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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 IκB 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.

Hyperpolarization-activated cyclic nucleotide-regulated (HCN) channels in the brain associate with their auxiliary subunit TRIP8b (also known as PEX5R), a cytoplasmic protein expressed as a family of alternatively spliced isoforms. Recent in vitro and in vivo studies have shown that association of TRIP8b with HCN subunits both inhibits channel opening and alters channel membrane trafficking, with some splice variants increasing and others decreasing channel surface expression. Here, we address the structural bases of the regulatory interactions between mouse TRIP8b and HCN1. We find that HCN1 and TRIP8b interact at two distinct sites: an upstream site where the C-linker/cyclic nucleotide-binding domain of HCN1 interacts with an 80 amino acid domain in the conserved central core of TRIP8b, and a downstream site where the C-terminal -SNL tripeptide of the channel interacts with the tetratricopeptide repeat domain of TRIP8b. These two interaction sites play distinct functional roles in the effects of TRIP8b on HCN1 trafficking and gating. Binding at the upstream site is both necessary and sufficient for TRIP8b to inhibit channel opening. It is also sufficient to mediate the trafficking effects of those TRIP8b isoforms that downregulate channel surface expression, in combination with the trafficking motifs present in the N-terminal region of TRIP8b. In contrast, binding at the downstream interaction site serves to stabilize the C-terminal domain of TRIP8b, allowing for optimal interaction between HCN1 and TRIP8b as well as for proper assembly of the molecular complexes that mediate the effects of TRIP8b on HCN1 channel trafficking.

Tumor necrosis factor (TNF) family ligands play essential roles in regulating a variety of cellular processes including proliferation, differentiation and survival. Expression of Drosophila TNF ortholog Eiger (Egr) induces JNK-dependent cell death, while the roles of caspases in this process remain elusive. To further delineate the Egr-triggered cell death pathway, we performed a genetic screen to identify dominant modifiers of the Egr-induced cell death phenotype. Here we report that Egr elicits a caspase-mediated cell death pathway independent of JNK signaling. Furthermore, we show NOPO, the Drosophila ortholog of TRIP (TRAF interacting protein) encoding an E3 ubiquitin ligase, modulates Egr-induced Caspase-mediated cell death through transcriptional activation of pro-apoptotic genes reaper and hid. Finally, we found Bendless and dUEV1a, an ubiquitin-conjugating E2 enzyme complex, regulates NOPO-triggered cell death. Our results indicate that the Ben-dUEV1a complex constitutes a molecular switch that bifurcates the Egr-induced cell death signaling into two pathways mediated by JNK and caspases respectively.

Summary

HCN1 channel subunits, which contribute to the hyperpolarization-activated cation current (Ih), are selectively targeted to distal apical dendrites of hippocampal CA1 pyramidal neurons. Here we addressed the importance of the brain-specific auxiliary subunit of HCN1, TRIP8b, in regulating HCN1 expression and localization. More than ten N-terminal splice variants of TRIP8b exist in brain and exert distinct effects on HCN1 trafficking when overexpressed. We found that isoform-wide disruption of the TRIP8b/HCN1 interaction caused HCN1 to be mistargeted throughout CA1 somatodendritic compartments. In contrast, HCN1 was targeted normally to CA1 distal dendrites in a TRIP8b knockout mouse that selectively lacked exons 1b and 2. Of the two remaining hippocampal TRIP8b isoforms, TRIP8b(1a-4) promoted HCN1 surface expression in dendrites whereas TRIP8b(1a) suppressed HCN1 misexpression in axons. Thus, proper subcellular localization of HCN1 depends on its differential additive and subtractive sculpting by two isoforms of a single auxiliary subunit gene.

Through their interaction with the TNF receptor–associated factor (TRAF) family, members of the tumor necrosis factor receptor (TNFR) superfamily elicit a wide range of biological effects including differentiation, proliferation, activation, or cell death. We have identified and characterized a novel component of the receptor–TRAF signaling complex, designated TRIP (TRAF-interacting protein), which contains a RING finger motif and an extended coiled-coil domain. TRIP associates with the TNFR2 or CD30 signaling complex through its interaction with TRAF proteins. When associated, TRIP inhibits the TRAF2-mediated NF-κB activation that is required for cell activation and also for protection against apoptosis. Thus, TRIP acts as a receptor–proximal regulator that may influence signals responsible for cell activation/proliferation and cell death induced by members of the TNFR superfamily.

The Fas/CD95 receptor mediates apoptosis but is also capable of triggering nonapoptotic signals. However, the mechanisms that selectively regulate these opposing effects are not yet fully understood. Here we demonstrate that the activation of Fas or stimulation with lysophosphatidic acid (LPA) induces cytoskeletal reorganization, leading to the association of Fas with actin stress fibers and the adaptor protein TRIP6. TRIP6 binds to the cytoplasmic juxtamembrane domain of Fas and interferes with the recruitment of FADD to Fas. Furthermore, through physical interactions with NF-κB p65, TRIP6 regulates nuclear translocation and the activation of NF-κB upon Fas activation or LPA stimulation. As a result, TRIP6 antagonizes Fas-induced apoptosis and further enhances the antiapoptotic effect of LPA in cells that express high levels of TRIP6. On the other hand, TRIP6 promotes Fas-mediated cell migration in apoptosis-resistant glioma cells. This effect is regulated via the Src-dependent phosphorylation of TRIP6 at Tyr-55. As TRIP6 is overexpressed in glioblastomas, this may have a significant impact on enhanced NF-κB activity, resistance to apoptosis, and Fas-mediated cell invasion in glioblastomas.

Ubiquitination and the degradation of the large subunit of RNA polymerase II, Rpb1, is not only involved in DNA damage-induced arrest but also in other transcription-obstructing events. However, the ubiquitin ligases responsible for DNA damage-independent processes in mammalian cells remain to be identified. Here, we identified Wwp2, a mouse HECT domain ubiquitin E3 ligase, as a novel ubiquitin ligase of Rpb1. We found that Wwp2 specifically interacted with mouse Rpb1 and targeted it for ubiquitination both in vitro and in vivo. Interestingly, the interaction with and ubiquitination of Rpb1 was dependent neither on its phosphorylation state nor on DNA damage. However, the enzymatic activity of Wwp2 was absolutely required for its ubiquitin modification of Rpb1. Furthermore, our study indicates that the interaction between Wwp2 and Rpb1 was mediated through WW domain of Wwp2 and C-terminal domain of Rpb1, respectively. Strikingly, downregulation of Wwp2 expression compromised Rpb1 ubiquitination and elevated its intracellular steady-state protein level significantly. Importantly, we identified six lysine residues in the C-terminal domain of Rpb1 as ubiquitin acceptor sites mediated by Wwp2. These results indicate that Wwp2 plays an important role in regulating expression of Rpb1 in normal physiological conditions.

Posttranslational modification of proteins by ubiquitin has emerged as a critical regulator of synapse development and function. Ubiquitination is a reversible modification mediated by the concerted action of a large number of specific ubiquitin ligases and ubiquitin proteases, called deubiquitinating enzymes (DUBs). The balance of activity of these enzymes determines the localization, function, and stability of target proteins. While some DUBs counter the action of specific ubiquitin ligases by removing ubiquitin and editing ubiquitin chains, other DUBs function more generally to maintain the cellular pool of free ubiquitin monomers. The importance of DUB function at the synapse is underscored by the association of specific mutations in DUB genes with several neurological disorders. Over the last decade, although much research has led to the identification and characterization of many ubiquitin ligases at the synapse, our knowledge of the relevant DUBs that act at the synapse has lagged. This review is focused on highlighting our current understanding of DUBs that regulate synaptic function and the diseases that result from dysfunction of these DUBs.

MAK-V/Hunk is a scantily characterized AMPK-like protein kinase. Recent findings identified MAK-V as a pro-survival and anti-apoptotic protein and revealed its role in embryonic development as well as in tumorigenesis and metastasis. However molecular mechanisms of MAK-V action and regulation of its activity remain largely unknown. We identified Nedd4 as an interaction partner for MAK-V protein kinase. However, this HECT-type E3 ubiquitin ligase is not involved in the control of MAK-V degradation by the ubiquitin-proteasome system that regulates MAK-V abundance in cells. However, Nedd4 in an ubiquitin ligase-independent manner rescued developmental defects in Xenopus embryos induced by MAK-V overexpression, suggesting physiological relevance of interaction between MAK-V and Nedd4. This identifies Nedd4 as the first known regulator of MAK-V function.

The human E3 ubiquitin ligase murine double minute 2 (MDM2) targets the tumor suppressor p53 for ubiquitination and degradation but also promotes its own ubiquitination and subsequent degradation. As the balance between MDM2 and p53 levels plays a crucial role in regulating cell proliferation and apoptosis, we sought to identify factors selectively inhibiting MDM2 self-ubiquitination. Here we have shown that the LIM domain protein Enigma directly interacts with MDM2 to form a ternary complex with p53 in vitro and in human hepatoma and colon carcinoma cell lines and mouse embryonic fibroblasts. We found that Enigma elicited p53 degradation by inhibiting MDM2 self-ubiquitination and increasing its ubiquitin ligase activity toward p53 in cells. Moreover, mitogenic stimuli such as serum, FGF, and HGF increased Enigma transcription via induction of serum response factor (SRF), leading to MDM2 stabilization and subsequent p53 degradation. We observed similar results in the livers of mice treated with HGF. In humans, we found SRF and Enigma coexpressed with MDM2 but not p53 in several liver and stomach tumors. Finally, we showed that Enigma promoted cell survival and chemoresistance by suppressing p53-mediated apoptosis in both cell lines and a mouse xenograft model. Our findings suggest a role for Enigma in tumorigenesis and uncover a mechanism whereby mitogens attenuate p53 antiproliferative activity through an SRF/Enigma/MDM2 pathway.

For successful mitotic entry and spindle assembly, mitosis-promoting factors are activated at the G2/M transition stage, followed by stimulation of the anaphase-promoting complex (APC), an E3 ubiquitin ligase, to direct the ordered destruction of several critical mitotic regulators. Given that inhibition of APC activity is important for preventing premature or improper ubiquitination and destruction of substrates, several modulators and their regulation mechanisms have been studied. Emi1, an early mitotic inhibitor, is one of these regulatory factors. Here we show, by analyzing Emi1-deficient embryos, that Emi1 is essential for precise mitotic progression during early embryogenesis. Emi1−/− embryos were found to be lethal due to a defect in preimplantation development. Cell proliferation appeared to be normal, but mitotic progression was severely defective during embryonic cleavage. Moreover, multipolar spindles and misaligned chromosomes were frequently observed in Emi1 mutant cells, possibly due to premature APC activation. Our results collectively suggest that the late prophase checkpoint function of Emi1 is essential for accurate mitotic progression and embryonic viability.

The ubiquitin ligase Smurf1 can target a handful of signaling proteins for ubiquitin-mediated proteasomal destruction or functional modification, including TGF-β receptors, Smads, transcription factors, RhoA and MEKK2. Smurf1 was initially implicated in BMP pathway regulation in embryonic development, but its potential role in vertebrate embryogenesis has yet to be clarified. Here we demonstrate that inhibition of Smurf1 in Xenopus laevis embryos with an antisense morpholino oligonucleotide or a dominant-negative protein disrupts early development, with the nervous system being the principal target. Smurf1 is enriched on the dorsal side of gastrula stage embryos, and blocking Smurf1 disturbs neural folding and neural, but not mesoderm differentiation, enhances BMP/Smad1 signaling, and elevates phospho-Smad1 levels in the dorsal ectoderm. We conclude that in Xenopus embryos, the BMP pathway is a major physiological target of Smurf1, and we propose that in normal development Smurf1 cooperates with secreted BMP antagonists to limit BMP signaling in dorsal ectoderm. Our data also reveal a novel role for Smurf1 and Smad1 in neural plate morphogenesis.

The structure and dynamics of actin cytoskeleton are factors important for regulation of cell adhesion, spreading, and migration. TRIP6 is a LIM domain-containing protein interacting with many actin cytoskeleton- associated proteins and modulating the activity of certain transcription factors. To study the functions of TRIP6, we inhibited its expression in A549 and A431 cells with short interfering RNAs (siRNAs). TRIP6 knockdown led to an increase in the number and length of stress fibers and acquisition of the locomotor phenotype. Staining for paxillin demonstrated a decrease in the number of focal adhesion zones and their reorganization, while staining for E-cadherin revealed a loss of cell-to-cell contacts. These morphological changes were accompanied by a twofold increase in cell motility rate, as determined by the wound-healing assay. Thus, downregulation of TRIP6 in the above cell lines led to development of more malignant phenotype of epithelial cells. Possible mechanisms underlying the effects observed are discussed.

Background

The Cdc42-interacting protein-4, Trip10 (also known as CIP4), is a multi-domain adaptor protein involved in diverse cellular processes, which functions in a tissue-specific and cell lineage-specific manner. We previously found that Trip10 is highly expressed in estrogen receptor-expressing (ER+) breast cancer cells. Estrogen receptor depletion reduced Trip10 expression by progressively increasing DNA methylation. We hypothesized that Trip10 functions as a tumor suppressor and may be involved in the malignancy of ER-negative (ER-) breast cancer. To test this hypothesis and evaluate whether Trip10 is epigenetically regulated by DNA methylation in other cancers, we evaluated DNA methylation of Trip10 in liver cancer, brain tumor, ovarian cancer, and breast cancer.

Methods

We applied methylation-specific polymerase chain reaction and bisulfite sequencing to determine the DNA methylation of Trip10 in various cancer cell lines and tumor specimens. We also overexpressed Trip10 to observe its effect on colony formation and in vivo tumorigenesis.

Results

We found that Trip10 is hypermethylated in brain tumor and breast cancer, but hypomethylated in liver cancer. Overexpressed Trip10 was associated with endogenous Cdc42 and huntingtin in IMR-32 brain tumor cells and CP70 ovarian cancer cells. However, overexpression of Trip10 promoted colony formation in IMR-32 cells and tumorigenesis in mice inoculated with IMR-32 cells, whereas overexpressed Trip10 substantially suppressed colony formation in CP70 cells and tumorigenesis in mice inoculated with CP70 cells.

Conclusions

Trip10 regulates cancer cell growth and death in a cancer type-specific manner. Differential DNA methylation of Trip10 can either promote cell survival or cell death in a cell type-dependent manner.

Partitioning of the Golgi membrane into daughter cells during mammalian cell division occurs through a unique disassembly and reassembly process that is regulated by ubiquitination. However, the identity of the ubiquitin ligase is unknown. Here we show that the Homologous to the E6-AP Carboxyl Terminus (HECT) domain containing ubiquitin ligase HACE1 is targeted to the Golgi membrane through interactions with Rab proteins. The ubiquitin ligase activity of HACE1 in mitotic Golgi disassembly is required for subsequent postmitotic Golgi membrane fusion. Depletion of HACE1 using small interfering RNAs or expression of an inactive HACE1 mutant protein in cells impaired postmitotic Golgi membrane fusion. The identification of HACE1 as a Golgi-localized ubiquitin ligase provides evidence that ubiquitin has a critical role in Golgi biogenesis during the cell cycle.

E3 ubiquitin ligases, which target specific molecules for proteolytic destruction, have emerged as key regulators of immune functions. Several E3 ubiquitin ligases, including c-Cbl, Cbl-b, GRAIL, Itch, and Nedd4, have been shown to negatively regulate T-cell activation. Here, we report that the HECT-type E3 ligase AIP2 positively regulates T-cell activation. Ectopic expression of AIP2 in mouse primary T cells enhances their proliferation and interleukin-2 production by suppressing the apoptosis of T cells. AIP2 interacts with and promotes ubiquitin-mediated degradation of EGR2, a zinc finger transcription factor that has been found to regulate Fas ligand (FasL) expression during activation-induced T-cell death. Suppression of AIP2 expression by small RNA interference upregulates EGR2, inhibits EGR2 ubiquitination and FasL expression, and enhances the apoptosis of T cells. Therefore, AIP2 regulates activation-induced T-cell death by suppressing EGR2-mediated FasL expression via the ubiquitin pathway.

The ubiquitin–proteasome pathway is the major nonlysosomal proteolytic system in eukaryotic cells responsible for regulating the level of many key regulatory molecules within the cells. Modification of cellular proteins by ubiquitin and ubiquitin-like proteins, such as small ubiquitin-like modifying protein (SUMO), plays an essential role in a number of biological schemes, and ubiquitin pathway enzymes have become important therapeutic targets. Ubiquitination is a dynamic reversible process; a multitude of ubiquitin ligases and deubiquitinases (DUBs) are responsible for the wide-ranging influence of this pathway as well as its selectivity. The DUB enzymes serve to maintain adequate pools of free ubiquitin and regulate the ubiquitination status of cellular proteins. Using SUMO fusions, a novel assay system, based on poliovirus RNA-dependent RNA polymerase activity, is described here. The method simplifies the isopeptidase assay and facilitates high-throughput analysis of these enzymes. The principle of the assay is the dependence of the viral polymerase on a free N terminus for activity; accordingly, the polymerase is inactive when fused at its N terminus to SUMO or any other ubiquitin-like protein. The assay is sensitive, reproducible, and adaptable to a high-throughput format for use in screens for inhibitors/activators of clinically relevant SUMO proteases and deubiquitinases.

E3 ubiquitin ligases target a growing number of pro- and anti-apoptotic proteins, including tumour suppressor p53, caspases, and the Bcl-2 family. The core apoptosis pathway is well conserved between mammals and Caenorhabditis elegans, but the extent to which ubiquitin ligases regulate apoptotic cell death is not known. To investigate the role of E3 ligases in apoptosis, we inhibited 108 of the 165 predicted E3 ubiquitin ligase genes by RNA interference and quantified apoptosis in the C. elegans germline after genotoxic stress. From this screen, we identified the homologous to E6-associated protein C terminus-domain E3 ligase EEL-1 as a positive regulator of apoptosis. Intriguingly, the human homologue of EEL-1, Huwe1/ARF-BP1/Mule/HectH9, has been reported to possess both pro- and anti-apoptotic functions through its ability to stimulate Mcl-1 and p53 degradation, respectively. Here, we demonstrate that eel-1 is required to promote DNA damage-induced germ cell apoptosis, but does not have a role in physiological germ cell apoptosis or developmental apoptosis in somatic tissue. Furthermore, eel-1 acts in parallel to the p53-like gene cep-1 and intersects the core apoptosis pathway upstream of the Bcl-2/Mcl-1 orthologue ced-9. Although ee1-1 mutants exhibit hypersensitivity to genotoxic stress they do not appear to be defective in DNA repair, suggesting a distinct role for EEL-1 in promoting damage-induced apoptosis in the germline.

The ubiquitination–proteasome and degradation system is an essential process that regulates protein homeostasis. This system is involved in the regulation of cell proliferation, differentiation and survival, and dysregulations in this system lead to pathologies including cancers. The ubiquitination system is an enzymatic cascade that mediates the marking of target proteins by an ubiquitin label and thereby directs their degradation through the proteasome pathway. The ubiquitination of proteins occurs through a three-step process involving ubiquitin activation by the E1 enzyme, allowing for the transfer to a ubiquitin-conjugated enzyme E2 and to the targeted protein via ubiquitin-protein ligases (E3), the most abundant group of enzymes involved in ubiquitination. Significant advances have been made in our understanding of the role of E3 ubiquitin ligases in the control of bone turnover and tumorigenesis. These ligases are implicated in the regulation of bone cells through the degradation of receptor tyrosine kinases, signaling molecules and transcription factors. Initial studies showed that the E3 ubiquitin ligase c-Cbl, a multi-domain scaffold protein, regulates bone resorption by interacting with several molecules in osteoclasts. Further studies showed that c-Cbl controls the ubiquitination of signaling molecules in osteoblasts and in turn regulates osteoblast proliferation, differentiation and survival. Recent data indicate that c-Cbl expression is decreased in primary bone tumors, resulting in excessive receptor tyrosine kinase signaling. Consistently, c-Cbl ectopic expression reduces bone tumorigenesis by promoting tyrosine kinase receptor degradation. Here, we review the mechanisms of action of E3 ubiquitin ligases in the regulation of normal and pathologic bone formation, and we discuss how targeting the interactions of c-Cbl with some substrates may be a potential therapeutic strategy to promote osteogenesis and to reduce tumorigenesis.

Summary

Endogenous 24-hour rhythms are generated by circadian clocks located in most tissues. The molecular clock mechanism is based on feedback loops involving clock genes and their protein products. Post-translational modifications, including ubiquitination, are important for regulating the clock feedback mechanism. Previous work has focused on the role of ubiquitin ligases in the clock mechanism. Here we show a role for the rhythmically-expressed deubiquitinating enzyme ubiquitin specific peptidase 2 (USP2) in clock function. Mice with a deletion of the Usp2 gene (Usp2 KO) display a longer free-running period of locomotor activity rhythms and altered responses of the clock to light. This was associated with altered expression of clock genes in synchronized Usp2 KO mouse embryonic fibroblasts and increased levels of clock protein PERIOD1 (PER1). USP2 can be coimmunoprecipitated with several clock proteins but directly interacts specifically with PER1 and deubiquitinates it. Interestingly, this deubiquitination does not alter PER1 stability. Taken together, our results identify USP2 as a new core component of the clock machinery and demonstrate a role for deubiquitination in the regulation of the circadian clock, both at the level of the core pacemaker and its response to external cues.

This study identifies the degradation pathway for the F-box protein Dia2, which plays an important role in maintaining genomic integrity. The Hect domain E3 ligase Tom1 recognizes a stretch of positively charged residues in Dia2, leading to Dia2 degradation by the ubiquitin proteasome system. Failure to degrade Dia2 disrupts cell cycle dynamics.

The ubiquitin proteasome system plays a pivotal role in controlling the cell cycle. The budding yeast F-box protein Dia2 is required for genomic stability and is targeted for ubiquitin-dependent degradation in a cell cycle–dependent manner, but the identity of the ubiquitination pathway is unknown. We demonstrate that the Hect domain E3 ubiquitin ligase Tom1 is required for Dia2 protein degradation. Deletion of DIA2 partially suppresses the temperature-sensitive phenotype of tom1 mutants. Tom1 is required for Dia2 ubiquitination and degradation during G1 and G2/M phases of the cell cycle, whereas the Dia2 protein is stabilized during S phase. We find that Tom1 binding to Dia2 is enhanced in G1 and reduced in S phase, suggesting a mechanism for this proteolytic switch. Tom1 recognizes specific, positively charged residues in a Dia2 degradation/NLS domain. Loss of these residues blocks Tom1-mediated turnover of Dia2 and causes a delay in G1–to–S phase progression. Deletion of DIA2 rescues a delay in the G1–to–S phase transition in the tom1Δ mutant. Together our results suggest that Tom1 targets Dia2 for degradation during the cell cycle by recognizing positively charged residues in the Dia2 degradation/NLS domain and that Dia2 protein degradation contributes to G1–to–S phase progression.

In eukaryotic cells an uncleavable ubiquitin moiety conjugated to the N-terminus of a protein signals the degradation of the fusion protein via the proteasome-dependent ubiquitin fusion degradation (UFD) pathway. In yeast the molecular mechanism of the UFD pathway has been well characterized. Recently the human E3 ubiquitin-protein ligase TRIP12 was connected with the UFD pathway, but little is otherwise known about this system in mammalian cells. In the present work, we utilized high-throughput imaging on cells transfected with a targeted siRNA library to identify components involved in degradation of the UFD substrate UbG76V-YFP. The most significant hits from the screen were the E3 ubiquitin-protein ligase HUWE1, as well as PSMD7 and PSMD14 that encode proteasome subunits. Accordingly, knock down of HUWE1 led to an increase in the steady state level and a retarded degradation of the UFD substrate. Knock down of HUWE1 also led to a stabilization of the physiological UFD substrate UBB+1. Precipitation experiments revealed that HUWE1 is associated with both the UbG76V-YFP substrate and the 26S proteasome, indicating that it functions late in the UFD pathway. Double knock down of HUWE1 and TRIP12 resulted in an additive stabilization of the substrate, suggesting that HUWE1 and TRIP12 function in parallel during UFD. However, even when both HUWE1 and TRIP12 are downregulated, ubiquitylation of the UFD substrate was still apparent, revealing functional redundancy between HUWE1, TRIP12 and yet other ubiquitin-protein ligases.

Accurate chromosome segregation during meiosis requires that homologous chromosomes pair and become physically connected so that they can orient properly on the meiosis I spindle. These connections are formed by homologous recombination closely integrated with the development of meiosis-specific, higher-order chromosome structures. The yeast Pch2 protein has emerged as an important factor with roles in both recombination and chromosome structure formation, but recent analysis suggested that TRIP13, the mouse Pch2 ortholog, is not required for the same processes. Using distinct Trip13 alleles with moderate and severe impairment of TRIP13 function, we report here that TRIP13 is required for proper synaptonemal complex formation, such that autosomal bivalents in Trip13-deficient meiocytes frequently displayed pericentric synaptic forks and other defects. In males, TRIP13 is required for efficient synapsis of the sex chromosomes and for sex body formation. Furthermore, the numbers of crossovers and chiasmata are reduced in the absence of TRIP13, and their distribution along the chromosomes is altered, suggesting a role for TRIP13 in aspects of crossover formation and/or control. Recombination defects are evident very early in meiotic prophase, soon after DSB formation. These findings provide evidence for evolutionarily conserved functions for TRIP13/Pch2 in both recombination and formation of higher order chromosome structures, and they support the hypothesis that TRIP13/Pch2 participates in coordinating these key aspects of meiotic chromosome behavior.

Author Summary

Meiosis is the specialized cell division that gives rise to reproductive cells such as sperm and eggs. During meiosis in most organisms, genetic information is exchanged between homologous maternal and paternal chromosomes through the process of homologous recombination. This recombination forms connections between homologous chromosomes that allow them to segregate accurately when the meiotic cell divides. Recombination defects can result in reproductive cells with abnormal chromosome numbers, which are a major cause of developmental disorders and spontaneous abortions in humans. Meiotic recombination is tightly controlled such that each pair of chromosomes undergoes at least one crossover recombination event despite a low average number of crossovers per chromosome. Recombination is coordinated with the development of specialized, meiosis-specific chromosome structures that stabilize pairing interactions between homologous maternal and paternal chromosomes. We show here that the mouse TRIP13 protein is required for normal execution of many aspects of meiotic recombination and chromosome structure development that it was not previously known to influence. Intriguingly, many of these new roles appear to parallel known functions of a homologous protein from budding yeast, called Pch2. These findings thus indicate that TRIP13/Pch2 functions are more widely conserved throughout evolution than thought before.

The ubiquitin ligase Nedd4 has been proposed to regulate a number of signaling pathways, but its physiological role in mammals has not been characterized. Here we present an analysis of Nedd4-null mice to show that loss of Nedd4 results in reduced insulin-like growth factor 1 (IGF-1) and insulin signaling, delayed embryonic development, reduced growth and body weight, and neonatal lethality. In mouse embryonic fibroblasts, mitogenic activity was reduced, the abundance of the adaptor protein Grb10 was increased, and the IGF-1 receptor, which is normally present on the plasma membrane, was mislocalized. However, surface expression of IGF-1 receptor was restored in homozygous mutant mouse embryonic fibroblasts after knockdown of Grb10, and Nedd4−/− lethality was rescued by maternal inheritance of a disrupted Grb10 allele. Thus, in vivo, Nedd4 appears to positively control IGF-1 and insulin signaling partly through the regulation of Grb10 function.

TRIP6 (thyroid receptor-interacting protein 6), also known as ZRP-1 (zyxin-related protein 1), is a member of the zyxin family that has been implicated in cell motility. Previously we have shown that TRIP6 binds to the LPA2 receptor and associates with several components of focal complexes in an agonist-dependent manner and, thus, enhances lysophosphatidic acid (LPA)-induced cell migration. Here we further report that the function of TRIP6 in LPA signaling is regulated by c-Src-mediated phosphorylation of TRIP6 at the Tyr-55 residue. LPA stimulation induces tyrosine phosphorylation of endogenous TRIP6 in NIH 3T3 cells and c-Src-expressing fibroblasts, which is virtually eliminated in Src-null fibroblasts. Strikingly, both phosphotyrosine-55 and proline-58 residues of TRIP6 are required for Crk binding in vitro and in cells. Mutation of Tyr-55 to Phe does not alter the ability of TRIP6 to localize at focal adhesions or associate with actin. However, it abolishes the association of TRIP6 with Crk and p130cas in cells and significantly reduces the function of TRIP6 to promote LPA-induced ERK activation. Ultimately, these signaling events control TRIP6 function in promoting LPA-induced morphological changes and cell migration.