ATP-Dependent inactivation and sequestration of ornithine decarboxylase by the 26S proteasome are prerequisites for degradation

Fully Human Bifunctional Intrabodies Achieve Graded Reduction of Intracellular Tau and Rescue Survival of MAPT Mutation iPSC-derived Neurons

Tau protein aggregation is a hallmark of several neurodegenerative diseases, including Alzheimer's disease, frontotemporal dementia (FTD) and progressive supranuclear palsy (PSP), spurring development of tau-lowering therapeutic strategies. Here, we report fully human bifunctional anti-tau-PEST intrabodies that bind the mid-domain of tau to block aggregation and degrade tau via the proteasome using the ornithine decarboxylase (ODC) PEST degron. They effectively reduced tau protein in human iPSC-derived cortical neurons in 2D cultures and 3D organoids, including those with the disease-associated tau mutations R5L, N279K, R406W, and V337M. Anti-tau-hPEST intrabodies facilitated efficient ubiquitin-independent proteolysis, in contrast to tau-lowering approaches that rely on the cell's ubiquitination system. Importantly, they counteracted the proteasome impairment observed in V337M patient-derived cortical neurons and significantly improved neuronal survival. By serial mutagenesis, we created variants of the PEST degron that achieved graded levels of tau reduction. Moderate reduction was as effective as high reduction against tau V337M-induced neural cell death.

Bifunctional anti-huntingtin proteasome-directed intrabodies mediate efficient degradation of mutant huntingtin exon 1 protein fragments

Huntington's disease (HD) is a fatal autosomal dominant neurodegenerative disorder caused by a trinucleotide (CAG)(n) repeat expansion in the coding sequence of the huntingtin gene, and an expanded polyglutamine (>37Q) tract in the protein. This results in misfolding and accumulation of huntingtin protein (htt), formation of neuronal intranuclear and cytoplasmic inclusions, and neuronal dysfunction/degeneration. Single-chain Fv antibodies (scFvs), expressed as intrabodies that bind htt and prevent aggregation, show promise as immunotherapeutics for HD. Intrastriatal delivery of anti-N-terminal htt scFv-C4 using an adeno-associated virus vector (AAV2/1) significantly reduces the size and number of aggregates in HDR6/1 transgenic mice; however, this protective effect diminishes with age and time after injection. We therefore explored enhancing intrabody efficacy via fusions to heterologous functional domains. Proteins containing a PEST motif are often targeted for proteasomal degradation and generally have a short half life. In ST14A cells, fusion of the C-terminal PEST region of mouse ornithine decarboxylase (mODC) to scFv-C4 reduces htt exon 1 protein fragments with 72 glutamine repeats (httex1-72Q) by ~80-90% when compared to scFv-C4 alone. Proteasomal targeting was verified by either scrambling the mODC-PEST motif, or via proteasomal inhibition with epoxomicin. For these constructs, the proteasomal degradation of the scFv intrabody proteins themselves was reduced<25% by the addition of the mODC-PEST motif, with or without antigens. The remaining intrabody levels were amply sufficient to target N-terminal httex1-72Q protein fragment turnover. Critically, scFv-C4-PEST prevents aggregation and toxicity of httex1-72Q fragments at significantly lower doses than scFv-C4. Fusion of the mODC-PEST motif to intrabodies is a valuable general approach to specifically target toxic antigens to the proteasome for degradation.

Ubiquitin-independent degradation of proteins by the proteasome

The proteasome is the main proteolytic machinery of the cell and constitutes a recognized drugable target, in particular for treating cancer. It is involved in the elimination of misfolded, altered or aged proteins as well as in the generation of antigenic peptides presented by MHC class I molecules. It is also responsible for the proteolytic maturation of diverse polypeptide precursors and for the spatial and temporal regulation of the degradation of many key cell regulators whose destruction is necessary for progression through essential processes, such as cell division, differentiation and, more generally, adaptation to environmental signals. It is generally believed that proteins must undergo prior modification by polyubiquitin chains to be addressed to, and recognized by, the proteasome. In reality, however, there is accumulating evidence that ubiquitin-independent proteasomal degradation may have been largely underestimated. In particular, a number of proto-oncoproteins and oncosuppressive proteins are privileged ubiquitin-independent proteasomal substrates, the altered degradation of which may have tumorigenic consequences. The identification of ubiquitin-independent mechanisms for proteasomal degradation also poses the paramount question of the multiplicity of catabolic pathways targeting each protein substrate. As this may help design novel therapeutic strategies, the underlying mechanisms are critically reviewed here.

Proteasome substrate degradation requires association plus extended peptide

To determine the minimum requirements for substrate recognition and processing by proteasomes, the functional elements of a ubiquitin-independent degradation tag were dissected. The 37-residue C-terminus of ornithine decarboxylase (cODC) is a native degron, which also functions when appended to diverse proteins. Mutating the cysteine 441 residue within cODC impaired its proteasome association in the context of ornithine decarboxylase and prevented the turnover of GFP-cODC in yeast cells. Degradation of GFP-cODC with C441 mutations was restored by providing an alternate proteasome association element via fusion to the Rpn10 proteasome subunit. However, Rpn10-GFP was stable, unless extended by cODC or other peptides of similar size. In vitro reconstitution experiments confirmed the requirement for both proteasome tethering and a loosely structured region. Therefore, cODC and degradation tags in general must serve two functions: proteasome association and a site, consisting of an extended peptide region, used for initiating insertion into the protease.

The 26S proteasome system degrades the ERM transcription factor and regulates its transcription-enhancing activity

ERM is a member of the ETS transcription factor family. High levels of the corresponding mRNA are detected in a variety of human breast cancer cell lines, as well as in aggressive human breast tumors. As ERM protein is almost undetectable in these cells, high degradation of this transcription factor has been postulated. Here we have investigated whether ERM degradation might depend on the proteasome pathway. We show that endogenous and ectopically expressed ERM protein is short-lived protein and undergoes proteasome-dependent degradation. Deletion mutagenesis studies indicate that the 61 C-terminal amino acids of ERM are critical for its proteolysis and serve as a degradation signal. Although ERM conjugates with ubiquitin, this post-translational modification does not depend on the C-terminal domain. We have used an Ets-responsive ICAM-1 reporter plasmid to show that the ubiquitin-proteasome pathway can affect transcriptional function of ERM. Thus, ERM is subject to degradation via the 26S proteasome pathway, and this pathway probably plays an important role in regulating ERM transcriptional activity.

Overproduction of cardiac S-adenosylmethionine decarboxylase in transgenic mice

The present study was designed to provide a better understanding of the role played by AdoMetDC (S-adenosylmethionine decarboxylase), the key rate-controlling enzyme in the synthesis of spermidine and spermine, in controlling polyamine levels and the importance of polyamines in cardiac physiology. The alphaMHC (alpha-myosin heavy chain) promoter was used to generate transgenic mice with cardiac-specific expression of AdoMetDC. A founder line (alphaMHC/AdoMetDC) was established with a >100-fold increase in AdoMetDC activity in the heart. Transgene expression was maximal by 1 week of age and remained constant into adulthood. However, the changes in polyamine levels were most pronounced during the first week of age, with a 2-fold decrease in putrescine and spermidine and a 2-fold increase in spermine. At later times, spermine returned to near control levels, whereas putrescine and spermidine levels remained lower, suggesting that compensatory mechanisms exist to limit spermine accumulation. The alphaMHC/AdoMetDC mice did not display an overt cardiac phenotype, but there was an increased cardiac hypertrophy after beta-adrenergic stimulation with isoprenaline ('isoproterenol'), as well as a small increase in spermine content. Crosses of the alphaMHC/AdoMetDC with alphaMHC/ornithine decarboxylase mice that have a >1000-fold increase in cardiac ornithine decarboxylase were lethal in utero, presumably due to increase in spermine to toxic levels. These findings suggest that cardiac spermine levels are highly regulated to avoid polyamine-induced toxicity and that homoeostatic mechanisms can maintain non-toxic levels even when one enzyme of the biosynthetic pathway is greatly elevated but are unable to do so when two biosynthetic enzymes are increased.

Large- and small-scale purification of mammalian 26S proteasomes

The 26S proteasome is an ATP-dependent protease known to collaborate with ubiquitin, whose polymerization acts as a marker for regulated and enforced destruction of unnecessary proteins in eukaryotic cells. It is an unusually large multi-subunit protein complex, consisting of a central catalytic machine (called the 20S proteasome or CP/core particle) and two terminal regulatory subcomplexes, termed PA700 or RP/regulatory particle, that are attached to both ends of the central portion in opposite orientations to form an enzymatically active proteasome. To date, proteolysis driven by the ubiquitin-proteasome system has been shown to be involved in a diverse array of biologically important processes, such as the cell cycle, immune response, signaling cascades, and developmental programs; and the field continues to expand rapidly. Whereas the proteasome complex has been highly conserved during evolution because of its fundamental roles in cells, it has also acquired considerable diversity in multicellular organisms, particularly in mammals, such as immunoproteasomes, PA28, S5b, and various alternative splicing forms of S5a (Rpm 10). However, the details of the ultimate pathophysiological roles of mammalian proteasomes have remained elusive. This article focuses on methods for assay and purification of 26S proteasomes from mammalian cells and tissues.

Determinants of proteasome recognition of ornithine decarboxylase, a ubiquitin-independent substrate

Ornithine decarboxylase (ODC) is regulated by its metabolic products through a feedback loop that employs a second protein, antizyme 1 (AZ1). AZ1 accelerates the degradation of ODC by the proteasome. We used purified components to study the structural elements required for proteasomal recognition of this ubiquitin-independent substrate. Our results demonstrate that AZ1 acts on ODC to enhance the association of ODC with the proteasome, not the rate of its processing. Substrate-linked or free polyubiquitin chains compete for AZ1-stimulated degradation of ODC. ODC-AZ1 is therefore recognized by the same element(s) in the proteasome that mediate recognition of polyubiquitin chains. The 37 C-terminal amino acids of ODC harbor an AZ1-modulated recognition determinant. Within the ODC C terminus, three subsites are functionally distinguishable. The five terminal amino acids (ARINV, residues 457-461) collaborate with residue C441 to constitute one recognition element, and AZ1 collaborates with additional constituents of the ODC C terminus to generate a second recognition element.

Antizyme is a target of sex-lethal in the Drosophila germline and appears to act downstream of hedgehog to regulate sex-lethal and cyclin B

The sex determination master switch, Sex-lethal, has been shown to regulate the mitosis of early germ cells in Drosophila melanogaster. Sex-lethal is an RNA binding protein that regulates splicing and translation of specific targets in the soma, but the germline targets are unknown. In an experiment aimed at identifying targets of Sex-lethal in early germ cells, the RNA encoded by gutfeeling, the Drosophila homolog of Ornithine Decarboxylase Antizyme, was isolated. gutfeeling interacts genetically with Sex-lethal. It is not only a target of Sex-lethal, but also appears to regulate the nuclear entry and overall levels of Sex-lethal in early germ cells. This regulation of Sex-lethal by gutfeeling appears to occur downstream of the Hedgehog signal. We also show that Hedgehog, Gutfeeling, and Sex-lethal function to regulate Cyclin B, providing a link between Sex-lethal and mitosis.

Protein degradation and the generation of MHC class I-presented peptides

Over the past decade there has been considerable progress in understanding how MHC class I-presented peptides are generated. The emerging theme is that the immune system has not evolved its own specialized proteolytic mechanisms but instead utilizes the phylogenetically ancient catabolic pathways that continually turnover proteins in all cells. Three distinct proteolytic steps have now been defined in MHC class I antigen presentation. The first step is the degradation of proteins by the ubiquitin-proteasome pathway into oligopeptides that either are of the correct size for presentation or are extended on their amino-termini. In the second step, aminopeptidases trim N-extended precursors into peptides of the correct length to be presented on class I molecules. The third step involves the destruction of peptides by endo- and exopeptidases, which limits antigen presentation, but is important for preventing the accumulation of peptides and recycling them back to amino acids for protein synthesis or production of energy. The immune system has evolved several components that modify the activity of these ancient pathways in ways that enhance the generation of class I-presented peptides. These include catalytically active subunits of the proteasome, the PA28 proteasome activator, and leucine aminopeptidase, all of which are upregulated by interferon-gamma. In addition to these pathways that operate in all cells, dendritic cells and macrophages can also generate class I-presented peptides from proteins internalized from the extracellular fluids by degrading them in endocytic compartments or transferring them to the cyotosol for degradation by proteasomes.

Decreased ornithine decarboxylase activity in the kidneys of Dahl salt-sensitive rats

To assess the roles of polyamines (putrescine, spermidine, and spermine) and ornithine decarboxylase (ODC), the rate-limiting enzyme of polyamine synthesis, in the development of salt-sensitive hypertension, we evaluated activity and expression of ODC, urinary polyamine excretion, and antizyme (endogenous ODC inhibitor protein) expression in Dahl salt-sensitive (SS) and salt-resistant (SR) rats after they were fed on a low (0.3%) or high (4%) salt diet for 4 weeks. We also examined the effects of spermidine and difluoromethylornithine (DFMO: a specific inhibitor of ODC) on the systolic blood pressure and ODC protein expression in SS rats fed a high salt diet. Renal ODC activity and urinary polyamine excretion in SS rats were lower than those in SR rats after 4 weeks treatment with a low or high salt diet. The renal ODC protein expression of SS rats was paradoxically increased as compared to the SR group. A high salt diet did not alter ODC activity but increased ODC protein only in SS rats. ODC mRNA and antizyme protein expressions were not significantly different among the four groups. Spermidine supplementation attenuated and DFMO exaggerated hypertension in SS rats fed a high salt diet. Spermidine down-regulated and DFMO up-regulated renal ODC protein in SS rats on a high salt diet. ODC activity was decreased but protein was paradoxically increased in kidneys of SS rats. ODC protein was suggested to increase in compensation for the inhibition of its activity. Impaired ODC activity and polyamine production in the kidney may exaggerate salt-sensitive hypertension in SS rats.

Proteasomal degradation of cytotoxic necrotizing factor 1-activated rac

The cytotoxic necrotizing factor 1 (CNF1) from Escherichia coli has been shown to activate members of the Rho family by deamidation of glutamine 63. This amino acid is essential for hydrolysis of GTP, and any substitution results in a constitutively active Rho. Activation of Rho induces the formation of stress fibers, filopodia, and membrane ruffles due to activation of RhoA, Cdc42, and Rac, respectively. Here we show that the level of endogenous Rac decreased in CNF1-treated HEK293 and HeLa cells. The amount of mRNA remained unaffected, leaving the possibility that Rac is subject to proteolytic degradation. Treatment of cells with lactacystin, an inhibitor of the 26S proteasome, protected Rac from degradation. We have previously shown that CNF1 activates the c-Jun N-terminal kinase (JNK) only transiently in HeLa cells (M. Lerm, J. Selzer, A. Hoffmeyer, U. R. Rapp, K. Aktories, and G. Schmidt, Infect. Immun. 67:496-503, 1998). Here we show that CNF1-induced JNK activation is stabilized in the presence of lactacystin. The data indicate that Rac is degraded by a proteasome-dependent pathway in CNF1-treated cells.

Natural substrates of the proteasome and their recognition by the ubiquitin system

The multitude of natural substrates of the 26S proteasome demonstrates convincingly the diversity and flexibility of the ubiquitin/proteasome system: at the same time, the number of pathways in which ubiquitin-dependent degradation is involved highlights the importance of regulated proteolysis for cellular metabolism. This review has addressed recent advances in our understanding of the principles that govern the recognition and targeting of potential substrates. While the mechanism of ubiquitin activation and conjugation is largely understood, the determination of substrate specificity by ubiquitin protein ligases remains a field of active research. Several conserved degradation signals within substrate proteins have been identified, and it is becoming increasingly clear that these serve as docking sites for specific sets of E3s, which in turn adhere to a number of well-defined strategies for the recognition of these motifs. In particular, RING finger proteins are now emerging as a new and apparently widespread class of ubiquitin ligases. The discovery of more and more E3s will undoubtedly reveal even better the common principles in architecture and mechanisms of this class of enzymes. In contrast to substrate recognition by the ubiquitin conjugation system, the way in which a ubiquitylated protein is delivered to the 26S proteasome is poorly understood. There is no doubt that multiubiquitin chains serve as the principal determinant for recognition by the proteasome, and a number of receptors and candidate targeting factors are known, some of which are associated with the proteasome itself; however, unresolved issues are the significance of the different geometries that alternatively linked multiubiquitin chains can adopt, the role of transport between subcellular compartments, as well as the participation of chaperones in the delivery step. Finally, the analysis of ubiquitin-independent, substrate-specific targeting mechanisms, such as the AZ-dependent degradation of ODC, may provide unexpected answers to questions about protein recognition by the 26S proteasome.

Selective proteasomal dysfunction in the hippocampal CA1 region after transient forebrain ischemia

Delayed neuronal death in the hippocampal CA1 region after transient forebrain ischemia may share its underlying mechanism with neurodegeneration and other modes of neuronal death. The precise mechanism, however, remains unknown. In the postischemic hippocampus, conjugated ubiquitin accumulates and free ubiquitin is depleted, suggesting impaired proteasome function. The authors measured regional proteasome activity after transient forebrain ischemia in male Mongolian gerbils. At 30 minutes after ischemia, proteasome activity was 40% of normal in the frontal cortex and hippocampus. After 2 hours of reperfusion, it had returned to normal levels in the frontal cortex, CA3 region, and dentate gyrus, but remained low for up to 48 hours in the CA1 region. Thus, the 26S proteasome was globally impaired in the forebrain during transient ischemia and failed to recover only in the CA1 region after reperfusion. The authors also measured 20S and 26S proteasome activities directly after decapitation ischemia (at 5 and 20 minutes) by fractionating the extracts with glycerol gradient centrifugation. Without adenosine triphosphate (ATP), only 20S proteasome activity was detected in extracts from both the hippocampus and frontal cortex. When the extracts were incubated with ATP in an ATP-regenerating system, 26S proteasome activity recovered almost fully in the frontal cortex but only partially in the hippocampus. Thus, after transient forebrain ischemia, ATP-dependent reassociation of the 20S catalytic and PA700 regulatory subunits to form the active 26S proteasome is severely and specifically impaired in the hippocampus. The irreversible loss of proteasome function underlies the delayed neuronal death induced by transient forebrain ischemia in the hippocampal CA1 region.

P elements inserted in the vicinity of or within the Drosophila snRNP SmD3 gene nested in the first intron of the Ornithine Decarboxylase Antizyme gene affect only the expression of SmD3

The Drosophila gene for snRNP SmD3 (SmD3) is contained in reverse orientation within the first intron of the Ornithine Decarboxylase Antizyme (AZ) gene. Previous studies show that two closely linked P elements cause the gutfeeling phenotype characterized by embryonic lethality and aberrant neuronal and muscle cell differentiation. However, the exact nature of the gene(s) affected in the gutfeeling phenotype remained unknown. This study shows that a series of P inserts located within the 5'-untranslated region (5'-UTR) of SmD3 or its promoter affects only the expression of SmD3. Our analysis reveals that the gutfeeling phenotype associated with P elements inserted in the 5'-UTR of SmD3 results from amorphic or strongly hypomorphic mutations. In contrast, P inserts in the SmD3 promoter region reduce the expression of SmD3 without abolishing it and produce larval lethality with overgrown imaginal discs, brain hemispheres, and hematopoietic organs. The lethality of these mutations could be rescued by an SmD3+ transgene. Finally, inactivation of AZ was obtained by complementing with SmD3+ the deficiency Df(2R)guf(lex47) that uncovers both SmD3 and AZ. Interestingly, AZ inactivation causes a new phenotype characterized by late larval lethality and atrophy of the brain, imaginal discs, hematopoietic organs, and salivary glands.

26 S proteasomes function as stable entities

Most proteins in eukaryotic cells are degraded by 26-S proteasomes, usually after being conjugated to ubiquitin. In the absence of ATP, 26-S proteasomes fall apart into their two sub-complexes, 20-S proteasomes and PA700, which reassemble upon addition of ATP. Conceivably, 26-S proteasomes dissociate and reassemble during initiation of protein degradation in a ternary complex with the substrate, as in the dissociation-reassembly cycles found for ribosomes and the chaperonin GroEL/GroES. Here we followed disassembly and assembly of 26-S proteasomes in cell extracts as the exchange of PA700 subunits between mouse and human 26-S proteasomes. Compared to the rate of proteolysis in the same extract, the disassembly-reassembly cycle was much too slow to present an obligatory step in a degradation cycle. It has been suggested that subunit S5a (Mcb1, Rpn10), which binds poly-ubiquitin substrates, shuttles between a free state and the 26-S proteasome, bringing substrate to the complex. However, S5a was not found in the free state in HeLa cells. Besides, all subunits in PA700, including S5a, exchanged at similar low rates. It therefore seems that 26-S proteasomes function as stable entities during degradation of proteins.

Proteasomal degradation of Smad1 induced by bone morphogenetic proteins

The bone morphogenetic proteins (BMPs) regulate early embryogenesis and morphogenesis of multiple organs, such as bone, kidney, limbs, and muscle. Smad1 is one of the key signal transducers of BMPs and is responsible for transducing receptor activation signals from the cytoplasm to the nucleus, where Smad1 serves as a transcriptional regulator of various BMP-responsive genes. Based upon the ability of Smad1 to bind multiple proteins involved in proteasome-mediated degradation pathway, we investigated whether Smad1 could be a substrate for proteasome. We found that Smad1 is targeted to proteasome for degradation in response to BMP type I receptor activation. The targeting of Smad1 to proteasome involves not only the receptor activation-induced Smad1 ubiquitination but also the targeting functions of the ornithine decarboxylase antizyme and the proteasome beta subunit HsN3. Our studies provide the first evidence for BMP-induced proteasomal targeting and degradation of Smad1 and also reveal new players and novel mechanisms involved in this important aspect of Smad1 regulation and function.

Immunoproteasome assembly and antigen presentation in mice lacking both PA28alpha and PA28beta

Two members of the proteasome activator, PA28alpha and PA28beta, form a heteropolymer that binds to both ends of the 20S proteasome. Evidence in vitro indicates that this interferon-gamma (IFN-gamma)-inducible heteropolymer is involved in the processing of intracellular antigens, but its functions in vivo remain elusive. To investigate the role of PA28alpha/beta in vivo, we generated mice deficient in both PA28alpha and PA28beta genes. The ATP-dependent proteolytic activities were decreased in PA28alpha(-/-)/beta(-/-) cells, suggesting that 'hybrid proteasomes' are involved in protein degradation. Treatment of PA28alpha(-/-)/beta(-/-) cells with IFN-gamma resulted in sufficient induction of the 'immunoproteasome'. Moreover, splenocytes from PA28alpha(-/-)/beta(-/-) mice displayed no apparent defects in processing of ovalbumin. These results are in marked contrast to the previous finding that immunoproteasome assembly and immune responses were impaired in PA28beta(-/-) mice. PA28alpha(-/-)/beta(-/-) mice also showed apparently normal immune responses against infection with influenza A virus. However, they almost completely lost the ability to process a melanoma antigen TRP2-derived peptide. Hence, PA28alpha/beta is not a prerequisite for antigen presentation in general, but plays an essential role for the processing of certain antigens.

Selective degradation of ubiquitinated Sic1 by purified 26S proteasome yields active S phase cyclin-Cdk

Selective degradation of single subunits of multimeric complexes by the ubiquitin pathway underlies multiple regulatory switches, including those involving cyclins and Cdk inhibitors. The machinery that segregates ubiquitinated proteins from unmodified partners prior to degradation remains undefined. We report that ubiquitinated Sic1 (Ub-Sic1) embedded within inactive S phase cyclin-Cdk (S-Cdk) complexes was rapidly degraded by purified 26S proteasomes, yielding active S-Cdk. Mutant proteasomes that failed to degrade Ub-Sic1 activated S-Cdk only partially in an ATP-dependent manner. Whereas Ub-Sic1 was degraded within approximately 2 min, spontaneous dissociation of Ub-Sic1 from S-Cdk was approximately 200-fold slower. We propose that the 26S proteasome has the intrinsic capability to extract, unfold, and degrade ubiquitinated proteins while releasing bound partners untouched. Activation of S-Cdk reported herein represents a complete reconstitution of the regulatory switch underlying the G1/S transition in budding yeast.

The NS2 protein generated by the parvovirus minute virus of mice is degraded by the proteasome in a manner independent of ubiquitin chain elongation or activation

The NS2 protein generated by the parvovirus minute virus of mice is very labile, having a half-life during infection of approximately 90 min. The degradation of NS2 is blocked by proteasome inhibitors but is likely ubiquitin independent: NS2 does not form detectable higher molecular weight ubiquitin-containing conjugates, and NS2 degradation requires neither ubiquitin chain elongation nor intracellular ubiquitin activation. We have also identified a region in the carboxyl half of NS2 that is required for its proteasome-mediated degradation.

Antizyme regulates the degradation of ornithine decarboxylase in fission yeast Schizosaccharomyces pombe. Study in the spe2 knockout strains

The mechanism of the regulatory degradation of ornithine decarboxylase (ODC) by polyamines was studied in fission yeast, Schizosaccharomyces pombe. To regulate cellular spermidine experimentally, we cloned and disrupted S-adenosylmethionine decarboxylase gene (spe2) in S. pombe. The null mutant of spe2 was devoid of spermidine and spermine, accumulated putrescine, and contained a high level of ODC. Addition of spermidine to the culture medium resulted in rapid decrease in the ODC activity caused by the acceleration of ODC degradation, which was dependent on de novo protein synthesis. A fraction of ODC forming an inactive complex concomitantly increased. The accelerated ODC degradation was prevented either by knockout of antizyme gene or by selective inhibitors of proteasome. Thus, unlike budding yeast, mammalian type antizyme-mediated ODC degradation by proteasome is operating in S. pombe.

Quaternary structure of the ATPase complex of human 26S proteasomes determined by chemical cross-linking

The 26S proteasome is the major protease responsible for nonlysosomal protein degradation in eukaryotic cells. The enzyme is composed of two subparticles: the 20S proteasome, and a 19S regulatory particle (PA700) which binds to the ends of the 20S proteasome cylinder and accounts for ATP dependence and substrate specificity. Among the approximately 18 subunits of PA700 regulator, six are ATPases. The ATPases presumably recognize, unfold, and translocate substrates into the interior of the 26S proteasome. It is generally believed that the ATPases form a hexameric ring. By means of chemical cross-linking, immunoprecipitation, and blotting, we have determined that the ATPases are organized in the order S6-S6'-S10b-S8-S4-S7. Additionally, we found cross-links between the ATPase S10b and the 20S proteasome subunit alpha6. Together with the previously known interaction between S8 and alpha1 and between S4 and alpha7, these data establish the relative orientations of ATPases with respect to the 20S proteasome.

Regulation of cellular polyamines by antizyme

Proteins that are degraded by the proteasome are first modified by a set of enzymes that attach multiple copies of ubiquitin to substrate lysines, but a tiny minority, including the polyamine-synthesizing enzyme ornithine decarboxylase, is handled differently. This enzyme is targeted for destruction by another protein--antizyme. Why does ornithine decarboxylase have its own dedicated destruction mechanism, how does it work, and is it the only protein to be targeted to the proteasome in this way?

Hepatocyte growth factor induces pro-apoptotic genes in HepG2 hepatoma but not in B16-F1 melanoma cells

Hepatocyte growth factor (HGF) exerts a cytostatic effect on HepG2 and B16-F1 cell lines. To evaluate the possible involvement of the apoptotic process in this effect, we performed studies at cellular and molecular levels. HGF induced apoptosis only in HepG2 hepatoma cells at day 3 in about 20% of the cells undergoing growth inhibition, while hallmarks of apoptosis did not occur in B16-F1 melanoma cells. During the first 24 h after HGF treatment, enhanced expression of the pro-apoptotic genes bax and c-Myc was observed at level of mRNA and protein. Concomitant induction of antizyme (AZ) might lower ornithine decarboxylase (ODC) protein level though a huge increase in ODC mRNA level took place. This was suggested as a signal for apoptosis decisional phase. The levels of the proteins examined except that of AZ fell down thereafter when HepG2 cells underwent apoptosis. In B16-F1 cells, only ODC and AZ protein levels were elevated probably in relation to the initial elevated growth rate and the absence of apoptosis involvement in the following cytostatic effect of HGF in melanoma cells. Consistent with this hypothesis, bax mRNA and protein levels were unchanged or even lower relative to control values.

Nuclear translocation of antizyme and expression of ornithine decarboxylase and antizyme are developmentally regulated

The polyamines are important regulators of cell growth and differentiation. Cells acquire polyamines by energy-dependent transport and by synthesis where the highly regulated ornithine decarboxylase (ODC) catalyzes the first and rate-controlling step. Inactivation of ODC is mainly exerted by antizyme (AZ), a 20--25 kDa polyamine-induced protein that binds to ODC, inactivates it, and targets it for degradation by the 26S proteasome without ubiquitination. In the present study, we have performed a systematic analysis of the expression of ODC and AZ, at the mRNA and protein levels, during mouse development. The expression patterns for ODC and AZ were found to be developmentally regulated, suggesting important functions for the polyamines in early embryogenesis, axonogenesis, epithelial-mesenchymal interaction, and in apoptosis. In addition, AZ protein was found to translocate to the nucleus in a developmentally regulated manner. The nuclear localization is consistent with the fact that the amino acid sequence of AZ exhibits features that characterize nuclear proteins. Interestingly, we found that cultivation of mandibular components of the first branchial arch in the presence of a selective proteasome inhibitor caused ODC accumulation in the nucleus of a subset of cells, suggesting that the observed nuclear translocation of AZ is linked to proteasome-mediated ODC degradation in the nucleus. The presence of AZ in the nucleus may suggest that nuclear ODC activity is under tight control, and that polyamine production can be rapidly interrupted when those developmental events, which depend on access to nuclear polyamines, have been completed.

Regulation of proteolysis

The mechanisms of proteolysis remain to be fully defined. This review focuses on recent advances in our understanding of the ubiquitin-proteasome-dependent pathway, which is involved in the control of many major biological functions. The ubiquitinylation/deubiquitinylation system is a complex machinery responsible for the specific tagging and proof-reading of substrates degraded by the 26S proteasome, as well as having other functions. The formation of a polyubiquitin degradation signal is required for proteasome-dependent proteolysis. Several families of enzymes, which may comprise hundreds of members to achieve high selectivity, control this process. The substrates tagged by ubiquitin are then recognized by the 26S proteasome and degraded into peptides. In addition, the 26S proteasome also recognizes and degrades some non-ubiquitinylated proteins. In fact, there are multiple ubiquitin- or proteasome-dependent pathways. These systems presumably degrade specific classes of substrates and single proteins by alternative mechanisms and could be interconnected. They may also interfere or cooperate with other proteolytic pathways.

Tissue distribution of constitutive proteasomes, immunoproteasomes, and PA28 in rats

We investigated the expression of standard proteasomes, immunoproteasomes, and their regulators, PA28, and PA700, in rat tissues. Immunoproteasomes (with subunits LMP2, LMP7, and MECL1) were abundant in the spleen but almost absent in the brain. In contrast, standard proteasomes (with X, Y, and Z) were highly expressed in the brain but not in the spleen. Both proteasome types were present in the lung and the liver. PA700 subunits (p112, S5a, and p45) were found in all tissues. PA28alpha, PA28beta, and PA28gamma were also expressed in all tissues, except for the brain which contained very little PA28beta. The results did not depend on rat sex or age. The cleavage specificity for peptide substrates differed greatly between brain and spleen proteasomes. Hybrid proteasomes, containing both PA28alphabeta and PA700, were not present in the brain but in all other tissues examined.

A nonproteolytic function of the proteasome is required for the dissociation of Cdc2 and cyclin B at the end of M phase

Inactivation of cyclin B-Cdc2 kinase at the exit from M phase depends on the specific proteolysis of the cyclin B subunit, whereas the Cdc2 subunit remains present at nearly constant levels throughout the cell cycle. It is unknown how Cdc2 escapes degradation when cyclin B is destroyed. In Xenopus egg extracts that reproduce the exit from M phase in vitro, we have found that dissociation of the cyclin B-Cdc2 complex occurred under conditions where cyclin B was tethered to the 26S proteasome but not yet degraded. The dephosphorylation of Thr 161 on Cdc2 was unlikely to be necessary for the dissociation of the two subunits. However, the dissociation was dependent on the presence of a functional destruction box in cyclin B. Cyclin B ubiquitination was also, by itself, not sufficient for separation of Cdc2 and cyclin B. The 26S proteasome, but not the 20S proteasome, was capable of dissociating the two subunits. These results indicate that the cyclin B and Cdc2 subunits are separated by the proteasome through a mechanism that precedes proteolysis of cyclin B and is independent of proteolysis. As a result, cyclin B levels decrease on exit from M phase but Cdc2 levels remain constant.

Antizyme expression: a subversion of triplet decoding, which is remarkably conserved by evolution, is a sensor for an autoregulatory circuit

The efficiency of programmed ribosomal frameshifting in decoding antizyme mRNA is the sensor for an autoregulatory circuit that controls cellular polyamine levels in organisms ranging from the yeast Schizosaccharomyces pombe to Drosophila to mammals. Comparison of the frameshift sites and flanking stimulatory signals in many organisms now permits a reconstruction of the likely evolutionary path of the remarkably conserved mRNA sequences involved in the frameshifting.

Hybrid proteasomes. Induction by interferon-gamma and contribution to ATP-dependent proteolysis

Eukaryotic cells contain various types of proteasomes. Core 20 S proteasomes (abbreviated 20 S below) have two binding sites for the regulatory particles, PA700 and PA28. PA700-20 S-PA700 complexes are known as 26 S proteasomes and are ATP-dependent machines that degrade cell proteins. PA28 is found both in previously described complexes of the type PA28-20 S-PA28 and in complexes that also contain PA700, as PA700-20 S-PA28. We refer to the latter as "hybrid proteasomes." The relative amounts of the various types of proteasomes in HeLa extracts were determined by a combination of immunoprecipitation and immunoblotting. Hybrid proteasomes accounted for about a fourth of all proteasomes in the extracts. Association of PA28 and proteasomes proved to be ATP-dependent. Hybrid proteasomes catalyzed ATP-dependent degradation of ornithine decarboxylase (ODC) without ubiquitinylation, as do 26 S proteasomes. In contrast, the homo-PA28 complex (PA28-20 S-PA28) was incapable of degrading ODC. Intriguingly, a major immunomodulatory cytokine, interferon-gamma, appreciably enhanced the ODC degradation in HeLa and SW620 cells through induction of the hybrid proteasome, which may also be responsible for the immunological processing of intracellular antigens. Taken together, we report here for the first time the existence of two types of ATP-dependent proteases, the 26 S proteasome and the hybrid proteasome, which appear to share the ATP-dependent proteolytic pathway in mammalian cells.

Conservation of polyamine regulation by translational frameshifting from yeast to mammals

Regulation of ornithine decarboxylase in vertebrates involves a negative feedback mechanism requiring the protein antizyme. Here we show that a similar mechanism exists in the fission yeast Schizosaccharomyces pombe. The expression of mammalian antizyme genes requires a specific +1 translational frameshift. The efficiency of the frameshift event reflects cellular polyamine levels creating the autoregulatory feedback loop. As shown here, the yeast antizyme gene and several newly identified antizyme genes from different nematodes also require a ribosomal frameshift event for their expression. Twelve nucleotides around the frameshift site are identical between S.pombe and the mammalian counterparts. The core element for this frameshifting is likely to have been present in the last common ancestor of yeast, nematodes and mammals.