Disruption of Glycosylation Enhances Ubiquitin-Mediated Proteasomal Degradation of Shadoo in Scrapie-Infected Rodents and Cultured Cells

Shadoo (Sho) is an N-glycosylated glycophosphatidylinositol-anchored protein that is expressed in the brain and exhibits neuroprotective properties. Recently, research has shown that a reduction of Sho levels may reflect the presence of PrP^Sc in the brain. However, the possible mechanism by which prion infection triggers down-regulation of Sho remains unclear. In the present study, Western blot and immunohistochemical assays revealed that Sho, especially glycosylated Sho, declined markedly in the brains of five scrapie agent-infected hamsters and mice at the terminal stages. Analyses of the down-regulation of Sho levels with the emergence of PrP^Sc C2 proteolytic fragments did not identify close association in all tested scrapie-infected models. To further investigate the mechanism of depletion of Sho in prion disease, a Sho-expressing plasmid with HA tag was introduced into a scrapie-infected cell line, SMB-S15, and its normal cell line, SMB-PS. Western blot assay revealed dramatically decreased Sho in SMB-S15 cells, especially its glycosylated form. Proteasome inhibitor MG132 reversed the decrease of nonglycosylated Sho, but had little effect on glycosylated Sho. N-acetylglucosamine transferase inhibitor tunicamycin efficiently reduced the glycosylations of Sho and PrP^C in SMB-PS cells, while two other endoplasmic reticulum stress inducers showed clear inhibition of diglycosylated PrP^C, but did not change the expression level and profile of Sho. Furthermore, immunoprecipitation of HA-Sho illustrated ubiquitination of Sho in SMB-S15 cells, but not in SMB-PS cells. We propose that the depletions of Sho in scrapie-infected cell lines due to inhibition of glycosylation mediate protein destabilization and subsequently proteasome degradation after modification by ubiquitination.     

FBXO32 Targets c-Myc for Proteasomal Degradation and Inhibits c-Myc Activity

FBXO32 (MAFbx/Atrogin-1) is an E3 ubiquitin ligase that is markedly up-regulated in muscle atrophy. Although some data indicate that FBXO32 may play an important role in tumorigenesis, the molecular mechanism of FBXO32 in tumorigenesis has been poorly understood. Here, we present evidence that FBXO32 targets the oncogenic protein c-Myc for ubiquitination and degradation through the proteasome pathway. Phosphorylation of c-Myc at Thr-58 and Ser-62 is dispensable for FBXO32 to induce c-Myc degradation. Mutation of the lysine 326 in c-Myc reduces c-Myc ubiquitination and prevents the c-Myc degradation induced by FBXO32. Furthermore, overexpression of FBXO32 suppresses c-Myc activity and inhibits cell growth, but knockdown of FBXO32 enhances c-Myc activity and promotes cell growth. Finally, we show that FBXO32 is a direct downstream target of c-Myc, highlighting a negative feedback regulation loop between c-Myc and FBXO32. Thus, FBXO32 may function by targeting c-Myc. This work explains the function of FBXO32 and highlights its mechanisms in tumorigenesis.     

Ubiquitin-Mediated Regulation of RIPK1 Kinase Activity Independent of IKK and MK2

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Serine 123 Phosphorylation Modulates p21 Protein Stability and Activity by Suppressing Ubiquitin-independent Proteasomal Degradation

The cyclin-dependent kinase inhibitor p21^Waf1/Cip1 is a major regulator of the cell cycle and plays an important role in many cellular processes, including differentiation, stress response, apoptosis, and tumorigenesis. We previously cloned the gene encoding dog p21 and found that unlike its human ortholog, dog p21 is expressed as two isoforms, one high molecular mass band of 23 kDa and one low molecular mass band of 19 kDa. In the current study, we found that the high molecular mass band is phosphorylated, whereas the low molecular mass band is hypophosphorylated. Moreover, by generating multiple mutants of dog p21, we found that serine 123 and proline 124, which form a consensus site for proline-directed phosphorylation, are required for expression of the high molecular mass p21 isoform through phosphorylation at serine 123. Most importantly, we showed that serine 123 phosphorylation inhibits ubiquitin-independent proteasomal degradation of p21 protein and subsequently, prolongs p21 protein half-life and enhances the ability of p21 to suppress cell proliferation. Taken together, these data reveal that serine 123 phosphorylation modulates p21 protein stability and activity by suppressing ubiquitin-independent proteasomal degradation.     

Atypical Ubiquitylation in Yeast Targets Lysine-less Asi2 for Proteasomal Degradation

Proteins are typically targeted for proteasomal degradation by the attachment of a polyubiquitin chain to ϵ-amino groups of lysine residues. Non-lysine ubiquitylation of proteasomal substrates has been considered an atypical and rare event limited to complex eukaryotes. Here we report that a fully functional lysine-less mutant of an inner nuclear membrane protein in yeast, Asi2, is polyubiquitylated and targeted for proteasomal degradation. Efficient degradation of lysine-free Asi2 requires E3-ligase Doa10 and E2 enzymes Ubc6 and Ubc7, components of the endoplasmic reticulum-associated degradation pathway. Together, our data suggest that non-lysine ubiquitylation may be more prevalent than currently considered.     

Proteasomal Degradation of Proinsulin Requires Derlin-2, HRD1 and p97

Patients with type 1 diabetes (T1D) suffer from beta-cell destruction by CD8⁺ T-cells that have preproinsulin as an important target autoantigen. It is of great importance to understand the molecular mechanism underlying the processing of preproinsulin into these CD8⁺ T-cell epitopes. We therefore studied a pathway that may contribute to the production of these antigenic peptides: degradation of proinsulin via ER associated protein degradation (ERAD). Analysis of the MHC class I peptide ligandome confirmed the presentation of the most relevant MHC class I-restricted diabetogenic epitopes in our cells: the signal peptide-derived sequence A15-A25 and the insulin B-chain epitopes H29-A38 and H34-V42. We demonstrate that specific silencing of Derlin-2, p97 and HRD1 by shRNAs increases steady state levels of proinsulin. This indicates that these ERAD constituents are critically involved in proinsulin degradation and may therefore also play a role in subsequent antigen generation. These ERAD proteins therefore represent interesting targets for novel therapies aiming at the reduction and possibly also prevention of beta-cell directed auto-immune reactions in T1D.     

An InCytes from MBC Selection: Vimentin Regulates Scribble Activity by Protecting It from Proteasomal Degradation

Scribble (Scrib), Discs large, and Lethal giant larvae form a protein complex that regulates different aspects of cell polarization, including apical–basal asymmetry in epithelial cells and anterior–posterior polarity in migrating cells. Here, we show that Scrib interacts with the intermediate filament cytoskeleton in epithelial Madin-Darby canine kidney (MDCK) cells and endothelial human umbilical vein endothelial cells. Scrib binds vimentin via its postsynaptic density 95/disc-large/zona occludens domains and in MDCK cells redistributes from filaments to the plasma membrane during the establishment of cell–cell contacts. RNA interference-mediated silencing of Scrib, vimentin, or both in MDCK cells results in defects in the polarization of the Golgi apparatus during cell migration. Concomitantly, wound healing is delayed due to the loss of directional movement. Furthermore, cell aggregation is dependent on both Scrib and vimentin. The similar phenotypes observed after silencing either Scrib or vimentin support a coordinated role for the two proteins in cell migration and aggregation. Interestingly, silencing of vimentin leads to an increased proteasomal degradation of Scrib. Thus, the upregulation of vimentin expression during epithelial to mesenchymal transitions may stabilize Scrib to promote directed cell migration.     

The Proteasome-associated Protein Ecm29 Inhibits Proteasomal ATPase Activity and in Vivo Protein Degradation by the Proteasome

Several proteasome-associated proteins regulate degradation by the 26 S proteasome using the ubiquitin chains that mark most substrates for degradation. The proteasome-associated protein Ecm29, however, has no ubiquitin-binding or modifying activity, and its direct effect on substrate degradation is unclear. Here, we show that Ecm29 acts as a proteasome inhibitor. Besides inhibiting the proteolytic cleavage of peptide substrates in vitro, it inhibits the degradation of ubiquitin-dependent and -independent substrates in vivo. Binding of Ecm29 to the proteasome induces a closed conformation of the substrate entry channel of the core particle. Furthermore, Ecm29 inhibits proteasomal ATPase activity, suggesting that the mechanism of inhibition and gate regulation by Ecm29 is through regulation of the proteasomal ATPases. Consistent with this, we identified through chemical cross-linking that Ecm29 binds to, or in close proximity to, the proteasomal ATPase subunit Rpt5. Additionally, we show that Ecm29 preferentially associates with both mutant and nucleotide depleted proteasomes. We propose that the inhibitory ability of Ecm29 is important for its function as a proteasome quality control factor by ensuring that aberrant proteasomes recognized by Ecm29 are inactive.     

AiiA Quorum-Sensing Quenching Controls Proteolytic Activity and Biofilm Formation by Enterobacter cloacae

The aim of this work was to evaluate a quorum-quenching approach to identify functions regulated by quorum sensing in Enterobacter cloacae. We employed an aiiA transconjugant strain of E. cloacae that synthesizes a lactonase enzyme that hydrolyzes N-acyl homoserine lactone signaling molecules to compare bacterial phenotypes in the presence and absence of quorum signals. The aiiA-expressing strain displayed increased proteolytic activity and intensity of a milk-clotting reaction when compared to the wild-type strain. Although both strains growing on polystyrene plates in rich media and a minimal medium of salts formed biofilms, the wild-type strain exhibited a higher number of adhered cells. On the surface of stainless steel coupons that were submerged in culture media, the number of adhered cells of the wild type contained up to one log more cells compared with the aiiA transconjugant. However, after 48?h of incubation, there was no significant difference between the strains. The results demonstrated that the quorum-sensing system negatively regulates proteolytic activity and is likely involved in the early steps of biofilm formation by E. cloacae 067.     

Gi-Coupled GPCR Signaling Controls the Formation and Organization of Human Pluripotent Colonies

Background

Reprogramming adult human somatic cells to create human induced pluripotent stem (hiPS) cell colonies involves a dramatic morphological and organizational transition. These colonies are morphologically indistinguishable from those of pluripotent human embryonic stem (hES) cells. G protein-coupled receptors (GPCRs) are required in diverse developmental processes, but their role in pluripotent colony morphology and organization is unknown. We tested the hypothesis that G_i-coupled GPCR signaling contributes to the characteristic morphology and organization of human pluripotent colonies.

Methodology/Principal Findings

Specific and irreversible inhibition of G_i-coupled GPCR signaling by pertussis toxin markedly altered pluripotent colony morphology. Wild-type hES and hiPS cells formed monolayer colonies, but colonies treated with pertussis toxin retracted inward, adopting a dense, multi-layered conformation. The treated colonies were unable to reform after a scratch wound insult, whereas control colonies healed completely within 48 h. In contrast, activation of an alternative GPCR pathway, G_s-coupled signaling, with cholera toxin did not affect colony morphology or the healing response. Pertussis toxin did not alter the proliferation, apoptosis or pluripotency of pluripotent stem cells.

Conclusions/Significance

Experiments with pertussis toxin suggest that G_i signaling plays a critical role in the morphology and organization of pluripotent colonies. These results may be explained by a G_i-mediated density-sensing mechanism that propels the cells radially outward. GPCRs are a promising target for modulating the formation and organization of hiPS and hES cell colonies and may be important for understanding somatic cell reprogramming and for engineering pluripotent stem cells for therapeutic applications.     

A Conformational Switch in the Scaffolding Protein NHERF1 Controls Autoinhibition and Complex Formation

The mammalian Na⁺/H⁺ exchange regulatory factor 1 (NHERF1) is a multidomain scaffolding protein essential for regulating the intracellular trafficking and macromolecular assembly of transmembrane ion channels and receptors. NHERF1 consists of tandem PDZ-1, PDZ-2 domains that interact with the cytoplasmic domains of membrane proteins and a C-terminal (CT) domain that binds the membrane-cytoskeleton linker protein ezrin. NHERF1 is held in an autoinhibited state through intramolecular interactions between PDZ2 and the CT domain that also includes a C-terminal PDZ-binding motif (-SNL). We have determined the structures of the isolated and tandem PDZ2CT domains by high resolution NMR using small angle x-ray scattering as constraints. The PDZ2CT structure shows weak intramolecular interactions between the largely disordered CT domain and the PDZ ligand binding site. The structure reveals a novel helix-turn-helix subdomain that is allosterically coupled to the putative PDZ2 domain by a network of hydrophobic interactions. This helical subdomain increases both the stability and the binding affinity of the extended PDZ structure. Using NMR and small angle neutron scattering for joint structure refinement, we demonstrate the release of intramolecular domain-domain interactions in PDZ2CT upon binding to ezrin. Based on the structural information, we show that human disease-causing mutations in PDZ2, R153Q and E225K, have significantly reduced protein stability. Loss of NHERF1 expressed in cells could result in failure to assemble membrane complexes that are important for normal physiological functions.     

Pirh2 E3 Ubiquitin Ligase Targets DNA Polymerase Eta for 20S Proteasomal Degradation

DNA polymerase eta (PolH), a Y family translesion polymerase, is required for repairing UV-induced DNA damage, and loss of PolH is responsible for early onset of malignant skin cancers in patients with xeroderma pigmentosum variant (XPV), an autosomal recessive disorder. Here, we show that PolH, a target of the p53 tumor suppressor, is a short-half-life protein. We found that PolH is degraded by proteasome, which is enhanced upon UV irradiation. We also found that PolH interacts with Pirh2 E3 ligase, another target of the p53 tumor suppressor, via the polymerase-associated domain in PolH and the RING finger domain in Pirh2. In addition, we show that overexpression of Pirh2 decreases PolH protein stability, whereas knockdown of Pirh2 increases it. Interestingly, we found that PolH is recruited by Pirh2 and degraded by 20S proteasome in a ubiquitin-independent manner. Finally, we observed that Pirh2 knockdown leads to accumulation of PolH and, subsequently, enhances the survival of UV-irradiated cells. We postulate that UV irradiation promotes cancer formation in part by destabilizing PolH via Pirh2-mediated 20S proteasomal degradation.Polymerase eta (PolH) is a member of the Y family translesion DNA polymerases and capable of translesion synthesis over UV-induced cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts (7). PolH is also involved in double-stranded break repair via homologous recombination (15, 23). Human PolH is the product of the xeroderma pigmentosum variant (XPV) gene (14, 22). XPV, an autosomal recessive disorder, exhibits clinical phenotypes of extreme sun sensibility, cutaneous and ocular deterioration, and early onset of malignant skin cancers. Thus, it is postulated that loss of PolH is responsible for accumulation of UV-induced lesions, which lead to early onset of multiple skin cancers in XPV patients.The ubiquitin-dependent degradation pathway plays a key role in many cellular processes, including cell proliferation, differentiation, and DNA repair (6, 10, 11). The pathway involves multiple enzymatic reactions catalyzed by a single ubiquitin-activating enzyme (E1), several ubiquitin-conjugating enzymes (E2s), and a large number of ubiquitin ligases (E3s). Protein polyubiquitination serves as a signal for rapid degradation by 26S proteasome, whereas monoubiquitination modulates protein function (3, 30). 26S proteasome is a multisubunit protease consisting of a core 20S proteasome and two 19S regulatory particles (24). 20S proteasome on its own is a broad-spectrum ATP- and ubiquitin-independent protease. 19S regulatory particles recognize and thread polyubiquitinated proteins into 20S proteasome for degradation in an ATP-dependent manner.The RING-H2 type E3 ligase (Pirh2) is regulated by p53 and targets p53 for degradation (19). Recently, studies showed that Pirh2 interacts with and potentially serves as an E3 ligase for TIP60 (21) and p27Kip1 (8). Here, we show that PolH protein stability is reduced by UV irradiation via Pirh2 in a ubiquitin-independent manner. We also showed that upon knockdown of Pirh2, PolH is accumulated and, consequently, desensitizes cells to UV-induced cell killing. Based on these observations, we postulate that UV irradiation promotes cancer formation in part by destabilizing PolH via Pirh2-mediated 20S proteasome degradation.