bims-proteo Biomed News
on Proteostasis
Issue of 2020‒11‒29
forty-one papers selected by
Eric Chevet

  1. J Cell Sci. 2020 Nov 27. pii: jcs251983. [Epub ahead of print]133(22):
      Protein synthesis is an energetically costly, complex and risky process. Aberrant protein biogenesis can result in cellular toxicity and disease, with membrane-embedded proteins being particularly challenging for the cell. In order to protect the cell from consequences of defects in membrane proteins, quality control systems act to maintain protein homeostasis. The majority of these pathways act post-translationally; however, recent evidence reveals that membrane proteins are also subject to co-translational quality control during their synthesis in the endoplasmic reticulum (ER). This newly identified quality control pathway employs components of the cytosolic ribosome-associated quality control (RQC) machinery but differs from canonical RQC in that it responds to biogenesis state of the substrate rather than mRNA aberrations. This ER-associated RQC (ER-RQC) is sensitive to membrane protein misfolding and malfunctions in the ER insertion machinery. In this Review, we discuss the advantages of co-translational quality control of membrane proteins, as well as potential mechanisms of substrate recognition and degradation. Finally, we discuss some outstanding questions concerning future studies of ER-RQC of membrane proteins.
    Keywords:  Endoplasmic reticulum; Membrane protein; Protein folding; Ribosome; Translation
  2. iScience. 2020 Nov 20. 23(11): 101708
      AMFR/gp78 and USP13 are a pair of ubiquitin ligase and deubiquitinase that ensure the accuracy of endoplasmic reticulum-associated degradation (ERAD). Depletion of USP13 leads to caspase activation and cleavage of the ERAD chaperone BAG6, which is reversed by knockdown of AMFR. However, the mechanism and physiological relevance of this regulation are still unclear. Here, by using the NEDDylator system, we screened out TXN as a substrate of AMFR and USP13 and showed its involvement in regulating CASP3 activation and BAG6 cleavage. Furthermore, we showed that the cleaved N-terminal BAG6 is located in the cytosol and interacts with both LC3B-I and unprocessed form of LC3B (Pro-LC3B) through the LIR1 motif to suppress autophagy. An NMR approach verified the direct interaction between BAG6 LIR1 and LC3B-I or Pro-LC3B. Collectively, our findings uncover a mechanism that converts BAG6 from an ERAD regulator to an autophagy tuner and apoptosis inducer during ER stress.
    Keywords:  Biochemistry; Biological Sciences; Cell Biology; Molecular Biology
  3. Eur J Pharmacol. 2020 Nov 24. pii: S0014-2999(20)30841-4. [Epub ahead of print] 173749
      Cellular stress and inflammation, establishing as disease pathology, have reached great heights in the last few decades. Stress conditions such as hyperglycemia, hyperlipidemia and lipoproteins are known to disturb proteostasis resulting in the accumulation of unfolded or misfolded proteins, alteration in calcium homeostasis culminating in unfolded protein response. Protein disulfide isomerase and endoplasmic reticulum oxidase-1 are the key players in protein folding. The protein folding process assisted by endoplasmic reticulum oxidase-1 results in the production of reactive oxygen species in the lumen of the endoplasmic reticulum. Production of reactive oxygen species beyond the quenching capacity of the antioxidant systems perturbs ER homeostasis. Endoplasmic reticulum stress also induces the production of cytokines leading to inflammatory responses. This has been proven to be the major causative factor for various pathophysiological states compared to other cellular triggers in diseases, which further manifests to increased oxidative stress, mitochondrial dysfunction, and altered inflammatory responses, deleterious to cellular physiology and homeostasis. Numerous studies have drawn correlations between the progression of several diseases in association with endoplasmic reticulum stress, redox protein folding, oxidative stress and inflammatory responses. This review aims to provide an insight into the role of protein disulfide isomerase and endoplasmic reticulum oxidase-1 in endoplasmic reticulum stress, unfolded protein response, mitochondrial dysfunction, and inflammatory responses, which exacerbate the progression of various diseases.
    Keywords:  Crosstalk; ER Stress; Endoplasmic Reticulum Oxidase; Oxidative Stress; Protein Disulphide Isomerase
  4. Elife. 2020 Nov 25. pii: e62611. [Epub ahead of print]9
      Membrane protein biogenesis in the endoplasmic reticulum (ER) is complex and failure-prone. The ER membrane protein complex (EMC), comprising eight conserved subunits, has emerged as a central player in this process. Yet, we have limited understanding of how EMC enables insertion and integrity of diverse clients, from tail-anchored to polytopic transmembrane proteins. Here, yeast and human EMC cryo-EM structures reveal conserved intricate assemblies and human-specific features associated with pathologies. Structure-based functional studies distinguish between two separable EMC activities, as an insertase regulating tail-anchored protein levels and a broader role in polytopic membrane protein biogenesis. These depend on mechanistically coupled yet spatially distinct regions including two lipid-accessible membrane cavities which confer client-specific regulation, and a non-insertase EMC function mediated by the EMC lumenal domain. Our studies illuminate the structural and mechanistic basis of EMC's multifunctionality and point to its role in differentially regulating the biogenesis of distinct client protein classes.
    Keywords:  S. cerevisiae; cell biology; human; molecular biophysics; structural biology
  5. EMBO J. 2020 Nov 23. e104948
      Autophagosome formation requires multiple autophagy-related (ATG) factors. However, we find that a subset of autophagy substrates remains robustly targeted to the lysosome in the absence of several core ATGs, including the LC3 lipidation machinery. To address this unexpected result, we performed genome-wide CRISPR screens identifying genes required for NBR1 flux in ATG7KO cells. We find that ATG7-independent autophagy still requires canonical ATG factors including FIP200. However, in the absence of LC3 lipidation, additional factors are required including TAX1BP1 and TBK1. TAX1BP1's ability to cluster FIP200 around NBR1 cargo and induce local autophagosome formation enforces cargo specificity and replaces the requirement for lipidated LC3. In support of this model, we define a ubiquitin-independent mode of TAX1BP1 recruitment to NBR1 puncta, highlighting that TAX1BP1 recruitment and clustering, rather than ubiquitin binding per se, is critical for function. Collectively, our data provide a mechanistic basis for reports of selective autophagy in cells lacking the lipidation machinery, wherein receptor-mediated clustering of upstream autophagy factors drives continued autophagosome formation.
    Keywords:  ATG7; NBR1; TAX1BP1; autophagosome; selective autophagy
  6. iScience. 2020 Nov 20. 23(11): 101725
      Upon heat shock, the fission yeast Hsp40 chaperone Mas5 drives temperature-sensitive proteins toward protein aggregate centers (PACs) to avoid their degradation until lower temperatures favor their refolding. We show here that cells lacking Mas5 are resistant to oxidative stress. Components of the general stress pathways, the MAP kinase Sty1 and the transcription factor Atf1, are suppressors of this phenotype. Strain Δmas5 expresses higher levels of Sty1- and Atf1-dependent stress genes than wild-type cells. Pyp1, the main tyrosine phosphatase maintaining Sty1 inactive in the absence of stress, is a temperature-sensitive protein that aggregates upon temperature up-shifts in a Mas5-dependent manner. In strain Δmas5, Pyp1 is sent to proteasomal degradation even in the absence of stress. We propose that Pyp1 is a thermo-sensitive phosphatase, which during heat stress coalescences into PACs in a Mas5-dependent manner, to promote full activation of the anti-stress Sty1-Atf1 cascade.
    Keywords:  Biological Sciences; Cell Biology; Molecular Biology
  7. Cell. 2020 Nov 17. pii: S0092-8674(20)31399-4. [Epub ahead of print]
      In order to analyze how a signal transduction network converts cellular inputs into cellular outputs, ideally one would measure the dynamics of many signals within the network simultaneously. We found that, by fusing a fluorescent reporter to a pair of self-assembling peptides, it could be stably clustered within cells at random points, distant enough to be resolved by a microscope but close enough to spatially sample the relevant biology. Because such clusters, which we call signaling reporter islands (SiRIs), can be modularly designed, they permit a set of fluorescent reporters to be efficiently adapted for simultaneous measurement of multiple nodes of a signal transduction network within single cells. We created SiRIs for indicators of second messengers and kinases and used them, in hippocampal neurons in culture and intact brain slices, to discover relationships between the speed of calcium signaling, and the amplitude of PKA signaling, upon receiving a cAMP-driving stimulus.
    Keywords:  cAMP; calcium imaging; fluorescent reporters; live-cell imaging; protein kinase; protein scaffold; signal transduction; signaling pathway; signaling reporter islands; spatial multiplexing
  8. Nat Commun. 2020 Nov 25. 11(1): 5975
      Hop/Stip1/Sti1 is thought to be essential as a co-chaperone to facilitate substrate transfer between the Hsp70 and Hsp90 molecular chaperones. Despite this proposed key function for protein folding and maturation, it is not essential in a number of eukaryotes and bacteria lack an ortholog. We set out to identify and to characterize its eukaryote-specific function. Human cell lines and the budding yeast with deletions of the Hop/Sti1 gene display reduced proteasome activity due to inefficient capping of the core particle with regulatory particles. Unexpectedly, knock-out cells are more proficient at preventing protein aggregation and at promoting protein refolding. Without the restraint by Hop, a more efficient folding activity of the prokaryote-like Hsp70-Hsp90 complex, which can also be demonstrated in vitro, compensates for the proteasomal defect and ensures the proteostatic equilibrium. Thus, cells may act on the level and/or activity of Hop to shift the proteostatic balance between folding and degradation.
  9. Mol Cell. 2020 Nov 17. pii: S1097-2765(20)30776-0. [Epub ahead of print]
      Breathing depends on pulmonary surfactant, a mixture of phospholipids and proteins, secreted by alveolar type II cells. Surfactant requires lamellar bodies (LBs), organelles containing densely packed concentric membrane layers, for storage and secretion. LB biogenesis remains mysterious but requires surfactant protein B (SP-B), which is synthesized as a precursor (pre-proSP-B) that is cleaved during trafficking into three related proteins. Here, we elucidate the functions and cooperation of these proteins in LB formation. We show that the N-terminal domain of proSP-B is a phospholipid-binding and -transfer protein whose activities are required for proSP-B export from the endoplasmic reticulum (ER) and sorting to LBs, the conversion of proSP-B into lipoprotein particles, and neonatal viability in mice. The C-terminal domain facilitates ER export of proSP-B. The mature middle domain, generated after proteolytic cleavage of proSP-B, generates the striking membrane layers characteristic of LBs. Together, our results lead to a mechanistic model of LB biogenesis.
    Keywords:  ARDS; AT2; DPPC; ER exit; alveoli; crystal structure; lipid transfer; lysosome-related organelle; multilamellar; saposin
  10. Cell Host Microbe. 2020 Nov 17. pii: S1931-3128(20)30620-X. [Epub ahead of print]
      Pathogenesis induced by SARS-CoV-2 is thought to result from both an inflammation-dominated cytokine response and virus-induced cell perturbation causing cell death. Here, we employ an integrative imaging analysis to determine morphological organelle alterations induced in SARS-CoV-2-infected human lung epithelial cells. We report 3D electron microscopy reconstructions of whole cells and subcellular compartments, revealing extensive fragmentation of the Golgi apparatus, alteration of the mitochondrial network and recruitment of peroxisomes to viral replication organelles formed by clusters of double-membrane vesicles (DMVs). These are tethered to the endoplasmic reticulum, providing insights into DMV biogenesis and spatial coordination of SARS-CoV-2 replication. Live cell imaging combined with an infection sensor reveals profound remodeling of cytoskeleton elements. Pharmacological inhibition of their dynamics suppresses SARS-CoV-2 replication. We thus report insights into virus-induced cytopathic effects and provide alongside a comprehensive publicly available repository of 3D datasets of SARS-CoV-2-infected cells for download and smooth online visualization.
    Keywords:  FIB-SEM; Golgi; coronavirus; cytoskeleton; electron tomography; intermediate filaments; live cell imaging; membrane remodeling; peroxisomes; viral replication organelles
  11. Nat Commun. 2020 Nov 27. 11(1): 6042
      NOD-like receptor protein 3 (NLRP3) detects microbial infections or endogenous danger signals and activates the NLRP3 inflammasome, which has important functions in host defense and contributes to the pathogenesis of inflammatory diseases, and thereby needs to be tightly controlled. Deubiquitination of NLRP3 is considered a key step in NLRP3 inflammasome activation. However, the mechanisms by which deubiquitination controls NLRP3 inflammasome activation are unclear. Here, we show that the UAF1/USP1 deubiquitinase complex selectively removes K48-linked polyubiquitination of NLRP3 and suppresses its ubiquitination-mediated degradation, enhancing cellular NLRP3 levels, which are indispensable for subsequent NLRP3 inflammasome assembly and activation. In addition, the UAF1/USP12 and UAF1/USP46 complexes promote NF-κB activation, enhance the transcription of NLRP3 and proinflammatory cytokines (including pro-IL-1β, TNF, and IL-6) by inhibiting ubiquitination-mediated degradation of p65. Consequently, Uaf1 deficiency attenuates NLRP3 inflammasome activation and IL-1β secretion both in vitro and in vivo. Our study reveals that the UAF1 deubiquitinase complexes enhance NLRP3 and pro-IL-1β expression by targeting NLRP3 and p65 and licensing NLRP3 inflammasome activation.
  12. Virology. 2020 Nov 13. pii: S0042-6822(20)30222-1. [Epub ahead of print]553 70-80
      Many viruses utilize molecular chaperone heat shock protein 90 (Hsp90) for protein folding and stabilization, however, the role of Hsp90 in herpesvirus lifecycle is obscure. Here, we provide evidence that Hsp90 participates in pseudorabies virus (PRV) replication. Viral growth kinetics assays show that Hsp90 inhibitor geldanamycin (GA) abrogates PRV replication at the post-penetration step. Transmission electron microscopy demonstrates that dysfunction of Hsp90 diminishes the quantity of PRV nucleocapsids. Overexpression and knockdown of Hsp90 suggest that de novo Hsp90 is involved in PRV replication. Mechanismly, dysfunction of Hsp90 inhibits PRV major capsid protein VP5 expression. Co-immunoprecipitation and indirect immunofluorescence assays indicate that Hsp90 interacts with VP5. Interestingly, Hsp70, a collaborator of Hsp90, also interacts with VP5, but doesn't affect PRV growth. Finally, inhibition of Hsp90 results in PRV VP5 degradation in a proteasome-dependent manner. Collectively, our data suggest that Hsp90 contributes to PRV virion assembly and replication via stabilization of VP5.
    Keywords:  Assembly; Capsid; Hsp70; Hsp90; Pseudorabies virus; VP5
  13. Dev Cell. 2020 Nov 16. pii: S1534-5807(20)30874-1. [Epub ahead of print]
      In C. elegans, expression of the UPRER transcription factor xbp-1s in neurons cell non-autonomously activates the UPRER in the intestine, leading to enhanced proteostasis and lifespan. To better understand this signaling pathway, we isolated neurons from animals expressing neuronal xbp-1s for transcriptomic analysis, revealing a striking remodeling of transcripts involved in neuronal signaling. We then identified signaling molecules required for cell non-autonomous intestinal UPRER activation, including the biogenic amine tyramine. Expression of xbp-1s in just two pairs of neurons that synthesize tyramine, the RIM and RIC interneurons, induced intestinal UPRER activation and extended longevity, and exposure to stress led to splicing and activation of xbp-1 in these neurons. In addition, we found that neuronal xbp-1s modulates feeding behavior and reproduction, dependent upon tyramine synthesis. XBP-1s therefore remodels neuronal signaling to coordinately modulate intestinal physiology and stress-responsive behavior, functioning as a global regulator of organismal responses to stress.
    Keywords:  C. elegans; ER stress; aging; neurobiology; proteostasis; signaling; stress response
  14. J Biol Chem. 2020 Nov 23. pii: jbc.RA120.016175. [Epub ahead of print]
      The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a β-coronavirus, is the causative agent of the COVID-19 pandemic. Like for other coronaviruses, its particles are composed of four structural proteins: Spike (S), Envelope (E), Membrane (M) and Nucleoprotein (N) proteins. The involvement of each of these proteins and their interactions are critical for assembly and production of β-coronavirus particles. Here, we sought to characterize the interplay of SARS-CoV-2 structural proteins during the viral assembly process. By combining biochemical and imaging assays in infected vs. transfected cells, we show that E and M regulate intracellular trafficking of S as well as its intracellular processing. Indeed, the imaging data reveal that S is re-localized at endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC) or Golgi compartments upon co-expression of E or M, as observed in SARS-CoV-2-infected cells, which prevents syncytia formation. We show that a C-terminal retrieval motif in the cytoplasmic tail of S is required for its M-mediated retention in the ERGIC, whereas E induces S retention by modulating the cell secretory pathway. We also highlight that E and M induce a specific maturation of N-glycosylation of S, independently of the regulation of its localization, with a profile that is observed both in infected cells and in purified viral particles. Finally, we show that E, M and N are required for optimal production of virus- like-particles. Altogether, these results highlight how E and M proteins may influence the properties of S proteins and promote the assembly of SARS-CoV-2 viral particles.
    Keywords:  COVID-19; SARS-CoV-2; glycoprotein; infectious disease; secretion; viral protein; virus assembly
  15. Front Endocrinol (Lausanne). 2020 ;11 588685
      The endoplasmic reticulum stress and the unfolded protein response are triggered following an imbalance between protein load and protein folding. Until recently, two possible outcomes of the unfolded protein response have been considered: life or death. We sought to substantiate a third alternative, dedifferentiation, mesenchymal shift, and activation of the antioxidant response by using typical endocrine cells, i.e. thyroid cells. The thyroid is a unique system both of endoplasmic reticulum stress (a single protein, thyroglobulin represents the majority of proteins synthesized in the endoplasmic reticulum by the thyrocyte) and of polarized epithelium (the single layer of thyrocytes delimiting the follicle). Following endoplasmic reticulum stress, in thyroid cells the folding of thyroglobulin was disrupted. The mRNAs of unfolded protein response were induced or spliced (X-box binding protein-1). Differentiation was inhibited: mRNA levels of thyroid specific genes, and of thyroid transcription factors were dramatically downregulated, at least in part, transcriptionally. The dedifferentiating response was accompanied by an upregulation of mRNAs of antioxidant genes. Moreover, cadherin-1, and the thyroid (and kidney)-specific cadherin-16 mRNAs were downregulated, vimentin, and SNAI1 mRNAs were upregulated. In addition, loss of cortical actin and stress fibers formation were observed. Together, these data indicate that ER stress in thyroid cells induces dedifferentiation, loss of epithelial organization, shift towards a mesenchymal phenotype, and activation of the antioxidant response, highlighting, at the same time, a new and wide strategy to achieve survival following ER stress, and, as a sort of the other side of the coin, a possible new molecular mechanism of decline/loss of function leading to a deficit of thyroid hormones formation.
    Keywords:  ER stress; antioxidant response; dedifferentiation; mesenchymal phenotype; thyroid
  16. Front Cell Dev Biol. 2020 ;8 604240
      The endoplasmic reticulum (ER) and mitochondria are physically connected to form dedicated structural domains known as mitochondria-associated ER membranes (MAMs), which participate in fundamental biological processes, including lipid and calcium (Ca2+) homeostasis, mitochondrial dynamics and other related cellular behaviors such as autophagy, ER stress, inflammation and apoptosis. Many studies have proved the importance of MAMs in maintaining the normal function of both organelles, and the abnormal amount, structure or function of MAMs is related to the occurrence of cardiovascular diseases. Here, we review the knowledge regarding the components of MAMs according to their different functions and the specific roles of MAMs in cardiovascular physiology and pathophysiology, focusing on some highly prevalent cardiovascular diseases, including ischemia-reperfusion, diabetic cardiomyopathy, heart failure, pulmonary arterial hypertension and systemic vascular diseases. Finally, we summarize the possible mechanisms of MAM in cardiovascular diseases and put forward some obstacles in the understanding of MAM function we may encounter.
    Keywords:  SR-mitochondrial contact; cardiovascular diseases; metabolic transition; mitochondria-associated ER membrane; mitochondrial bioenergetics
  17. Biochem Biophys Res Commun. 2020 Nov 23. pii: S0006-291X(20)32082-9. [Epub ahead of print]
      To elucidate the regulation of serotonin transporter (SERT) function via its membrane trafficking, we investigated the involvement of the ubiquitin E3 ligase HRD1 (HMG-CoA reductase degradation protein), which participates in endoplasmic reticulum (ER)-associated degradation (ERAD), in the functional regulation of SERT. Cells transiently expressing wild-type SERT or a SERT C-terminal deletion mutant (SERTΔCT), a SERT protein predicted to be misfolded, were used for experiments. Studies using HRD1-overexpressing or HRD1-knockdown cells demonstrated that HRD1 is involved in SERT proteolysis. Overexpression of HRD1 promoted SERT ubiquitination, the effect of which was augmented by treatment with the proteasome inhibitor MG132. Immunoprecipitation studies revealed that HRD1 interacts with SERT in the presence of MG132. In addition, HRD1 was intracellularly colocalized with SERT, especially with aggregates of SERTΔCT in the ER. HRD1 also affected SERT uptake activity in accordance with the expression levels of the SERT protein. These results suggest that HRD1 contributes to the membrane trafficking and functional regulation of SERT through its involvement in ERAD-mediated SERT degradation.
    Keywords:  E3 ubiquitin ligase; Endoplasmic reticulum-associated degradation (ERAD); HRD1; Serotonin transporter
  18. Am J Physiol Lung Cell Mol Physiol. 2020 Nov 25.
      The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor whose physiological function is poorly understood. The AhR is highly expressed in barrier organs such as the skin, intestine and lung. The lungs are continuously exposed to environmental pollutants such as cigarette smoke (CS) that can induce cell death mechanisms such as apoptosis, autophagy and endoplasmic reticulum (ER) stress. CS also contains toxicants that are AhR ligands. We have previously shown that the AhR protects against apoptosis, but whether the AhR also protects against autophagy or ER stress is not known. Using cigarette smoke extract (CSE) as our in vitro surrogate of environmental tobacco exposure, we first assessed the conversion of LC3I to LC3II, a classic feature of both autophagic and ER stress-mediated cell death pathways. LC3II was elevated in CSE-exposed lung structural cells (mouse lung fibroblasts [MLFs], MLE12 and A549 cells) when AhR was absent. However, this heightened LC3II expression could not be explained by increased expression of key autophagy genes (Gabarapl1, Becn1, Map1lc3b), upregulation of upstream autophagic machinery (Atg5-12, Atg3) or by impaired autophagic flux, suggesting that LC3II may be autophagy-independent. This was further supported by the absence of autophagosomes in Ahr-/- lung cells. However, Ahr-/- lung cells had widespread ER-dilation, elevated expression of the ER stress markers CHOP and GADD34 and an accumulation of ubiquitinated proteins. These findings collectively illustrate a novel role for the AhR in attenuating ER stress by a mechanism that may be autophagy-independent.
    Keywords:  COPD; ER stress; autophagy; cigarette smoke; fibroblasts
  19. Cell Rep. 2020 Nov 24. pii: S2211-1247(20)31420-0. [Epub ahead of print]33(8): 108431
      Exposure to excessive sound causes noise-induced hearing loss through complex mechanisms and represents a common and unmet neurological condition. We investigate how noise insults affect the cochlea with proteomics and functional assays. Quantitative proteomics reveals that exposure to loud noise causes proteotoxicity. We identify and confirm hundreds of proteins that accumulate, including cytoskeletal proteins, and several nodes of the proteostasis network. Transcriptomic analysis reveals that a subset of the genes encoding these proteins also increases acutely after noise exposure, including numerous proteasome subunits. Global cochlear protein ubiquitylation levels build up after exposure to excess noise, and we map numerous posttranslationally modified lysines residues. Several collagen proteins decrease in abundance, and Col9a1 specifically localizes to pillar cell heads. After two weeks of recovery, the cochlea selectively elevates the abundance of the protein synthesis machinery. We report that overstimulation of the auditory system drives a robust cochlear proteotoxic stress response.
    Keywords:  cochlear synaptopathy; neurodegeneration; noise-induced hearing loss; proteostasis; quantitative proteomics
  20. J Leukoc Biol. 2020 Nov 22.
      Pseudomonas aeruginosa is a severe Gram-negative opportunistic bacterium that causes a spectrum of organ system diseases, particularly in immunocompromised patients. This bacterium has been shown to induce unfolded protein response (UPR) during mammalian infection. Annexin A2 (AnxA2) is a multicompartmental protein relating to a number of cellular processes; however, it remains unknown whether AnxA2 coordinates a UPR pathway under bacterial infection conditions. Here, we report that the endoplasmic reticulum stress inositol-requiring enzyme 1 (IRE1)-X-box binding protein 1 (XBP1) pathway was up-regulated by AnxA2 through p38 MAPK signaling following P. aeruginosa infection in macrophages, whereas ATF4 and ATF6 not. In addition, XBP1 was found as a positive regulator of innate immunity to tame P. aeruginosa challenges by enhancing autophagy and bacterial clearance. XBP1 also facilitated NF-κB activation to elicit the release of proinflammatory cytokines predominantly in macrophages. Together, our findings identify AnxA2 as a regulator for XBP1-mediated UPR pathway.
    Keywords:  Annexin A2; ER stress; Pseudomonas aeruginosa; XBP1
  21. Proc Natl Acad Sci U S A. 2020 Nov 23. pii: 202020124. [Epub ahead of print]
      The Arg/N-degron pathway targets proteins for degradation by recognizing their N-terminal or internal degrons. Our previous work produced double-knockout (2-KO) HEK293T human cell lines that lacked the functionally overlapping UBR1 and UBR2 E3 ubiquitin ligases of the Arg/N-degron pathway. Here, we studied these cells in conjunction with RNA-sequencing, mass spectrometry (MS), and split-ubiquitin binding assays. 1) Some mRNAs, such as those encoding lactate transporter MCT2 and β-adrenergic receptor ADRB2, are strongly (∼20-fold) up-regulated in 2-KO cells, whereas other mRNAs, including those encoding MAGEA6 (a regulator of ubiquitin ligases) and LCP1 (an actin-binding protein), are completely repressed in 2-KO cells, in contrast to wild-type cells. 2) Glucocorticoid receptor (GR), an immunity-modulating transcription factor (TF), is up-regulated in 2-KO cells and also physically binds to UBR1, strongly suggesting that GR is a physiological substrate of the Arg/N-degron pathway. 3) PREP1, another TF, was also found to bind to UBR1. 4) MS-based analyses identified ∼160 proteins whose levels were increased or decreased by more than 2-fold in 2-KO cells. For example, the homeodomain TF DACH1 and the neurofilament subunits NF-L (NFEL) and NF-M (NFEM) were expressed in wild-type cells but were virtually absent in 2-KO cells. 5) The disappearance of some proteins in 2-KO cells took place despite up-regulation of their mRNAs, strongly suggesting that the Arg/N-degron pathway can also modulate translation of specific mRNAs. In sum, this multifunctional proteolytic system has emerged as a regulator of mammalian gene expression, in part through conditional targeting of TFs that include ATF3, GR, and PREP1.
    Keywords:  GR; PREP1; UBR1; degradation; transcription
  22. Redox Biol. 2020 Sep 29. pii: S2213-2317(20)30944-7. [Epub ahead of print]37 101739
      Redox homeostasis regulates key cellular signaling in both physiology and pathology. While perturbations result in shifting the redox homeostasis towards oxidative stress are well documented, the influence of reductive stress (RS) in neurodegenerative diseases and its mechanisms are unknown. Here, we postulate that a redox shift towards the reductive arm (through the activation of Nrf2 signaling) will damage neurons and impair neurogenesis. In proliferating and differentiating neuroblastoma (Neuro 2a/N2a) cells, sulforaphane-mediated Nrf2 activation resulted in increased transcription/translation of antioxidants and glutathione (GSH) production along with significantly declined ROS in a dose-dependent manner leading to a reductive-redox state (i.e. RS). Interestingly, this resulted in endoplasmic reticulum (ER) stress leading to subsequent protein aggregation/proteotoxicity in neuroblastoma cells. Under RS, we also observed elevated Tau/α-synuclein and their co-localization with other protein aggregates in these cells. Surprisingly, we noticed that acute RS impaired neurogenesis as evidenced from reduced neurite outgrowth/length. Furthermore, maintaining the cells in a sustained RS condition (for five consecutive generations) dramatically reduced their differentiation and prevented the formation of axons (p < 0.05). This impairment in RS mediated neurogenesis occurs through the alteration of Tau dynamics i.e. RS activates the pathogenic GSK3β/Tau cascade thereby promoting the phosphorylation of Tau leading to proteotoxicity. Of note, intermittent withdrawal of sulforaphane from these cells suppressed the proteotoxic insult and re-activated the differentiation process. Overall, this results suggest that either acute or chronic RS could hamper neurogenesis through GSK3β/TAU signaling and proteotoxicity. Therefore, investigations identifying novel redox mechanisms impacting proteostasis are crucial to preserve neuronal health.
    Keywords:  ER stress; Glutathione; Neurogenesis; Nrf2; Proteotoxicity; Reductive stress
  23. J Gen Appl Microbiol. 2020 Nov 21.
      O-GlcNAc modification mediated by O-GlcNAc transferase (OGT) is a reversible protein modification in which O-GlcNAc moieties are attached to target proteins in the cytosol, nucleus, and mitochondria. O-GlcNAc moieties attached to proteins can be removed by O-GlcNAcase (OGA). The addition of an O-GlcNAc moiety can influence several aspects of protein function, and aberrant O-GlcNAc modification is linked to a number of diseases. While OGT and OGA are conserved across eukaryotic cells, yeasts lack these enzymes. Previously, we reported that protein O-GlcNAc modification occurred in the budding yeast Saccharomyces cerevisiae when OGT was ectopically expressed. Because yeast cells lack OGA, O-GlcNAc moieties are stably attached to target proteins. Thus, the yeast system may be useful for finding novel OST substrates. By proteomic analysis, we identified 468 O-GlcNAcylated proteins in yeast cells expressing human OGT. Among these proteins, 13 have human orthologues that show more than 30% identity to their corresponding yeast orthologue, and possible glycosylation residues are conserved in these human orthologues. In addition, the orthologues have not been reported as substrates of OGT. We verified that some of these human orthologues are O-GlcNAcylated in cultured human cells. These proteins include an ubiquitin-conjugating enzyme, UBE2D1, and an eRF3-similar protein, HBS1L. Thus, the yeast system would be useful to find previously unknown O-GlcNAcylated proteins and regulatory mechanisms.
    Keywords:  O-GlcNAc; O-GlcNAc transferase; branched-chain amino acid aminotransferase; ubiquitin-conjugating enzyme; yeast
  24. iScience. 2020 Nov 20. 23(11): 101713
      Nonsense-mediated mRNA decay (NMD) is a biological surveillance mechanism that eliminates mRNA transcripts with premature termination codons. In Caenorhabditis elegans, NMD contributes to longevity by enhancing RNA quality. Here, we aimed at identifying NMD-modulating factors that are crucial for longevity in C. elegans by performing genetic screens. We showed that knocking down each of algn-2/asparagine-linked glycosylation protein, zip-1/bZIP transcription factor, and C44B11.1/FAS apoptotic inhibitory molecule increased the transcript levels of NMD targets. Among these, algn-2 exhibited an age-dependent decrease in its expression and was required for maintaining normal lifespan and for longevity caused by various genetic interventions. We further demonstrated that upregulation of ALGN-2 by inhibition of daf-2/insulin/IGF-1 receptor contributed to longevity in an NMD-dependent manner. Thus, algn-2, a positive regulator of NMD, plays a crucial role in longevity in C. elegans, likely by enhancing RNA surveillance. Our study will help understand how NMD-mediated mRNA quality control extends animal lifespan.
    Keywords:  Cell Biology; Genetics; Molecular Biology
  25. Hepatology. 2020 Nov 22.
      BACKGROUND & AIMS: Cholangiocarcinoma (CCA) is a devastating disease often detected at advanced stages when surgery cannot be performed. Conventional and targeted systemic therapies perform poorly and therefore effective drugs are urgently needed. Different epigenetic modifications occur in CCA and contribute to malignancy. Targeting epigenetic mechanisms may thus open new therapeutic opportunities. However, modifications such as DNA and histone methylation often co-exist and cooperate in carcinogenesis. We tested the therapeutic efficacy and mechanism of action of a new class of dual G9a histone-methyltransferase and DNA-methyltransferase 1 (DNMT1) inhibitors.APPROACH & RESULTS: Expression of G9a, DNMT1 and their molecular adaptor ubiquitin-like with PHD and RING finger domains-1 (UHRF1) was determined in human CCA. We evaluated the effect of individual and combined pharmacological inhibition of G9a and DNMT1 on CCA cell growth. Our lead G9a/DNMT1 inhibitor, CM272, was tested in human CCA cells, patients-derived tumoroids and xenograft, and a mouse model of cholangiocarcinogenesis with hepatocellular deletion of c-Jun-N-terminal-kinase-(Jnk) 1/2 and diethyl-nitrosamine (DEN) plus CCl4 treatment (JnkΔhepa +DEN+CCl4 mice). We found an increased and correlative expression of G9a, DNMT1 and UHRF1 in CCAs. Co-treatment with independent pharmacological inhibitors G9a and DNMT1 synergistically inhibited CCA cell growth. CM272 markedly reduced CCA cells proliferation and synergized with Cisplatin and the ERBB-targeted inhibitor Lapatinib. CM272 inhibited CCA tumoroids and xenograft growth, and significantly antagonized CCA progression in JnkΔhepa +DEN+CCl4 mice without apparent toxicity. Mechanistically, CM272 reprogrammed the tumoral metabolic transcriptome and phenotype towards a differentiated and quiescent status.
    CONCLUSIONS: Dual targeting of G9a and DNMT1 with epigenetic small molecule inhibitors such as CM272 is a potential novel strategy to treat CCA and/or to enhance the efficacy of other systemic therapies.
    Keywords:  Biliary cancer; epigenetics; therapy; tumor differentiation; tumor metabolism
  26. Aging (Albany NY). 2020 Nov 20. 12
      Ubiquitination-mediated protein degradation is the selective degradation of diverse forms of damaged proteins that are tagged with ubiquitin, while deubiquitinating enzymes reverse ubiquitination-mediated protein degradation by removing the ubiquitin chain from the target protein. The interactions of ubiquitinating and deubiquitinating enzymes are required to maintain protein homeostasis. The ubiquitin-specific protease USP7 is a deubiquitinating enzyme that indirectly plays a role in repairing DNA damage and development. However, the mechanism of its participation in aging has not been fully explored. Regarding this issue, we found that USP7 was necessary to maintain the normal lifespan of Drosophila melanogaster, and knockdown of dusp7 shortened the lifespan and reduced the ability of Drosophila to cope with starvation, oxidative stress and heat stress. Furthermore, we showed that the ability of USP7 to regulate aging depends on the autophagy and ubiquitin signaling pathways. Furthermore, 2,5-dimethyl-celecoxib (DMC), a derivative of celecoxib, can partially restore the shortened lifespan and aberrant phenotypes caused by dusp7 knockdown. Our results suggest that USP7 is an important factor involved in the regulation of aging, and related components in this regulatory pathway may become new targets for anti-aging treatments.
    Keywords:  DMC; Drosophila; USP7; aging; autophagy
  27. Cell Chem Biol. 2020 Nov 18. pii: S2451-9456(20)30434-7. [Epub ahead of print]
      Functional analysis of lysine 27-linked ubiquitin chains (K27Ub) is difficult due to the inability to make them through enzymatic methods and due to a lack of model tools and substrates. Here we generate a series of ubiquitin (Ub) tools to study how the deubiquitinase UCHL3 responds to K27Ub chains in comparison to lysine 63-linked chains and mono-Ub. From a crystal structure of a complex between UCHL3 and synthetic K27Ub2, we unexpectedly discover that free K27Ub2 and K27Ub2-conjugated substrates are natural inhibitors of UCHL3. Using our Ub tools to profile UCHL3's activity, we generate a quantitative kinetic model of the inhibitory mechanism and we find that K27Ub2 can inhibit UCHL3 covalently, by binding to its catalytic cysteine, and allosterically, by locking its catalytic loop tightly in place. Based on this inhibition mechanism, we propose that UCHL3 and K27Ub chains likely sense and regulate each other in cells.
    Keywords:  Lys27-linked diubiquitin; UCHL3; deubiquitination; kinetic modeling; synthetic substrates; ubiquitin; ubiquitin probe
  28. Biochemistry. 2020 Nov 25.
      Histone deacetylase 6 is a class II histone deacetylase primarily present in the cytoplasm and involved in the regulation of various cellular functions. It consists of two catalytic deacetylase domains and a unique zinc finger ubiquitin binding protein domain, which sets it apart from other HDACs. HDAC6 is known to regulate cellular activities by modifying the function of microtubules, HSP90, and cortactin through deacetylation. Apart from the catalytic activity of HDAC6, it interacts with other proteins through either the SE14 domain or the ZnF UBP domain to modulate their functions. Here, we have studied the role of the HDAC6 ZnF UBP domain as a modifier of Tau aggregation by its direct interaction with the polyproline region/repeat region of Tau. Interaction of HDAC6 ZnF UBP with Tau was found to reduce the propensity of Tau to self-aggregate and to disaggregate preformed aggregates in a concentration-dependent manner and also bring about the conformational changes in Tau protein. The interaction of HDAC6 ZnF UBP with Tau results in its degradation, suggesting either proteolytic activity of HDAC6 ZnF UBP or its role in enhancing autoproteolysis of Tau.
  29. ACS Chem Biol. 2020 Nov 23.
      Arginylation is a protein post-translational modification catalyzed by arginyl-tRNA transferases (ATE1s), which are critical enzymes conserved across all eukaryotes. Arginylation is a key step in the Arg N-degron pathway, a hierarchical cellular signaling pathway that links the ubiquitin-dependent degradation of a protein to the identity of its N-terminal amino acid side chain. The fidelity of ATE1-catalyzed arginylation is imperative, as this post-translational modification regulates several essential biological processes such as cardiovascular maturation, chromosomal segregation, and even the stress response. While the process of ATE1-catalyzed arginylation has been studied in detail at the cellular level, much remains unknown about the structure of this important enzyme, its mechanism of action, and its regulation. In this work, we detail the current state of knowledge on ATE1-catalyzed arginylation, and we discuss both ongoing and future directions that will reveal the structural and mechanistic details of this essential eukaryotic cellular regulator.
  30. Methods Mol Biol. 2021 ;2233 253-264
      Proteins destined to be exposed to the extracellular space enter the secretory pathway at the level of the endoplasmic reticulum. Proteins are then transported to the Golgi apparatus and addressed to their destination compartment, such as the plasma membrane for exocytic cargos. Exocytosis constitutes the last step of the anterograde transport of secretory cargos. Exocytic vesicles fuse with the plasma membrane, releasing soluble proteins to the extracellular milieu and transmembrane proteins to the plasma membrane. In order to monitor local exocytosis of cargos, we describe in this chapter how to perform synchronization of the anterograde transport of an exocytic cargo of interest using the retention using selective hooks (RUSH) assay in combination with selective protein immobilization (SPI). SPI is based on the coating of coverslips with anti-green fluorescent protein (GFP) antibodies, which capture the GFP-tagged RUSH cargos once exposed to the cell surface after its release by the addition of biotin.
    Keywords:  Antibody coating; Exocytosis; Protein immobilization; RUSH; Real-time imaging; Secretory transport
  31. Science. 2020 Nov 27. 370(6520): 1105-1110
      The human mitochondrial ribosome (mitoribosome) and associated proteins regulate the synthesis of 13 essential subunits of the oxidative phosphorylation complexes. We report the discovery of a mitoribosome-associated quality control pathway that responds to interruptions during elongation, and we present structures at 3.1- to 3.3-angstrom resolution of mitoribosomal large subunits trapped during ribosome rescue. Release factor homolog C12orf65 (mtRF-R) and RNA binding protein C6orf203 (MTRES1) eject the nascent chain and peptidyl transfer RNA (tRNA), respectively, from stalled ribosomes. Recruitment of mitoribosome biogenesis factors to these quality control intermediates suggests additional roles for these factors during mitoribosome rescue. We also report related cryo-electron microscopy structures (3.7 to 4.4 angstrom resolution) of elongating mitoribosomes bound to tRNAs, nascent polypeptides, the guanosine triphosphatase elongation factors mtEF-Tu and mtEF-G1, and the Oxa1L translocase.
  32. Curr Opin Struct Biol. 2020 Nov 22. pii: S0959-440X(20)30182-2. [Epub ahead of print]66 104-111
      Protein folding in vivo has been largely understood in the context of molecular chaperones preventing aggregation of nascent polypeptides in the crowded cellular environment. Nascent chains utilize the crowded environment in favor of productive folding by direct physical connection with cellular macromolecules. The intermolecular repulsive forces by large excluded volume and surface charges of interacting cellular macromolecules, exerting 'social distancing' measure among folding intermediates, could play an important role in stabilizing their physically connected polypeptides against aggregation regardless of the physical connection types. The generic intrinsic chaperone activity of cellular macromolecules likely provides a robust cellular environment for the productive protein folding and solubility maintenance at the whole proteome level.
  33. Biochem Biophys Res Commun. 2020 Nov 19. pii: S0006-291X(20)32072-6. [Epub ahead of print]
      Ubiquitin Conjugating Enzyme E2 C (UBE2C) has a key oncogenic role in many human malignancies, including gastric cancer. However, it remains largely unknow at which level UBE2C expression is altered, as well as what are the downstream targets of UBE2C. In this study, we show that UBE2C is frequently overexpressed in gastric cancer patients. Interestingly, high expression of UBE2C mRNA instead of genome amplification is the predominant alterations observed in both stomach adenocarcinoma. We then confirmed that silencing UBE2C not only suppresses gastric cancer colony formation, but also inhibits DNA biosynthesis. Furthermore, we discovered that microRNA-300 is able to suppress gastric cancer progression through reducing UBE2C mRNA abundance, which is protected by an RNA binding protein HuR. Lastly, through an analysis of genes whose expressions correlate with that of UBE2C from gastric cancer cell lines, we have proposed several key genes that can be regulated by UBE2C, contributing to its oncogenic activity.
    Keywords:  Gastric cancer; HuR; UBE2C; mRNA expression; miR-300
  34. Mol Cell. 2020 Nov 11. pii: S1097-2765(20)30738-3. [Epub ahead of print]
      The small molecule ISRIB antagonizes the activation of the integrated stress response (ISR) by phosphorylated translation initiation factor 2, eIF2(αP). ISRIB and eIF2(αP) bind distinct sites in their common target, eIF2B, a guanine nucleotide exchange factor for eIF2. We have found that ISRIB-mediated acceleration of eIF2B's nucleotide exchange activity in vitro is observed preferentially in the presence of eIF2(αP) and is attenuated by mutations that desensitize eIF2B to the inhibitory effect of eIF2(αP). ISRIB's efficacy as an ISR inhibitor in cells also depends on presence of eIF2(αP). Cryoelectron microscopy (cryo-EM) showed that engagement of both eIF2B regulatory sites by two eIF2(αP) molecules remodels both the ISRIB-binding pocket and the pockets that would engage eIF2α during active nucleotide exchange, thereby discouraging both binding events. In vitro, eIF2(αP) and ISRIB reciprocally opposed each other's binding to eIF2B. These findings point to antagonistic allostery in ISRIB action on eIF2B, culminating in inhibition of the ISR.
    Keywords:  CRISPR/Cas9-homologous recombination; cell stress; drug action; eukaryotic initiation factor-2B; mRNA translation; phosphorylation; protein binding; protein biosynthesis/drug effects; protein conformation
  35. Front Cell Dev Biol. 2020 ;8 600868
      The Drosophila hyperplastic disc (hyd) gene is the ortholog of mammalian tumor suppressor EDD, which is implicated in a wide variety of cellular processes, and its regulation is impaired in various tumors. It is a member of the highly conserved HECT family of E3 ubiquitin ligases, which directly attach ubiquitin to targeted substrates. In early works, it was shown that Drosophila Hyd may be a tumor suppressor because it is involved in the control of imaginal-disc cell proliferation and growth. In this study, we demonstrated that Hyd is also important for the regulation of female germ cell proliferation and that its depletion leads to additional germline cell mitoses. Furthermore, we revealed a previously unknown Hyd function associated with the maintenance of germ cells' viability. A reduction in hyd expression by either mutations or RNA interference resulted in large-scale germ cell death at different stages of oogenesis. Thus, the analysis of phenotypes arising from the hyd deficiency points to Hyd's role in the regulation of germline metabolic processes during oogenesis.
    Keywords:  Drosophila; E3 ubiquitin ligase; cell death; germ cells; hyperplastic disc gene; oogenesis
  36. FEBS Lett. 2020 Nov 22.
      Synaptic vesicle fusion is mediated by SNARE proteins - VAMP2 on the vesicle and Syntaxin-1/SNAP25 on the pre-synaptic membrane. Chaperones Munc18-1 and Munc13-1 co-operatively catalyze SNARE assembly via an intermediate 'template' complex containing Syntaxin-1 and VAMP2. How SNAP25 enters this reaction remains a mystery. Here we report that Munc13-1 recruits SNAP25 to initiate the ternary SNARE complex assembly by direct binding, as judged by bulk FRET spectroscopy and single-molecule optical tweezer studies. Detailed structure-function analyses show that the binding is mediated by the Munc13-1 MUN domain and is specific for the SNAP25 'linker' region that connects the two SNARE motifs. Consequently, freely diffusing SNAP25 molecules on phospholipid bilayers are concentrated and bound in ~1:1 stoichiometry by the self-assembled Munc13-1 nano-clusters.
  37. EMBO J. 2020 Nov 23. e106990
      LC3/GABARAP (hereafter ATG8) conjugation machineries have long been thought to play an essential role in autophagy by driving ATG8 lipidation on autophagosomal membranes. In this issue, Ohnstad et al (2020) describe an ATG8 lipidation bypass pathway which governs autophagy-dependent turnover of NBR1, highlighting that there is more than one road to autophagic degradation.
  38. Biomolecules. 2020 Nov 20. pii: E1581. [Epub ahead of print]10(11):
      The Blm10/PA200 family of proteasome activators modulates the peptidase activity of the core particle (20S CP). They participate in opening the 20S CP gate, thus facilitating the degradation of unstructured proteins such as tau and Dnm1 in a ubiquitin- and ATP-independent manner. Furthermore, PA200 also participates in the degradation of acetylated histones. In our study, we use a combination of yeast and human cell systems to investigate the role of Blm10/PA200 in the degradation of N-terminal Huntingtin fragments (N-Htt). We demonstrate that the human PA200 binds to N-Htt. The loss of Blm10 in yeast or PA200 in human cells results in increased mutant N-Htt aggregate formation and elevated cellular toxicity. Furthermore, Blm10 in vitro accelerates the proteasomal degradation of soluble N-Htt. Collectively, our data suggest N-Htt as a new substrate for Blm10/PA200-proteasomes and point to new approaches in Huntington's disease (HD) research.
    Keywords:  Huntington’s disease; degradation; huntingtin protein (Htt), Blm10/PA200-proteasomes; mutant N-Htt aggregates; soluble N-Htt
  39. Dev Cell. 2020 Nov 23. pii: S1534-5807(20)30837-6. [Epub ahead of print]55(4): 385-386
      Oligogenic inheritance makes the etiology of developmental diseases challenging to determine. In this issue of Developmental Cell, Kong et al., 2020 identify members of a membrane-tethered ubiquitin complex that attenuates Hedgehog signaling strength and genetically interact to regulate digit number, body patterning, and cardiac development.
  40. J Proteome Res. 2020 Nov 26.
      Autophagy is a catabolic process facilitating the degradation of cytoplasmic proteins and organelles in a lysosome- or vacuole-dependent manner in plants, animals, and fungi. Proteomic studies have demonstrated that autophagy controls and shapes the proteome and has identified both receptor and cargo proteins inside autophagosomes. In a smaller selection of studies, proteomics has been used for the analysis of post-translational modifications that target proteins for elimination and protein-protein interactions between receptors and cargo, providing a better understanding of the complex regulatory processes controlling autophagy. In this perspective, we highlight how proteomic studies have contributed to our understanding of autophagy in plants against the backdrop of yeast and animal studies. We then provide a framework for how the future application of proteomics in plant autophagy can uncover the mechanisms and outcomes of sculpting organelles during plant development, particularly through the identification of autophagy receptors and cargo in plants.
    Keywords:  autophagy; organelle development; protein degradation; proteomics
  41. Int J Mol Sci. 2020 Nov 23. pii: E8880. [Epub ahead of print]21(22):
      Protein trafficking is altered when normal cells acquire a tumor phenotype. A key subcellular compartment in regulating protein trafficking is the Golgi apparatus, but its role in carcinogenesis is still not well defined. Golgi phosphoprotein 3 (GOLPH3), a peripheral membrane protein mostly localized at the trans-Golgi network, is overexpressed in several tumor types including glioblastoma multiforme (GBM), the most lethal primary brain tumor. Moreover, GOLPH3 is currently considered an oncoprotein, however its precise function in GBM is not fully understood. Here, we analyzed in T98G cells of GBM, which express high levels of epidermal growth factor receptor (EGFR), the effect of stable RNAi-mediated knockdown of GOLPH3. We found that silencing GOLPH3 caused a significant reduction in the proliferation of T98G cells and an unexpected increase in total EGFR levels, even at the cell surface, which was however less prone to ligand-induced autophosphorylation. Furthermore, silencing GOLPH3 decreased EGFR sialylation and fucosylation, which correlated with delayed ligand-induced EGFR downregulation and its accumulation at endo-lysosomal compartments. Finally, we found that EGF failed at promoting EGFR ubiquitylation when the levels of GOLPH3 were reduced. Altogether, our results show that GOLPH3 in T98G cells regulates the endocytic trafficking and activation of EGFR likely by affecting its extent of glycosylation and ubiquitylation.
    Keywords:  EGFR; GOLPH3; Golgi apparatus; T98G; glioblastoma multiforme; glycosylation; protein trafficking; ubiquitylation