Showing papers on "Proteasome published in 2008"

Misfolded proteins partition between two distinct quality control compartments

Abstract: The accumulation of misfolded proteins in intracellular amyloid inclusions, typical of many neurodegenerative disorders including Huntington's and prion disease, is thought to occur after failure of the cellular protein quality control mechanisms. Here we examine the formation of misfolded protein inclusions in the eukaryotic cytosol of yeast and mammalian cell culture models. We identify two intracellular compartments for the sequestration of misfolded cytosolic proteins. Partition of quality control substrates to either compartment seems to depend on their ubiquitination status and aggregation state. Soluble ubiquitinated misfolded proteins accumulate in a juxtanuclear compartment where proteasomes are concentrated. In contrast, terminally aggregated proteins are sequestered in a perivacuolar inclusion. Notably, disease-associated Huntingtin and prion proteins are preferentially directed to the perivacuolar compartment. Enhancing ubiquitination of a prion protein suffices to promote its delivery to the juxtanuclear inclusion. Our findings provide a framework for understanding the preferential accumulation of amyloidogenic proteins in inclusions linked to human disease.

Proteasome subunit Rpn13 is a novel ubiquitin receptor

Abstract: Proteasomal receptors that recognize ubiquitin chains attached to substrates are key mediators of selective protein degradation in eukaryotes. Here we report the identification of a new ubiquitin receptor, Rpn13/ARM1, a known component of the proteasome. Rpn13 binds ubiquitin through a conserved amino-terminal region termed the pleckstrin-like receptor for ubiquitin (Pru) domain, which binds K48-linked diubiquitin with an affinity of approximately 90 nM. Like proteasomal ubiquitin receptor Rpn10/S5a, Rpn13 also binds ubiquitin-like (UBL) domains of UBL-ubiquitin-associated (UBA) proteins. In yeast, a synthetic phenotype results when specific mutations of the ubiquitin binding sites of Rpn10 and Rpn13 are combined, indicating functional linkage between these ubiquitin receptors. Because Rpn13 is also the proteasomal receptor for Uch37, a deubiquitinating enzyme, our findings suggest a coupling of chain recognition and disassembly at the proteasome.

The proteasome inhibitor bortezomib depletes plasma cells and protects mice with lupus-like disease from nephritis.

Abstract: Autoantibody-mediated diseases like myasthenia gravis, autoimmune hemolytic anemia and systemic lupus erythematosus represent a therapeutic challenge. In particular, long-lived plasma cells producing autoantibodies resist current therapeutic and experimental approaches. Recently, we showed that the sensitivity of myeloma cells toward proteasome inhibitors directly correlates with their immunoglobulin synthesis rates. Therefore, we hypothesized that normal plasma cells are also hypersensitive to proteasome inhibition owing to their extremely high amount of protein biosynthesis. Here we show that the proteasome inhibitor bortezomib, which is approved for the treatment of multiple myeloma, eliminates both short- and long-lived plasma cells by activation of the terminal unfolded protein response. Treatment with bortezomib depleted plasma cells producing antibodies to double-stranded DNA, eliminated autoantibody production, ameliorated glomerulonephritis and prolonged survival of two mouse strains with lupus-like disease, NZB/W F1 and MRL/lpr mice. Hence, the elimination of autoreactive plasma cells by proteasome inhibitors might represent a new treatment strategy for antibody-mediated diseases.

Ubiquitin, the proteasome and protein degradation in neuronal function and dysfunction

Abstract: Eukaryotic protein degradation by the proteasome and the lysosome is a dynamic and complex process in which ubiquitin has a key regulatory role. The distinctive morphology of the postmitotic neuron creates unique challenges for protein degradation systems with respect to cell-surface protein turnover and substrate delivery to proteolytic machineries that are required for both synaptic plasticity and self-renewal. Moreover, the discovery of ubiquitin-positive protein aggregates in a wide spectrum of neurodegenerative diseases underlines the importance and vulnerability of the degradative system in neurons. In this article, we discuss the molecular mechanism of protein degradation in the neuron with respect to both its function and its dysfunction.

Arabidopsis DREB2A-Interacting Proteins Function as RING E3 Ligases and Negatively Regulate Plant Drought Stress–Responsive Gene Expression

Abstract: The DEHYDRATION-RESPONSIVE ELEMENT BINDING PROTEIN2A (DREB2A) transcription factor controls water deficit–inducible gene expression and requires posttranslational modification for its activation. The activation mechanism is not well understood; however, the stability of this protein in the nucleus was recently found to be important for its activation. Here, we report the isolation of Arabidopsis thaliana DREB2A-INTERACTING PROTEIN1 (DRIP1) and DRIP2, C3HC4 RING domain–containing proteins that interact with the DREB2A protein in the nucleus. An in vitro ubiquitination assay showed that they function as E3 ubiquitin ligases and are capable of mediating DREB2A ubiquitination. Overexpression of DRIP1 in Arabidopsis delayed the expression of DREB2A-regulated drought-responsive genes. Drought-inducible gene expression was slightly enhanced in the single T-DNA mutants of drip1-1 and drip2-1. By contrast, significantly enhanced gene expression was revealed in the drip1 drip2 double mutant under dehydration stress. Collectively, these data imply that DRIP1 and DRIP2 function negatively in the response of plants to drought stress. Moreover, overexpression of full-length DREB2A protein was more stable in drip1-1 than in the wild-type background. These results suggest that DRIP1 and DRIP2 act as novel negative regulators in drought-responsive gene expression by targeting DREB2A to 26S proteasome proteolysis.

Lysine 63-linked ubiquitination promotes the formation and autophagic clearance of protein inclusions associated with neurodegenerative diseases

Abstract: Although ubiquitin-enriched protein inclusions represent an almost invariant feature of neurodegenerative diseases, the mechanism underlying their biogenesis remains unclear. In particular, whether the topology of ubiquitin linkages influences the dynamics of inclusions is not well explored. Here, we report that lysine 48 (K48)- and lysine 63 (K63)-linked polyubiquitination, as well as monoubiquitin modification contribute to the biogenesis of inclusions. K63-linked polyubiquitin is the most consistent enhancer of inclusions formation. Under basal conditions, ectopic expression of K63 mutant ubiquitin in cultured cells promotes the accumulation of proteins and the formation of intracellular inclusions in the apparent absence of proteasome impairment. When co-expressed with disease-associated tau and SOD1 mutants, K63 ubiquitin mutant facilitates the formation of tau- and SOD-1-positive inclusions. Moreover, K63-linked ubiquitination was found to selectively facilitate the clearance of inclusions via autophagy. These data indicate that K63-linked ubiquitin chains may represent a common denominator underlying inclusions biogenesis, as well as a general cellular strategy for defining cargo destined for the autophagic system. Collectively, our results provide a novel mechanistic route that underlies the life cycle of an inclusion body. Harnessing this pathway may offer innovative approaches in the treatment of neurodegenerative disorders.

Mechanisms of glucocorticoid-induced myopathy.

Abstract: Glucocorticoid-induced muscle atrophy is characterized by fast-twitch or type II muscle fiber atrophy illustrated by decreased fiber cross-sectional area and reduced myofibrillar protein content. Muscle proteolysis, in particular through the ubiquitin- proteasome system (UPS), is considered to play a major role in the catabolic action of glucocorticoids. The stimulation by glucocorticoids of the UPS is mediated through the increased expression of several atrogenes ('genes involved in atrophy'), such as atrogin-1 and MuRF-1, two ubiquitin ligases involved in the targeting of protein to be degraded by the proteasome machinery. Glucocorticoids also exert an anti-anabolic action by blunting muscle protein synthesis. These changes in protein turnover may result from changes in the production of two growth factors which control muscle mass, namely IGF-I and myostatin respectively anabolic and catabolic toward the skeletal muscle. The decreased production of IGF-I as well as the increased production of myostatin have been both demonstrated to contribute to the muscle atrophy caused by glucocorticoids. At the molecular level, IGF-I antagonizes the catabolic action of glucocorticoids by inhibiting, through the PI3-kinase/Akt pathway, the activity of the transcription factor FOXO, a major switch for the stimulation of several atrogenes. These recent progress in the understanding of the glucocorticoid-induced muscle atrophy should allow to define new therapies aiming to minimize this myopathy. Promising new therapeutic approaches for treating glucocorticoid-induced muscle atrophy are also presented in this review.

Sorting, recognition and activation of the misfolded protein degradation pathways through macroautophagy and the proteasome

Abstract: Based on a functional categorization, proteins may be grouped into three types and sorted to either the proteasome or the macroautophagy pathway for degradation. The two pathways are mechanistically connected but their capacity seems different. Macroautophagy can degrade all forms of misfolded proteins whereas proteasomal degradation is likely limited to soluble ones. Unlike the bulk protein degradation that occurs during starvation, autophagic degradation of misfolded proteins can have a degree of specificity, determined by ubiquitin modification and the interactions of p62/SQSTM1 and HDAC6. Macroautophagy is initiated in response to endoplasmic reticulum (ER) stress caused by misfolded proteins, via the ER-activated autophagy (ERAA) pathway, which activates a partial unfolded protein response involving PERK and/or IRE1, and a calcium-mediated signaling cascade. ERAA serves the function of mitigating ER stress and suppressing cell death, which may be explored for controlling protein conformational diseases. Conversely, inhibition of ERAA may be explored for sensitizing resistant tumor cells to cytotoxic agents.

Ubiquitin-like protein involved in the proteasome pathway of Mycobacterium tuberculosis.

Abstract: The protein modifier ubiquitin is a signal for proteasome-mediated degradation in eukaryotes. Proteasome-bearing prokaryotes have been thought to degrade proteins via a ubiquitin-independent pathway. We have identified a prokaryotic ubiquitin-like protein, Pup (Rv2111c), which was specifically conjugated to proteasome substrates in the pathogen Mycobacterium tuberculosis. Pupylation occurred on lysines and required proteasome accessory factor A (PafA). In a pafA mutant, pupylated proteins were absent and substrates accumulated, thereby connecting pupylation with degradation. Although analogous to ubiquitylation, pupylation appears to proceed by a different chemistry. Thus, like eukaryotes, bacteria may use a small-protein modifier to control protein stability.

Ubiquitin docking at the proteasome through a novel pleckstrin-homology domain interaction

Abstract: Targeted protein degradation is largely performed by the ubiquitin-proteasome pathway, in which substrate proteins are marked by covalently attached ubiquitin chains that mediate recognition by the proteasome. It is currently unclear how the proteasome recognizes its substrates, as the only established ubiquitin receptor intrinsic to the proteasome is Rpn10/S5a (ref. 1), which is not essential for ubiquitin-mediated protein degradation in budding yeast. In the accompanying manuscript we report that Rpn13 (refs 3-7), a component of the nine-subunit proteasome base, functions as a ubiquitin receptor, complementing its known role in docking de-ubiquitinating enzyme Uch37/UCHL5 (refs 4-6) to the proteasome. Here we merge crystallography and NMR data to describe the ubiquitin-binding mechanism of Rpn13. We determine the structure of Rpn13 alone and complexed with ubiquitin. The co-complex reveals a novel ubiquitin-binding mode in which loops rather than secondary structural elements are used to capture ubiquitin. Further support for the role of Rpn13 as a proteasomal ubiquitin receptor is demonstrated by its ability to bind ubiquitin and proteasome subunit Rpn2/S1 simultaneously. Finally, we provide a model structure of Rpn13 complexed to diubiquitin, which provides insights into how Rpn13 as a ubiquitin receptor is coupled to substrate deubiquitination by Uch37.

Depletion of 26S proteasomes in mouse brain neurons causes neurodegeneration and Lewy-like inclusions resembling human pale bodies

Abstract: Ubiquitin-positive intraneuronal inclusions are a consistent feature of the major human neurodegenerative diseases, suggesting that dysfunction of the ubiquitin proteasome system is central to disease etiology. Research using inhibitors of the 20S proteasome to model Parkinson's disease is controversial. We report for the first time that specifically 26S proteasomal dysfunction is sufficient to trigger neurodegenerative disease. Here, we describe novel conditional genetic mouse models using the Cre/loxP system to spatially restrict inactivation of Psmc1 (Rpt2/S4) to neurons of either the substantia nigra or forebrain (e.g., cortex, hippocampus, and striatum). PSMC1 is an essential subunit of the 26S proteasome and Psmc1 conditional knock-out mice display 26S proteasome depletion in targeted neurons, in which the 20S proteasome is not affected. Impairment of specifically ubiquitin-mediated protein degradation caused intraneuronal Lewy-like inclusions and extensive neurodegeneration in the nigrostriatal pathway and forebrain regions. Ubiquitin and alpha-synuclein neuropathology was evident, similar to human Lewy bodies, but interestingly, inclusion bodies contained mitochondria. We support this observation by demonstrating mitochondria in an early form of Lewy body (pale body) from Parkinson's disease patients. The results directly confirm that 26S dysfunction in neurons is involved in the pathology of neurodegenerative disease. The model demonstrates that 26S proteasomes are necessary for normal neuronal homeostasis and that 20S proteasome activity is insufficient for neuronal survival. Finally, we are providing the first reproducible genetic platform for identifying new therapeutic targets to slow or prevent neurodegeneration.

BRCA1-Associated Protein-1 Is a Tumor Suppressor that Requires Deubiquitinating Activity and Nuclear Localization

Abstract: BRCA1-associated protein-1 (BAP1), a deubiquitinating enzyme of unknown cellular function, is mutated in breast and lung cancers In this study, we have shown for the first time that BAP1 has tumor suppressor activity in vivo by showing that BAP1 can suppress tumorigenicity of lung cancer cells in athymic nude mice We show that BAP1 fulfills another criterion of a genuine tumor suppressor because cancer-associated BAP1 mutants are deficient in deubiquitinating activity We show for the first time that one of the two predicted nuclear targeting motifs is required for nuclear localization of BAP1 and that a truncation mutant found in a lung cancer cell line results in BAP1 that fails to localize to the nucleus Furthermore, we show that deubiquitinating activity and nuclear localization are both required for BAP1-mediated tumor suppression in nude mice We show that BAP1 exerts its tumor suppressor functions by affecting the cell cycle, speeding the progression through the G(1)-S checkpoint, and inducing cell death via a process that has characteristics of both apoptosis and necrosis Surprisingly, BAP1-mediated growth suppression is independent of wild-type BRCA1 Because deubiquitinating enzymes are components of the ubiquitin proteasome system, this pathway has emerged as an important target for anticancer drugs The identification of the deubiquitinating enzyme BAP1 as a tumor suppressor may lead to further understanding of how the ubiquitin proteasome system contributes to cancer and aid in the identification of new targets for cancer therapy

A plant pathogen virulence factor inhibits the eukaryotic proteasome by a novel mechanism

Abstract: Pathogenic bacteria often use effector molecules to increase virulence. In most cases, the mode of action of effectors remains unknown. Strains of Pseudomonas syringae pv. syringae (Pss) secrete syringolin A (SylA), a product of a mixed non-ribosomal peptide/polyketide synthetase, in planta1. Here we identify SylA as a virulence factor because a SylA-negative mutant in Pss strain B728a obtained by gene disruption was markedly less virulent on its host, Phaseolus vulgaris (bean). We show that SylA irreversibly inhibits all three catalytic activities of eukaryotic proteasomes, thus adding proteasome inhibition to the repertoire of modes of action of virulence factors. The crystal structure of the yeast proteasome in complex with SylA revealed a novel mechanism of covalent binding to the catalytic subunits. Thus, SylA defines a new class of proteasome inhibitors that includes glidobactin A (GlbA), a structurally related compound from an unknown species of the order Burkholderiales2, for which we demonstrate a similar proteasome inhibition mechanism. As proteasome inhibitors are a promising class of anti-tumour agents, the discovery of a novel family of inhibitory natural products, which we refer to as syrbactins, may also have implications for the development of anti-cancer drugs3. Homologues of SylA and GlbA synthetase genes are found in some other pathogenic bacteria, including the human pathogen Burkholderia pseudomallei, the causative agent of melioidosis4. It is thus possible that these bacteria are capable of producing proteasome inhibitors of the syrbactin class.

Polyubiquitin chains: functions, structures, and mechanisms.

Abstract: Ubiquitin is a highly conserved 76-aminoacid polypeptide that is found throughout the eukaryotic kingdom. The covalent conjugation of ubiquitin (often in the form of a polymer) to substrates governs a variety of biological processes ranging from proteolysis to DNA damage tolerance. The functional flexibility of this post-translational modification has its roots in the existence of a large number of ubiquitinating enzymes that catalyze the formation of distinct ubiquitin polymers, which in turn encode different signals. This review summarizes recent advances in the field with an emphasis on the non-canonical functions of polyubiquitination. We also discuss the potential mechanism of chain linkage specification as well as how structural disparity in ubiquitin polymers may be distinguished by ubiquitin receptors to translate the versatile ubiquitin signals into various cellular functions.

The HECT-domain ubiquitin ligase Huwe1 controls neural differentiation and proliferation by destabilizing the N-Myc oncoprotein.

Abstract: Development of the nervous system requires that timely withdrawal from the cell cycle be coupled with initiation of differentiation. Ubiquitin-mediated degradation of the N-Myc oncoprotein in neural stem/progenitor cells is thought to trigger the arrest of proliferation and begin differentiation. Here we report that the HECT-domain ubiquitin ligase Huwe1 ubiquitinates the N-Myc oncoprotein through Lys 48-mediated linkages and targets it for destruction by the proteasome. This process is physiologically implemented by embryonic stem (ES) cells differentiating along the neuronal lineage and in the mouse brain during development. Genetic and RNA interference-mediated inactivation of the Huwe1 gene impedes N-Myc degradation, prevents exit from the cell cycle by opposing the expression of Cdk inhibitors and blocks differentiation through persistent inhibition of early and late markers of neuronal differentiation. Silencing of N-myc in cells lacking Huwe1 restores neural differentiation of ES cells and rescues cell-cycle exit and differentiation of the mouse cortex, demonstrating that Huwe1 restrains proliferation and enables neuronal differentiation by mediating the degradation of N-Myc. These findings indicate that Huwe1 links destruction of N-Myc to the quiescent state that complements differentiation in the neural tissue.

Curcumin Inhibits the Proteasome Activity in Human Colon Cancer Cells In vitro and In vivo

Abstract: Curcumin (diferuloylmethane) is the major active ingredient of turmeric (Curcuma longa) used in South Asian cuisine for centuries. Curcumin has been shown to inhibit the growth of transformed cells and to have a number of potential molecular targets. However, the essential molecular targets of curcumin under physiologic conditions have not been completely defined. Herein, we report that the tumor cellular proteasome is most likely an important target of curcumin. Nucleophilic susceptibility and in silico docking studies show that both carbonyl carbons of the curcumin molecule are highly susceptible to a nucleophilic attack by the hydroxyl group of the NH(2)-terminal threonine of the proteasomal chymotrypsin-like (CT-like) subunit. Consistently, curcumin potently inhibits the CT-like activity of a purified rabbit 20S proteasome (IC(50) = 1.85 micromol/L) and cellular 26S proteasome. Furthermore, inhibition of proteasome activity by curcumin in human colon cancer HCT-116 and SW480 cell lines leads to accumulation of ubiquitinated proteins and several proteasome target proteins, and subsequent induction of apoptosis. Furthermore, treatment of HCT-116 colon tumor-bearing ICR SCID mice with curcumin resulted in decreased tumor growth, associated with proteasome inhibition, proliferation suppression, and apoptosis induction in tumor tissues. Our study shows that proteasome inhibition could be one of the mechanisms for the chemopreventive and/or therapeutic roles of curcumin in human colon cancer. Based on its ability to inhibit the proteasome and induce apoptosis in both HCT-116 and metastatic SW480 colon cancer cell lines, our study suggests that curcumin could potentially be used for treatment of both early-stage and late-stage/refractory colon cancer.

SEL1L nucleates a protein complex required for dislocation of misfolded glycoproteins

Abstract: Membrane and secretory proteins that fail to pass quality control in the endoplasmic reticulum are discharged into the cytosol and degraded by the proteasome. Many of the mammalian components involved in this process remain to be identified. We performed a biochemical search for proteins that interact with SEL1L, a protein that is part of the mammalian HRD1 ligase complex and involved in substrate recognition. SEL1L is crucial for dislocation of Class I major histocompatibility complex heavy chains by the human cytomegalovirus US11 protein. We identified AUP1, UBXD8, UBC6e, and OS9 as functionally important components of this degradation complex in mammalian cells, as confirmed by mutagenesis and dominant negative versions of these proteins.

Regulation of TGF‐β family signaling by E3 ubiquitin ligases

Abstract: Members of the transforming growth factor-beta (TGF-beta) family, including TGF-beta, activin and bone morphogenetic proteins (BMPs), are multifunctional proteins that regulate a wide variety of cellular responses, such as proliferation, differentiation, migration and apoptosis. Alterations in their downstream signaling pathways are associated with a range of human diseases like cancer. TGF-beta family members transduce signals through membrane serine/threonine kinase receptors and intracellular Smad proteins. The ubiquitin-proteasome pathway, an evolutionarily conserved cascade, tightly regulates TGF-beta family signaling. In this pathway, E3 ubiquitin ligases play a crucial role in the recognition and degradation of target proteins by the 26S proteasomes. Smad degradation regulates TGF-beta family signaling; HECT (homologous to the E6-accessory protein C-terminus)-type E3 ubiquitin ligases, Smad ubiquitin regulatory factor 1 (Smurf1), Smurf2, and a RING-type E3 ubiquitin ligase, ROC1-SCF(Fbw1a) have been implicated in Smad degradation. Smurf1 and Smurf2 bind to TGF-beta family receptors via the inhibitory Smads, Smad6 and Smad7, to induce their ubiquitin-dependent degradation. Arkadia, a RING-type E3 ubiquitin ligase, induces the ubiquitination and degradation of Smad7 and corepressors, c-Ski and SnoN, to enhance TGF-beta family signaling. Abnormalities in E3 ubiquitin ligases that control components of TGF-beta family signaling may lead to the development and progression of various cancers.

Myofibrillar protein turnover: The proteasome and the calpains

Abstract: Metabolic turnover of myofibrillar proteins in skeletal muscle requires that, before being degraded to AA, myofibrillar proteins be removed from the myofibril without disrupting the ability of the myofibril to contract and develop tension. Skeletal muscle contains 4 proteolytic systems in amounts such that they could be involved in metabolic protein turnover: 1) the lysosomal system, 2) the caspase system, 3) the calpain system, and 4) the proteasome. The catheptic proteases in lysosomes are not active at the neutral pH of the cell cytoplasm, so myofibrillar proteins would have to be degraded inside lysosomes if the lysosomal system were involved. Lysosomes could not engulf a myofibril without destroying it, so the lysosomal system is not involved to a significant extent in metabolic turnover of myofibrillar proteins. The caspases are not activated until initiation of apoptosis, and, therefore, it is unlikely that the caspases are involved to a significant extent in myofibrillar protein turnover. The calpains do not degrade proteins to AA or even to small peptides and do not catalyze bulk degradation of the sarcoplasmic proteins, so they cannot be the only proteolytic system involved in myofibrillar protein turnover. Research during the past 20 yr has shown that the proteasome is responsible for 80 to 90% of total intracellular protein turnover, but the proteasome degrades peptide chains only after they have been unfolded, so that they can enter the catalytic chamber of the proteasome. Thus, although the proteasome can degrade sarcoplasmic proteins, it cannot degrade myofibrillar proteins until they have been removed from the myofibril. It remains unclear how this removal is done. The calpains degrade those proteins that are involved in keeping the myofibrillar proteins assembled in myofibrils, and it was proposed over 30 yr ago that the calpains initiated myofibrillar protein turnover by disassembling the outer layer of proteins from the myofibril and releasing them as myofilaments. Such myofilaments have been found in skeletal muscle. Other studies have indicated that individual myofibrillar proteins can exchange with their counterparts in the cytoplasm; it is unclear whether this can be done to an extent that is consistent with the rate of myofibrillar protein turnover in living muscle. It seems that both the calpains and the proteasome are responsible for myofibrillar protein turnover, but the mechanism is still unknown.

Arabidopsis PUB22 and PUB23 Are Homologous U-Box E3 Ubiquitin Ligases That Play Combinatory Roles in Response to Drought Stress

Abstract: Ubiquitination is involved in diverse cellular processes in higher plants. In this report, we describe Arabidopsis thaliana PUB22 and PUB23, two homologous U-box-containing E3 ubiquitin (Ub) ligases. The PUB22 and PUB23 genes were rapidly and coordinately induced by abiotic stresses but not by abscisic acid. PUB22- and PUB23-overexpressing transgenic plants were hypersensitive to drought stress. By contrast, loss-of-function pub22 and pub23 mutant plants were significantly more drought-tolerant, and a pub22 pub23 double mutant displayed even greater drought tolerance. These results indicate that PUB22 and PUB23 function as negative regulators in the water stress response. Yeast two-hybrid, in vitro pull-down, and in vivo coimmunoprecipitation experiments revealed that PUB22 and PUB23 physically interacted with RPN12a, a subunit of the 19S regulatory particle (RP) in the 26S proteasome. Bacterially expressed RPN12a was effectively ubiquitinated in a PUB-dependent fashion. RPN12a was highly ubiquitinated in 35S:PUB22 plants, but not in pub22 pub23 double mutant plants, consistent with RPN12a being a substrate of PUB22 and PUB23 in vivo. In water-stressed wild-type and PUB-overexpressing plants, a significant amount of RPN12a was dissociated from the 19S RP and appeared to be associated with small-molecular-mass protein complexes in cytosolic fractions, where PUB22 and PUB23 are localized. Overall, our results suggest that PUB22 and PUB23 coordinately control a drought signaling pathway by ubiquitinating cytosolic RPN12a in Arabidopsis.

The ubiquitin‐proteasome system in Alzheimer's disease

Abstract: Accumulation of proteins is a recurring event in many neurodegenerative diseases, including Alzheimer's disease (AD).Evidence has suggested that protein accumulation may result from a dysfunction in the ubiquitin proteasome system (UPS). Indeed, there is clear genetic and biochemical evidence of an involvement of the ubiquitin proteasome system in AD. This review summarizes the data supporting an involvement of the UPS in the pathogenesis of AD, focusing on the data showing the relationship between Aβ and tau, the two hallmark lesions of AD, and the UPS.