus Int6 and the COP9 signalosome play opposing roles in regulation of cullin neddylation.

CSN5 isopeptidase activity links COP9 signalosome activation to breast cancer progression

CSN5 has been implicated as a candidate oncogene in human breast cancers by genetic linkage with activation of the poor-prognosis, wound response gene expression signature. CSN5 is a subunit of the eight-protein COP9 signalosome, a signaling complex with multiple biochemical activities; the mechanism of CSN5 action in cancer development remains poorly understood. Here, we show that CSN5 isopeptidase activity is essential for breast epithelial transformation and progression. Amplification of CSN5 is required for transformation of primary human breast epithelial cells by defined oncogenes. The transforming effects of CSN5 require CSN subunits for assembly of the full COP9 signalosome and the isopeptidase activity of CSN5, which potentiates the transcriptional activity of MYC. Transgenic inhibition of CSN5 isopeptidase activity blocks breast cancer progression evoked by MYC and RAS in vivo. These results highlight CSN5 isopeptidase activity in breast cancer progression, suggesting it as a therapeutic target in aggressive human breast cancers.

Downregulation of COP9 signalosome subunits differentially affects the CSN complex and target protein stability

BACKGROUND

The COP9 signalosome (CSN) is a conserved protein complex in eukaryotic cells consisting of eight subunits (CSN1 to CSN8). Recent data demonstrate that the CSN is a regulator of the ubiquitin (Ub) proteasome system (UPS). It controls substrate ubiquitination by cullin-RING Ub ligases (CRLs), a process that determines substrate specificity of the UPS. The intrinsic deneddylating activity localized to CSN5 as well as the associated kinases and deubiquitinating activity are involved in the regulatory function of CSN. The exact mechanisms are unclear. In this study we knocked down CSN1 (siCSN1), CSN3 (siCSN3) and CSN5 (siCSN5) by specific siRNA oligos permanently expressed in HeLa cells. The analysis and comparison of siRNA cells revealed differential impact of individual subunits on CSN structure and function.

RESULTS

Permanent knockdowns of CSN1 and CSN3 led to a reduction of the subunits to approximately 40%, which is accompanied by a proportional decrease of the CSN holocomplex. In contrast, downregulation of CSN5 in HeLa cells reduced the CSN5 protein below 20% without significant effects on the remaining complex. The CRL component Rbx1 was characterized by accelerated proteolysis in siCSN1 and siCSN3 and also in siCSN5 cells, however, with lesser extent. Immunoprecipitated CSN complex from siCSN5 cells was less effective in phosphorylating c-Jun and p27. Accelerated degradation of c-Jun in siCSN5 cells was rescued by overexpression of CSN5 as well as of the deneddylation mutant CSN5D151N. Overexpression of CSN5 cannot rescue c-Jun destabilization in siCSN1.

CONCLUSION

There exists a coordinated downregulation of CSN subunits in the CSN1 and CSN3 knockdowns. The underlying regulatory mechanisms are obscure. CSN5 seems to possess a specific status in HeLa cells. Its reduction is not connected with coordinated downregulation of other subunits. CSN knockdowns confirm that the stabilization of the CRL component Rbx1 is a major CSN function. In addition, downregulation of CSN subunits influences the stability of important cellular regulators such as c-Jun and p27.

The coregulator Alien

Alien has characteristics of a corepressor for selected members of the nuclear hormone receptor (NHR) superfamily and also for transcription factors involved in cell cycle regulation and DNA repair. Alien mediates gene silencing and represses the transactivation of specific NHRs and other transcription factors to modulate hormone response and cell proliferation. Alien is a highly conserved protein and is expressed in a wide variety of tissues. Knockout of the gene encoding Alien in mice is embryonic lethal at a very early stage, indicating an important evolutionary role in multicellular organisms. From a mechanistic perspective, the corepressor function of Alien is in part mediated by histone deacetylase (HDAC) activity. In addition, Alien seems to modulate nucleosome assembly activity. This suggests that Alien is acting on chromatin not only through recruitment of histone-modifying activities, but also through enhancing nucleosome assembly.

COP9 signalosome subunit 8 is essential for peripheral T cell homeostasis and antigen receptor-induced entry into the cell cycle from quiescence

Engagement of antigen receptors triggers the proliferation and functional activation of lymphocytes. Here we report that T cell homeostasis and antigen-induced responses require the COP9 signalosome (CSN), a regulator of the ubiquitin-proteasome system. Conditional deletion of the CSN subunit Csn8 in peripheral T lymphocytes disrupted formation of the CSN complex, reduced T cell survival and proliferation in vivo and impaired antigen-induced production of interleukin 2. Moreover, Csn8-deficient T cells showed defective entry into the cell cycle from the G0 quiescent state. This phenotype was associated with a lack of signal-induced expression of cell cycle-related genes, including G1 cyclins and cyclin-dependent kinases, and with excessive induction of p21(Cip1). Our data define a CSN-dependent pathway of transcriptional control that is essential for antigen-induced initiation of T cell proliferation.

Genomic analysis of COP9 signalosome function in Drosophila melanogaster reveals a role in temporal regulation of gene expression

The COP9 signalosome (CSN), an eight-subunit protein complex, is conserved in all higher eukaryotes. CSN intersects the ubiquitin–proteasome pathway, modulating signaling pathways controlling various aspects of development. We are using Drosophila as a model system to elucidate the function of this important complex. Transcriptome data were generated for four csn mutants, sampled at three developmental time points. Our results are highly reproducible, being confirmed using two different experimental setups that entail different microarrays and different controls. Our results indicate that the CSN acts as a transcriptional repressor during development of Drosophila, resulting in achronic gene expression in the csn mutants. ‘Time shift' analysis with the publicly available Drosophila transcriptome data indicates that genes repressed by the CSN are normally induced primarily during late embryogenesis or during metamorphosis. These temporal shifts are likely due to the roles of the CSN in regulating transcription factors. A null mutation in CSN subunit 4 and hypomorphic mutations in csn5 lead to more severe defects than seen in the csn5-null mutants strain, suggesting that CSN5 carries only some of the CSN function.

An eight-subunit COP9 signalosome with an intact JAMM motif is required for fungal fruit body formation

Fruit body formation in filamentous fungi is a complex and yet hardly understood process. We show here that protein turnover control is crucial for Aspergillus nidulans development. Deletion of genes encoding COP9 signalosome (CSN) subunits 1, 2, 4, or 5 resulted in identical blocks in fruit body formation. The CSN multiprotein complex controls ubiquitin-dependent protein degradation in eukaryotes. Six CSN subunits interacted in a yeast two-hybrid analysis, and the complete eight-subunit CSN was recruited by a functional tandem affinity purification tag fusion of subunit 5 (CsnE). The tagged CsnE was unable to recruit any CSN subunit in a strain deleted for subunit 1 or subunit 4. Mutations in the JAMM metalloprotease core of CsnE resulted in mutant phenotypes identical to those of csn deletion strains. We propose that a correctly assembled CSN including a functional JAMM links protein turnover to fungal sexual development.

COP9 signalosome subunit 5 (CSN5/Jab1) regulates the development of the Drosophila immune system: effects on Cactus, Dorsal and hematopoiesis

The COP9 signalosome is a multifunctional regulator essential for Drosophila development. A loss-of-function mutant in Drosophila COP9 signalosome subunit 5 (CSN5) develops melanotic bodies, a phenotype common to mutants in immune signaling. csn5(null) larvae accumulated high levels of Cactus that co-localizes with Dorsal to the nucleus. However, Dorsal-dependent transcriptional activity remained repressed in the absence of an inducing signal, despite its nuclear localization. Dorsal activity in mutant larvae and NFkappaB activity in CSN5 down-regulated mammalian cells can be induced following activation of the Toll/IL-1 pathway. csn5(null) larvae contained more hemocytes than wild-type (wt) larvae. A large portion of these cells have differentiated to lamellocytes (LM), a hemocyte cell type rarely seen in normal larvae. The results presented here indicate that CSN5 is a negative regulator of Dorsal subcellular localization, and of hemocyte proliferation and differentiation. These results further indicate that nuclear localization of Dorsal can be uncoupled from its activation. Surprisingly, CSN5 is not necessary for immune-induced degradation of Cactus.

Role of the MPN subunits in COP9 signalosome assembly and activity, and their regulatory interaction with Arabidopsis Cullin3-based E3 ligases

The COP9 signalosome (CSN) is an evolutionarily conserved multisubunit protein complex that regulates a variety of biological processes. Among its eight subunits, CSN5 and CSN6 contain a characteristic MPN (for Mpr1p and Pad1p N-terminal) domain and, in Arabidopsis thaliana, are each encoded by two genes: CSN5A, CSN5B and CSN6A, CSN6B, respectively. We characterized both MPN subunits using a series of single and double mutants within each gene family. Our results indicate that although CSN6A and CSN6B retain mostly redundant functions, CSN5A and CSN5B play unequal roles in the regulation of plant development. Complete depletion of either of the two MPN members results in CSN instability and the decay of various CSN components, along with the complete loss of CUL1, CUL3, and CUL4 derubylation. Furthermore, we demonstrate that CSN interacts with CUL3, in addition to CUL1 and CUL4, and that the lack of CSN activity differentially affects the stability of those three cullins. Interestingly, we also show that optimal CUL3 activity is required to maintain the cellular pool of CSN5, through a posttranscriptional mechanism. Our data suggest the existence of reciprocal regulation between CUL3 and CSN5 accumulation. This study thus completes the genetic analysis of all CSN subunits and confirms the structural interdependence between PCI and MPN subunits in functional CSN complex formation.

Preparation and characterization of monoclonal antibodies against mouse Jab1/CSN5 protein

Jab1, also known as the fifth component of the COP9 signalosome complex (CSN5), directly interacts with and regulates the activity and stability of multiple intracellular regulatory molecules, such as c-Jun, p27, p53, Cullin, Smad4, and HIF1alpha. In addition, a high level of Jab1 is observed in a variety of human cancers and is sometimes correlated with a poor prognosis, suggesting that Jab1 contributes to cancer cell proliferation and survival and could be a novel target of cancer therapy. In this report, we generated five mouse monoclonal antibodies to a bacterially produced recombinant mouse Jab1 protein and examined their capabilities and limitations in commonly used assays, including enzyme linked immunosorbent assay (ELISA), Western blotting with denatured and native polyacrylamide gel electrophoresis (PAGE), immunoprecipitation, and immunofluorescence microscopy, finding the most suitable antibody for each application. Because these antibodies proved useful for immunohistochemical staining for Jab1 in fixed sections of human cancer samples, they should be useful in determining the expression and subcellular distribution of Jab1 in human tumors.

Retinoblastoma protein regulation by the COP9 signalosome

Similar to their human counterparts, the Drosophila Rbf1 and Rbf2 Retinoblastoma family members control cell cycle and developmentally regulated gene expression. Increasing evidence suggests that Rbf proteins rely on multiprotein complexes to control target gene transcription. We show here that the developmentally regulated COP9 signalosome (CSN) physically interacts with Rbf2 during embryogenesis. Furthermore, the CSN4 subunit of the COP9 signalosome co-occupies Rbf target gene promoters with Rbf1 and Rbf2, suggesting an active role for the COP9 signalosome in transcriptional regulation. The targeted knockdown of individual CSN subunits leads to diminished Rbf1 and Rbf2 levels and to altered cell cycle progression. The proteasome-mediated destruction of Rbf1 and Rbf2 is increased in cells and embryos with diminished COP9 activity, suggesting that the COP9 signalosome protects Rbf proteins during embryogenesis. Previous evidence has linked gene activation to protein turnover via the promoter-associated proteasome. Our findings suggest that Rbf repression may similarly involve the proteasome and the promoter-associated COP9 signalosome, serving to extend Rbf protein lifespan and enable appropriate programs of retinoblastoma gene control during development.

Depletion of Jab1 inhibits proliferation of pancreatic cancer cell lines

Jab1 overexpression is observed in many human cancers, but its physiological significance remains to be investigated. We reduced the level of Jab1 expression in pancreatic cancer cell lines, MIA PaCa-2 and PANC-1 by the RNA interference and found that Jab1-knockdown resulted in impaired cell proliferation and enhanced apoptosis regardless of the genotype of the tumor suppressor p53. This growth inhibition was rescued by the introduction of siRNA-resistant mouse Jab1 cDNA. Jab1-knocked-down cells expressed a higher level of c-myc, and additional depletion of c-myc rescued cells from Jab1-knockdown-mediated growth suppression. Thus, Jab1 overexpression contributes to pancreatic cancer cell proliferation and survival. Jab1 could be a novel target in cancer therapy.

Life Is Degrading—Thanks to Some Zomes

Three structurally related protein complexes, the COP9 signalosome, the proteasome lid, and the eukaryotic translation initiation factor 3, are revealing new insights into developmental processes and into cell cycle control in healthy cells and cells exposed to genotoxic stress. Newly discovered cullin-RING E3 ubiquitin ligases assembled on the CUL4 platform may provide links between DNA replication, chromatin, and proteolysis.

The COP9 signalosome regulates Skp2 levels and proliferation of human cells

The COP9 signalosome (CSN) is a conserved, multisubunit complex first identified as a developmental regulator in plants. Gene inactivation of single CSN subunits results in early embryonic lethality in mice, indicating that the CSN is essential for mammalian development. The pleiotropic function of the CSN may be related to its ability to remove the ubiquitin-like peptide Nedd8 from cullin-RING ubiquitin ligases, such as the SCF complex, and therefore regulate their activity. However, the mechanism of CSN regulatory action on cullins has been debated, since, paradoxically, the CSN has an inhibitory role in vitro, while genetic evidence supports a positive regulatory role in vivo. We have targeted expression of CSN subunits 4 and 5 in human cells by lentivirus-mediated small hairpin RNA delivery. Down-regulation of either subunit resulted in disruption of the CSN complex and in Cullin1 hyperneddylation. Functional consequences of CSN down-regulation were decreased protein levels of Skp2, the substrate recognition subunit of SCF(Skp2), and stabilization of a Skp2 target, the cyclin-dependent kinase inhibitor p27(Kip1). CSN down-regulation caused an impairment in cell proliferation, which could be partially reversed by suppression of p27(Kip1). Moreover, restoring Skp2 levels in CSN-deficient cells recovered cell cycle progression, indicating that loss of Skp2 in these cells plays an important role in their proliferation defect. Our data indicate that the CSN is necessary to ensure the assembly of a functional SCF(Skp2) complex and therefore contributes to cell cycle regulation of human cells.

COPing with hypoxia

To understand how cells respond to altered oxygenation, a frequent experimental paradigm is to isolate known components of bona fide oxygen responsive proteins. Recent studies have shown that a protein known as CSN5 or JAB1 interacts with both the HIF-1alpha oxygen-responsive transcription factor and its oxygen-dependent regulator, the Von Hippel-Lindau (pVHL) tumor suppressor. CSN5 is a component of the COP9 Signalosome (CSN) which is a multi-subunit protein that has high homology to the lid of the 19S lid of 26S proteasome. The exact function of the CSN5 interaction with pVHL and HIF-1alpha remains to be fully elucidated, but it is clear that the interaction is both oxygen dependent and that CSN5 may play different roles under oxic and hypoxic responses. Further, evidence has also been published indicating that pVHL can be potentially post-translationally modified by CSN5 (de-neddylation) and that CSN5 transcription is regulated by hypoxia as are many of the key pVHL/HIF-1alpha regulatory genes such as the PHDs and OS-9. This review will give a broad overview of known CSN5 and COP9 Signalosome functions and how these functions impact the pVHL/HIF-1alpha signaling complex and potentially other oxygen-sensitive response networks.

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.

The hepatitis B virus X protein enhances AP-1 activation through interaction with Jab1

Hepatitis B virus X protein (HBx) has many cellular functions and is a major factor in hepatitis and hepatocellular carcinoma caused by HBV infection. A proteomic approach was used to search for HBx-interacting proteins in order to elucidate the molecular mechanism of hepatocarcinogenesis. HBx was attached to myc and flag tags (MEF tags) and expressed in 293T cells; the protein complex formed within the cells was purified and characterized by mass spectrometry. COP9 signalosome (CSN) subunits 3 and 4 were subsequently identified as HBx-interacting proteins. In addition, CSN subunit 5, Jun activation domain-binding protein 1 (Jab1), was shown to be a novel cellular target of HBx. In vivo and in vitro interactions between HBx and Jab1 were confirmed by standard immunoprecipitation and GST pull-down assays. An analysis of HBx deletion constructs showed that amino acids 30-125 of HBx were responsible for binding to Jab1. Confocal laser microscopy demonstrated that HBx was mainly localized in the cytoplasm, while Jab1 was found mainly in the nucleus and partially in the cytoplasm, and that the two proteins colocalized in the cytoplasm. The cotransfection of HBx and Jab1 resulted in substantial activator protein 1 (AP-1) activation and knockdown of endogenous Jab1 attenuated AP-1 activation caused by HBx. In addition, the coexpression of HBx and Jab1 potentiated phosphorylation of JNK, leading to the subsequent phosphorylation of c-Jun, whereas the level of c-Jun and JNK phosphorylation induced by HBx was decreased in Jab1 knockdown cells. These results suggest that the interaction between HBx and Jab1 enhances HBx-mediated AP-1 activation.

Identification of the down-regulated genes in a mat1-2-deleted strain of Gibberella zeae, using cDNA subtraction and microarray analysis

Gibberella zeae (anamorph: Fusarium graminearum), a self-fertile ascomycete, is an important pathogen of cereal crops. Here, we have focused on the genes specifically controlled by the mating type (MAT) locus, a master regulator of sexual developmental process in G. zeae. To identify these genes, we employed suppression subtractive hybridization between a G. zeae wild-type strain Z03643 and the isogenic self-sterile mat1-2 strain T43deltaM2-2. Both reverse Northern and cDNA microarray analyses using 291 subtractive unigenes confirmed that 58.8% (171 genes) were significantly down-regulated in T43deltaM2-2. Among these, 98 could be either manually or automatically annotated based on known functions of their possible homologs. Northern blot analysis revealed that all of the genes examined were differentially regulated by MAT1-2 during sexual development. This study is the first report on the set of genes that are transcriptionally altered by the deletion of MAT1-2 during sexual reproduction in G. zeae.

Jab1 is a specificity factor for E2F1-induced apoptosis

The members of the E2F family of transcription factors are key regulators of genes involved in cell cycle progression, cell fate determination, DNA damage repair, and apoptosis. Many cell-based experiments suggest that E2F1 is a stronger inducer of apoptosis than the other E2Fs. Our previous work identified the E2F1 marked box and flanking region as critical for the specificity in E2F1 apoptosis induction. We have now used a yeast two-hybrid screen to identify proteins that bind the E2F1 marked box and flanking regions, with a potential role in E2F1 apoptosis induction. We identified Jab1 as an E2F1-specific binding protein and showed that Jab1 and E2F1 coexpression synergistically induce apoptosis, coincident with an induction of p53 protein accumulation. In contrast, Jab1 does not synergize with E2F1 to promote cell cycle entry. Cells depleted of Jab1 are deficient for both E2F1-induced apoptosis and induction of p53 accumulation. We suggest that Jab1 is an essential cofactor for the apoptotic function of E2F1.

The emerging role of the COP9 signalosome in cancer

In the last several years, multiple lines of evidence have suggested that the COP9 signalosome (CSN) plays a significant role in the regulation of multiple cancers and could be an attractive target for therapeutic intervention. First, the CSN plays a key role in the regulation of Cullin-containing ubiquitin E3 ligases that are central mediators of a variety of cellular functions essential during cancer progression. Second, several studies suggest that the individual subunits of the CSN, particularly CSN5, might regulate oncogenic and tumor suppressive functions independently of, or coordinately with, the CSN holocomplex. Thus, deregulation of CSN subunit function can have a dramatic effect on diverse cellular functions, including the maintenance of DNA fidelity, cell cycle control, DNA repair, angiogenesis, and microenvironmental homeostasis that are critical for tumor development. Additionally, clinical studies have suggested that the expression or localization of some CSN subunits correlate to disease progression or clinical outcome in a variety of tumor types. Although the study of CSN function in relation to tumor progression is in its infancy, this review will address current studies in relation to cancer initiation, progression, and potential for therapeutic intervention.

Targeted silencing of Jab1/Csn5 in human cells downregulates SCF activity through reduction of F-box protein levels

Background

SCF ubiquitin ligases target numerous proteins for ubiquitin dependent proteolysis, including p27 and cyclin E. SCF and other cullin-RING ligases (CRLs) are regulated by the ubiquitin-like protein Nedd8 that covalently modifies the cullin subunit. The removal of Nedd8 is catalyzed by the Jab1/MPN domain metalloenzyme (JAMM) motif within the Csn5 subunit of the Cop9 Signalosome.

Results

Here, we conditionally knock down Csn5 expression in HEK293 human cells using a doxycycline-inducible shRNA system. Cullin levels were not altered in CSN-deficient human cells, but the levels of multiple F-box proteins were decreased. Molecular analysis indicates that this decrease was due to increased Cul1- and proteasome-dependent turnover. Diminished F-box levels resulted in reduced SCF activity, as evidenced by accumulation of two substrates of the F-box protein Fbw7, cyclin E and c-myc, in Csn5-depleted cells.

Conclusion

We propose that deneddylation of Cul1 is required to sustain optimal activity of SCF ubiquitin ligases by repressing 'autoubiquitination' of F-box proteins within SCF complexes, thereby rescuing them from premature degradation.

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.

The csnD/csnE signalosome genes are involved in the Aspergillus nidulans DNA damage response

The signalosome (CSN) is a conserved multiprotein complex involved in regulation of eukaryotic development and is also required to activate ribonucleotide reductase for DNA synthesis. In Aspergillus nidulans, csnD/csnE are key regulators of sexual development. Here, we investigated whether the csnD/csnE genes are involved in the DNA damage response in this fungus. The growth of the csnD/csnE deletion mutants was reduced by subinhibitory concentrations of hydroxyurea, camptothecin, 4-nitroquinoline oxide, and methyl methanesulfonate. A. nidulans increases csnD/csnE mRNA levels when it is challenged by different DNA-damaging agents. There is no significant transcriptional induction of the csnE promoter fused with lacZ gene in the presence of DNA-damaging agents, suggesting that increased mRNA accumulation is due to increased mRNA stability. Septation was not inhibited in the csnD/csnE deletion mutants while DeltauvsB DeltacsnE presented an increase in septation upon DNA damage caused by methyl methanesulfonate, suggesting that uvsB(ATR) and csnE genetically interact during checkpoint-dependent inhibition of septum formation. The double DeltacsnD/DeltacsnE DeltanpkA mutants were more sensitive to DNA-damaging agents than were the respective single mutants. Our results suggest that csnD/csnE genes are involved in the DNA damage response and that NpkA and UvsB(ATR) genetically interact with the signalosome.

Growth suppression induced by the TRC8 hereditary kidney cancer gene is dependent upon JAB1/CSN5

TRC8 encodes an E3-ubiquitin ligase disrupted in a family with hereditary renal cell carcinoma (RCC). We previously reported that Drosophila Trc8 (DTrc8) overexpression inhibits growth and that human and fly proteins interact with with the COP9 signalosome (CSN) subunit JAB1/CSN5. However, further mechanistic evidence linking DTrc8 growth suppression to CSN5 was lacking. Here, we show that haploinsufficiency of CSN5, or a T100I point mutation (CSN5(3)), relieved growth suppression by DTrc8, whereas CSN5(1) (E160V) and CSN5(2) (G147D) mutations had no effect. The strength of yeast two-hybrid interactions between DTrc8 and CSN5 were in complete agreement with the observed phenotypes. DTrc8 overexpression resulted in elevated levels of CSN5 and CSN7, but had no effect on NEDD8-modified Cul-1. In contrast to CSN5, heterozygosity for CSN4null had no effect on the DTrc8 phenotype. We also looked for genetic interactions between DTrc8 and other MPN domain proteins in the CSN and 26S proteasome lid. CSN6 haploinsufficiency restored growth, whereas reduction of proteasome subunits RPN8 or RPN11 had no effect. DTrc8 expression increased the level of digitonin-extractable CSN complex, consistent with elevated levels of CSN5 and 7. Our genetic results confirm that DTrc8-induced growth suppression is CSN5 (and CSN6) dependent. While there was no obvious influence on CSN deneddylation activity, the increase in CSN subunits and holocomplex suggests that TRC8 modulates signalosome levels or compartmentalization.

Expression pattern of the JAB1/CSN5 gene during murine embryogenesis: colocalization with NEDD8

The COP9 signalosome (CSN) is a conserved multiprotein complex, with an important developmental role in several organisms, ranging from plants to mammalians. The influence of the CSN on several signaling and developmental processes has been ascribed to its ability to regulate degradation of a number of signaling proteins by the ubiquitin-proteasome system. The CSN controls the function of the SCF ubiquitin-ligase complex through an enzymatic activity that removes the small ubiquitin-like molecule NEDD8 from the cullin component of the SCF and that requires subunit 5 of the CSN (JAB1/CSN5). Mutants of the CSN display early embryonic lethality, a feature that has hindered further characterization of the role of the CSN at later stages of mammalian development. Here we report the analysis of JAB1/CSN5 expression pattern in the mouse embryo. At early stages of development, JAB1/CSN5 transcripts were present with low expression levels in all tissues. Preferential expression in selected tissues was detected starting at E11.5, with higher levels in dorsal root ganglia; at later stages, prominent expression of JAB1/CSN5 transcripts was observed in cranial nerve, spinal and sympathetic ganglia, as well as in selected epithelia, such as the oral and the olfactory epithelium. In the adult brain, additional areas of JAB1/CSN5 expression were the hippocampus and the Purkinjie layer of the cerebellum. We also analyzed the temporal and spatial expression pattern of NEDD8, and found that it substantially overlapped JAB1/CSN5 expression at all stages analyzed, supporting the model of a functional interaction between the two proteins during developmental processes.

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.

The Jab1/COP9 signalosome subcomplex is a downstream mediator of Bcr-Abl kinase activity and facilitates cell-cycle progression

Jab1 is a multifunctional protein associated with the signaling pathway, cell-cycle regulation, and development, and acts as a key subunit of COP9 signalosome (CSN). Jab1 promotes degradation of the cyclin-dependent kinase inhibitor p27Kip1 by transportation from the nucleus to the cytoplasm. However, there has been no clear evidence for whether and how Jab1 contributes to malignant transformation in human cancers. Here we show that Bcr-Abl tyrosine kinase facilitates the down-regulation of p27 by modulating complex formation of Jab1/CSN through the mitogen-activated protein (MAP) kinase and phosphatidylinositol 3 (PI3) kinase signaling pathways. Nearly half of the chronic myelogenous leukemia cell lines and the murine hematopoietic precursor cells expressing Bcr-Abl exhibited a marked increase in the small loose Jab1 complex located in the cytoplasm. Inhibition of Bcr-Abl kinase by STI571 induced G1 arrest and caused a recovery of the p27 level with reduction of the small Jab1 complex from the cytoplasm. Either blockade of the MAP kinase and PI3 kinase pathways by specific inhibitors or Jab1 knockdown by small interfering RNA (siRNA) prevented p27 down-regulation as well as formation of the small complex. Thus, regulation of p27 via modulation of the Jab1 subcomplex is a novel mechanism whereby Bcr-Abl oncogenic signals accelerate abnormal cell proliferation.

Small Jab1-containing subcomplex is regulated in an anchorage- and cell cycle-dependent manner, which is abrogated by ras transformation

Jab1 interacts with a variety of cell cycle and signal transduction regulators to control cell proliferation, differentiation, and tumorigenesis. In this study, we employed a non-denaturing gel electrophoresis method to separate different Jab1-containing complexes, the COP9 signalosome complex and the small Jab1-containing subcomplex. The formation of the small Jab1 complex was dependent on a low cell density and anchorage to a solid support, and enhanced during the early G1 phase of the cell cycle, which was abrogated in ras-transformed cells. The small Jab1-containing subcomplex may be a novel mediator of anchorage and cell-cell contact-dependent signal transduction.

The COP9 signalosome (CSN): an evolutionary conserved proteolysis regulator in eukaryotic development

The COP9 signalosome (CSN) is a multiprotein complex of the ubiquitin-proteasome pathway. CSN is typically composed of eight subunits, each of which is related to one of the eight subunits that form the lid of the 26S proteasome regulatory particle. CSN was first identified in Arabidopsis where it is required for the repression of photomorphogenic seedling development in the dark. CSN or CSN-related complexes have by now been reported from most eukaryotic model organisms and CSN has been implicated in a vast array of biological processes. It is widely accepted that CSN directly interacts with cullin-containing E3 ubiquitin ligases, and that CSN is required for their proper function. The requirement of CSN for proper E3 function may at least in part be explained by the observation that CSN subunit 5 (CSN5) is the isopeptidase that deconjugates the essential ubiquitin-like Nedd8 modification from the E3 cullin subunit. In addition to its interaction with E3s, CSN may also regulate proteolysis by its association with protein kinases and deubiquitylating enzymes. This review provides a summary of the role of CSN in regulating protein degradation and in eukaryotic development.

Multiple Functions of Jab1 Are Required for Early Embryonic Development and Growth Potential in Mice

Jab1 interacts with a variety of signaling molecules and regulates their stability in mammalian cells. As the fifth component of the COP9 signalosome (CSN) complex, Jab1 (CSN5) plays a central role in the deneddylation of the cullin subunit of the Skp1-Cullin-F box protein ubiquitin ligase complex. In addition, a CSN-independent function of Jab1 is suggested but is less well characterized. To elucidate the function of Jab1, we targeted the Jab1 locus by homologous recombination in mouse embryonic stem cells. Jab1-null embryos died soon after implantation. Jab1-/- embryonic cells, which lacked other CSN components, expressed higher levels of p27, p53, and cyclin E, resulting in impaired proliferation and accelerated apoptosis. Jab1 heterozygous mice were healthy and fertile but smaller than their wild-type littermates. Jab1+/- mouse embryonic fibroblast cells, in which the amount of Jab1-containing small subcomplex, but not that of CSN, was selectively reduced, proliferated poorly, showed an inefficient down-regulation of p27 during G1, and was delayed in the progression from G0 to S phase by 3 h compared with the wild-type cells. Most interestingly, in Jab1+/- mouse embryonic fibroblasts, the levels of cyclin E and deneddylated Cul1 were unchanged, and p53 was not induced. Thus, Jab1 controls cell cycle progression and cell survival by regulating multiple cell cycle signaling pathways.

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

The COP9 signalosome is an eight-subunit protein complex that regulates protein ubiquitination and protein turnover in a variety of plant developmental and physiological contexts, including light-regulated development, hormone signaling, and defense against pathogens. In all eukaryotes tested, the COP9 signalosome is able to posttranslationally modify the cullin subunit of E3-ubiquitin-ligase complexes by cleaving off the covalently coupled peptide, Nedd8. Two contrasting models ascribe stimulatory or inhibitory roles to the modification of cullin/E3 that is mediated by the COP9 signalosome. There is considerable disagreement as to whether Nedd8 cleavage underlies all of the COP9 signalosome's numerous cellular and phenotypic effects. This is because macroscopic phenotypes do not always correlate with biochemical defects in COP9 signalosome mutants. Additional biochemical activities, including protein interactions with the cellular machineries for protein phosphorylation, protein turnover, and protein translation, have been proposed to account for the role of the COP9 signalosome in development and disease.

The COP9 Signalosome

The COP9 signalosome (CSN) is composed of eight distinct subunits and is highly homologous to the lid sub-complex of the 26S proteasome. CSN was initially defined as a repressor of photomorphogenesis in Arabidopsis, and it has now been found to participate in diverse cellular and developmental processes in various eukaryotic organisms. Recently, CSN was revealed to have a metalloprotease activity centered in the CSN5/Jab1 subunit, which removes the post-translational modification of a ubiquitin-like protein, Nedd8/Rub1, from the cullin component of SCF ubiquitin E3 ligase (i.e., de-neddylation). In addition, CSN is associated with de-ubiquitination activity and protein kinase activities capable of phosphorylating important signaling regulators. The involvement of CSN in a number of cellular and developmental processes has been attributed to its control over ubiquitin-proteasome-mediated protein degradation.

COP9 signalosome subunit 3 is essential for maintenance of cell proliferation in the mouse embryonic epiblast

Csn3 (Cops3) maps to the mouse chromosome 11 region syntenic to the common deletion interval for the Smith-Magenis syndrome, a contiguous gene deletion syndrome. It encodes the third subunit of an eight-subunit protein complex, the COP9 signalosome (CSN), which controls a wide variety of molecules of different functions. Mutants of this complex caused lethality at early development of both plants and Drosophila melanogaster. CSN function in vivo in mammals is unknown. We disrupted the murine Csn3 gene in three independent ways with insertional vectors, including constructing a approximately 3-Mb inversion chromosome. The heterozygous mice appeared normal, although the protein level was reduced. Csn3(-/-) embryos arrested after 5.5 days postcoitum (dpc) and resorbed by 8.5 dpc. Mutant embryos form an abnormal egg cylinder which does not gastrulate. They have reduced numbers of epiblast cells, mainly due to increased cell death. In the Csn3(-/-) mice, subunit 8 of the COP9 complex was not detected by immunohistochemical techniques, suggesting that the absence of Csn3 may disrupt the entire COP9 complex. Therefore, Csn3 is important for maintaining the integrity of the COP9 signalosome and is crucial to maintain the survival of epiblast cells and thus the development of the postimplantation embryo in mice.

Disruption of the COP9 signalosome Csn2 subunit in mice causes deficient cell proliferation, accumulation of p53 and cyclin E, and early embryonic death

Csn2 (Trip15/Cops2/Alien) encodes the second subunit of the COP9 signalosome (CSN), an eight-subunit heteromeric complex homologous to the lid subcomplex of the 26S proteasome. CSN is a regulator of SCF (Skp1-cullin-F-box protein)ubiquitin ligases, mostly through the enzymatic activity that deconjugates the ubiquitin-like protein Nedd8 from the SCF Cul1 component. In addition, CSN associates with protein kinase activities targeting p53, c-Jun, and IkappaB for phosphorylation. Csn2 also interacts with and regulates a subset of nuclear hormone receptors and is considered a novel corepressor. We report that targeted disruption of Csn2 in mice caused arrest of embryo development at the peri-implantation stage. Csn2(-/-) blastocysts failed to outgrow in culture and exhibited a cell proliferation defect in inner cell mass, accompanied by a slight decrease in Oct4. In addition, lack of Csn2 disrupted the CSN complex and resulted in a drastic increase in cyclin E, supporting a role for CSN in cooperating with the SCF-ubiquitin-proteasome system to regulate protein turnover. Furthermore, Csn2(-/-) embryos contained elevated levels of p53 and p21, which may contribute to premature cell cycle arrest of the mutant.

COP9 Signalosome

COP9 Signalosome (CSN) is a fascinating protein complex whose biochemical and physiological functions are only beginning to be understood. It is conserved throughout eukaryotes and is critical to the proper development of all multicellular organisms in which its function has been explored. Recent work suggests that CSN plays a key role in sustaining the activity of SCF and other cullin-based ubiquitin ligases, which may account for its essential roles in development. Here, we summarize what is known about CSN, and discuss hypotheses for how CSN promotes the activity of SCF ubiquitin ligases.

The COP9 signalosome is an essential regulator of development in the filamentous fungus Aspergillus nidulans

The COP9 signalosome (CSN) is a conserved multiprotein complex involved in regulation of eukaryotic development. The deduced amino acid sequences of two Aspergillus nidulans genes, csnD and csnE, show high identities to the fourth and fifth CSN subunits of higher eukaryotes. The csnD transcript is abundant during vegetative growth as well as development and the corresponding protein accumulates in the nucleus. Strains deleted for either csn gene are viable and show identical mutant phenotypes at conditions that allow development: hyphae appear partly red and contain cells of reduced size. Additionally, light dependence of propagation onset is affected. The Delta csn mutants are capable of initiating the sexual cycle and develop primordia, but maturation to sexual fruit bodies is blocked. This developmental arrest could not be overcome by overexpression of the sexual activator velvet (VEA). We conclude that the COP9 signalosome in A. nidulans is a key regulator of sexual development, and its proposed structural and functional conservation to the CSN of higher eukaryotes enables studies on this regulatory complex in a genetically amenable organism.

The COP9 signalosome-like complex in S. cerevisiae and links to other PCI complexes

The COP9 signalosome (CSN), the lid subcomplex of the proteasome and translational initiation factor 3 (eIF3) share structural similarities and are often referred to as the PCI family of complexes. In multicellular eukaryotes, the CSN is highly conserved as an 8-subunit complex but in Saccharomyces cerevisiae the complex is rather divergent. We further characterize the composition and properties of the CSN in budding yeast and its interactions with these related complexes. Using the generalized profile method we identified CSN candidates, four with PCI domains: Csn9, Csn10, Pci8/Csn11, and Csn12, and one with an MPN domain, Csn5/Rri1. These proteins and an additional interactor, Csi1, were tested for pairwise interactions by yeast two-hybrid and were found to form a cluster surrounding Csn12. Csn5 and Csn12 cofractionate in a complexed form with an apparent molecular weight of roughly 250kDa. However, Csn5 migrates as a monomer in Deltacsn12 supporting the pivotal role of Csn12 in stabilizing the complex. Confocal fluorescence microscopy detects GFP-tagged Csn5 preferentially in the nucleus, whereas in absence of Csn12, Csn10, Pci8/Csn11, or Csi1, Csn5 is delocalized throughout the cell, indicating that multiple subunits are required for nuclear localization of Csn5. Two CSN subunits, Csn9 and Csi1, interact with the proteasome lid subunit Rpn5. Pci8/Csn11 has previously been shown to interact with eIF3. Together, these results point to a network of interactions between these three structurally similar, yet functionally diverse, complexes.

The COP9 signalosome promotes degradation of Cyclin E during early Drosophila oogenesis

The COP9 signalosome (CSN) is an eight-subunit complex that regulates multiple signaling and cell cycle pathways. Here we link the CSN to the degradation of Cyclin E, which promotes the G1-S transition in the cell cycle and then is rapidly degraded by the ubiquitin-proteasome pathway. Using CSN4 and CSN5/Jab1 mutants, we show that the CSN acts during Drosophila oogenesis to remove Nedd8 from Cullin1, a subunit of the SCF ubiquitin ligase. Overexpression of Cyclin E causes similar defects as mutations in CSN or SCF(Ago) subunits: extra divisions or, in contrast, cell cycle arrest and polyploidy. Because the phenotypes are so similar and because CSN and Cyclin E mutations reciprocally suppress each other, Cyclin E appears to be the major target of the CSN during early oogenesis. Genetic interactions among CSN, SCF, and proteasome subunits further confirm CSN involvement in ubiquitin-mediated Cyclin E degradation.

COP9 signalosome components play a role in the mating pheromone response of S. cerevisiae

A family of genetically and structurally homologous complexes, the proteasome lid, Cop9 signalosome (CSN) and eukaryotic translation initiation factor 3, mediate different regulatory pathways. The CSN functions in numerous eukaryotes as a regulator of development and signaling, yet until now no evidence for a complex has been found in Saccharomyces cerevisiae. We identified a group of proteins, including a homolog of Csn5/Jab1 and four uncharacterized PCI components, that interact in a manner suggesting they form a complex analogous to the CSN in S. cerevisiae. These newly identified subunits play a role in adaptation to pheromone signaling. Deletants for individual subunits enhance pheromone response and increase mating efficiency. Overexpression of individual subunits or a human homolog mitigates sst2-induced pheromone sensitivity. Csi1, a novel CSN interactor, exhibits opposite phenotypes. Deletants also accumulate Cdc53/cullin in a Rub1-modified form; however, this role of the CSN appears to be distinct from that in the mating pathway.