The proteasome 11S regulator subunit REG alpha (PA28 alpha) is a heptamer

Inhibition of the Proteasome Regulator PA28 Aggravates Oxidized Protein Overload in the Diabetic Rat Brain

Oxidized protein overloading caused by diabetes is one accelerating pathological pathway in diabetic encephalopathy development. To determine whether the PA28-regulated function of the proteasome plays a role in diabetes-induced oxidative damaged protein degradation, brain PA28α and PA28β interference experiments were performed in a high-fat diet (HFD) and streptozotocin (STZ)-induced rat model. The present results showed that proteasome activity was changed in the brains of diabetic rats, but the constitutive subunits were not. In vivo PA28α and PA28β inhibition via adeno-associated virus (AAV) shRNA infection successfully decreased PA28 protein levels and further exacerbated oxidized proteins load by regulating proteasome catalytic activity. These findings suggest that the proteasome plays a role in the elimination of oxidized proteins and that PA28 is functionally involved in the regulation of proteasome activity in vivo. This study suggests that abnormal protein turbulence occurring in the diabetic brain could be explained by the proteasome-mediated degradation pathway. Changes in proteasome activity regulator PA28 could be a reason to induce oxidative aggregation in diabetic brain. Proteasome regulator PA28 inhibition in vivo by AAV vector injection could aggravate oxidized proteins abundance in brain of HFD-STZ diabetic rat model.

PA28γ: New Insights on an Ancient Proteasome Activator

PA28 (also known as 11S, REG or PSME) is a family of proteasome regulators whose members are widely present in many of the eukaryotic supergroups. In jawed vertebrates they are represented by three paralogs, PA28α, PA28β, and PA28γ, which assemble as heptameric hetero (PA28αβ) or homo (PA28γ) rings on one or both extremities of the 20S proteasome cylindrical structure. While they share high sequence and structural similarities, the three isoforms significantly differ in terms of their biochemical and biological properties. In fact, PA28α and PA28β seem to have appeared more recently and to have evolved very rapidly to perform new functions that are specifically aimed at optimizing the process of MHC class I antigen presentation. In line with this, PA28αβ favors release of peptide products by proteasomes and is particularly suited to support adaptive immune responses without, however, affecting hydrolysis rates of protein substrates. On the contrary, PA28γ seems to be a slow-evolving gene that is most similar to the common ancestor of the PA28 activators family, and very likely retains its original functions. Notably, PA28γ has a prevalent nuclear localization and is involved in the regulation of several essential cellular processes including cell growth and proliferation, apoptosis, chromatin structure and organization, and response to DNA damage. In striking contrast with the activity of PA28αβ, most of these diverse biological functions of PA28γ seem to depend on its ability to markedly enhance degradation rates of regulatory protein by 20S proteasome. The present review will focus on the molecular mechanisms and biochemical properties of PA28γ, which are likely to account for its various and complex biological functions and highlight the common features with the PA28αβ paralog.

Proteasomes and Several Aspects of Their Heterogeneity Relevant to Cancer

The life of every organism is dependent on the fine-tuned mechanisms of protein synthesis and breakdown. The degradation of most intracellular proteins is performed by the ubiquitin proteasome system (UPS). Proteasomes are central elements of the UPS and represent large multisubunit protein complexes directly responsible for the protein degradation. Accumulating data indicate that there is an intriguing diversity of cellular proteasomes. Different proteasome forms, containing different subunits and attached regulators have been described. In addition, proteasomes specific for a particular tissue were identified. Cancer cells are highly dependent on the proper functioning of the UPS in general, and proteasomes in particular. At the same time, the information regarding the role of different proteasome forms in cancer is limited. This review describes the functional and structural heterogeneity of proteasomes, their association with cancer as well as several established and novel proteasome-directed therapeutic strategies.

A Practical Review of Proteasome Pharmacology

The ubiquitin proteasome system (UPS) degrades individual proteins in a highly regulated fashion and is responsible for the degradation of misfolded, damaged, or unneeded cellular proteins. During the past 20 years, investigators have established a critical role for the UPS in essentially every cellular process, including cell cycle progression, transcriptional regulation, genome integrity, apoptosis, immune responses, and neuronal plasticity. At the center of the UPS is the proteasome, a large and complex molecular machine containing a multicatalytic protease complex. When the efficiency of this proteostasis system is perturbed, misfolded and damaged protein aggregates can accumulate to toxic levels and cause neuronal dysfunction, which may underlie many neurodegenerative diseases. In addition, many cancers rely on robust proteasome activity for degrading tumor suppressors and cell cycle checkpoint inhibitors necessary for rapid cell division. Thus, proteasome inhibitors have proven clinically useful to treat some types of cancer, especially multiple myeloma. Numerous cellular processes rely on finely tuned proteasome function, making it a crucial target for future therapeutic intervention in many diseases, including neurodegenerative diseases, cystic fibrosis, atherosclerosis, autoimmune diseases, diabetes, and cancer. In this review, we discuss the structure and function of the proteasome, the mechanisms of action of different proteasome inhibitors, various techniques to evaluate proteasome function in vitro and in vivo, proteasome inhibitors in preclinical and clinical development, and the feasibility for pharmacological activation of the proteasome to potentially treat neurodegenerative disease.

Structural Analysis of the Bacterial Proteasome Activator Bpa in Complex with the 20S Proteasome

Mycobacterium tuberculosis harbors proteasomes that recruit substrates for degradation through an ubiquitin-like modification pathway. Recently, a non-ATPase activator termed Bpa (bacterial proteasome activator) was shown to support an alternate proteasomal degradation pathway. Here, we present the cryo-electron microscopy (cryo-EM) structure of Bpa in complex with the 20S core particle (CP). For docking into the cryo-EM density, we solved the X-ray structure of Bpa, showing that it forms tight four-helix bundles arranged into a 12-membered ring with a 40 Å wide central pore and the C-terminal helix of each protomer protruding from the ring. The Bpa model was fitted into the cryo-EM map of the Bpa-CP complex, revealing its architecture and striking symmetry mismatch. The Bpa-CP interface was resolved to 3.5 Å, showing the interactions between the C-terminal GQYL motif of Bpa and the proteasome α-rings. This docking mode is related to the one observed for eukaryotic activators with features specific to the bacterial complex.

Nitric oxide differentially affects proteasome activator 28 after arterial injury in type 1 and type 2 diabetic rats

BACKGROUND

Diabetic patients display aggressive restenosis after vascular interventions, likely because of proproliferative influences of hyperglycemia and hyperinsulinemia. We have shown that nitric oxide (NO) inhibits neointimal hyperplasia in type 2, but not in type 1, diabetic rats. Here, we examined proteasome activator 28 (PA28) after arterial injury in different diabetic environments, with or without NO. We hypothesize that NO differentially affects PA28 levels based on metabolic environment.

MATERIALS AND METHODS

Vascular smooth muscle cell (VSMC) lysates from male, nondiabetic Lean Zucker (LZ) and Zucker Diabetic Fatty (ZDF) rats were assayed for 26S proteasome activity with or without PA28 and S-nitroso-N-acetylpenicillamine. LZ and ZDF VSMCs were treated with (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate for 24 h. Balloon-injured carotid arteries from LZ, streptozotocin-injected LZ (STZ, type 1), and ZDF (type 2) rats treated with disodium 1-[2-(carboxylato)pyrrolidin-1-iyl]diazen-1-ium-1,2-diolate were harvested at 3 or 14 d. PA28α was assessed by Western blotting and immunofluorescent staining.

RESULTS

S-nitroso-N-acetylpenicillamine reversed PA28-stimulated increases in 26S proteasome activity in LZ and ZDF VSMCs. Increased (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate lowered PA28α in LZ VSMCs but increased PA28α in ZDF VSMCs. At 3 d after injury, disodium 1-[2-(carboxylato)pyrrolidin-1-iyl]diazen-1-ium-1,2-diolate potentiated injury-induced PA28α decreases in LZ, STZ, and ZDF rats, suggesting VSMCs, depleted at this early time point, are major sources of PA28α. At 14 d after injury, total PA28α staining returned to baseline. However, although intimal and medial PA28α staining increased in injured STZ rats, adventitial PA28α staining increased in injured ZDF rats.

CONCLUSIONS

PA28 dysregulation may explain the differential ability of NO to inhibit neointimal hyperplasia in type 1 versus type 2 diabetes.

Evolution of proteasome regulators in eukaryotes

All living organisms require protein degradation to terminate biological processes and remove damaged proteins. One such machine is the 20S proteasome, a specialized barrel-shaped and compartmentalized multicatalytic protease. The activity of the 20S proteasome generally requires the binding of regulators/proteasome activators (PAs), which control the entrance of substrates. These include the PA700 (19S complex), which assembles with the 20S and forms the 26S proteasome and allows the efficient degradation of proteins usually labeled by ubiquitin tags, PA200 and PA28, which are involved in proteolysis through ubiquitin-independent mechanisms and PI31, which was initially identified as a 20S inhibitor in vitro. Unlike 20S proteasome, shown to be present in all Eukaryotes and Archaea, the evolutionary history of PAs remained fragmentary. Here, we made a comprehensive survey and phylogenetic analyses of the four types of regulators in 17 clades covering most of the eukaryotic supergroups. We found remarkable conservation of each PA700 subunit in all eukaryotes, indicating that the current complex PA700 structure was already set up in the last eukaryotic common ancestor (LECA). Also present in LECA, PA200, PA28, and PI31 showed a more contrasted evolutionary picture, because many lineages have subsequently lost one or two of them. The paramount conservation of PA700 composition in all eukaryotes and the dynamic evolution of PA200, PA28, and PI31 are discussed in the light of current knowledge on their physiological roles.

PA28αβ: the enigmatic magic ring of the proteasome?

PA28αβ is a γ-interferon-induced 11S complex that associates with the ends of the 20S proteasome and stimulates in vitro breakdown of small peptide substrates, but not proteins or ubiquitin-conjugated proteins. In cells, PA28 also exists in larger complexes along with the 19S particle, which allows ATP-dependent degradation of proteins; although in vivo a large fraction of PA28 is present as PA28αβ-20S particles whose exact biological functions are largely unknown. Although several lines of evidence strongly indicate that PA28αβ plays a role in MHC class I antigen presentation, the exact molecular mechanisms of this activity are still poorly understood. Herein, we review current knowledge about the biochemical and biological properties of PA28αβ and discuss recent findings concerning its role in modifying the spectrum of proteasome's peptide products, which are important to better understand the molecular mechanisms and biological consequences of PA28αβ activity.

Expanded polyglutamine-containing N-terminal huntingtin fragments are entirely degraded by mammalian proteasomes

Huntington disease is a neurodegenerative disorder caused by an expanded polyglutamine (polyQ) repeat within the protein huntingtin (Htt). N-terminal fragments of the mutant Htt (mHtt) proteins containing the polyQ repeat are aggregation-prone and form intracellular inclusion bodies. Improving the clearance of mHtt fragments by intracellular degradation pathways is relevant to obviate toxic mHtt species and subsequent neurodegeneration. Because the proteasomal degradation pathway has been the subject of controversy regarding the processing of expanded polyQ repeats, we examined whether the proteasome can efficiently degrade Htt-exon1 with an expanded polyQ stretch both in neuronal cells and in vitro. Upon targeting mHtt-exon1 to the proteasome, rapid and complete clearance of mHtt-exon1 was observed. Proteasomal degradation of mHtt-exon1 was devoid of polyQ peptides as partial cleavage products by incomplete proteolysis, indicating that mammalian proteasomes are capable of efficiently degrading expanded polyQ sequences without an inhibitory effect on the proteasomal activity.

The Ubiquitin-Proteasome System in Huntington's Disease: Are Proteasomes Impaired, Initiators of Disease, or Coming to the Rescue?

Huntington's disease is a progressive neurodegenerative disease, caused by a polyglutamine expansion in the huntingtin protein. A prominent hallmark of the disease is the presence of intracellular aggregates initiated by N-terminal huntingtin fragments containing the polyglutamine repeat, which recruit components of the ubiquitin-proteasome system. While it is commonly thought that proteasomes are irreversibly sequestered into these aggregates leading to impairment of the ubiquitin-proteasome system, the data on proteasomal impairment in Huntington's disease is contradictory. In addition, it has been suggested that proteasomes are unable to actually cleave polyglutamine sequences in vitro, thereby releasing aggregation-prone polyglutamine peptides in cells. Here, we discuss how the proteasome is involved in the various stages of polyglutamine aggregation in Huntington's disease, and how alterations in activity may improve clearance of mutant huntingtin fragments.

Ubiquitin-independent degradation of hepatitis C virus F protein

Hepatitis C virus (HCV) F protein is encoded by the +1 reading frame of the viral genome. It overlaps with the core protein coding sequence, and multiple mechanisms for its expression have been proposed. The full-length F protein that is synthesized by translational ribosomal frameshift at codons 9 to 11 of the core protein sequence is a labile protein. By using a combination of genetic, biochemical, and cell biological approaches, we demonstrate that this HCV F protein can bind to the proteasome subunit protein alpha3, which reduces the F-protein level in cells in a dose-dependent manner. Deletion-mapping analysis identified amino acids 40 to 60 of the F protein as the alpha3-binding domain. This alpha3-binding domain of the F protein together with its upstream sequence could significantly destabilize the green fluorescent protein, an otherwise stable protein. Further analyses using an F-protein mutant lacking lysine and a cell line that contained a temperature-sensitive E1 ubiquitin-activating enzyme indicated that the degradation of the F protein was ubiquitin independent. Based on these observations as well as the observation that the F protein could be degraded directly by the 20S proteasome in vitro, we propose that the full-length HCV F protein as well as the F protein initiating from codon 26 is degraded by an ubiquitin-independent pathway that is mediated by the proteasome subunit alpha3. The ability of the F protein to bind to alpha3 raises the possibility that the HCV F protein may regulate protein degradation in cells.

Purification and Analysis of Recombinant 11S Activators of the 20S Proteasome: Trypanosoma brucei PA26 and Human PA28α, PA28β, and PA28γ

Proteasomes perform the bulk of nonlysosomal degradation of aberrant, damaged, misfolded, and naturally short-lived regulatory proteins in eukaryotic cells. They are approximately 700-kDa assemblies whose hollow architecture sequesters the proteolytic sites inside a central chamber, thereby ensuring that the activity of isolated proteasomes is repressed. In vivo, proteasomes are activated by protein complexes, including the 11S activators (PA28 and PA26), which bind to one or both ends of the barrel-shaped structure. This chapter describes protocols for the purification of recombinant 11S regulators, characterization of their ability to stimulate proteasome activity, and crystallization of proteasome complexes.

Drosophila proteasome regulator REGgamma: transcriptional activation by DNA replication-related factor DREF and evidence for a role in cell cycle progression

The proteasome regulator REG (PA28gamma) is a conserved complex present in metazoan nuclei and is able to stimulate the trypsin-like activity of the proteasome in a non-ATP dependent manner. However, the in vivo function for REGgamma in metazoan cells is currently unknown. To understand the role of Drosophila REGgamma we have attempted to identify the type of promoter elements regulating its transcription. Mapping the site of the transcription initiation revealed a TATA-less promoter, and a sequence search identified elements found typically in Drosophila genes involved in cell cycle progression and DNA replication. In order to test the relevance of the motifs, REGgamma transcriptional assays were carried out with mutations in the proposed promoter. Our results indicate that a single Drosophila replication-related element sequence, DRE, is essential for REGgamma transcription. To confirm that REGgamma has a role in cell cycle progression, the effect of removing REGgamma from S2 cells was tested using RNA interference. Drosophila cells depleted of REGgamma showed partial arrests in G1/S cell cycle transition. Immuno-staining of Drosophila embryos revealed that REGgamma is typically localized to the nucleus during embryogenesis with increased levels present in invaginating cells during gastrulation. The REGgamma was found dispersed throughout the cell volume within mitotic domains undergoing cell division. Finally, database searches suggest that the DRE system may regulate key members of the proteasome system in Drosophila.

The 11S regulators of 20S proteasome activity

Although substantial progress has been made in understanding the biochemical properties of 11S regulators since their discovery in 1992, we still only have a rudimentary understanding of their biological role. As discussed above, we have proposed a model in which the alpha/beta complex promotes the production of antigenic peptides by opening the exit port of the 20S proteasome (Whitby et al. 2000). There are other possibilities, however, that are not exclusive of the exit port hypothesis. For example the alpha/beta complex may promote assembly of immunoproteasome as suggested by Preckel et al. 1999, or it may function as a docking module and conduit for the delivery of peptides to the ER lumen (Realini et al. 1994b). There are also unanswered structural and mechanistic questions. Higher resolution data are needed to discern important structural details of the PA26/20S proteasome complex. The models for binding and activation that are suggested from the structural data have to be tested by mutagenesis and biochemical analysis. What is the role of homolog-specific inserts? Will cognate regulator/proteasome complexes show conformational changes that are not apparent in the currently available crystal structures, including perhaps signs of allosteric communication between the regulator and the proteasome active sites?

Lysine 188 substitutions convert the pattern of proteasome activation by REGgamma to that of REGs alpha and beta

11S REGs (PA28s) are multimeric rings that bind proteasomes and stimulate peptide hydrolysis. Whereas REGalpha activates proteasomal hydrolysis of peptides with hydrophobic, acidic or basic residues in the P1 position, REGgamma only activates cleavage after basic residues. We have isolated REGgamma mutants capable of activating the hydrolysis of fluorogenic peptides diagnostic for all three active proteasome beta subunits. The most robust REGgamma specificity mutants involve substitution of Glu or Asp for Lys188. REGgamma(K188E/D) variants are virtually identical to REGalpha in proteasome activation but assemble into less stable heptamers/hexamers. Based on the REGalpha crystal structure, Lys188 of REGgamma faces the aqueous channel through the heptamer, raising the possibility that REG channels function as substrate-selective gates. However, covalent modification of proteasome chymotrypsin-like subunits by 125I-YL3-VS demonstrates that REGgamma(K188E)'s activation of all three proteasome active sites is not due to relaxed gating. We propose that decreased stability of REGgamma(K188E) heptamers allows them to change conformation upon proteasome binding, thus relieving inhibition of the CT and PGPH sites normally imposed by the wild-type REGgamma molecule.

Properties of the beta subunit of the proteasome activator PA28 (11S REG)

The proteasome activator PA28 (11S REG) is composed of two homologous subunits termed alpha and beta. The properties of the recombinant beta-subunit were explored and compared to the properties of the recombinant alpha-subunit. PA28beta produced in an Escherichia coli expression system migrates on a calibrated gel filtration column as an apparent heptamer (Mr = 250,000). Low concentrations of SDS (0.005%), dissociate the protein to a monomer (Mr = 33,000). PA28beta, has a complex effect on proteasome activity. At concentrations which favor oligomerization (> 2 microM), PA28beta is a strong proteasome activator although its affinity for the proteasome is about 10-fold less than recombinant PA28alpha. The catalytic properties of the PA28alpha and PA28beta-activated proteasome are similar. At low concentrations, PA28beta is a monomer and a potent allosteric proteasome inhibitor. These studies show that oligomerization of PA28beta is required for proteasome activation and that PA28beta monomers are potent proteasome inhibitors.

Properties of the nuclear proteasome activator PA28gamma (REGgamma)

PA28 or 11S REG is a proteasome activator composed of homologous alpha- and beta-subunits and predominantly found in the cytosol. A homologous protein originally known as the Ki antigen but now called PA28gamma or REGgamma is predominantly localized in the nucleus. To further characterize the biochemical properties of PA28gamma, we expressed and purified homogenous recombinant human protein with and without an N-terminal 6-His extension. PA28gamma is a heptamer based on the molecular masses of the native and monomeric proteins. The heptameric 6-His fusion protein can dimerize. Recombinant PA28y stimulates the proteasome-mediated hydrolysis of synthetic substrates containing hydrophobic, basic, and acidic amino acids in the P1 position. Stimulation is dependent on substrate size. PA28y only minimally stimulates degradation of the oxidized B chain of insulin. PA28gamma may facilitate the later stages of protein metabolism in the nucleus and/or have a more specialized role in controlling the levels of biologically active peptides in the nucleus.

Kinetic evidences for facilitation of peptide channelling by the proteasome activator PA28

The activation kinetics of constitutive and IFNgamma-stimulated 20S proteasomes obtained with homomeric (recPA28alpha, recPA28beta) and heteromeric (recPA28alphabeta) forms of recombinant 11S regulator PA28 was analysed by means of kinetic modelling. The activation curves obtained with increasing concentrations of the individual PA28 subunits (RecP28alpha/RecP28beta/RecP28alpha + RecP28beta) exhibit biphasic characteristics which can be attributed to a low-level activation by PA28 monomers and full proteasome activation by assembled activator complexes. The dissociation constants do not reveal significant differences between the constitutive and the immunoproteasome. Intriguingly, the affinity of the proteasome towards the recPA28alphabeta complex is about two orders of magnitude higher than towards the homomeric PA28alpha and PA28beta complexes. Striking similarities can been revealed in the way how PA28 mediates the kinetics of latent proteasomes with respect to three different fluorogenic peptides probing the chymotrypsin-like, trypsin-like and peptidylglutamyl-peptide hydrolyzing like activity: (a) positive cooperativity disappears as indicated by a lack of sigmoid initial parts of the kinetic curves, (b) substrate affinity is increased, whereby (c), the maximal activity remains virtually constant. As these kinetic features are independent of the peptide substrates, we conclude that PA28 exerts its activating influence on the proteasome by enhancing the uptake (and release) of shorter peptides.

The 26S proteasome: a molecular machine designed for controlled proteolysis

In eukaryotic cells, most proteins in the cytosol and nucleus are degraded via the ubiquitin-proteasome pathway. The 26S proteasome is a 2.5-MDa molecular machine built from approximately 31 different subunits, which catalyzes protein degradation. It contains a barrel-shaped proteolytic core complex (the 20S proteasome), capped at one or both ends by 19S regulatory complexes, which recognize ubiquitinated proteins. The regulatory complexes are also implicated in unfolding and translocation of ubiquitinated targets into the interior of the 20S complex, where they are degraded to oligopeptides. Structure, assembly and enzymatic mechanism of the 20S complex have been elucidated, but the functional organization of the 19S complex is less well understood. Most subunits of the 19S complex have been identified, however, specific functions have been assigned to only a few. A low-resolution structure of the 26S proteasome has been obtained by electron microscopy, but the precise arrangement of subunits in the 19S complex is unclear.

The proteasome activator 11 S REG or PA28: chimeras implicate carboxyl-terminal sequences in oligomerization and proteasome binding but not in the activation of specific proteasome catalytic subunits

The REG homologs, alpha, beta and gamma, activate mammalian proteasomes in distinct ways. REGalpha and REGbeta activate the trypsin-like, chymotrypsin-like and peptidylglutamyl-preferring active sites, whereas REGgamma only activates the proteasome's trypsin-like subunit. The three REG homologs differ in carboxyl-terminal sequences that are located next to activation loops on their proteasome binding surface. To assess the importance of these carboxyl-terminal sequences in the activation of specific proteasome beta catalytic subunits, we characterized chimeras in which 8 or 12 residues were exchanged among the three proteins. Like the wild-type molecule, REGalpha chimeras activated all three proteasome catalytic subunits regardless of the carboxyl-terminal sequence. However, REGalpha-beta chimeras activated the proteasome at lower concentrations than wild-type REGalpha and higher levels of REGalpha-gamma chimeras were needed for maximal activation because exchanged carboxyl-terminal sequences can stabilize (REGalpha-beta) or destabilize (REGalpha-gamma) the REGalpha heptamer. REGgamma chimeras were equivalent to REGgamma in their activation properties, but they bound the proteasome less tightly than the wild-type molecule. REGbeta chimeras also bound the proteasome more weakly than wild-type REGbeta and were virtually unable to activate it. Our findings demonstrate that the carboxyl-terminal sequences of REG subunits can affect heptamer stability and proteasome affinity, but they do not determine which proteasome beta subunits become activated.

The proteasome activator 11 S REG (PA28) and class I antigen presentation

There are two immune responses in vertebrates: humoral immunity is mediated by circulating antibodies, whereas cytotoxic T lymphocytes (CTL) confer cellular immunity. CTL lyse infected cells upon recognition of cell-surface MHC Class I molecules complexed with foreign peptides. The displayed peptides are produced in the cytosol by degradation of host proteins or proteins from intracellular pathogens that might be present. Proteasomes are cylindrical multisubunit proteases that generate many of the peptides eventually transferred to the cell surface for immune surveillance. In mammalian proteasomes, six active sites face a central chamber. As this chamber is sealed off from the enzyme's surface, there must be mechanisms to promote entry of substrates. Two protein complexes have been found to bind the ends of the proteasome and activate it. One of the activators is the 19 S regulatory complex of the 26 S proteasome; the other activator is '11 S REG' [Dubiel, Pratt, Ferrell and Rechsteiner (1992) J. Biol. Chem. 267, 22369-22377] or 'PA28' [Ma, Slaughter and DeMartino (1992) J. Biol. Chem. 267, 10515-10523]. During the past 7 years, our understanding of the structure of REG molecules has increased significantly, but much less is known about their biological functions. There are three REG subunits, namely alpha, beta and gamma. Recombinant REGalpha forms a ring-shaped heptamer of known crystal structure. 11 S REG is a heteroheptamer of alpha and beta subunits. REGgamma is also presumably a heptameric ring, and it is found in the nuclei of the nematode work Caenorhabditis elegans and higher organisms, where it may couple proteasomes to other nuclear components. REGalpha and REGbeta, which are abundant in vertebrate immune tissues, are located mostly in the cytoplasm. Synthesis of REG alpha and beta subunits is induced by interferon-gamma, and this has led to the prevalent hypothesis that REG alpha/beta hetero-oligomers play an important role in Class I antigen presentation. In the present review we focus on the structural properties of REG molecules and on the evidence that REGalpha/beta functions in the Class I immune response.

The proteasome activator 11 S REG (PA28) and class I antigen presentation

There are two immune responses in vertebrates: humoral immunity is mediated by circulating antibodies, whereas cytotoxic T lymphocytes (CTL) confer cellular immunity. CTL lyse infected cells upon recognition of cell-surface MHC Class I molecules complexed with foreign peptides. The displayed peptides are produced in the cytosol by degradation of host proteins or proteins from intracellular pathogens that might be present. Proteasomes are cylindrical multisubunit proteases that generate many of the peptides eventually transferred to the cell surface for immune surveillance. In mammalian proteasomes, six active sites face a central chamber. As this chamber is sealed off from the enzyme's surface, there must be mechanisms to promote entry of substrates. Two protein complexes have been found to bind the ends of the proteasome and activate it. One of the activators is the 19 S regulatory complex of the 26 S proteasome; the other activator is '11 S REG' [Dubiel, Pratt, Ferrell and Rechsteiner (1992) J. Biol. Chem. 267, 22369-22377] or 'PA28' [Ma, Slaughter and DeMartino (1992) J. Biol. Chem. 267, 10515-10523]. During the past 7 years, our understanding of the structure of REG molecules has increased significantly, but much less is known about their biological functions. There are three REG subunits, namely alpha, beta and gamma. Recombinant REGalpha forms a ring-shaped heptamer of known crystal structure. 11 S REG is a heteroheptamer of alpha and beta subunits. REGgamma is also presumably a heptameric ring, and it is found in the nuclei of the nematode work Caenorhabditis elegans and higher organisms, where it may couple proteasomes to other nuclear components. REGalpha and REGbeta, which are abundant in vertebrate immune tissues, are located mostly in the cytoplasm. Synthesis of REG alpha and beta subunits is induced by interferon-gamma, and this has led to the prevalent hypothesis that REG alpha/beta hetero-oligomers play an important role in Class I antigen presentation. In the present review we focus on the structural properties of REG molecules and on the evidence that REGalpha/beta functions in the Class I immune response.

Structural and functional characterizations of the proteasome-activating protein PA26 from Trypanosoma brucei

The activated 20 S proteasome, which has been found only in mammalian cells, is composed of two heptamer rings of an activator protein on each end of the 20 S proteasome and is inducible by interferon-gamma. A 20 S proteasome has been recently identified in a protozoan pathogen Trypanosoma brucei, but there has been no experimental evidence yet for the presence of a 26 S proteasome. Instead, an activated form of 20 S proteasome was isolated from this organism, which has significantly enhanced peptidase activities. It consists of an additional activator protein with an estimated molecular mass of 26 kDa (PA26) (To, W. Y., and Wang, C. C. (1997) FEBS Lett. 404, 253-262). The profile and sequences of tryptic peptides from PA26 were determined by mass spectrometry; no matches were found in the data base. The peptide sequences were used in reverse transcriptase-polymerase chain reaction to isolate a full-length cDNA clone encoding PA26. The protein sequence thus derived from it indicates little sequence identity with those of mammalian activator proteins PA28 alpha, beta, or gamma. There is only a single copy of PA26 gene in T. brucei. Purified recombinant PA26 polymerizes spontaneously to form heptamer ring with an outer diameter of 8.5 nm. The ring binds and activates 20 S proteasomes from T. brucei as well as rat, whereas human PA28alpha can neither bind nor activate T. brucei 20 S proteasome. The former is thus apparently more ubiquitous than PA28 in its capability of binding to and activating 20 S proteasomes. Its presence in T. brucei may also suggest a more ancient origin of proteasome activator proteins and a much wider involvement in protein degradation among other eukaryotic organisms than was originally envisaged.

The Leishmania mexicana proteasome

As a start to understanding the importance of intracellular proteolysis in the protozoon Leishmania mexicana, the parasite proteasome has been purified and characterised. The L. mexicana proteasome is similar to proteasomes from other eukaryotes. It is soluble, and the 20S form has a mass of around 670 kDa, composed of at least 10 distinct subunits in the 22 to 32 kDa size range. The molecular mass of the L. mexicana proteasome increases to 1200 kDa in the presence of adenosine-5'-triphosphate, consistent with there being a 26S proteasome in the parasite. The purified 20S proteasome has activity towards substrates with hydrophobic, basic and acidic P, residues, and is sensitive to a range of peptide aldehyde inhibitors, as well as the proteasome-specific inhibitor lactacystin. The peptide aldehydes are able to arrest parasite growth in vitro with the same relative effectiveness as against the purified proteasome activity. The parasite population arrests with an increased 4N DNA content, indicating that, in part, the essential nature of the proteasome for L. mexicana proliferation is due to a role in the parasite cell cycle. Surprisingly, lactacystin is a relatively inefficient inhibitor of L. mexicana growth in vitro.

A second gene encoding the mouse proteasome activator PA28beta subunit is part of a LINE1 element and is driven by a LINE1 promoter

The PMSE2 gene encodes the beta-subunit of the proteasome activator PA28 and, as shown by genomic Southern blot analysis, there probably exist four copies sharing sequence homology with PMSE2. Here, we report that in the mouse genome there exist two different chromosomal loci for PA28beta, both of which are transcribed and and which encode a functional PA28beta subunit. One of these represents the previously described PMSE2 gene possessing an intron-exon structure and a gamma interferon (IFNgamma)-inducible promoter. The second one, named PMSE2b, which we describe here, exhibits all the characteristics of an expressed retrotransposon. Our data show that the PA28beta retrotransposon is inserted into a transcriptional active LINE1 element and is driven by a LINE1 F-type monomer promoter as revealed by luciferase assays. The resulting PMSE2b mRNA encodes a protein which is indistinguishable from that encoded by the IFNgamma-inducible PMSE2 gene. Since PA28 plays an important role in major histocompatibility complex class I antigen presentation, the implications for the mouse immune system through a constitutively expressed PA28beta subunit and the biological relevance of this finding are discussed.

Antigen presentation by MHC class I and its regulation by interferon gamma

Antigen processing by MHC class I molecules begins with the generation of peptides by proteolytic breakdown of proteins. IFN-gamma upregulates gene expression of several proteasomal subunits as well as the proteasome regulator PA28; this implicated their role in antigen degradation. Crystallographic, mutational and biochemical studies contributed to our understanding of the basic principles of proteasomal protein degradation and the consequences of IFN-gamma induction for proteasome function. In addition, nonproteasomal mechanisms seem to be involved in antigen degradation. Leucine aminopeptidase, which is also upregulated by IFN-gamma, was shown to collaborate with the proteasome for epitope production and unknown proteases seem to compensate for the loss of proteasomal degradation in the presence of proteasome inhibitors. Thus, a rather complex picture emerges for the rules governing peptide production in the presence or absence of IFN-gamma.

Properties of the proteasome activator subunit PA28 alpha and its des-tyrosyl analog

The proteasome activator protein PA28 or 11 S regulator may play an important role in facilitating the generation of peptides for presentation by the MHC class I system. PA28 is composed of two homologous subunits termed alpha and beta. Removal of the carboxyl terminal tyrosine of the alpha subunit of PA28 abolishes activity (X. Song et al., 1997, J. Biol. Chem. 272, 27994-28000). To explore the structural basis of this effect the des-tyrosyl analog of PA28alpha prepared by site-directed mutagenesis and PA28alpha were expressed at high levels in a baculovirus system and purified by FPLC. Des-tyrosyl-PA28alpha neither stimulated the proteasome nor competed with PA28alpha for binding to the proteasome. Hydrophobic interaction chromatography revealed that the hydrophobicity of the mutant protein was considerably greater than PA28alpha. When the mutant protein was chromatographed on a calibrated Superose 6 column a mixture of approximately 25% oligomer and 75% monomer was found. The oligomer weakly stimulated the proteasome but this molecule was labile. Very low concentrations of SDS (0.005%) dissociated PA28alpha and abolished its stimulatory activity. It is concluded that the lack of activity of des-tyrosyl-PA28alpha is due to conformational changes resulting in dissociation and that the oligomeric form of PA28alpha is required for activation.

The proteasome activator 11 S regulator or PA28. Contribution By both alpha and beta subunits to proteasome activation

The proteasome 11 S regulator (REG) consists of two homologous subunits, REGalpha and REGbeta. Each subunit is capable of activating the proteasome, and when combined, they form superactive REGalpha/REGbeta complexes. We have previously shown that a highly conserved loop in the REGalpha crystal structure is critical for proteasome activation. We now show that hetero-oligomers formed from REGalpha activation loop mutants and wild-type REGbeta or vice versa are partially active. By contrast, hetero-oligomers bearing mutations in the activation loops of REGalpha and REGbeta subunits are inactive, demonstrating that both alpha and beta subunits contribute to proteasome activation. We have also characterized REG proteins with mutations near or at their C termini. Partially active REGalpha(Y249C) and REGalpha(M247V) and an inactive REGalpha subunit bearing five additional C-terminal amino acids formed active hetero-oligomers with REGbeta. REGbeta subunits lacking 1, 2, or 9 C-terminal amino acids did not bind or activate the proteasome, but each of these mutants formed partially active hetero-oligomers with the monomer REGalpha(N50Y). However, hetero-oligomers formed from REG subunits lacking the last 14 amino acids were unable to bind the proteasome. Thus, C-terminal regions of both alpha and beta subunits are required for hetero-oligomers to bind the proteasome.

Structure of the proteasome activator REGalpha (PA28alpha)

The specificity of the 20S proteasome, which degrades many intracellular proteins, is regulated by protein complexes that bind to one or both ends of the cylindrical proteasome structure. One of these regulatory complexes, the 11S regulator (known as REG or PA28), stimulates proteasome peptidase activity and enhances the production of antigenic peptides for presentation by class I molecules of the major histocompatibility complex (MHC). The three REG subunits that have been identified, REGalpha, REGbeta and REGgamma (also known as the Ki antigen), share extensive sequence similarity, apart from a highly variable internal segment of 17-34 residues which may confer subunit-specific properties. REGalpha and REGbeta preferentially form a heteromeric complex, although purified REGalpha forms a heptamer in solution and has biochemical properties similar to the heteromeric REGalpha/REGbeta complex. We have now determined the crystal structure of human recombinant REGalpha at 2.8 A resolution. The heptameric barrel-shaped assembly contains a central channel that has an opening of 20 A diameter at one end and another of 30 A diameter at the presumed proteasome-binding surface. The binding of REG probably causes conformational changes that open a pore in the proteasome alpha-subunits through which substrates and products can pass.