Temperature-sensitive (ts) CHO-K1 mutant tsTM3 exhibits chromosomal instability and cell-cycle arrest in the S to G2 phases with decreased DNA synthesis at the nonpermissive temperature, 39°C. Previously, complementation tests with other mutants showed that tsTM3 harbors a genetic defect in the ubiquitin-activating enzyme Uba1. Sequence comparison of the Uba1 gene between wild-type and mutant cells in this study revealed that the mutant phenotype is caused by a G-to-A transition that yields a Met-to-Ile substitution at position 256 in hamster Uba1. The ts defects in tsTM3 were complemented by expression of the wild-type Uba1 tagged with green fluorescent protein. Expression of the Uba1 primarily in the nucleus appeared to rescue tsTM3 cells. Incubation at 39°C resulted in a decrease of nuclear Uba1 in tsTM3 cells, suggesting that loss of Uba1 in the nucleus may lead to the ts defects. Analyses with the fluorescent ubiquitination-based cell cycle indicator revealed that loss of function of Uba1 leads to failure of the ubiquitin system in the nucleus. Incubation at 39°C caused an increase in endogenous geminin in tsTM3 cells. A ts mutation of Uba1 found in tsTM3 cells appears to be a novel mutation reflecting the important roles of Uba1 in nucleus.
Post-translational protein modifications are systems designed to expand restricted genomic information through functional conversion of target molecules. Ubiquitin-like post-translational modifiers regulate numerous cellular events through their covalent linkages to target protein(s) by an enzymatic cascade analogous to ubiquitylation consisting of E1 (activating), E2 (conjugating) and E3 (ligating) enzymes. In this study, we report the essential role of Uba5, a specific activating enzyme for the ubiquitin-like modifier, Ufm1, in erythroid development. Mice lacking Uba5 exhibited severe anaemia, followed by death in utero. Although Uba5 was dispensable for the production of erythropoietin, its genetic loss led to impaired development of megakaryocyte and erythroid progenitors from common myeloid progenitors. Intriguingly, transgenic expression of Uba5 in the erythroid lineage rescued the Uba5-deficient embryos from anaemia and prolonged their survival, demonstrating the importance of Uba5 in cell-autonomous erythroid differentiation. Our results suggest that one of the ubiquitin-like protein modification systems, the Ufm1 system, is involved in the regulation of haematopoiesis.
Post-translational modifications are important in regulating protein function and turnover, and Ufm1 is part of a recently identified protein modification system. In this study, the authors show that Uba5, a component of the Ufm1 system, is important for regulating haematopoiesis and the differentiation of erythroid cells.
E1 ubiquitin activating enzyme catalyzes the initial step in all ubiquitin-dependent processes. We report the isolation of uba1-204, a temperature-sensitive allele of the essential Saccharomyces cerevisiae E1 gene, UBA1. Uba1-204 cells exhibit dramatic inhibition of the ubiquitin–proteasome system, resulting in rapid depletion of cellular ubiquitin conjugates and stabilization of multiple substrates. We have employed the tight phenotype of this mutant to investigate the role ubiquitin conjugates play in the dynamic interaction of the UbL/UBA adaptor proteins Rad23 and Dsk2 with the proteasome. Although proteasomes purified from mutant cells are intact and proteolytically active, they are depleted of ubiquitin conjugates, Rad23, and Dsk2. Binding of Rad23 to these proteasomes in vitro is enhanced by addition of either free or substrate-linked ubiquitin chains. Moreover, association of Rad23 with proteasomes in mutant and wild-type cells is improved upon stabilizing ubiquitin conjugates with proteasome inhibitor. We propose that recognition of polyubiquitin chains by Rad23 promotes its shuttling to the proteasome in vivo.
Poly-ubiquitination of target proteins typically marks them for destruction via the proteasome and provides an essential mechanism for the dynamic control of protein levels. The E1 ubiquitin-activating enzyme lies at the apex of the ubiquitination cascade, and its activity is necessary for all subsequent steps in the reaction. We have isolated a temperature-sensitive mutation in the Caenorhabditis elegans uba-1 gene, which encodes the sole E1 enzyme in this organism. Manipulation of UBA-1 activity at different developmental stages reveals a variety of functions for ubiquitination, including novel roles in sperm fertility, control of body size, and sex-specific development. Levels of ubiquitin conjugates are substantially reduced in the mutant, consistent with reduced E1 activity. The uba-1 mutation causes delays in meiotic progression in the early embryo, a process that is known to be regulated by ubiquitin-mediated proteolysis. The uba-1 mutation also demonstrates synthetic lethal interactions with alleles of the anaphase-promoting complex, an E3 ubiquitin ligase. The uba-1 mutation provides a sensitized genetic background for identifying new in vivo functions for downstream components of the ubiquitin enzyme cascade, and it is one of the first conditional mutations reported for the essential E1 enzyme in a metazoan animal model.
Proteins that control an organism's development must first be turned on at the proper time and place, and then turned off when they are no longer needed. One of the “off” signals occurs through the attachment of a small protein, known as ubiquitin, to the target protein, which typically leads to the destruction of the target. Attachment of ubiquitin is controlled by a series of enzymes, the first of which is known as E1. Most organisms have a single gene for the E1 enzyme, and its activity is crucial for the degradation of a wide range of target proteins throughout development. We have identified a temperature-sensitive mutation in the E1 enzyme of the nematode Caenorhabditis elegans. By manipulating the growth temperature, we have determined the various functions of E1 at different stages of development. We find that this enzyme controls embryonic and larval development, sperm fertility, and body size. We also characterized sex-specific roles for E1; males exhibit progressive paralysis and defects in the tail, which is used for mating. In addition to the knowledge gained, this mutation provides a means of identifying both the functions of other ubiquitin enzymes during development as well as the target proteins that are marked for destruction.
The autosomal recessive neurodegenerative disease spinal muscular atrophy (SMA) results from low levels of survival motor neuron (SMN) protein; however, it is unclear how reduced SMN promotes SMA development. Here, we determined that ubiquitin-dependent pathways regulate neuromuscular pathology in SMA. Using mouse models of SMA, we observed widespread perturbations in ubiquitin homeostasis, including reduced levels of ubiquitin-like modifier activating enzyme 1 (UBA1). SMN physically interacted with UBA1 in neurons, and disruption of Uba1 mRNA splicing was observed in the spinal cords of SMA mice exhibiting disease symptoms. Pharmacological or genetic suppression of UBA1 was sufficient to recapitulate an SMA-like neuromuscular pathology in zebrafish, suggesting that UBA1 directly contributes to disease pathogenesis. Dysregulation of UBA1 and subsequent ubiquitination pathways led to β-catenin accumulation, and pharmacological inhibition of β-catenin robustly ameliorated neuromuscular pathology in zebrafish, Drosophila, and mouse models of SMA. UBA1-associated disruption of β-catenin was restricted to the neuromuscular system in SMA mice; therefore, pharmacological inhibition of β-catenin in these animals failed to prevent systemic pathology in peripheral tissues and organs, indicating fundamental molecular differences between neuromuscular and systemic SMA pathology. Our data indicate that SMA-associated reduction of UBA1 contributes to neuromuscular pathogenesis through disruption of ubiquitin homeostasis and subsequent β-catenin signaling, highlighting ubiquitin homeostasis and β-catenin as potential therapeutic targets for SMA.
In budding yeast, anaphase initiation is controlled by ubiquitin-dependent degradation of Pds1p. Analysis of pds1 mutants implicated Pds1p in the DNA damage, spindle assembly, and S-phase checkpoints. Though some components of these pathways are known, others remain to be identified. Moreover, the essential function of Pds1p, independent of its role in checkpoint control, has not been elucidated. To identify loci that genetically interact with PDS1, we screened for dosage suppressors of a temperature-sensitive pds1 allele, pds1-128, defective for checkpoint control at the permissive temperature and essential for viability at 37°C. Genetic and functional interactions of two suppressors are described. RAD23 and DDI1 suppress the temperature and hydroxyurea, but not radiation or nocodazole, sensitivity of pds1-128. rad23 and ddi1 mutants are partially defective in S-phase checkpoint control but are proficient in DNA damage and spindle assembly checkpoints. Therefore, Rad23p and Ddi1p participate in a subset of Pds1p-dependent cell cycle controls. Both Rad23p and Ddi1p contain ubiquitin-associated (UBA) domains which are required for dosage suppression of pds1-128. UBA domains are found in several proteins involved in ubiquitin-dependent proteolysis, though no function has been assigned to them. Deletion of the UBA domains of Rad23p and Ddi1p renders cells defective in S-phase checkpoint control, implicating UBA domains in checkpoint signaling. Since Pds1p destruction, and thus checkpoint regulation of mitosis, depends on ubiquitin-dependent proteolysis, we propose that the UBA domains functionally interact with the ubiquitin system to control Pds1p degradation in response to checkpoint activation.
In this report, we demonstrate the existence of the ubiquitin fold modifier-1 (Ufm1) and its conjugation pathway in trypanosomatid parasite Leishmania donovani. LdUfm1 is activated by E1-like enzyme LdUba5. LdUfc1 (E2) specifically interacted with LdUfm1 and LdUba5 to conjugate LdUfm1 to proteinaceous targets. Mass spectrometry analysis revealed that LdUfm1 is conjugated to Leishmania protein targets that are associated with mitochondria. Immunofluorescence experiments showed that Leishmania Ufm1, Uba5 and Ufc1 are associated with the mitochondria. The demonstration that all the components of this system as well as the substrates are associated with mitochondrion suggests it may have physiological roles not yet described in any other organism. Overexpression of a non-conjugatable form of LdUfm1 and an active site mutant of LdUba5 resulted in reduced survival of Leishmania in the macrophage. Since mitochondrial activities are developmentally regulated in the life cycle of trypanosomatids, Ufm1 mediated modifications of mitochondrial proteins may be important in such regulation. Thus, Ufm1 conjugation pathway in Leishmania could be explored as a potential drug target in the control of Leishmaniasis.
The covalent attachment of ubiquitin to target proteins influences various cellular processes, including DNA repair, NF-κB signalling and cell survival1. The most common mode of regulation by ubiquitin-conjugation involves specialized ubiquitin-binding proteins that bind to ubiquitylated proteins and link them to downstream biochemical processes. Unravelling how the ubiquitin-message is recognized is essential because aberrant ubiquitin-mediated signalling contributes to tumour formation2. Recent evidence indicates that inhibitor of apoptosis (IAP) proteins are frequently overexpressed in cancer and their expression level is implicated in contributing to tumorigenesis, chemoresistance, disease progression and poor patient-survival3. Here, we have identified an evolutionarily conserved ubiquitin-associated (UBA) domain in IAPs, which enables them to bind to Lys 63-linked polyubiquitin. We found that the UBA domain is essential for the oncogenic potential of cIAP1, to maintain endothelial cell survival and to protect cells from TNF-α-induced apoptosis. Moreover, the UBA domain is required for XIAP and cIAP2–MALT1 to activate NF-κB. Our data suggest that the UBA domain of cIAP2–MALT1 stimulates NF-κB signalling by binding to polyubiquitylated NEMO. Significantly, 98% of all cIAP2–MALT1 fusion proteins retain the UBA domain, suggesting that ubiquitin-binding contributes to the oncogenic potential of cIAP2–MALT1 in MALT lymphoma. Our data identify IAPs as ubiquitin-binding proteins that contribute to ubiquitin-mediated cell survival, NF-κB signalling and oncogenesis.
Studies on asbestos-induced tumourigenesis have indicated the role of, e.g., reactive oxygen/nitrogen species, mitochondria, as well as NF-κB and MAPK signalling pathways. The exact molecular mechanisms contributing to asbestos-mediated carcinogenesis are, however, still to be characterized.
In this study, gene expression data analyses together with gene annotation data from the Gene Ontology (GO) database were utilized to identify pathways that are differentially regulated in lung and tumour tissues between asbestos-exposed and non-exposed lung cancer patients. Differentially regulated pathways were identified from gene expression data from 14 asbestos-exposed and 14 non-exposed lung cancer patients using custom-made software and Iterative Group Analysis (iGA). Western blotting was used to further characterize the findings, specifically to determine the protein levels of UBA1 and UBA7.
Differences between asbestos-related and non-related lung tumours were detected in pathways associated with, e.g., ion transport, NF-κB signalling, DNA repair, as well as spliceosome and nucleosome complexes. A notable fraction of the pathways down-regulated in both normal and tumour tissue of the asbestos-exposed patients were related to protein ubiquitination, a versatile process regulating, for instance, DNA repair, cell cycle, and apoptosis, and thus being also a significant contributor of carcinogenesis. Even though UBA1 or UBA7, the early enzymes involved in protein ubiquitination and ubiquitin-like regulation of target proteins, did not underlie the exposure-related deregulation of ubiquitination, a difference was detected in the UBA1 and UBA7 levels between squamous cell carcinomas and respective normal lung tissue (p = 0.02 and p = 0.01) without regard to exposure status.
Our results indicate alterations in protein ubiquitination related both to cancer type and asbestos. We present for the first time pathway analysis results on asbestos-associated lung cancer, providing important insight into the most relevant targets for future research.
Canonical ubiquitin-like proteins (UBLs) such as ubiquitin, Sumo, NEDD8, and ISG15 are ligated to targets by E1-E2-E3 multienzyme cascades. The Sumo cascade, conserved among all eukaryotes, regulates numerous biological processes including protein localization, transcription, DNA replication, and mitosis. Sumo conjugation is initiated by the heterodimeric Aos1-Uba2 E1 enzyme (in humans called Sae1-Uba2), which activates Sumo's C-terminus, binds the dedicated E2 enzyme Ubc9, and promotes Sumo C-terminal transfer between the Uba2 and Ubc9 catalytic cysteines. To gain insights into details of E1-E2 interactions in the Sumo pathway, we determined crystal structures of the C-terminal ubiquitin fold domain (ufd) from yeast Uba2 (Uba2ufd), alone and in complex with Ubc9. The overall structures of both yeast Uba2ufd and Ubc9 superimpose well on their individual human counterparts, suggesting conservation of fundamental features of Sumo conjugation. Docking the Uba2ufd-Ubc9 and prior full-length human Uba2 structures allows generation of models for steps in Sumo transfer from Uba2 to Ubc9, and supports the notion that Uba2 undergoes remarkable conformational changes during the reaction. Comparisons to previous structures from the NEDD8 cascade demonstrate that UBL cascades generally utilize some parallel E1-E2 interaction surfaces. In addition, the structure of the Uba2ufd-Ubc9 complex reveals interactions unique to Sumo E1 and E2. Comparison with a previous Ubc9-E3 complex structure demonstrates overlap between Uba2 and E3 binding sites on Ubc9, indicating that loading with Sumo and E3-catalyzed transfer to substrates are strictly separate steps. The results suggest mechanisms establishing specificity and order in Sumo conjugation cascades.
Although APOBEC3G protein is a potent and innate anti-HIV-1 cellular factor, HIV-1 Vif counteracts the effect of APOBEC3G by promoting its degradation through proteasome-mediated proteolysis. Thus, any means that could prevent APOBEC3G degradation could potentially enhance its anti-viral effect. The UBA2 domain has been identified as an intrinsic stabilization signal that protects protein from proteasomal degradation. In this pilot study, we tested whether APOBEC3G, when it is fused with UBA2, can resist Vif-mediated proteasomal degradation and further inhibit HIV-1 infection.
APOBEC3G-UBA2 fusion protein is indeed more resistant to Vif-mediated degradation than APOBEC3G. The ability of UBA2 domain to stabilize APOBEC3G was diminished when polyubiquitin was over-expressed and the APOBEC3G-UBA2 fusion protein was found to bind less polyubiquitin than APOBEC3G, suggesting that UBA2 stabilizes APOBEC3G by preventing ubiquitin chain elongation and proteasome-mediated proteolysis. Consistently, treatment of cells with a proteasome inhibitor MG132 alleviated protein degradation of APOBEC3G and APOBEC3G-UBA2 fusion proteins. Analysis of the effect of APOBEC3G-UBA2 fusion protein on viral infectivity indicated that infection of virus packaged from HEK293 cells expressing APOBEC3G-UBA2 fusion protein is significantly lower than those packaged from HEK293 cells over-producing APOBEC3G or APOBEC3G-UBA2 mutant fusion proteins.
Fusion of UBA2 to APOBEC3G can make it more difficult to be degraded by proteasome. Thus, UBA2 could potentially be used to antagonize Vif-mediated APOBEC3G degradation by preventing polyubiquitination. The stabilized APOBEC3G-UBA2 fusion protein gives stronger inhibitory effect on viral infectivity than APOBEC3G without UBA2.
Genetic immunization is a promising new technology for developing vaccines against tuberculosis that are more effective. In the present study, we evaluated the effects of intracellular turnover of antigens expressed by DNA vaccines on the immune response induced by these vaccines in a mouse model of pulmonary tuberculosis. The mycobacterial culture filtrate protein MPT64 was expressed as a chimeric protein fused to one of three variants of the ubiquitin protein (UbG, UbA, and UbGR) known to differentially affect the intracellular processing of the coexpressed antigens. Immunoblot analysis of cell lysates of in vitro-transfected cells showed substantial differences in the degradation rate of ubiquinated MPT64 (i.e., UbG64 < UbA64 < UbGR64). The specific immune response generated in mice correlated with the stability of the ubiquitin-conjugated antigen. The UbA64 DNA vaccine induced a weak humoral response compared to UbG64, and a mixed population of interleukin-4 (IL-4)- and gamma interferon (IFN-γ)-secreting cells. Vaccination with the UbGR64 plasmid generated a strong Th1 cell response (high IFN-γ, low IL-4) in the absence of a detectable humoral response. Aerogenic challenge of vaccinated mice with Mycobacterium tuberculosis indicated that immunization with both the UbA64- and UbGR64-expressing plasmids evoked an enhanced protective response compared to the vector control. The expression of mycobacterial antigens from DNA vaccines as fusion proteins with a destabilizing ubiquitin molecule (UbA or UbGR) shifted the host response toward a stronger Th1-type immunity which was characterized by low specific antibody levels, high numbers of IFN-γ-secreting cells, and significant resistance to a tuberculous challenge.
Vertebrates express two enzymes for activation of ubiquitin – UBA1, which is responsible for activation of the vast majority of E2 conjugating enzymes, and UBA6, which uses the dedicated E2, USE1. However, targets and E3s for UBA6-USE1 are unknown. Here, we demonstrate that UBA6-USE1 functions with the UBR1-3 subfamily of N-recognin E3s to degrade the N-end rule substrates RGS4, RGS5, and Arg(R)-GFP. This pathway functions in the cytoplasm in parallel with the UBA1-UBE2A/B-UBR2 cascade, which promotes turnover of nuclear RGS4/5 proteins and an apparently phenotypically distinct pool of cytoplasmic RGS4/5. UBR2 promotes Lys48 (K48)-specific ubiquitin discharge from, and RGS4 ubiquitylation by, both USE1 and UBE2A in vitro. This work provides insight into the machinery employed by the UBA6-USE1 cascade to promote protein turnover, and suggests that the UBA6 and UBA1 pathways can function in parallel with the same E3 to degrade the same targets in a spatially distinct manner.
SQSTM1 mutations are common in patients with Paget disease of bone (PDB), with most affecting the C-terminal ubiquitin-associated (UBA) domain of the SQSTM1 protein. We performed structural and functional analyses of two UBA domain mutations, an I424S mutation relatively common in UK PDB patients, and an A427D mutation associated with a severe phenotype in Southern Italian patients. Both impaired SQSTM1's ubiquitin-binding function in pull-down assays and resulted in activation of basal NF-κB signalling, compared to wild-type, in reporter assays. We found evidence for a relationship between the ability of different UBA domain mutants to activate NF-κB signalling in vitro and number of affected sites in vivo in 1152 PDB patients from the UK and Italy, with A427D-SQSTM1 producing the greatest level of activation (relative to wild-type) of all PDB mutants tested to date. NMR and isothermal titration calorimetry studies were able to demonstrate that I424S is associated with global structural changes in the UBA domain, resulting in 10-fold weaker UBA dimer stability than wild-type and reduced ubiquitin-binding affinity of the UBA monomer. Our observations provide insights into the role of SQSTM1-mediated NF-κB signalling in PDB aetiology, and demonstrate that different mutations in close proximity within loop 2/helix 3 of the SQSTM1 UBA domain exert distinct effects on protein structure and stability, including indirect effects at the UBA/ubiquitin-binding interface.
•The I424S and A427D SQSTM1 mutations affect ubiquitin-binding and NF-κB signalling.•Mutant SQSTM1's ability to activate NF-κB signalling may be related to disease extent in PDB.•A427D-SQSTM1 produces the greatest activation (relative to wild-type) of all PDB mutants.•The I424S mutant destabilises the UBA dimer causing unfolding of the monomer.•UBA domain mutations of SQSTM1 exert distinct effects on protein structure and stability.
Sequestosome 1; SQSTM1; p62; Paget disease of bone; NF-κB; Ubiquitin
Cytosolic and peroxisomal enzymes necessary for methanol assimilation are synthesized when Pichia pastoris is grown in methanol. Upon adaptation from methanol to a glucose environment, these enzymes are rapidly and selectively sequestered and degraded within the yeast vacuole. Sequestration begins when the vacuole changes shape and surrounds the peroxisomes. The opposing membranes then fuse, engulfing the peroxisome. In this study, we have characterized a mutant cell line (glucose-induced selective autophagy), gsa7, which is defective in glucose-induced selective autophagy of peroxisomes, and have identified the GSA7 gene. Upon glucose adaptation, gsa7 cells were unable to degrade peroxisomal alcohol oxidase. We observed that the peroxisomes were surrounded by the vacuole, but complete uptake into the vacuole did not occur. Therefore, we propose that GSA7 is not required for initiation of autophagy but is required for bringing the opposing vacuolar membranes together for homotypic fusion, thereby completing peroxisome sequestration. By sequencing the genomic DNA fragment that complemented the gsa7 phenotype, we have found that GSA7 encodes a protein of 71 kDa (Gsa7p) with limited sequence homology to a family of ubiquitin-activating enzymes, E1. The knockout mutant gsa7Δ had an identical phenotype to gsa7, and both mutants were rescued by an epitope-tagged Gsa7p (Gsa7-hemagglutinin [HA]). In addition, a GSA7 homolog, APG7, a protein required for autophagy in Saccharomyces cerevisiae, was capable of rescuing gsa7. We have sequenced the human homolog of GSA7 and have shown many regions of identity between the yeast and human proteins. Two of these regions align to the putative ATP-binding domain and catalytic site of the family of ubiquitin activating enzymes, E1 (UBA1, UBA2, and UBA3). When either of these sites was mutated, the resulting mutants [Gsa7(ΔATP)-HA and Gsa7(C518S)-HA] were unable to rescue gsa7 cells. We provide evidence to suggest that Gsa7-HA formed a thio-ester linkage with a 25–30 kDa protein. This conjugate was not observed in cells expressing Gsa7(ΔATP)-HA or in cells expressing Gsa7(C518S)-HA. Our results suggest that this unique E1-like enzyme is required for homotypic membrane fusion, a late event in the sequestration of peroxisomes by the vacuole.
E2-25K is an ubiquitin-conjugating enzyme with the ability to synthesize Lys48-linked polyubiquitin chains. E2-25K and its homologues represent the only known E2 enzymes which contain a C-terminal ubiquitin-associated (UBA) domain as well as the conserved catalytic ubiquitin-conjugating (UBC) domain. As an additional non-covalent binding surface for ubiquitin, the UBA domain must provide some functional specialization. We mapped the protein-protein interface involved in the E2-25K UBA/ubiquitin complex by solution nuclear magnetic resonance (NMR) spectroscopy and subsequently modeled the structure of the complex. Domain-domain interactions between the E2-25K catalytic UBC domain and the UBA domain do not induce significant structural changes in the UBA domain or alter the affinity of the UBA domain for ubiquitin. We determined that one of the roles of the C-terminal UBA domain, in the context of E2-25K, is to increase processivity in Lys48-linked polyubiquitin chain synthesis, possibly through increased binding to the ubiquitinated substrate. Additionally, we see evidence that the UBA domain directs specificity in polyubiquitin chain linkage.
Ubiquitin; Ubiquitin-conjugating enzyme; E2-25K; UBA domain
Ubiquilin/PLIC proteins belong to the family of UBL-UBA proteins implicated in the regulation of the ubiquitin-dependent proteasomal degradation of cellular proteins. A human presenilin-interacting protein, ubiquilin-1 has been suggested as potential therapeutic target for treating Huntington’s disease. Ubiquilin’s interactions with mono- and polyubiquitins are mediated by its UBA domain which is one of the tightest ubiquitin binders among known ubiquitin-binding domains. Here we report the three-dimensional structure of the UBA domain of ubiquilin-1 (UQ1-UBA) free in solution and in complex with ubiquitin. UQ1-UBA forms a compact three-helix bundle structurally similar to other known UBAs, and binds to the hydrophobic patch on ubiquitin with a Kd of 20 μM. To gain structural insights into UQ1-UBA’s interactions with polyubiquitin chains, we have mapped the binding interface between UQ1-UBA and Lys48- and Lys63-linked di-ubiquitins and characterized the strength of UQ1-UBA binding to these chains. Our NMR data show that UQ1-UBA interacts with the individual ubiquitin units in both chains in a mode similar to its interaction with mono-ubiquitin, though with an improved binding affinity for the chains. Our results indicate that, in contrast to UBA2 of hHR23A that has strong binding preference for Lys48-linked chains, UQ1-UBA shows little or no binding selectivity toward a particular chain linkage or between the two ubiquitin moieties in the same chain. The structural data obtained in this study provide insights into the possible structural reasons for the diversity of polyubiquitin chain recognition by UBA domains.
ubiquilin; ubiquitin; polyubiquitin; UBA domain; protein-protein interaction
Inhibitor of apoptosis proteins (IAPs) belong to a pivotal antiapoptotic protein family that plays a crucial role in tumorigenesis, cancer progression, chemoresistance and poor patient-survival. X-linked inhibitor of apoptosis protein (XIAP) is a prominent member of IAPs attracting intense research because it has been demonstrated to be a physiological inhibitor of caspases and apoptosis. Recently, an evolutionarily conserved ubiquitin-associated (UBA) domain was identified in XIAP and a number of RING domain-bearing IAPs. This has placed the IAPs in the group of ubiquitin binding proteins. Here, we explore the three-dimensional structure of the XIAP UBA domain (XIAP-UBA) and how it interacts with mono-ubiquitin and diubiquitin conjugates.
The solution structure of the XIAP-UBA domain was determined by NMR spectroscopy. XIAP-UBA adopts a typical UBA domain fold of three tightly packed α-helices but with an additional N-terminal 310 helix. The XIAP-UBA binds mono-ubiquitin as well as Lys48-linked and linear-linked diubiquitins at low-micromolar affinities. NMR analysis of the XIAP-UBA–ubiquitin interaction reveals that it involves the classical hydrophobic patches surrounding Ile44 of ubiquitin and the conserved MGF/LV motif surfaces on XIAP-UBA. Furthermore, dimerization of XIAP-UBA was observed. Mapping of the self-association surface of XIAP-UBA reveals that the dimerization interface is formed by residues in the N-terminal 310 helix, helix α1 and helix α2, separate from the ubiquitin-binding surface.
Our results provide the first structural information of XIAP-UBA and map its interaction with mono-ubiquitin, Lys48-linked and linear-linked diubiquitins. The notion that XIAP-UBA uses different surfaces for ubiquitin-binding and self-association provides a plausible model to explain the reported selectivity of XIAP in binding polyubiquitin chains with different linkages.
Rad23 contains a ubiquitin-like domain (UbLR23) that interacts with catalytically active proteasomes and two ubiquitin (Ub)-associated (UBA) sequences that bind Ub. The UBA domains can bind Ub in vitro, although the significance of this interaction in vivo is poorly understood. Rad23 can interfere with the assembly of multi-Ub chains in vitro, and high-level expression caused stabilization of proteolytic substrates in vivo. We report here that Rad23 interacts with ubiquitinated cellular proteins through the synergistic action of its UBA domains. Rad23 plays an overlapping role with Rpn10, a proteasome-associated multi-Ub chain binding protein. Mutations in the UBA domains prevent efficient interaction with ubiquitinated proteins and result in poor suppression of the growth and proteolytic defects of a rad23Δ rpn10Δ mutant. High-level expression of Rad23 revealed, for the first time, an interaction between ubiquitinated proteins and the proteasome. This increase was not observed in rpn10Δ mutants, suggesting that Rpn10 participates in the recognition of proteolytic substrates that are delivered by Rad23. Overexpression of UbLR23 caused stabilization of a model substrate, indicating that an unregulated UbLR23-proteasome interaction can interfere with the efficient delivery of proteolytic substrates by Rad23. Because the suppression of a rad23Δ rpn10Δ mutant phenotype required both UbLR23 and UBA domains, our findings support the hypothesis that Rad23 encodes a novel regulatory factor that translocates ubiquitinated substrates to the proteasome.
Proteins containing ubiquitin-like (UBL) and ubiquitin associated (UBA) domains have been suggested to shuttle ubiquitinated substrates to the proteasome for degradation. There are three UBL-UBA containing proteins in budding yeast: Ddi1, Dsk2 and Rad23, which have been demonstrated to play regulatory roles in targeting ubiquitinated substrates to the proteasome for degradation. An involvement of these proteins in cell cycle related events has also been reported. We tested whether these three proteins act redundantly in the cell cycle.
Here we show that the UBL-UBA proteins are partially redundant for cell cycle related roles. RAD23 is redundant with DDI1 and DSK2, but DDI1 and DSK2 are not redundant with each other and the triple deletion shows a synthetic effect, suggesting the existence of at least two roles for RAD23 in cell cycle control. The rad23Δddi1Δdsk2Δ triple deletion strain delays both in G2/M-phase and in mid-anaphase at high temperatures with duplicated spindle pole bodies. Cell cycle progression in the triple deletion strain can only be partially rescued by a rad23 allele lacking the c-terminal UBA domain, suggesting that RAD23 requires its c-terminal UBA domain for full function. In addition to their ability to bind ubiquitin and the proteasome, the UBL-UBA proteins also share the ability to homodimerize. Rad23 and Dsk2 dimerization requires their UBL and/or UBA domains whereas Ddi1 dimerization does not. Here we show that Ddi1 homodimerization is necessary for its cell cycle related functions.
The three yeast UBL-UBA proteins have partially redundant roles required for progression through mitosis.
Extensive genome-wide remodeling occurs during the formation of the somatic macronuclei from the germ line micronuclei in ciliated protozoa. This process is limited to sexual reproduction and includes DNA amplification, chromosome fragmentation, and the elimination of internal segments of DNA. Our efforts to define the pathways regulating these events revealed a gene encoding a homologue of ubiquitin activating enzyme 2 (UBA2) that is upregulated at the onset of macronuclear development in Paramecium tetraurelia. Uba2 enzymes are known to activate the protein called small ubiquitin-related modifier (SUMO) that is covalently attached to target proteins. Consistent with this relationship, Northern analysis showed increased abundance of SUMO transcripts during sexual reproduction in Paramecium. RNA interference (RNAi) against UBA2 or SUMO during vegetative growth had little effect on cell survival or fission rates. In contrast, RNAi of mating cells resulted in failure to form a functional macronucleus. Despite normal amplification of the genome, excision of internal eliminated sequences was completely blocked. Additional experiments showed that the homologous UBA2 and SUMO genes in Tetrahymena thermophila are also upregulated during conjugation. These results provide evidence for the developmental regulation of the SUMO pathway in ciliates and suggest a key role for the pathway in controlling genome remodeling.
Human Fas associated factor 1 (hFAF1) is a pro-apoptotic scaffolding protein containing ubiquitin-associating (UBA), ubiquitin like 1 and 2 (UBL1, UBL2), and ubiquitin regulatory X (UBX) domains. hFAF1 interacts with polyubiquitinated proteins via its N-terminal UBA domain and with valosin containing protein (VCP) via its C-terminal UBX domain. Overexpression of hFAF1 or its N-terminal UBA domain significantly increases cell death by increasing the degradation of polyubiquitinated proteins. In this study, we investigated whether hFAF1, whose expression level is reduced in cervical cancer, plays a role in tumor formation. We found that HeLa cells overexpressing full-length hFAF1 or the hFAF1 UBA domain alone, significantly suppressed the anchorage independent tumor growth in soft agar colony formation, increased cell death, and activated JNK and caspase 3. Employing UBA-specific tandem immunoprecipitation, we identified moieties specifically interacting with UBA domain of hFAF1, and found that polyubiquitinated Hsp70s are recruited to UBA domain. We also demonstrated that hFAF1 overexpression promotes Hsp70 degradation via the proteasome. We further found that mutating the UBA domain (I41N), as well as knocking down hFAF1 with specific RNAi, abolishs its ability to increase the proteasomal degradation of Hsp70. These findings suggest that hFAF1 inhibits tumor formation by increasing the degradation of Hsp70 mediated via its UBA domain.
SUMOylation is an essential modification that regulates predominantly nuclear proteins. Here we describe two pathways for the generation of nuclear SUMO E1 enzyme, import of individual subunits, and transport of the holo-enzyme. The NLS in Uba2 is required for transport of the complex; the c-Myc–like NLS in Aos1 functions only in the free subunit.
SUMOylation, reversible attachment of small ubiquitin-related modifier (SUMO), serves to regulate hundreds of proteins. Consistent with predominantly nuclear targets, enzymes required for attachment and removal of SUMO are highly enriched in this compartment. This is true also for the first enzyme of the SUMOylation cascade, the SUMO E1 enzyme heterodimer, Aos1/Uba2 (SAE1/SAE2). This essential enzyme serves to activate SUMO and to transfer it to the E2-conjugating enzyme Ubc9. Although the last 40 amino acids in yeast Uba2 have been implicated in its nuclear localization, little was known about the import pathways of Aos1, Uba2, and/or of the assembled E1 heterodimer. Here we show that the mammalian E1 subunits can be imported separately, identify nuclear localization signals (NLSs) in Aos1 and in Uba2, and demonstrate that their import is mediated by importin α/β in vitro and in intact cells. Once assembled into a stable heterodimer, the E1 enzyme can still be efficiently imported by importin α/β, due to the Uba2 NLS that is still accessible. These pathways may serve distinct purposes: import of nascent subunits prior to assembly and reimport of stable E1 enzyme complex after mitosis.
NEDD8/Rub1 is a ubiquitin (Ub)-like molecule that covalently ligates to target proteins through an enzymatic cascade analogous to ubiquitylation. This modifier is known to target all cullin (Cul) family proteins. The latter are essential components of Skp1/Cul-1/F-box protein (SCF)–like Ub ligase complexes, which play critical roles in Ub-mediated proteolysis. To determine the role of the NEDD8 system in mammals, we generated mice deficient in Uba3 gene that encodes a catalytic subunit of NEDD8-activating enzyme. Uba3−/− mice died in utero at the periimplantation stage. Mutant embryos showed selective apoptosis of the inner cell mass but not of trophoblastic cells. However, the mutant trophoblastic cells could not enter the S phase of the endoreduplication cycle. This cell cycle arrest was accompanied with aberrant expression of cyclin E and p57Kip2. These results suggested that the NEDD8 system is essential for both mitotic and the endoreduplicative cell cycle progression. β-Catenin, a mediator of the Wnt/wingless signaling pathway, which degrades continuously in the cytoplasm through SCF Ub ligase, was also accumulated in the Uba3−/− cytoplasm and nucleus. Thus, the NEDD8 system is essential for the regulation of protein degradation pathways involved in cell cycle progression and morphogenesis, possibly through the function of the Cul family proteins.
NEDD8; ubiquitin; cullin; knock-out; cell cycle
Meiosis is unique to germ cells and occurs in a sex-specific manner. The genes regulating meiotic initiation in either sex are yet to be fully elucidated. Recent studies have revealed the importance of retinoic acid and one of its target genes, Stra8, in meiotic initiation in both sexes. Microarray analysis of whole murine embryonic ovary and postnatal testis time course data revealed a single peak of Stra8 expression in each organ at the onset of meiosis; at Embryonic Day 14.5 in the ovary and 10 days postpartum in the testis. In order to identify other genes involved in the initiation of meiosis in mammals, murine testis and ovary microarray data were examined more closely for transcripts with expression profiles similar to Stra8. Three such candidates include establishment of cohesion 1 homolog 2 (Esco2), encoding a protein essential for sister chromatid cohesion; SET domain, bifurcated 2 (Setdb2), the mouse ortholog of Eggless, which is essential for oogenesis in Drosophila; and ubiquitin-activating enzyme 6 (Uba6), a gene with fivefold higher expression in human and mouse testes than any other organ. In situ hybridization and immunohistochemistry or immunofluorescence were performed to localize Esco2, Setbd2, and Uba6 expression in the developing testis. The cellular expression pattern localized all three of these transcripts and their respective proteins to germ cells transitioning from mitosis to meiosis, hence supporting the hypothesis of their involvement in the initiation of meiosis. Future research will be directed at determining a specific role for these three proteins in germ cell differentiation.
Three genes, Esco2, Setdb2, and Uba6 show a Stra8-like microarray expression profile, and their proteins localize specifically to germ cells during the mitotic-to-meiotic transition.
gamete biology; germ cells; meiosis; spermatogenesis; testis