COP9 complex homolog subunit 5
The Constitutive Photomorphogenic9 (COP9) complex is a nuclear localized, multisubunit protein complex essential for repression of light-mediated development in Arabidopsis. Mutations that abolish the complex result in constitutive photomorphogenic development in darkness and pleiotropic developmental defects in both light and darkness. Two apparently redundant genes, AJH1 and AJH2, encode a subunit of the COP9 complex. Both AJH1 and AJH2 share high amino acid sequence identity (62 and 63%, respectively) with JAB1, a specific mammalian coactivator of AP-1 transcription. The proteins encoded by these two genes are present in both complex and monomeric forms, whereas complex formation is in part mediated by the direct interaction with FUSCA6. In addition, the stability of the monomeric AJH proteins requires functional COP1 and DEETIOLATED1 loci. Together with the fact that the previously known subunit FUSCA6 is an Arabidopsis homolog of human GPS1, a negative regulator of AP-1 transcription, these data suggest that the COP9 complex may contain both negative and positive regulators of transcription. Therefore, the COP9 complex may achieve its pleiotropic effects on Arabidopsis development by modulating activities of transcription factors in response to environmental stimuli (Kwok, 1998).
The COP9 signalosome is a highly conserved eight-subunit protein complex initially defined as a repressor of photomorphogenic development in Arabidopsis. It has recently been suggested that the COP9 signalosome directly interacts and regulates SCF type E3 ligases, implying a key role in ubiquitin-proteasome mediated protein degradation. Arabidopsis FUS11 gene encodes the subunit 3 of the COP9 signalosome (CSN3). The fus11 mutant is defective in the COP9 signalosome and accumulates significant amount of multi-ubiquitinated proteins. The same mutant is specifically impaired in the 26S proteasome-mediated degradation of HY5 but not PHYA, indicating a selective involvement in protein degradation. Reduction-of-function transgenic lines of CSN3 produced through gene co-suppression also accumulate multi-ubiquitinated proteins and exhibit diverse developmental defects. This result substantiates a hypothesis that the COP9 signalosome is involved in multifaceted developmental processes through regulating proteasome-mediated protein degradation (Peng, 2001).
The COP9 signalosome (CSN) is an evolutionarily conserved multiprotein complex that mediates the repression of photomorphogenesis in the dark in Arabidopsis through the degradation of transcription factors such as HY5 and HYH. CSN-mediated HY5 and HYH degradation also requires the activity of the putative E3 ubiquitin ligase (E3) component COP1 and the E2-conjugating enzyme variant COP10. CSN is also required for auxin responses mediated by the SCF-type E3 SCF(TIR1). To determine whether Arabidopsis CSN is required for E3-mediated processes in a more general manner, plants were generated with reduced E3 function by suppressing AtRBX1, an essential core subunit of SCF-type E3s. AtRBX1 transgenic plants share multiple phenotypes with CSN reduced-function plants, such as morphological defects and reduced responses to auxin, jasmonic acid, and cold stress, suggesting that CSN is required for multiple AtRBX1-mediated processes. Furthermore, mutants with defects in AXR1, a protein that had been described only as a regulator of SCF(TIR1) function, also is required for other E3-mediated processes and for the COP1/COP10/CSN-mediated repression of photomorphogenesis in the dark. It is concluded that CSN and AXR1 are of general importance for different pathways that are controlled by E3-mediated protein degradation (Schweichheimer, 2002).
The COP9 signalosome (CSN) promotes the function of SCF-type cullin-based ubiquitin ligase complexes in vivo. Paradoxically, removal of the Nedd8 modification of cullins by CSN inhibits the ubiquitin ligase activity of SCF complexes in vitro. Ubiquitination-mediated degradation of the Neurospora circadian clock protein Frequency (Frq) is critical for clock function. Ubiquitination of Frq requires FWD-1, the substrate-recruiting subunit of an SCF complex. Disruption of a subunit of CSN (csn-2) impairs the degradation of Frq and compromises its normal circadian expression. A Frq-independent oscillator drives conidiation in the csn-2 mutant, resulting in a 2-d conidiation rhythm that persists in constant darkness (DD), constant light (LL), light-to-dark (LD) transitions, and temperature cycles. Strikingly, the levels of FWD-1 are drastically reduced in csn-2 mutant, explaining the impaired degradation of Frq. Reduction of FWD-1 levels in the mutant requires its F-box, suggesting that its degradation is due to autoubiquitination. In addition, SKP-1 and CUL-1 of the SCFFWD-1 complex are also unstable in the mutant. Therefore, these results establish an important role of CSN in the circadian clock of Neurospora. These findings also reconcile the CSN paradox and suggest that a major function of CSN is to maintain the stability of SCF ubiquitin ligases in vivo (He, 2005).
The function of the fission yeast cullins Pcu1p and Pcu4p requires modification by the ubiquitin-related peptide Ned8p. The COP9/signalosome (CSN) has been shown to regulate Ned8p modification of Pcu1p. Disruption of caa1/csn1, which encodes subunit 1 of the putative S. pombe CSN, results in accumulation of Pcu1p exclusively in the modified form. However, it remained unclear whether this reflects global control of all cullins by the entire CSN complex. Multiple CSN subunits control Ned8p modification of Pcu3p, another fission yeast cullin, which, in complex with the RING domain protein Pip1p, forms a ubiquitin ligase that functions in cellular stress response. Pcu3p is modified by Ned8p on Lys 729 and accumulates exclusively in the neddylated form in cells lacking the CSN subunits 1, 3, 4, and 5. These CSN subunits co-elute with Pcu3p in gel filtration fractions corresponding to approximately 550 kDa and specifically bind both native and Ned8p-modified Pcu3p in vivo. While CSN does not influence the subcellular localization of Pcu3p, Pcu3p-associated in vitro ubiquitin ligase activity is stimulated in the absence of CSN. Taken together, these data suggest that CSN is a global regulator of Ned8p modification of multiple cullins and potentially other proteins involved in cellular regulation (Zhou, 2001).
The COP9/signalosome complex is highly conserved in evolution and possesses significant structural similarity to the 19S regulatory lid complex of the proteasome. It also shares limited similarity to the translation initiation factor eIF3. The signalosome interacts with multiple cullins in mammalian cells. In the fission yeast Schizosaccharomyces pombe, the Csn1 subunit is required for the removal of covalently attached Nedd8 from Pcu1, one of three S. pombe cullins. It remains unclear whether this activity is required for all the functions ascribed to the signalosome. Csn1 and Csn2 have been identified as signalosome subunits in S. pombe. csn1 and csn2 null mutants are DNA damage sensitive and exhibit slow DNA replication. Two further putative subunits, Csn4 and Csn5, were identified from the S. pombe genome database. Null mutations of csn4 and csn5 have been characterized; both genes are required for removal of Nedd8 from the S. pombe cullin Pcu1 and their protein products associate with Csn1 and Csn2. However, neither csn4 nor csn5 null mutants share the csn1 and csn2 mutant phenotypes. The data suggest that the subunits of the signalosome cannot be considered as a distinct functional unit and imply that different subunits of the signalosome mediate distinct functions (Mundt, 2002).
The COP9/signalosome (CSN) is known to remove the stimulatory NEDD8 modification from cullins. The activity of the fission yeast cullins Pcu1p and Pcu3p is dramatically stimulated when retrieved from csn mutants but inhibited by purified CSN. This inhibition is independent of cullin deneddylation but mediated by the CSN-associated deubiquitylating enzyme Ubp12p, which forms a complex with Pcu3p in a CSN-dependent manner. In ubp12 mutants, as in csn mutants, Pcu3p activity is stimulated. CSN is required for efficient targeting of Ubp12p to the nucleus, where both cullins reside. Finally, the CSN/Ubp12p pathway maintains the stability of the Pcu1p-associated substrate-specific adaptor protein Pop1p. It is proposed that CSN/Ubp12p-mediated deubiquitylation creates an environment for the safe de novo assembly of cullin complexes by counteracting the autocatalytic destruction of adaptor proteins (Zhou, 2003).
The GLH proteins belong to a family of four germline RNA helicases in Caenorhabditis elegans. These putative ATP-dependent enzymes localize to the P granules, which are nonmembranous complexes of protein and RNA exclusively found in the cytoplasm of all C. elegans germ cells and germ cell precursors. To determine what proteins the GLHs bind, C. elegans cDNA libraries were screened by the yeast two-hybrid method, using GLHs as bait. Three interacting proteins, CSN-5, KGB-1, and ZYX-1, were identified and further characterized. GST pull-down assays independently established that these proteins bind GLHs. CSN-5 is closely related to the subunit 5 protein of COP9 signalosomes, conserved multiprotein complexes of plants and animals. RNA interference (RNAi) with csn-5 results in sterile worms with small gonads and no oocytes, a defect essentially identical to that produced by RNAi with a combination of glh-1 and glh-4. KGB-1 is a putative JNK MAP kinase that GLHs bind. A kgb-1 deletion strain has a temperature-sensitive, sterile phenotype characterized by the absence of mature oocytes and the presence of trapped, immature oocytes that have undergone endoreplication. ZYX-1 is a LIM domain protein most like vertebrate Zyxin, a cytoskeletal adaptor protein. In C. elegans, while zyx-1 appears to be a single copy gene, neither RNAi depletion nor a zyx-1 deletion strain results in an obvious phenotype. These three conserved proteins are the first members in each of their families reported to associate with germline helicases. Similar to the loss of GLH-1 and GLH-4, loss of either CSN-5 or KGB-1 causes oogenesis to cease, but does not affect the initial assembly of P granules (Smith, 2002).
The GLHs (germline RNA helicases), homologs of Drosophila Vasa, are constitutive components of the germline-specific P granules in the nematode C. elegans and are essential for fertility, yet how GLH proteins are regulated remains unknown. KGB-1 and CSN-5 are both GLH binding partners, previously identified by two-hybrid interactions. KGB-1 is a MAP kinase in the Jun N-terminal kinase (JNK) subfamily, whereas CSN-5 is a subunit of the COP9 signalosome. Intriguingly, although loss of either KGB-1 or CSN-5 results in sterility, their phenotypes are strikingly different. Whereas csn-5 RNA interference (RNAi) results in under-proliferated germlines, similar to glh-1/glh-4(RNAi), the kgb-1(um3) loss-of-function mutant exhibits germline over-proliferation. When kgb-1(um3) mutants are compared with wild-type C. elegans, GLH-1 protein levels are as much as 6-fold elevated and the organization of GLH-1 in P granules is grossly disrupted. A series of additional in vivo and in vitro tests indicates that KGB-1 and CSN-5 regulate GLH-1 levels, with GLH-1 targeted for proteosomal degradation by KGB-1 and stabilized by CSN-5. It is proposed the 'good cop: bad cop' team of CSN-5 and KGB-1 imposes a balance on GLH-1 levels, resulting in germline homeostasis. In addition, both KGB-1 and CSN-5 bind Vasa, a Drosophila germ granule component; therefore, similar regulatory mechanisms might be conserved from worms to flies (Orsborn, 2007).
The basic region-leucine zipper transcription factor c-Jun regulates gene expression and cell function. It participates in the formation of homo- or hetero-dimeric complexes that specifically bind to DNA sequences called activating protein 1 (AP-1) sites. The stability and activity of c-Jun is regulated by phosphorylation within the N-terminal activation domain. Mitogen- and stress-activated c-Jun N-terminal kinases (JNKs) were previously the only described enzymes phosphorylating c-Jun at the N terminus in vivo. A JNK-independent activation of c-Jun in vivo directed by the constitutive photomorphogenesis (COP9) signalosome has been demonstrated. The overexpression of signalosome subunit 2 (Sgn2), a subunit of the COP9 signalosome, leads to de novo assembly of the complex with a partial incorporation of the overexpressed subunit. The de novo formation of COP9 signalosome parallels an increase of c-Jun protein resulting in elevated AP-1 transcriptional activity. The c-Jun activation caused by Sgn2 overexpression is independent of JNK and mitogen-activated protein kinase kinase 4. The data demonstrate the existence of a novel COP9 signalosome-directed c-Jun activation pathway (Naumann, 1999).
The COP9 signalosome is an evolutionary conserved multiprotein complex of unknown function that acts as a negative regulator of photomorphogenic seedling development in Arabidopsis. Plants with reduced COP9 signalosome levels had decreased auxin response similar to loss-of-function mutants of the E3 ubiquitin ligase SCFTIR1. The COP9 signalosome and SCFTIR1 interact in vivo and the COP9 signalosome was required for efficient degradation of PSIAA6, a candidate substrate of SCFTIR1. Thus, the COP9 signalosome may play an important role in mediating E3 ubiquitin ligase-mediated responses (Schwechheimer, 2001a).
Nucleotide excision repair (NER) is a major cellular defense against the carcinogenic effects of ultraviolet light from the sun. Mutational inactivation of NER proteins, like DDB and CSA, leads to hereditary diseases such as xeroderma pigmentosum (XP) and Cockayne syndrome (CS). DDB2 and CSA are each integrated into nearly identical complexes via interaction with DDB1. Both complexes contain cullin 4A and Roc1 and display ubiquitin ligase activity. They also contain the COP9 signalosome (CSN), a known regulator of cullin-based ubiquitin ligases. Strikingly, CSN differentially regulates ubiquitin ligase activity of the DDB2 and CSA complexes in response to UV irradiation. Knockdown of CSN with RNA interference leads to defects in NER. These results suggest that the distinct UV response of the DDB2 and CSA complexes is involved in diverse mechanisms of NER (Groisman, 2003).
SCF ubiquitin ligases control various processes by marking regulatory proteins for ubiquitin-dependent proteolysis. To illuminate how SCF complexes are regulated, proteins that interact with the human SCF component CUL1 were sought. The COP9 signalosome (CSN), a suppressor of plant photomorphogenesis, associates with multiple cullins and promoted cleavage of the ubiquitin-like protein NEDD8 from Schizosaccharomyces pombe CUL1 in vivo and in vitro. Multiple NEDD8-modified proteins uniquely accumulated in CSN-deficient S. pombe cells. It is proposed that the broad spectrum of activities previously attributed to CSN subunits -- including repression of photomorphogenesis, activation of JUN, and activation of p27 nuclear export -- underscores the importance of dynamic cycles of NEDD8 attachment and removal in biological regulation (Lyapina, 2001).
Cullin proteins assemble a large number of RING E3 ubiquitin ligases and regulate various physiological processes. Covalent modification of cullins by the ubiquitin-like protein NEDD8 activates cullin ligases through an as yet undefined mechanism. p120(CAND1) selectively binds to unneddylated CUL1 and is dissociated by CUL1 neddylation. CAND1 formed a ternary complex with CUL1 and ROC1. CAND1 dissociates SKP1 from CUL1 and inhibits SCF ligase activity in vitro. Suppression of CAND1 in vivo increases the level of the CUL1-SKP1 complex. It is suggested that by restricting SKP1-CUL1 interaction, CAND1 regulated the assembly of productive SCF ubiquitin ligases, allowing a common CUL1-ROC core to be utilized by a large number of SKP1-F box-substrate subcomplexes (Liu, 2002).
The proliferation of mammalian cells is under strict control, and the cyclin-dependent-kinase inhibitory protein p27Kip1 is an essential participant in this regulation both in vitro and in vivo. Although mutations in p27Kip1 are rarely found in human tumors, reduced expression of the protein correlates well with poor survival among patients with breast or colorectal carcinomas, suggesting that disruption of the p27Kip1 regulatory mechanisms contributes to neoplasia. The abundance of p27Kip1 in the cell is determined either at or after translation, for example as a result of phosphorylation by cyclinE/Cdk2 complexes, degradation by the ubiquitin/proteasome pathway, sequestration by unknown Myc-inducible proteins, binding to cyclinD/Cdk4 complexes, or inactivation by the viral E1A oncoprotein. A mouse 38K protein (p38) encoded by the Jab1 gene interacts specifically with p27Kip1. Overexpression of p38 in mammalian cells causes the translocation of p27Kip1 from the nucleus to the cytoplasm, decreasing the amount of p27Kip1 in the cell by accelerating its degradation. Ectopic expression of p38 in mouse fibroblasts partially overcomes p27Kip1-mediated arrest in the G1 phase of the cell cycle and markedly reduces their dependence on serum. These findings indicate that p38 functions as a negative regulator of p27Kip1 by promoting its degradation (Tomoda, 1999).
The COP9 signalosome (CSN) is a conserved protein complex with homologies to the lid subcomplex of the 26S proteasome. It promotes cleavage of the Nedd8 conjugate (deneddylation) from the cullin component of SCF ubiquitin ligases. Evidence suggests that both cullin neddylation and deneddylation are highly dynamic, and that equilibrium for neddylation can be effectively modulated by CSN, and that neddylation allows Cul1 to form larger protein complexes. CSN2 integrates into the CSN complex via its C-terminal region and its N-terminal half region is necessary for direct interaction with Cul1. The polyclonal antibodies against CSN2 but not other CSN subunits cause accumulation of neddylated Cul1/Cul2 in HeLa cell extract, indicating that CSN2 is essential in cullin deneddylation. Further, CSN inhibits ubiquitination and degradation of the cyclin-dependent kinase inhibitor p27(kip1) in vitro. Microinjection of the CSN complex impedes the G1 cells from entering the S phase. Moreover, anti-CSN2 antibodies negate the CSN-dependent p27 stabilization and the G1/S blockage, suggesting that these functions require the deneddylation activity. It is concluded that CSN inhibits SCF ubiquitin ligase activity in targeting p27 proteolysis and negatively regulates cell cycle at the G1 phase by promoting deneddylation of Cul1 (Yang, 2002).
In higher eukaryotic cells, the p53 protein is degraded by the ubiquitin-26S proteasome system mediated by Mdm2 or the human papilloma virus E6 protein. COP9 signalosome (CSN)-specific phosphorylation targets human p53 to ubiquitin-26S proteasome-dependent degradation. As visualized by electron microscopy, p53 binds with high affinity to the native CSN complex. p53 interacts via its N-terminus with CSN subunit 5/Jab1 as shown by far-western and pull-down assays. The CSN-specific phosphorylation sites map to the core domain of p53 including Thr155. A phosphorylated peptide, Deltap53(145-164), specifically inhibits CSN-mediated phosphorylation and p53 degradation. Curcumin, a CSN kinase inhibitor, blocks E6-dependent p53 degradation in reticulocyte lysates. Mutation of Thr155 to valine is sufficient to stabilize p53 against E6-dependent degradation in reticulocyte lysates and to reduce binding to Mdm2. The p53T155V mutant accumulates in both HeLa and HL 60 cells and exhibits a mutant conformation. The mutant protein induces the cyclin-dependent inhibitor p21. In HeLa and MCF-7 cells, inhibition of CSN kinase by curcumin or Deltap53(145-164) results in accumulation of endogenous p53 (Bech-Otschir, 2001).
Integrin adhesion receptors transduce signals that control complex cell functions which require the regulation of gene expression, such as proliferation, differentiation and survival. The integrin intracellular domain has no catalytic function, indicating that interaction with other transducing molecules is crucial for integrin-mediated signaling. A protein has been identified that interacts with the cytoplasmic domain of the beta2 subunit of the alphaL/beta2 integrin LFA-1. This protein is JAB1 (Jun activation domain-binding protein 1), a coactivator of the c-Jun transcription factor. JAB1 is present both in the nucleus and in the cytoplasm of cells and a fraction of JAB1 colocalizes with LFA-1 at the cell membrane. LFA-1 engagement is followed by an increase of the nuclear pool of JAB1, paralleled by enhanced binding of c-Jun-containing AP-1 complexes to their DNA consensus site and increased transactivation of an AP-1-dependent promoter. It is suggested that signaling through the LFA-1 integrin may affect c-Jun-driven transcription by regulating JAB1 nuclear localization. This represents a new pathway for integrin-dependent modulation of gene expression (Bianchi, 2000).
Home page: The Interactive Fly © 1995, 1996 Thomas B. Brody, Ph.D.
The Interactive Fly resides on the
Society for Developmental Biology's Web server.