Proteolytic regulation of metabolic enzymes by E3 ubiquitin ligase complexes: lessons from yeast

Metabolomics analysis of an AAA-ATPase Cdc48-deficient yeast strain

The ubiquitin-specific chaperone AAA-ATPase Cdc48 and its orthologs p97/valosin-containing protein (VCP) in mammals play crucial roles in regulating numerous intracellular pathways via segregase activity, which separates polyubiquitinated targets from membranes or binding partners. Interestingly, high-throughput experiments show that a vast number of metabolic enzymes are modified with ubiquitin. Therefore, Cdc48 may regulate metabolic pathways, for example by acting on the polyubiquitin chains of metabolic enzymes; however, the role of Cdc48 in metabolic regulation remains largely unknown. To begin to analyze the role of Cdc48 in metabolic regulation in yeast, we performed a metabolomics analysis of temperature-sensitive cdc48-3 mutant cells. We found that the amount of metabolites in the glycolytic pathway was altered. Moreover, the pool of nucleotides, as well as the levels of metabolites involved in the tricarboxylic acid cycle and oxidative phosphorylation, increased, whereas the pool of amino acids decreased. These results suggest the involvement of Cdc48 in metabolic regulation in yeast. In addition, because of the roles of p97/VCP in regulating multiple cellular pathways, its inhibition is being considered as a promising anticancer drug target. We propose that the metabolomics study of Cdc48-deficient yeast will be useful as a complement to p97/VCP-related pathological and therapeutic studies.

Triacylglycerol lipase Tgl4 is a stable protein and its dephosphorylation is regulated in a cell cycle-dependent manner in Saccharomyces cerevisiae

Triacylglycerols (TGs) serve as reservoirs for diacylglycerols and fatty acids, which play important roles in synthesizing energy and membrane lipids that are required for cell cycle progression. In the yeast, Saccharomyces cerevisiae, Tgl4, the functional ortholog of murine adipose triacylglycerol lipase (ATGL), is activated by Cdk1/Cdc28-mediated phosphorylation and facilitates the G1/S transition. However, little is known about how Tgl4 is inactivated during the cell cycle. To monitor the phosphorylation status and the stability of endogenous Tgl4, we raised a specific antibody against Tgl4. We found that in contrast to the previous suggestion, Tgl4 was a stable protein throughout the cell cycle. We also showed that Tgl4 was dephosphorylated upon entry into G1 phase. These results suggest that Tgl4 is a stable protein and is inactivated during G1 phase by dephosphorylation.

Spatiotemporal Regulation of Astaxanthin Synthesis in S. cerevisiae

As a high-valued antioxidant, astaxanthin biosynthesis using microbial cell factories has attracted increasing attention. However, its lipophilic nature conflicts with the limited storage capacity for lipophilic substances of model microorganisms such as Saccharomyces cerevisiae. Expansion of lipid droplets by enhancing lipid synthesis provides more storage room while diverting the metabolic flux from the target pathway. Therefore, proper spatial regulation is required. In this study, a library of genes related to lipid metabolism were screened using the trifunctional CRISPR system, identifying opi3 and hrd1 as new engineering targets to promote astaxanthin synthesis by moderately rather than excessively upregulating lipid synthesis. The astaxanthin yield reached 9.79 mg/g DCW after lipid engineering and was further improved to 10.21 mg/g DCW by balancing the expression of β-carotene hydroxylase and ketolase. Finally, by combining spatial regulation through lipid droplet engineering and temporal regulation via temperature-responsive pathway expression, 446.4 mg/L astaxanthin was produced in fed-batch fermentation.

Protein phosphatase 2A regulates cytotoxicity and drug resistance by dephosphorylating xenobiotic metabolism enzymes AHR and MDR1

Protein phosphatase 2A (PP2A) is a serine/threonine dephosphorylating enzyme complex that plays numerous roles in biological processes, including cell growth and metabolism. However, its specific actions in many of these critical pathways are unclear. To explore mechanisms underlying metabolic enzyme regulation in the liver, we investigated the key pathways involved in regulation of xenobiotic-metabolizing enzymes in a mouse model with hepatocyte-specific deletion of Ppp2r1a, encoding the Aα subunit of PP2A. We performed transcriptome and phosphoproteome analysis in mouse livers at the age of 3 months and identified 2695 differentially expressed genes and 549 upregulated phosphoproteins in homozygous knockout mouse livers compared with WT littermates. In particular, the expression of metabolic enzymes Cyp2e1, Cyp1a1, Cyp1a2, Mdr1a, and Abcg2 was dramatically altered in homozygous knockout mouse livers. We also demonstrated that activation of PP2A reversed the decline of metabolic enzyme expression in primary mouse hepatocytes. We found that specific PP2A holoenzymes were involved in metabolic enzyme induction through dephosphorylation of transcription factors, nuclear receptors, or the target enzymes themselves, leading to dysregulation of xenobiotic metabolism or drug-induced hepatotoxicity. Notably, we confirmed that a regulatory axis, PP2A B56α–aryl hydrocarbon receptor–Cyp1a1, was involved in benzo(a)pyrene-induced cytotoxicity through dephosphorylation of the metabolic nuclear receptor, aryl hydrocarbon receptor, at serine 36. In addition, we showed that PP2A B56δ complexes directly dephosphorylated the multidrug efflux pump MDR1 (encoded by multi-drug resistance gene 1), contributing to drug resistance against the chemotherapeutic 5-fluorouracil. Taken together, these novel findings demonstrate the involvement of PP2A in the regulation of liver metabolism.

Dot6/Tod6 degradation fine-tunes the repression of ribosome biogenesis under nutrient-limited conditions

Ribosome biogenesis (Ribi) is a complex and energy-consuming process, and should therefore be repressed under nutrient-limited conditions to minimize unnecessary cellular energy consumption. In yeast, the transcriptional repressors Dot6 and Tod6 are phosphorylated and inactivated by the TORC1 pathway under nutrient-rich conditions, but are activated and repress ∼200 Ribi genes under nutrient-limited conditions. However, we show that in the presence of rapamycin or under nitrogen starvation conditions, Dot6 and Tod6 were readily degraded by the proteasome in a SCF^Grr1 and Tom1 ubiquitin ligase-dependent manner, respectively. Moreover, promiscuous accumulation of Dot6 and Tod6 excessively repressed Ribi gene expression as well as translation activity and caused a growth defect in the presence of rapamycin. Thus, we propose that degradation of Dot6 and Tod6 is a novel mechanism to ensure an appropriate level of Ribi gene expression and thereby fine-tune the repression of Ribi and translation activity for cell survival under nutrient-limited conditions.

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Dot6 and Tod6 repress Ribi gene expression under nutrient-limited conditions

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Dot6 and Tod6 are degraded by the proteasome

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Excess repression of Ribi causes a growth defect in the presence of rapamycin

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Dot6 and Tod6 degradation fine-tunes the repression of Ribi and translation activity

Mechanisms and regulation of organic acid accumulation in plant vacuoles

In fleshy fruits, organic acids are the main source of fruit acidity and play an important role in regulating osmotic pressure, pH homeostasis, stress resistance, and fruit quality. The transport of organic acids from the cytosol to the vacuole and their storage are complex processes. A large number of transporters carry organic acids from the cytosol to the vacuole with the assistance of various proton pumps and enzymes. However, much remains to be explored regarding the vacuolar transport mechanism of organic acids as well as the substances involved and their association. In this review, recent advances in the vacuolar transport mechanism of organic acids in plants are summarized from the perspectives of transporters, channels, proton pumps, and upstream regulators to better understand the complex regulatory networks involved in fruit acid formation.

Divergent patterns of selection on metabolite levels and gene expression

Background

Natural selection can act on multiple genes in the same pathway, leading to polygenic adaptation. For example, adaptive changes were found to down-regulate six genes involved in ergosterol biosynthesis—an essential pathway targeted by many antifungal drugs—in some strains of the yeast Saccharomyces cerevisiae. However, the impact of this polygenic adaptation on metabolite levels was unknown. Here, we performed targeted mass spectrometry to measure the levels of eight metabolites in this pathway in 74 yeast strains from a genetic cross.

Results

Through quantitative trait locus (QTL) mapping we identified 19 loci affecting ergosterol pathway metabolite levels, many of which overlap loci that also impact gene expression within the pathway. We then used the recently developed v-test, which identified selection acting upon three metabolite levels within the pathway, none of which were predictable from the gene expression adaptation.

Conclusions

These data showed that effects of selection on metabolite levels were complex and not predictable from gene expression data. This suggests that a deeper understanding of metabolism is necessary before we can understand the impacts of even relatively straightforward gene expression adaptations on metabolic pathways.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01915-5.

GID E3 ligase supramolecular chelate assembly configures multipronged ubiquitin targeting of an oligomeric metabolic enzyme

How are E3 ubiquitin ligases configured to match substrate quaternary structures? Here, by studying the yeast GID complex (mutation of which causes deficiency in glucose-induced degradation of gluconeogenic enzymes), we discover supramolecular chelate assembly as an E3 ligase strategy for targeting an oligomeric substrate. Cryoelectron microscopy (cryo-EM) structures show that, to bind the tetrameric substrate fructose-1,6-bisphosphatase (Fbp1), two minimally functional GID E3s assemble into the 20-protein Chelator-GIDSR4, which resembles an organometallic supramolecular chelate. The Chelator-GIDSR4 assembly avidly binds multiple Fbp1 degrons so that multiple Fbp1 protomers are simultaneously ubiquitylated at lysines near the allosteric and substrate binding sites. Importantly, key structural and biochemical features, including capacity for supramolecular assembly, are preserved in the human ortholog, the CTLH E3. Based on our integrative structural, biochemical, and cell biological data, we propose that higher-order E3 ligase assembly generally enables multipronged targeting, capable of simultaneously incapacitating multiple protomers and functionalities of oligomeric substrates.

In-depth proteomic analysis of proteasome inhibitors bortezomib, carfilzomib and MG132 reveals that mortality factor 4-like 1 (MORF4L1) protein ubiquitylation is negatively impacted

Proteasome inhibitors are an important class of chemotherapeutic drugs. In this study, we performed a large-scale ubiquitylome analysis of the three proteasome inhibitors MG132, bortezomib and carfilzomib. Although carfilzomib is currently being used for the treatment of multiple myeloma, it has not yet been subjected to a global ubiquitylome analysis. In this study, we identified more than 14,000 unique sites of ubiquitylation in more than 4400 protein groups. We introduced stringent criteria to determine the correct ubiquitylation site ratios and used five biological replicates to achieve increased statistical power. With the vast amount of data acquired, we made proteome-wide comparisons between the proteasome inhibitors and indicate candidate proteins that will benefit from further study. We find that in addition to the expected increase in ubiquitylation in the majority of proteins, unexpectedly a select few are specifically and significantly decreased in ubiquitylation at specific sites after treatment with proteasome inhibitors. We chose to follow-up on Mortality factor 4-like 1 (MORF4L1), which was significantly decreased in ubiquitylation at lysine 187 and lysine 104 upon proteasome inhibition, but increased in protein abundance by approximately two-fold. We demonstrate that the endogenous protein level of MORF4L1 is highly regulated by the ubiquitin proteasome system. SIGNIFICANCE: This study provides a highly curated dataset of more than 14,000 unique sites of ubiquitylation in more than 4400 protein groups. For the proper quantification of ubiquitylation sites, we introduced a higher standard by quantifying only those ubiquitylation sites that are not flanked by neighboring ubiquitylation, thereby avoiding the report of incorrect ratios. The sites identified will serve to identify important targets of the ubiquitin proteasome system and aid to better understand the repertoire of proteins that are affected by inhibiting the proteasome with MG132, bortezomib, and carfilzomib. In addition, we investigated the unusual observation that ubiquitylation of the tumor suppressor Mortality factor 4-like (MORF4L1) protein decreases rather than increases upon proteasome inhibition, which may contribute to an additional anti-tumor effect of bortezomib and carfilzomib.

The deubiquitinase JOSD2 is a positive regulator of glucose metabolism

Cancer cells undergo complex metabolic alterations. The mechanisms underlying the tuning of cancer metabolism are under active investigation. Here, we identify the uncharacterized deubiquitinase JOSD2 as a positive regulator of cancer cell proliferation by displaying comprehensive effects on glucose catabolism. We found that JOSD2 directly controls a metabolic enzyme complex that includes Aldolase A, Phosphofructokinase-1 and Phosphoglycerate dehydrogenase, in vitro and in vivo. Further, JOSD2 expression, but not a catalytically inactive mutant, deubiquitinates and stabilizes the enzyme complex, thereby enhancing their activities and the glycolytic rate. This represents a selective JOSD2 feature that is not shared among other Machado-Joseph disease DUBs or observed in nontransformed cells. JOSD2 deficiency displays cytostatic effects and reduces glycolysis in a broad spectrum of tumor cells of distinct origin and its expression correlates with poor prognosis in non-small cell lung cancer. Overall, our study provides evidence for a previously unknown biological mechanism in which JOSD2 integrates glucose and serine metabolism with potential therapeutic implications.

Deubiquitinase Ubp3 enhances the proteasomal degradation of key enzymes in sterol homeostasis

Sterol homeostasis is tightly controlled by molecules that are highly conserved from yeast to humans, the dysregulation of which plays critical roles in the development of antifungal resistance and various cardiovascular diseases. Previous studies have shown that sterol homeostasis is regulated by the ubiquitin–proteasome system. Two E3 ubiquitin ligases, Hrd1 and Doa10, are known to mediate the proteasomal degradation of 3-hydroxy-3-methylglutaryl-CoA reductase Hmg2 and squalene epoxidase Erg1 with accumulation of the toxic sterols in cells, but the deubiquitinases (DUBs) involved are unclear. Here, we screened for DUBs responsible for sterol homeostasis using yeast strains from a DUB-deletion library. The defective growth observed in ubp3-deleted (ubp3Δ) yeast upon fluconazole treatment suggests that lack of Ubp3 disrupts sterol homeostasis. Deep-coverage quantitative proteomics reveals that ergosterol biosynthesis is rerouted into a sterol pathway that generates toxic products in the absence of Ubp3. Further genetic and biochemical analysis indicated that Ubp3 enhances the proteasome's ability to degrade the ergosterol biosynthetic enzymes Erg1 and Erg3. The retardation of ergosterol enzyme degradation in the ubp3Δ strain resulted in the severe accumulation of the intermediate lanosterol and a branched toxic sterol, and ultimately disrupted sterol homeostasis and led to the fluconazole susceptibility. Our findings uncover a role for Ubp3 in sterol homeostasis and highlight its potential as a new antifungal target.

Potential Physiological Relevance of ERAD to the Biosynthesis of GPI-Anchored Proteins in Yeast

Misfolded and/or unassembled secretory and membrane proteins in the endoplasmic reticulum (ER) may be retro-translocated into the cytoplasm, where they undergo ER-associated degradation, or ERAD. The mechanisms by which misfolded proteins are recognized and degraded through this pathway have been studied extensively; however, our understanding of the physiological role of ERAD remains limited. This review describes the biosynthesis and quality control of glycosylphosphatidylinositol (GPI)-anchored proteins and briefly summarizes the relevance of ERAD to these processes. While recent studies suggest that ERAD functions as a fail-safe mechanism for the degradation of misfolded GPI-anchored proteins, several pieces of evidence suggest an intimate interaction between ERAD and the biosynthesis of GPI-anchored proteins.

Mechanisms of productive folding and endoplasmic reticulum-associated degradation of glycoproteins and non-glycoproteins

BACKGROUND

The quality of proteins destined for the secretory pathway is ensured by two distinct mechanisms in the endoplasmic reticulum (ER): productive folding of newly synthesized proteins, which is assisted by ER-localized molecular chaperones and in most cases also by disulfide bond formation and transfer of an oligosaccharide unit; and ER-associated degradation (ERAD), in which proteins unfolded or misfolded in the ER are recognized and processed for delivery to the ER membrane complex, retrotranslocated through the complex with simultaneous ubiquitination, extracted by AAA-ATPase to the cytosol, and finally degraded by the proteasome.

SCOPE OF REVIEW

We describe the mechanisms of productive folding and ERAD, with particular attention to glycoproteins versus non-glycoproteins, and to yeast versus mammalian systems.

MAJOR CONCLUSION

Molecular mechanisms of the productive folding of glycoproteins and non-glycoproteins mediated by molecular chaperones and protein disulfide isomerases are well conserved from yeast to mammals. Additionally, mammals have gained an oligosaccharide structure-dependent folding cycle for glycoproteins. The molecular mechanisms of ERAD are also well conserved from yeast to mammals, but redundant expression of yeast orthologues in mammals has been encountered, particularly for components involved in recognition and processing of glycoproteins and components of the ER membrane complex involved in retrotranslocation and simultaneous ubiquitination of glycoproteins and non-glycoproteins. This may reflect an evolutionary consequence of increasing quantity or quality needs toward mammals.

GENERAL SIGNIFICANCE

The introduction of innovative genome editing technology into analysis of the mechanisms of mammalian ERAD, as exemplified here, will provide new insights into the pathogenesis of various diseases.

SCFCdc4 ubiquitin ligase regulates synaptonemal complex formation during meiosis

During meiosis, homologous chromosomes pair to form the synaptonemal complex (SC). This study showed that SCF^Cdc4 ubiquitin ligase is required for and works with Pch2 AAA+ ATPase for SC assembly.

Homologous chromosomes pair with each other during meiosis, culminating in the formation of the synaptonemal complex (SC), which is coupled with meiotic recombination. In this study, we showed that a meiosis-specific depletion mutant of a cullin (Cdc53) in the SCF (Skp-Cullin-F-box) ubiquitin ligase, which plays a critical role in cell cycle regulation during mitosis, is deficient in SC formation. However, the mutant is proficient in forming crossovers, indicating the uncoupling of meiotic recombination with SC formation in the mutant. Furthermore, the deletion of the PCH2 gene encoding a meiosis-specific AAA+ ATPase suppresses SC-assembly defects induced by CDC53 depletion. On the other hand, the pch2 cdc53 double mutant is defective in meiotic crossover formation, suggesting the assembly of SC with unrepaired DNA double-strand breaks. A temperature-sensitive mutant of CDC4, which encodes an F-box protein of SCF, shows meiotic defects similar to those of the CDC53-depletion mutant. These results suggest that SCF^Cdc4, probably SCF^Cdc4-dependent protein ubiquitylation, regulates and collaborates with Pch2 in SC assembly and meiotic recombination.

Blistering1 Modulates Penicillium expansum Virulence Via Vesicle-mediated Protein Secretion

The blue mold fungus, Penicillium expansum, is a postharvest apple pathogen that contributes to food waste by rotting fruit and by producing harmful mycotoxins (e.g. patulin). To identify genes controlling pathogen virulence, a random T-DNA insertional library was created from wild-type P. expansum strain R19. One transformant, T625, had reduced virulence in apples, blistered mycelial hyphae, and a T-DNA insertion that abolished transcription of the single copy locus in which it was inserted. The gene, Blistering1, encodes a protein with a DnaJ domain, but otherwise has little homology outside the Aspergillaceae, a family of fungi known for producing antibiotics, mycotoxins, and cheese. Because protein secretion is critical for these processes and for host infection, mass spectrometry was used to monitor proteins secreted into liquid media during fungal growth. T625 failed to secrete a set of enzymes that degrade plant cell walls, along with ones that synthesize the three final biosynthetic steps of patulin. Consequently, the culture broth of T625 had significantly reduced capacity to degrade apple tissue and contained 30 times less patulin. Quantitative mass spectrometry of 3,282 mycelial proteins revealed that T625 had altered cellular networks controlling protein processing in the endoplasmic reticulum, protein export, vesicle-mediated transport, and endocytosis. T625 also had reduced proteins controlling mRNA surveillance and RNA processing. Transmission electron microscopy of hyphal cross sections confirmed that T625 formed abnormally enlarged endosomes or vacuoles. These data reveal that Blistering1 affects internal and external protein processing involving vesicle-mediated transport in a family of fungi with medical, commercial, and agricultural importance.

The E3 ubiquitin ligase Pib1 regulates effective gluconeogenic shutdown upon glucose availability

Cells use multiple mechanisms to regulate their metabolic states in response to changes in their nutrient environment. One example is the response of cells to glucose. In Saccharomyces cerevisiae growing in glucose-depleted medium, the re-availability of glucose leads to the down-regulation of gluconeogenesis and the activation of glycolysis, leading to "glucose repression." However, our knowledge of the mechanisms mediating the glucose-dependent down-regulation of the gluconeogenic transcription factors is limited. Using the major gluconeogenic transcription factor Rds2 as a candidate, we identify here a novel role for the E3 ubiquitin ligase Pib1 in regulating the stability and degradation of Rds2. Glucose addition to cells growing under glucose limitation results in a rapid ubiquitination of Rds2, followed by its proteasomal degradation. Through in vivo and in vitro experiments, we establish Pib1 as the ubiquitin E3 ligase that regulates Rds2 ubiquitination and stability. Notably, this Pib1-mediated Rds2 ubiquitination, followed by proteasomal degradation, is specific to the presence of glucose. This Pib1-mediated ubiquitination of Rds2 depends on the phosphorylation state of Rds2, suggesting a cross-talk between ubiquitination and phosphorylation to achieve a metabolic state change. Using stable isotope-based metabolic flux experiments, we find that the loss of Pib1 results in an imbalanced gluconeogenic state, regardless of glucose availability. Pib1 is required for complete glucose repression and enables cells to optimally grow in competitive environments when glucose again becomes available. Our results reveal the existence of a Pib1-mediated regulatory program that mediates glucose repression when glucose availability is restored.

Proteasome-Dependent Regulation of Distinct Metabolic States During Long-Term Culture of Human iPSC-Derived Cardiomyocytes

RATIONALE

The immature presentation of human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) is currently a challenge for their application in disease modeling, drug screening, and regenerative medicine. Long-term culture is known to achieve partial maturation of iPSC-CMs. However, little is known about the molecular signaling circuitries that govern functional changes, metabolic output, and cellular homeostasis during long-term culture of iPSC-CMs.

OBJECTIVE

We aimed to identify and characterize critical signaling events that control functional and metabolic transitions of cardiac cells during developmental progression, as recapitulated by long-term culture of iPSC-CMs.

METHODS AND RESULTS

We combined transcriptomic sequencing with pathway network mapping in iPSC-CMs that were cultured until a late time point, day 200, in comparison to a medium time point, day 90, and an early time point, day 30. Transcriptomic landscapes of long-term cultured iPSC-CMs allowed mapping of distinct metabolic stages during development of maturing iPSC-CMs. Temporally divergent control of mitochondrial metabolism was found to be regulated by cAMP/PKA (protein kinase A)- and proteasome-dependent signaling events. The PKA/proteasome-dependent signaling cascade was mediated downstream by Hsp90 (heat shock protein 90), which in turn modulated mitochondrial respiratory chain proteins and their metabolic output. During long-term culture, this circuitry was found to initiate upregulation of iPSC-CM metabolism, resulting in increased cell contractility that reached a maximum at the day 200 time point.

CONCLUSIONS

Our results reveal a PKA/proteasome- and Hsp90-dependent signaling pathway that regulates mitochondrial respiratory chain proteins and determines cardiomyocyte energy production and functional output. These findings provide deeper insight into signaling circuitries governing metabolic homeostasis in iPSC-CMs during developmental progression.

ER-associated ubiquitin ligase HRD1 programs liver metabolism by targeting multiple metabolic enzymes

The HMG-CoA reductase degradation protein 1 (HRD1) has been identified as a key enzyme for endoplasmic reticulum-associated degradation of misfolded proteins, but its organ-specific physiological functions remain largely undefined. Here we show that mice with HRD1 deletion specifically in the liver display increased energy expenditure and are resistant to HFD-induced obesity and liver steatosis and insulin resistance. Proteomic analysis identifies a HRD1 interactome, a large portion of which includes metabolic regulators. Loss of HRD1 results in elevated ENTPD5, CPT2, RMND1, and HSD17B4 protein levels and a consequent hyperactivation of both AMPK and AKT pathways. Genome-wide mRNA sequencing revealed that HRD1-deficiency reprograms liver metabolic gene expression profiles, including suppressing genes involved in glycogenesis and lipogenesis and upregulating genes involved in glycolysis and fatty acid oxidation. We propose HRD1 as a liver metabolic regulator and a potential drug target for obesity, fatty liver disease, and insulin resistance associated with the metabolic syndrome.

The multi-subunit GID/CTLH E3 ubiquitin ligase promotes cell proliferation and targets the transcription factor Hbp1 for degradation

In yeast, the glucose-induced degradation-deficient (GID) E3 ligase selectively degrades superfluous gluconeogenic enzymes. Here, we identified all subunits of the mammalian GID/CTLH complex and provide a comprehensive map of its hierarchical organization and step-wise assembly. Biochemical reconstitution demonstrates that the mammalian complex possesses inherent E3 ubiquitin ligase activity, using Ube2H as its cognate E2. Deletions of multiple GID subunits compromise cell proliferation, and this defect is accompanied by deregulation of critical cell cycle markers such as the retinoblastoma (Rb) tumor suppressor, phospho-Histone H3 and Cyclin A. We identify the negative regulator of pro-proliferative genes Hbp1 as a bonafide GID/CTLH proteolytic substrate. Indeed, Hbp1 accumulates in cells lacking GID/CTLH activity, and Hbp1 physically interacts and is ubiquitinated in vitro by reconstituted GID/CTLH complexes. Our biochemical and cellular analysis thus demonstrates that the GID/CTLH complex prevents cell cycle exit in G1, at least in part by degrading Hbp1.

Ubiquitin Ligase COP1 Controls Hepatic Fat Metabolism by Targeting ATGL for Degradation

Optimal control of hepatic lipid metabolism is critical for organismal metabolic fitness. In liver, adipose triglyceride lipase (ATGL) serves as a major triacylglycerol (TAG) lipase and controls the bulk of intracellular lipid turnover. However, regulation of ATGL expression and its functional implications in hepatic lipid metabolism, particularly in the context of fatty liver disease, is unclear. We show that E3 ubiquitin ligase COP1 (also known as RFWD2) binds to the consensus VP motif of ATGL and targets it for proteasomal degradation by K-48 linked polyubiquitination, predominantly at the lysine 100 residue. COP1 thus serves as a critical regulator of hepatocyte TAG content, fatty acid mobilization, and oxidation. Moreover, COP1-mediated regulation of hepatic lipid metabolism requires optimum ATGL expression for its metabolic outcome. In vivo, adenovirus-mediated depletion of COP1 ameliorates high-fat diet-induced steatosis in mouse liver and improves liver function. Our study thus provides new insights into the regulation of hepatic lipid metabolism by the ubiquitin-proteasome system and suggests COP1 as a potential therapeutic target for nonalcoholic fatty liver disease.

Subcellular Fractionation Analysis of the Extraction of Ubiquitinated Polytopic Membrane Substrate during ER-Associated Degradation

During ER-associated degradation (ERAD), misfolded polytopic membrane proteins are ubiquitinated and retrotranslocated to the cytosol for proteasomal degradation. However, our understanding as to how polytopic membrane proteins are extracted from the ER to the cytosol remains largely unclear. To better define the localization and physical properties of ubiquitinated polytopic membrane substrates in vivo, we performed subcellular fractionation analysis of Ste6*, a twelve transmembrane protein that is ubiquitinated primarily by Doa10 E3 ligase in yeast. Consistent with previous in vitro studies, ubiquitinated Ste6* was extracted from P20 (20,000 g pellet) fraction to S20 (20,000 g supernatant) fraction in a Cdc48/p97-dependent manner. Similarly, Ubx2p, which recruits Cdc48/p97 to the ER, facilitated the extraction of Ste6*. By contrast, lipid droplet formation, which was suggested to be dispensable for the degradation of Hrd1-substrates in yeast, was not required for the degradation of Ste6*. Intriguingly, we found that ubiquitinated Ste6* in the S20 fraction could be enriched by further centrifugation at 100,000 g. Although it is currently uncertain whether ubiquitinated Ste6* in P100 fraction is completely free from any lipids, membrane flotation analysis suggested the existence of two distinct populations of ubiquitinated Ste6* with different states of membrane association. Together, these results imply that ubiquitinated Ste6* may be sequestered into a putative quality control sub-structure by Cdc48/p97. Fractionation assays developed in the present study provide a means to further dissect the ill-defined post-ubiquitination step during ERAD of polytopic membrane substrates.