Illuminating the ubiquitin/proteasome system

The ubiquitin/proteasome system (UPS) is responsible for the regulated processive degradation of proteins residing in the cytosol, nucleus, and endoplasmic reticulum. The two central players are ubiquitin, a small protein that is conjugated to substrates, and the proteasome, a large multi-subunit proteolytic complex that executes degradation of ubiquitylated proteins. Ubiquitylation and proteasomal degradation are highly dynamic processes. During the last decade, many researchers have started taking advantage of fluorescent proteins, which allow studying the dynamic nature of this system in the context of its natural environment: the living cell. In this review, we will summarize studies that have implemented this approach to examine the UPS and discuss novel insights in the dynamic organization of the UPS.     

Cooperation of molecular chaperones with the ubiquitin/proteasome system

Molecular chaperones and energy-dependent proteases have long been viewed as opposing forces that control protein biogenesis. Molecular chaperones are specialized in protein folding, whereas energy-dependent proteases such as the proteasome mediate efficient protein degradation. Recent data, however, suggest that molecular chaperones directly cooperate with the ubiquitin/proteasome system during protein quality control in eukaryotic cells. Modulating the intracellular balance of protein folding and protein degradation may open new strategies for the treatment of human diseases that involve chaperone pathways such as cancer and diverse amyloid diseases.     

Synaptic protein degradation by the ubiquitin proteasome system

Synaptic plasticity -- the modulation of synaptic strength between a presynaptic terminal and a postsynaptic dendrite -- is thought to be a mechanism that underlies learning and memory. It has become increasingly clear that regulated protein synthesis is an important mechanism used to regulate the protein content of synapses that results in changes in synaptic strength. Recent experiments have highlighted a role for the opposing process, that is, regulated protein degradation via the ubiquitin-proteasome system, in synaptic plasticity. These recent findings raise exciting questions as to how proteasomal activity can regulate synapses over different temporal and spatial scales.     

泛素-蛋白酶体途径及其生物学作用的研究进展

泛素-蛋白酶体途径是细胞内重要的非溶酶体蛋白降解途径,是调节各种细胞生物学过程的重要机制,参与调节细胞周期进程、细胞增生与分化以及信号转导等各种细胞生理过程,对维持细胞正常生理功能具有十分重要的意义。本文简要介绍了泛素-蛋白酶体途径的作用过程,并从其对某些抑癌基因、转录因子和细胞周期素依赖性激酶抑制蛋白的调节,参与肿瘤及癌症的发生和发展,讨论其生物学作用,并指出其在药物研究方面的重要作用。     

Stabilization signals: a novel regulatory mechanism in the ubiquitin/proteasome system

The turnover of cellular proteins is a highly organized process that involves spatially and temporally regulated degradation by the ubiquitin/proteasome system. It is generally acknowledged that the specificity of the process is determined by constitutive or conditional protein domains, the degradation signals, that target the substrate for proteasomal degradation. In this review, we discuss a new type of regulatory domain: the stabilization signal. A model is proposed according to which protein half-lives are determined by the interplay of counteracting degradation and stabilization signals.     

Regulation of apoptosis by E3 ubiquitin ligases in ubiquitin proteasome system

Apoptosis is an organised ATP‐dependent programmed cell death that organisms have evolved to maintain homoeostatic cell numbers and eliminate unnecessary or unhealthy cells from the system. Dysregulation of apoptosis can have serious manifestations culminating into various diseases, especially cancer. Accurate control of apoptosis requires regulation of a wide range of growth enhancing as well as anti‐oncogenic factors. Appropriate regulation of magnitude and temporal expression of key proteins is vital to maintain functional apoptotic signalling. Controlled protein turnover is thus critical to the unhindered operation of the apoptotic machinery, disruption of which can have severe consequences, foremost being oncogenic transformation of cells. The ubiquitin proteasome system (UPS) is one such major cellular pathway that maintains homoeostatic protein levels. Recent studies have found interesting links between these two fundamental cellular processes, wherein UPS depending on the cue can either inhibit or promote apoptosis. A diverse range of E3 ligases are involved in regulating the turnover of key proteins of the apoptotic pathway. This review summarises an overview of key E3 ubiquitin ligases involved in the regulation of the fundamental proteins involved in apoptosis, linking UPS to apoptosis and attempts to emphasize the significance of this relationship in context of cancer.     

Identification and radiation hybrid mapping of members of the porcine proteasome/ubiquitin system

We report the identification and radiation hybrid mapping of members of the proteasome/ubiquitin system in pigs that, so far, have only been identified in humans and cattle. Expressed sequence tags (ESTs) were constructed from ten oligo(dT)-primed individually tagged, directionally cloned and normalized cDNA libraries from peripheral blood cells (PBC), spleen (Sp), thymus (Th), lymph node (LN) and bone marrow (BM) from immunologically naive and challenged pigs as part of an implant-associated orthopedic infection model. The ESTs mapped using the 7000 rad IMpRH panel (Hawken et al., 1999) were ubiquitin fusion-degradation 1 like protein (UFD1L), ubiquitin activating enzyme E1 and ubiquitin-S27a fusion protein which mapped to porcine chromosomes 14, 7 and X, respectively.     

The regulation of mitochondrial homeostasis by the ubiquitin proteasome system

From mitochondrial quality control pathways to the regulation of specific functions, the Ubiquitin Proteasome System (UPS) could be compared to a Swiss knife without which mitochondria could not maintain its integrity in the cell. Here, we review the mechanisms that the UPS employs to regulate mitochondrial function and efficiency. For this purpose, we depict how Ubiquitin and the Proteasome participate in diverse quality control pathways that safeguard entry into the mitochondrial compartment. A focus is then achieved on the UPS-mediated control of the yeast mitofusin Fzo1 which provides insights into the complex regulation of this particular protein in mitochondrial fusion. We ultimately dissect the mechanisms by which the UPS controls the degradation of mitochondria by autophagy in both mammalian and yeast systems. This organization should offer a useful overview of this abundant but fascinating literature on the crosstalks between mitochondria and the UPS.     

p97/VCP at the intersection of the autophagy and the ubiquitin proteasome system

A feature of aged onset degenerative disease is ubiquitinated protein inclusions. Similar inclusions are found in different tissues ranging from the central nervous, cardiovascular, musculoskeletal and gastrointestinal systems; whether, the same pathomechanism is responsible for the similar pathology in these disparate tissues is not known. To address this question, we explored the pathogenesis of a multi-system degenerative disorder, IBMPFD or inclusion body myopathy (IBM), paget's disease of the bone (PDB) and fronto-temporal dementia (FTD) of which ubiquitinated inclusions are a key pathological feature in muscle, brain and bone tissue. IBMPFD is caused by mutations in the ubiquitin proteasome system (UPS) chaperone p97/VCP. Previous reports suggest dysfunctional UPS in IBMPFD, however, we find that autophagic protein degradation and autophagosome maturation are diminished in IBMPFD mutant-expressing mice, patients and cell models. Moreover, a loss of p97/VCP function recapitulates the same effects, suggesting that p97/VCP is essential for autophagy. Thus, the degenerative phenotype in IBMPFD and its phenotypic components (IBM, PDB and FTD) may be disorders of impaired autophagy. p97/VCP is likely important in regulating both UPS- and autophagy-mediated protein degradation. This places p97/VCP in a key regulatory position at the intersection of these two proteolytic pathways.     

Coordinate regulation of autophagy and the ubiquitin proteasome system by MTOR

Proteins in eukaryotic cells are continually being degraded to amino acids either by the ubiquitin proteasome system (UPS) or by the autophagic-lysosomal pathway. The breakdown of proteins by these 2 degradative pathways involves totally different enzymes that function in distinct subcellular compartments. While most studies of the UPS have focused on the selective ubiquitination and breakdown of specific cell proteins, macroautophagy/autophagy is a more global nonselective process. Consequently, the UPS and autophagy were traditionally assumed to serve distinct physiological functions and to be regulated in quite different manners. However, recent findings indicate that protein breakdown by these 2 systems is coordinately regulated by important physiological stimuli. The activation of MTORC1 by nutrients and hormones rapidly suppresses proteolysis by both proteasomes and autophagy, which helps promote protein accumulation, whereas in nutrient-poor conditions, MTORC1 inactivation causes the simultaneous activation of these 2 degradative pathways to supply the deprived cells with a source of amino acids. Also this selective breakdown of key anabolic proteins by the UPS upon MTORC1 inhibition can help limit growth-related processes (e.g., cholesterol biosynthesis). Thus, the collaboration of these 2 degradative systems, together with the simultaneous control of protein translation by MTORC1, provide clear advantages to the organism in both growth and starvation conditions.     

Degradation of mutant huntingtin via the ubiquitin/proteasome system is modulated by FE65

An unstable expansion of the polyglutamine repeat within exon 1 of the protein Htt (huntingtin) causes HD (Huntington's disease). Mounting evidence shows that accumulation of N-terminal mutant Htt fragments is the source of disruption of normal cellular processes which ultimately leads to neuronal cell death. Understanding the degradation mechanism of mutant Htt and improving its clearance has emerged as a new direction in developing therapeutic approaches to treat HD. In the present study we show that the brain-enriched adaptor protein FE65 is a novel interacting partner of Htt. The binding is mediated through WW-polyproline interaction and is dependent on the length of the polyglutamine tract. Interestingly, a reduction in mutant Htt protein level was observed in FE65-knockdown cells, and the process requires the UPS (ubiquitin/proteasome system). Moreover, the ubiquitination level of mutant Htt was found to be enhanced when FE65 is knocked down. Immunofluroescence staining revealed that FE65 associates with mutant Htt aggregates. Additionally, we demonstrated that overexpression of FE65 increases mutant Htt-induced cell death both in vitro and in vivo. These results suggest that FE65 facilitates the accumulation of mutant Htt in cells by preventing its degradation via the UPS, and thereby enhances the toxicity of mutant Htt.     

A humanized BAC transgenic/knockout mouse model for HbE/beta-thalassemia

Hemoglobin E (HbE) is caused by a G-->A mutation at codon 26 of the beta-globin gene, which substitutes Glu-->Lys. This mutation gives rise to functional but unstable hemoglobin and activates a cryptic splice site causing mild anemia. HbE reaches a carrier frequency of 60-80% in some Southeast Asian populations. HbE causes serious disease when co-inherited with a beta-thalassemia mutation. In this study, we report the creation and evaluation of humanized transgenic mice containing the beta(E) mutation in the context of the human beta-globin locus. Developmental expression of the human beta(E) locus transgene partially complements the hematological abnormalities in heterozygous knockout mice ((mu)beta(th-3/+)) and rescues the embryonic lethality of homozygous knockout mice ((mu)beta(th-3/th-3)). The phenotype of rescued mice was dependent on the transgene copy number. This mouse model displays hematological abnormalities similar to HbE/beta-thalassemia patients and represent an ideal in vivo model system for pathophysiological studies and evaluation of novel therapies.