ER-aminopeptidase 1 determines the processing and presentation of an immunotherapy-relevant melanoma epitope

Dissecting the different steps of the processing and presentation of tumor-associated antigens is a key aspect of immunotherapies enabling to tackle the immune response evasion attempts of cancer cells. The immunodominant glycoprotein gp100_209-217 epitope, which is liberated from the melanoma differentiation antigen gp100^PMEL17 , is part of immunotherapy trials. By analyzing different human melanoma cell lines, we here demonstrate that a pool of N-terminal extended peptides sharing the common minimal epitope is generated by melanoma proteasome subtypes. In vitro and in cellulo experiments indicate that ER-resident aminopeptidase 1 (ERAP1)-but not ERAP2-defines the processing of this peptide pool thereby modulating the T-cell recognition of melanoma cells. By combining the outcomes of our studies and others, we can sketch the complex processing and endogenous presentation pathway of the gp100_209-217 -containing epitope/peptides, which are produced by proteasomes and are translocated to the vesicular compartment through different pathways, where the precursor peptides that reach the endoplasmic reticulum are further processed by ERAP1. The latter step enhances the activation of epitope-specific T lymphocytes, which might be a target to improve the efficiency of anti-melanoma immunotherapy.

The proteasome immunosubunits, PA28 and ER-aminopeptidase 1 protect melanoma cells from efficient MART-126-35 -specific T-cell recognition

The immunodominant MART-1(26(27)-35) epitope, liberated from the differentiation antigen melanoma antigen recognized by T cells/melanoma antigen A (MART-1/Melan-A), has been frequently targeted in melanoma immunotherapy, but with limited clinical success. Previous studies suggested that this is in part due to an insufficient peptide supply and epitope presentation, since proteasomes containing the immunosubunits β5i/LMP7 (LMP, low molecular weight protein) or β1i/LMP2 and β5i/LMP7 interfere with MART-1(26-35) epitope generation in tumor cells. Here, we demonstrate that in addition the IFN-γ-inducible proteasome subunit β2i/MECL-1 (multicatalytic endopeptidase complex-like 1), proteasome activator 28 (PA28), and ER-resident aminopeptidase 1 (ERAP1) impair MART-1(26-35) epitope generation. β2i/MECL-1 and PA28 negatively affect C- and N-terminal cleavage and therefore epitope liberation from the proteasome, whereas ERAP1 destroys the MART-1(26-35) epitope by overtrimming activity. Constitutive expression of PA28 and ERAP1 in melanoma cells indicate that both interfere with MART-1(26-35) epitope generation even in the absence of IFN-γ. In summary, our results provide first evidence that activities of different antigen-processing components contribute to an inefficient MART-1(26-35) epitope presentation, suggesting the tumor cell's proteolytic machinery might have an important impact on the outcome of epitope-specific immunotherapies.

Regulation of immunoproteasome function in the lung

Impaired immune function contributes to the development of chronic obstructive pulmonary disease (COPD). Disease progression is further exacerbated by pathogen infections due to impaired immune responses. Elimination of infected cells is achieved by cytotoxic CD8⁺  T cells that are activated by MHC I-mediated presentation of pathogen-derived antigenic peptides. The immunoproteasome, a specialized form of the proteasome, improves generation of antigenic peptides for MHC I presentation thereby facilitating anti-viral immune responses. However, immunoproteasome function in the lung has not been investigated in detail yet. In this study, we comprehensively characterized the function of immunoproteasomes in the human and murine lung. Parenchymal cells of the lung express low constitutive levels of immunoproteasomes, while they are highly and specifically expressed in alveolar macrophages. Immunoproteasome expression is not altered in whole lung tissue of COPD patients. Novel activity-based probes and native gel analysis revealed that immunoproteasome activities are specifically and rapidly induced by IFNγ treatment in respiratory cells in vitro and by virus infection of the lung in mice. Our results suggest that the lung is potentially capable of mounting an immunoproteasome-mediated efficient adaptive immune response to intracellular infections.

Adult human liver contains intermediate-type proteasomes with different enzymatic properties

BACKGROUND

The 20S proteasome is the proteolytic core of the major intracellular protein degradative system, the ubiquitin-proteasome system. Since little is known about proteasomes of human liver, we have investigated the proteasome spectrum in adult human liver.

MATERIAL AND METHODS

20S proteasomes were chromatographically purified from adult human liver and from HuH7 cells. They were divided into subpopulations and subtypes and characterized with regard to their proteolytic activities using short fluorogenic oligo- and long poly-peptide substrates. Their subunit composition was studied by immunoblotting.

RESULTS

Proteasomes from adult human liver tissue can be separated into three subpopulations (I, II, III), each of which is composed of several subtypes, which total to a spectrum of 14 different subtypes. Two minor subtypes contain only the immuno-subunits β1i and β5i but not their standard counterparts; all others are intermediate subtypes containing β1 and β5 standard- and β1i and β5i immuno-subunits in various compositions. With regard to the proteolytic activities we observed that a decreasing content of subunit β1i in the subtypes goes along with a decreasing ratio of chymotrypsin-like/caspase-like activity, whereas the degradation rate of a 30 mer polypeptide substrate increased with decreasing β1i content. By comparison, 20S proteasomes from HuH7 cells do not contain immuno-subunits but are pure standard proteasomes, which can be separated into three subtypes.

CONCLUSION

These findings suggest that adult human liver contains a spectrum of 14 different 20S proteasome subtypes with different enzymatic properties reflecting most probably an adaptive response of liver cell functions to challenging factors during lifetime.

Proteasome isoforms exhibit only quantitative differences in cleavage and epitope generation

Immunoproteasomes are considered to be optimised to process Ags and to alter the peptide repertoire by generating a qualitatively different set of MHC class I epitopes. Whether the immunoproteasome at the biochemical level, influence the quality rather than the quantity of the immuno-genic peptide pool is still unclear. Here, we quantified the cleavage-site usage by human standard- and immunoproteasomes, and proteasomes from immuno-subunit-deficient mice, as well as the peptides generated from model polypeptides. We show in this study that the different proteasome isoforms can exert significant quantitative differences in the cleavage-site usage and MHC class I restricted epitope production. However, independent of the proteasome isoform and substrates studied, no evidence was obtained for the abolishment of the specific cleavage-site usage, or for differences in the quality of the peptides generated. Thus, we conclude that the observed differences in MHC class I restricted Ag presentation between standard- and immunoproteasomes are due to quantitative differences in the proteasome-generated antigenic peptides.

Lifelong maintenance of composition, function and cellular/subcellular distribution of proteasomes in human liver

Owing to organ shortage, livers from old donors are increasingly used for transplantation. The function and duration of such transplanted livers are apparently comparable to those from young donors, suggesting that, despite some morphological and structural age-related changes, no major functional changes do occur in liver with age. We tested this hypothesis by performing a comprehensive study on proteasomes, major cell organelles responsible for proteostasis, in liver biopsies from heart-beating donors. Oxidized and poly-ubiquitin conjugated proteins did not accumulate with age and the three major proteasome proteolytic activities were similar in livers from young and old donors. Analysis of proteasomes composition showed an age-related increased of β5i/α4 ratio, suggesting a shift toward proteasomes containing inducible subunits and a decreased content of PA28α subunit, mainly in the cytosol of hepatocytes. Thus our data suggest that, proteasomes activity is well preserved in livers from aged donors, concomitantly with subtle changes in proteasome subunit composition which might reflect the occurrence of a functional remodelling to maintain an efficient proteostasis. Gender differences are emerging and they deserve further investigations owing to the different aging trajectories between men and women. Finally, our data support the safe use of livers from old donors for transplantation.

Two inhibitors of the ubiquitin proteasome system enhance long-term memory formation upon olfactory conditioning in the honeybee (Apis mellifera)

In honeybees (Apis mellifera), the proteasome inhibitor Z-Leu-Leu-Leu-CHO (MG132) enhances long-term memory (LTM) formation. Studies in vertebrates using different inhibitors of the proteasome demonstrate the opposite, namely an inhibition of memory formation. The reason for this contradiction remains unclear. MG132 is an inhibitor of the proteasome, but also blocks other proteases. Accordingly, one possible explanation might be that other proteases affected by MG132 are responsible for the enhancement of LTM formation. We test this hypothesis by comparing the effect of MG132 and the more specific proteasome inhibitor clasto-lactacystin beta-lactone (β-lactone). We show that these two inhibitors block the activity of the proteasome in honeybee brains to a similar extent, do not affect the animals' survival but do enhance LTM retention upon olfactory conditioning. Thus, the enhancement of LTM formation is not due to MG132-specific side effects, but to inhibition of a protease targeted by MG132 and β-lactone, i.e. the proteasome.

Molecular alterations in proteasomes of rat liver during aging result in altered proteolytic activities

Aging induces alterations of tissue protein homoeostasis. To investigate one of the major systems catalysing intracellular protein degradation we have purified 20S proteasomes from rat liver of young (2 months) and aged (23 months) animals and separated them into three subpopulations containing different types of intermediate proteasomes with standard- and immuno-subunits. The smallest subpopulation ΙΙΙ and the major subpopulation Ι comprised proteasomes containing immuno-subunits β1i and β5i beside small amounts of standard-subunits, whereas proteasomes of subpopulation ΙΙ contained only β5i beside standard-subunits. In favour of a relative increase of the major subpopulation Ι, subpopulation ΙΙ and ΙΙΙ were reduced for about 55 % and 80 %, respectively, in aged rats. Furthermore, in all three 20S proteasome subpopulations from aged animals standard-active site subunits were replaced by immuno-subunits. Overall, this transformation resulted in a relative increase of immuno-subunit-containing proteasomes, paralleled by reduced activity towards short fluorogenic peptide substrates. However, depending on the substrate their hydrolysing activity of long polypeptide substrates was significantly higher or unchanged. Furthermore, our data revealed an altered MHC class I antigen-processing efficiency of 20S proteasomes from liver of aged rats. We therefore suggest that the age-related intramolecular alteration of hepatic proteasomes modifies its cleavage preferences without a general decrease of its activity. Such modifications could have implications on protein homeostasis as well as on MHC class I antigen presentation as part of the immunosenescence process.

T lymphocytes export proteasomes by way of microparticles: a possible mechanism for generation of extracellular proteasomes

The 20S proteasome is almost exclusively localized within cells. High levels of extracellular proteasomes are also found circulating in the blood plasma of patients suffering from a variety of inflammatory, autoimmune and neoplastic diseases. However, the origin of these proteasomes remained enigmatic. Since the proteome of microparticles, small membrane enclosed vesicles released from cells, was shown to contain proteasomal subunits, we studied whether intact proteasomes are actively released into the extracellular space. Using human primary T lymphocytes stimulated with CaCl2 and the calcium ionophore A23187 to induce membrane blebbing we demonstrate that microparticles contain proteolytically active 20S proteasomes as well as the proteasome activator PA28 and subunits of the 19S proteasome regulator. Furthermore, our experiments reveal that incubation of in vitro generated T lymphocyte-microparticles with sphingomyelinase results in the hydrolysis of the microparticle membranes and subsequent release of proteasomes from the vesicles. Thus, we here show for the first time that functional proteasomes can be exported from activated immune cells by way of microparticles, the dissolution of which may finally lead to the generation of extracellular proteasomes.

Driving forces of proteasome-catalyzed peptide splicing in yeast and humans

Proteasome-catalyzed peptide splicing (PCPS) represents an additional activity of mammalian 20S proteasomes recently identified in connection with antigen presentation. We show here that PCPS is not restricted to mammalians but that it is also a feature of yeast 20S proteasomes catalyzed by all three active site β subunits. No major differences in splicing efficiency exist between human 20S standard- and immuno-proteasome or yeast 20S proteasome. Using H(2)(18)O to monitor the splicing reaction we also demonstrate that PCPS occurs via direct transpeptidation that slightly favors the generation of peptides spliced in cis over peptides spliced in trans. Splicing efficiency itself is shown to be controlled by proteasomal cleavage site preference as well as by the sequence characteristics of the spliced peptides. By use of kinetic data and quantitative analyses of PCPS obtained by mass spectrometry we developed a structural model with two PCPS binding sites in the neighborhood of the active Thr1.

Circulating 20S proteasome in patients with non-metastasized breast cancer

BACKGROUND

Recent data suggest a role of the ubiquitin-proteasome system in various malignancies. In patients with neoplasms, increased extracellular concentrations of circulating 20S proteasome (c-proteasome) have been detected in blood plasma. We tested the hypothesis that the plasma c-proteasome concentration is a biomarker associated with tumor stage and nodal status in patients with the primary diagnosis of non-metastatic breast cancer.

PATIENTS AND METHODS

Venous plasma concentration of 20S proteasome was measured by ELISA technique in 224 non-metastatic breast cancer patients and in 50 healthy volunteers. To assess the relation of proteasome expression to c-proteasome concentration, tumor specimens from 32 patients were immunohistochemically stained for 20S proteasome using an antibody directed against the core subunits of the catalytic domain of the 20S proteasome.

RESULTS

The median c-proteasome concentration was higher (p<0.0001) in breast cancer patients (397.5 ng/ml, range: 200-50,000 ng/ml) than in healthy controls (305 ng/ml, range: 140-425 ng/ml). There was no significant correlation between c-proteasome concentration and strength of proteasomal staining in tumor specimens. Neither tumor size, nor nodal status, nor any other prognostically important clinical parameter, including the presence of disseminated tumor cells in the bone marrow, correlated with high c-proteasome concentrations.

CONCLUSION

Circulating proteasome concentrations appear to be higher in patients presenting with primary breast cancer than in healthy controls. Thus, the ubiquitin-proteasome system might represent a potential target in breast cancer treatment.

The prognostic impact of circulating proteasome concentrations in patients with epithelial ovarian cancer

BACKGROUND

Intracellularly, the ubiquitin-proteasome system participates in crucial functions such as cell cycling, differentiation, proliferation, gene transcription, and apoptosis. However, in malignancies including ovarian cancer increased extracellular concentrations of circulating 20S proteasomes (c-proteasomes) have been detected in blood. We tested the hypothesis that the c-proteasome plasma concentration is a biomarker associated with the clinical course of ovarian cancer patients.

METHODS

20S-proteasome venous plasma concentration was measured by ELISA in patients presenting with ovarian cancer before (n=120) and after (n=68) primary treatment, and in healthy volunteers (n=55). The median follow-up time was 19 months. To assess the relation of proteasome expression with c-proteasome concentration, tumor specimens from 27 patients were immunohistochemically stained for 20S proteasome using an antibody directed against the core subunits of the catalytic domain of the 20S proteasome.

RESULTS

Median c-proteasome concentration was higher (p<0.0001) in untreated ovarian cancer patients (457.5 ng/ml, range: 200-12540 ng/ml) than in healthy controls 290 ng/ml, range: 140-425 ng/ml). Following completion of primary treatment, the median c-proteasome concentration increased (p=0.003) relative to baseline (595 ng/ml, range: 200-20000 ng/ml) and concentrations positively correlated (p=0.031) with residual disease left at primary surgery. Patients with post-treatment c-proteasome concentrations exceeding the cohort's median showed a diminished survival (p=0.045). We found no correlation between c-proteasome concentration and strength of proteasomal staining in tumor specimens.

CONCLUSIONS

Circulating proteasome concentrations correlate with residual tumor mass and might be a prognostic variable in ovarian cancer following primary therapy.

Quantitative proteome analysis of the 20S proteasome of apoptotic Jurkat T cells

Regulated proteolysis plays important roles in cell biology and pathological conditions. A crosstalk exists between apoptosis and the ubiquitin-proteasome system, two pathways responsible for regulated proteolysis executed by different proteases. To investigate whether the apoptotic process also affects the 20S proteasome, we performed three independent SILAC-based quantitative proteome approaches: 1-DE/MALDI-MS, small 2-DE/MALDI-MS and large 2-DE/nano-LC-ESI-MS. Taking the results of all experiments together, no quantitative changes were observed for the α- and β-subunits of the 20S proteasome except for subunit α7. This protein was identified in two protein spots with a down-regulation of the more acidic protein species (α7a) and up-regulation of the more basic protein species (α7b) during apoptosis. The difference in these two α7 protein species could be attributed to oxidation of cysteine-41 to cysteine sulfonic acid and phosphorylation at serine-250 near the C terminus in α7a, whereas these modifications were missing in α7b. These results pointed to the biological significance of posttranslational modifications of proteasome subunit α7 after induction of apoptosis.

Multiple cardiac proteasome subtypes differ in their susceptibility to proteasome inhibitors

AIMS

The proteasome is the proteolytically active core of the ubiquitin-proteasome system, which regulates vital processes and which can cause various diseases when it malfunctions. Therefore, the proteasome has become an attractive target for pharmaceutical interventions. Inhibition of the cardiac proteasome by specific proteasome inhibitors has been shown to attenuate cardiac hypertrophy and ischaemia reperfusion injury of the heart. We have resolved the cardiac proteasome into its subtypes and have addressed the key question of how proteasome inhibitors affect single cardiac proteasomal subtypes.

METHODS AND RESULTS

The 20S proteasome from rat heart was dissected into three different subpopulations (groups I-III), each comprising 4-7 different subtypes. The major group (group II) comprises standard proteasome subtypes; the two minor subpopulations (groups I and III) contain intermediate proteasome subtypes. All subtypes exhibit chymotrypsin-, trypsin-, and caspase-like activity but to different degrees. We have tested the effect of two common proteasome inhibitors on the chymotrypsin-like activity of all subtypes: 20-30 nmol/L MG132 caused 50% inhibition of all subtypes from groups I and II, whereas 100 nmol/L was necessary to affect group III subtypes to the same extent. However, another inhibitor, bortezomib (VELCADE), already used clinically, inhibited 50% of the activity of group III proteasome subtypes even below 20 nmol/L, a concentration showing almost no effect on group I and II proteasome subtypes. The caspase-like activity of group II proteasome subtypes was not affected by MG132 and was inhibited by bortezomib only at concentrations above 100 nmol/L.

CONCLUSION

These data show that different inhibitors have differential inhibitory effects on the various cardiac proteasome subtypes. Different cardiac subtypes are inhibited by the same dose of proteasome inhibitor to a different extent.

Alveolar extracellular 20S proteasome in patients with acute respiratory distress syndrome

RATIONALE

Repair mechanisms resulting in alveolar protein degradation in acute respiratory distress syndrome (ARDS) are largely unknown.

OBJECTIVES

To test whether the 20S proteasome is present and functional in the alveolar space in patients with ARDS.

METHODS

Proteasome antigenic concentration in bronchoalveolar lavage (BAL) supernatants was measured by ELISA in patients with ARDS (n = 64), acute lung injury (ALI) (n = 8), sarcoidosis (n = 13), and in healthy subjects (n = 8). Cleavage of specific fluorogenic substrates (+/-epoxomicin), I(125) albumin degradation rate, and gel filtration were used to quantify and characterize proteasomal activity. The presence of proteasomes was confirmed independently by electron microscopic techniques.

MEASUREMENTS AND MAIN RESULTS

Proteasome concentrations in patients with ARDS were markedly increased (1,069 +/- 1,194 ng/ml) in comparison to healthy subjects (60.8 +/- 49.8; P < 0.001), ALI (154 +/- 43; P = 0.006), and sarcoidosis (97.6 +/- 42.2; P = 0.037). All fluorogenic substrates were hydrolyzed (Suc-LLVY-AMC, 3.6 +/- 8.8 pkat/mg; BZ-VGR-AMC, 1.8 +/- 3.1; Suc-LLE-AMC, 1 +/- 1.7) by BAL supernatants of patients with ARDS, with inhibition by epoxomicin (P = 0.0001), and the majority of proteolytic activity was detected in BAL supernatant. Maximum hydrolyzing activity occurred at 660 kD and 20S proteasome was seen microscopically after purification and being released by pneumocytes type II. Proteasomal activity and albumin degradation rate in patients with ARDS were approximately 17-fold lower than in healthy subjects. Proteasomal activity in normal BAL was inhibited by BAL aliquots from patients with ARDS but not by denatured BAL, and returned to normal by purification.

CONCLUSIONS

For the first time, we identified extracellular, biologically active 20S proteasome in the alveolar space of patients with ARDS in concentrations much higher than in normal subjects or in those with ALI.

Polyubiquitin substrates allosterically activate their own degradation by the 26S proteasome

The 26S proteasome degrades polyubiquitylated (polyUb) proteins by an ATP-dependent mechanism. Here we show that binding of model polyUb substrates to the 19S regulator of mammalian and yeast 26S proteasomes enhances the peptidase activities of the 20S proteasome about two-fold in a process requiring ATP hydrolysis. Monoubiquitylated proteins or tetraubiquitin alone exert no effect. However, 26S proteasomes from the yeast alpha3DeltaN open-gate mutant and the rpt2YA and rpt5YA mutants with impaired gating can still be activated (approximately 1.3-fold to 1.8-fold) by polyUb-protein binding. Thus, binding of polyUb substrates to the 19S regulator stabilizes gate opening of the 20S proteasome and induces conformational changes of the 20S proteasome that facilitate channeling of substrates and their access to active sites. In consequence, polyUb substrates will allosterically stimulate their own degradation.

The cell-penetrating peptide octa-arginine is a potent inhibitor of proteasome activities

Oligo-arginines are cell-penetrating peptides and find use as carriers for transportation of various membrane-impermeable biopharmaceuticals into target cells. We have found that oligo-arginines of a length of 4-10 amino acids, but especially (Arg)(8), are able to inhibit the major intracellular proteolytic system, the proteasome, with mixed-type inhibition characteristics. The IC(50) values of (Arg)(8) for the proteasomal chymotrypsin-like and caspase-like activities are approximately 100 and 200 nM, respectively. The inhibition of the trypsin-like activity never exceeds 50% even at micromolar concentrations. (Arg)(8) also inhibits 20S proteasome/PA28 complexes as well as 26S proteasomes, although with a decreased efficiency. Due to its cell membrane-penetrating capability, incubation of HeLa cells in the presence of (Arg)(8) resulted in an impaired activity of proteasomes going along with an accumulation of high-molecular mass ubiquitin-conjugated proteins, the preferred substrates of 26S proteasomes. The in vivo susceptibility of the three proteasome activities resembles that found in vitro with chymotrypsin-like>caspase-like>trypsin-like activities. Since inhibition of the proteasome system might affect fundamental basic cellular processes but on the other side might also prevent the degradation of a proteinacous cargo, we suggest that this proteasome inhibitory activity should be taken into account when oligo-arginines are being considered for use as vectors for the intracellular delivery of pharmaceuticals.

Extracellular, circulating proteasomes and ubiquitin - incidence and relevance

The ubiquitin-proteasome system is the major pathway for intracellular protein degradation and is also deeply involved in the regulation of most basic cellular processes. Its proteolytic core, the 20S proteasome, has found to be attached also to the cell plasma membrane and certain observations are interpreted as to suggest that they may be released into the extracellular medium, e.g. in the alveolar lining fluid, epididymal fluid and possibly during the acrosome reaction. Proteasomes have also been detected in normal human blood plasma and designated circulating proteasomes; these have a comparatively low specific activity, a distinct pattern of subtypes and their exact origin is still enigmatic. In patients suffering from autoimmune diseases, malignant myeloproliferative syndromes, multiple myeloma, acute and chronic lymphatic leukaemia, solid tumour, sepsis or trauma, respectively, the concentration of circulating proteasomes has been found to be elevated, to correlate with the disease state and has even prognostic significance. Similarly, ubiquitin has been discovered as a normal component of human blood and seminal plasma and in ovarian follicular fluid. Increased concentrations were measured in diverse pathological situations, not only in blood plasma but also in cerebrospinal fluid, where it may have neuroprotective effects. As defective spermatozoa are covered with ubiquitin in the epididymal fluid, extracellular ubiquitination is proposed to be a mechanism for quality control in spermatogenesis. Growing evidence exists also for a participation of extracellular proteasomes and ubiquitin in the fertilization process.

Role of proteasomes in disease

A functional ubiquitin proteasome system is essential for all eukaryotic cells and therefore any alteration to its components has potential pathological consequences. Though the exact underlying mechanism is unclear, an age-related decrease in proteasome activity weakens cellular capacity to remove oxidatively modified proteins and favours the development of neurodegenerative and cardiac diseases. Up-regulation of proteasome activity is characteristic of muscle wasting conditions including sepsis, cachexia and uraemia, but may not be rate limiting. Meanwhile, enhanced presence of immunoproteasomes in aging brain and muscle tissue could reflect a persistent inflammatory defence and anti-stress mechanism, whereas in cancer cells, their down-regulation reflects a means by which to escape immune surveillance. Hence, induction of apoptosis by synthetic proteasome inhibitors is a potential treatment strategy for cancer, whereas for other diseases such as neurodegeneration, the use of proteasome-activating or -modulating compounds could be more effective. Publication history: Republished from Current BioData's Targeted Proteins database (TPdb; http://www.targetedproteinsdb.com).

Intermediate-type 20 S proteasomes in HeLa cells: "asymmetric" subunit composition, diversity and adaptation

The 20 S proteasomes are cylinder-shaped heteromeric dimers with a subunit configuration of alpha7, beta7, beta7, alpha7. Replacement of the three active site-containing standard beta-subunits (beta1, beta2, beta5) by immuno-beta-subunits (beta1i, beta2i, beta5i) results in formation of 20 S immuno-proteasomes, while only partial replacement leads to intermediate-type proteasomes. Synthesis of immuno-subunits can be induced by interferon-gamma, which causes a complete transformation of three subtypes of standard proteasomes into three subtypes of intermediate-type proteasomes in HeLa cells, a process that results in a change in the proteolytic activities of the enzymes. HeLa cells producing the proteasome beta1-subunit tagged with the Fc region-binding ZZ domain of protein A were grown in the presence of interferon-gamma. From these cells, we have purified 20 S proteasomes by using IgG-affinity resin and analysed them by 2D PAGE. Our study showed that subunit replacement can be confined to one half of the proteasome cylinder, resulting in the formation of intermediate-type proteasomes with "asymmetric" subunit composition. Analysis of proteasomes purified from the cytoplasm, nucleoplasm, and microsomes of HeLa S3 cells reveals that all three compartments are furnished with intermediate-type proteasomes of different subtype and subunit composition, exhibiting different specific proteolytic activities.

The 20S proteasome isolated from Alzheimer's disease brain shows post-translational modifications but unchanged proteolytic activity

Chronic neurodegenerative diseases are characterized by the accumulation of aggregated protein species, and functional impairment of the ubiquitin proteasome system has been hypothesized to contribute to neuronal cell loss. Decreased proteolytic activity of the 20S proteasome has been shown postmortem in crude brain lysates from Alzheimer's disease (AD) patients. In the present study, we demonstrate, however, that catalytic activity of the 20S proteasome increases during chromatographic purification from AD brains as compared with age-matched controls. By two-dimensional difference gel electrophoresis we detected pI shifts in several proteasome subunits in AD samples pointing to differential post-translational modifications. Moreover, we identified N-terminal acetylation and dephosphorylation of subunit alpha7 in AD by tandem mass spectrometry. Thus, reduced peptidase activity in AD brain extracts is not an intrinsic property of the 20S proteasome, but may be resulting from the presence of endogenous inhibitory proteins or substrates. Post-translational modifications of non-catalytic subunits in situ may contribute to the trend towards enhanced hydrolytic activity of the isolated 20S proteasome after removal of the endogenous inhibitors.

Circulating proteasomes are functional and have a subtype pattern distinct from 20S proteasomes in major blood cells

BACKGROUND

20S proteasomes, the proteolytic core particles of the major intracellular protein degradative pathway, are potential disease markers because they are detectable in human plasma as circulating proteasomes and their concentrations are increased in patients suffering from various diseases. To investigate the origin of circulating proteasomes, we compared some of their features with those of proteasomes isolated from major blood cells.

METHODS

We isolated circulating proteasomes from the plasma of 2 patients with rheumatoid arthritis and 2 with systemic lupus erythematosus and from human plasma from healthy donors. We purified the proteasomes to apparent homogeneity and then used electron microscopy for imaging and chromatography for subtype spectrum analysis. We compared subtype results with those from 20S proteasomes purified from 4 major blood cell populations. We also tested proteasomes for enzymatic activity and immunosubunit content.

RESULTS

Circulating proteasomes from plasma of healthy donors and from patients with autoimmune disease were found to have the same size and shape as erythrocyte proteasomes, be proteolytically active, and contain standard- and immunosubunits. Chromatography revealed 6 circulating proteasome subtype peaks in healthy donor plasma and 7 in patient donor plasma. Proteasomes from erythrocytes had 3 subtype peaks and those of monocytes, T-lymphocytes, and thrombocytes each had 5 different subtype peaks.

CONCLUSION

Circulating proteasomes were intact and enzymatically active in plasma from healthy donors and from patients with autoimmune disease. Because the subtype patterns of circulating proteasomes clearly differ from those of proteasomes from blood cells, these cells cannot be regarded as a major source of circulating proteasomes.

Comprehensive quantitative proteome analysis of 20S proteasome subtypes from rat liver by isotope coded affinity tag and 2-D gel-based approaches

Quantitative protein profiling is an essential part of proteomics and requires technologies that accurately, reproducibly, and comprehensively identify and quantify proteins. Over the past years, many quantitative proteomic methods have been developed. Here, 20S proteasome subtypes isolated from rat were compared by four approaches based on the combination of isotope-coded affinity tag (ICAT), 2-DE, LC and ESI and MALDI MS: (i) 2-DE, (ii) ICAT/2-DE MALDI-MS, (iii) ICAT/LC-ESI-MS, (iv) ICAT/LC-MALDI-MS. A definite qualitative advantage of 2-DE gels was the separation of all known protein species, the identification of cysteine sulfoxide of alpha-4 (RC6-IS) and N-terminal acetylation of several subunits. Furthermore, quantitative differences between the standard subunits beta-2, and beta-5 and their immunosubunits were only detected by 2-DE image analysis revealing a higher replacement of standard- by immuno-beta-subunits in subtype IV. It was obvious that for relative quantification only protein spot and mass peaks with a certain level of intensity displayed acceptable values of SD. However, ICAT in conjunction with LC/MALDI-MS was the most accurate method for quantification. The experimental data of this investigation are accessible via http://www.mpiib-berlin.mpg.de/2D-PAGE/.

Proteasomes

The major enzyme system catalysing the degradation of intracellular proteins is the proteasome system. A central inner chamber of the cylinder-shaped 20 S proteasome contains the active site, formed by N-terminal threonine residues. The 20 S proteasomes are extremely inefficient in degrading folded protein substrates and therefore one or two multisubunit 19 S regulatory particles bind to one or both ends of the 20 S proteasome cylinder, forming 26 S and 30 S proteasomes respectively. These regulatory complexes are able to bind proteins marked as proteasome substrates by prior conjugation with polyubiquitin chains, and initiate their unfolding and translocation into the proteolytic chamber of the 20 S proteasome, where they are broken down into peptides of 3-25 amino acids. The polyubiquitin tag is removed from the substrate protein by the deubiquitinating activity of the 19 S regulator complex. Under conditions of an intensified immune response, many eukaryotic cells adapt by replacing standard 20 S proteasomes with immuno-proteasomes and/or generating the proteasome activator complex, PA28. Both of these adaptations change the protein-breakdown process for optimized generation of antigenic peptide epitopes that are presented by the class I MHCs. Hybrid proteasomes (19 S regulator-20 S proteasome-PA28) may have a special function during the immune response. The functions of other proteasome accessory complexes, such as PA200 and PI31 are still under investigation.

Alteration of 20S proteasome-subtypes and proteasome activator PA28 in skeletal muscle of rat after induction of diabetes mellitus

Insulin-dependent diabetes mellitus is known to go along with enhanced muscle protein breakdown. Since evidence has been presented that the ubiquitin-proteasome system is significantly involved in muscle wasting under this condition, we have investigated, whether this biological role goes along with alterations of the proteasome system in skeletal muscle of streptozotocin-diabetic rats. Previously, we have found a drop of overall proteasome activity in muscle extracts of rats after induction of diabetes but no change in total amount of 20S proteasome was detected. In the present investigation under the same diabetic conditions we have measured a significant decrease in the amount of proteasome activator PA28, a finding that explains the loss of total proteasome activity. Since increased mRNA levels of proteasome subunits have been measured in muscle tissue of rats after induction of diabetes, we have isolated and purified 20S proteasomes from muscle tissue of control and 6 days diabetic rats. The specific chymotrypsin-like, trypsin-like, and peptidylglutamylpeptide-hydrolysing activities of proteasomes from diabetic and control rats were found to be not significantly different. Therefore, we have fractionated 20S proteasomes into their subtypes and detected that induction of diabetes mellitus effects a redistribution of subtypes of all three proteasome populations but only the increase in subtype V (immuno-subtype) was statistically significant. This altered subtype pattern obviously meets the requirements to the system under wasting conditions. Since this process goes along with de novo biogenesis of 20S proteasomes, it most likely explains the phenomenon of elevated mRNA concentrations of proteasome subunits after induction of diabetes mellitus.

Reconstitution of hybrid proteasomes from purified PA700-20 S complexes and PA28alphabeta activator: ultrastructure and peptidase activities

The activity of the proteasome, the major non-lysosomal proteinase in eukaryotes, is stimulated by two activator complexes, PA700 and PA28. PA700-20 S-PA700 proteasome complexes, generally designated as 26 S proteasomes, degrade proteins, whereas complexes of the type PA28-20 S-PA28 degrade only peptides. We report, for the first time, the in vitro reconstitution of previously identified hybrid proteasomes (PA700-20 S-PA28) from purified PA700-20 S proteasome complexes and PA28 activator. In electron micrographs, the hybrid appears as a corkscrew-shaped particle with a PA700 and a PA28 activator each bound to a terminal alpha-disk of the 20 S core proteasome. The multiple peptidase activities of hybrid proteasomes are not different from those of PA28-20 S-PA28 or PA700-20 S-PA700 complexes.

Subtypes of 20S proteasomes from skeletal muscle

20S proteasomes from tissues and cells are a mixture of several subtypes. From rat skeletal muscle we have tentatively separated six different subtypes of 20S proteasomes purified from rat skeletal muscle by high-resolution anion exchange chromatography. Immunoblot analysis using antibodies to the beta-subunits LMP2, LMP7 and their constitutive counterparts delta and MB1 revealed that two of the three major subtypes (subtypes I and II) are constitutive proteasomes, whereas two of the three minor subtypes belong to the subpopulation of immuno-proteasomes. Subtype III and IV are intermediate-type proteasomes. Enzymological characterisation of the six subtypes revealed clearly different V(max) values for hydrolysis of fluorogenic peptide substrates as well as significantly different activities measured with a 25-mer polypeptide of the murine cytomegalovirus IE pp89 protein as substrate. Our data show that the properties of 20S proteasomes isolated from a given tissue or cells are always the average of the properties of the whole set of proteasome subtypes.

Different proteasome subtypes in a single tissue exhibit different enzymatic properties

It is concluded from many experiments that mammalian tissues and cells must contain a heterogeneous population of 20 S proteasome complexes. We describe the purification and separation by chromatographic procedures of constitutive 20 S proteasomes, 20 S immuno-proteasomes and intermediate-type 20 S proteasomes from a given tissue. Our data demonstrate that each of these three groups comprises more than one subtype and that the relative ratios of the subtypes differ between different rat tissues. Thus, six subtypes could be identified in rat muscle tissue. Subtypes I and II are constitutive proteasomes, while subtypes V and VI comprise immuno-proteasomes. Subtypes III and IV belong to a group of intermediate-type proteasomes. The subtypes differ with regard to their enzymatic characteristics. Subtypes I-III exhibit high chymotrypsin-like activity and high peptidylglutamylpeptide hydrolysing activity, while these activities are depressed in subtypes IV-VI. In contrast, trypsin-like activity of subtypes IV-VI is enhanced in comparison to subtypes I-III. Importantly, the subtypes also differ in their preferential cleavage site usage when tested by digestion of a synthetic 25mer polypeptide substrate. Therefore, the characteristics of proteasomes purified from tissues or cells represent the average of the different subtype activities which in turn may have different functions in vivo.

The base of the proteasome regulatory particle exhibits chaperone-like activity

Protein substrates of the proteasome must apparently be unfolded and translocated through a narrow channel to gain access to the proteolytic active sites of the enzyme. Protein folding in vivo is mediated by molecular chaperones. Here, to test for chaperone activity of the proteasome, we assay the reactivation of denatured citrate synthase. Both human and yeast proteasomes stimulate the recovery of the native structure of citrate synthase. We map this chaperone-like activity to the base of the regulatory particle of the proteasome, that is, to the ATPase-containing assembly located at the substrate-entry ports of the channel. Denatured but not native citrate synthase is bound by the base complex. Ubiquitination of citrate synthase is not required for its binding or refolding by the base complex of the proteasome. These data suggest a model in which ubiquitin-protein conjugates are initially tethered to the proteasome by specific recognition of their ubiquitin chains; this step is followed by a nonspecific interaction between the base and the target protein, which promotes substrate unfolding and translocation.

Alterations of proteasome activities in skeletal muscle tissue of diabetic rats

During the last years many investigations have shown that a major catalyst within the mechanism of skeletal muscle wasting occurring under conditions like sepsis, injuries, trauma, cancer cachexia, chronic acidosis, fasting, glucocorticoid treatment, and insulinopenia is the ubiquitin-proteasome system. Evidence for this was obtained by findings that the rate of ATP-dependent protein degradation is increased, that m-RNA concentrations of several proteasome subunits and ubiquitin are increased and the amount of ubiquitin-protein conjugates is elevated under these conditions. Additionally, the enhanced protein breakdown was shown to be suppressed by proteasome inhibitors. In the present report we show that most but not all of the proteolytic activities of partially purified 20S/26S proteasomes from skeletal muscle of rats increase after induction of Diabetes mellitus. This finding suggests that part of the mechanism of acceleration of muscle protein breakdown is due to changes in proteasome activities.

Identical subunit topographies of human and yeast 20S proteasomes

The arrangement of subunits in human 20S proteasomes was recently determined by us by immunoelectron microscopy and chemical cross-linking. The positions of 4 of the 14 subunits differed from those found in the yeast proteasome by X-ray crystallography. Double labeling of human 20S proteasomes with antibodies to subunits C2 and C5 has now shown that these subunits are nearest neighbors. The result contradicts our published model for the human proteasome but is in accordance with the subunit arrangement in yeast proteasomes, suggesting that yeast and human proteasomes most probably have identical subunit arrangements. Immunoelectron microscopy also showed that the C-terminal extension at the human C2 subunit is flexible but takes up a well-defined position in the proteasome.

Involvement of proteasomal subunits zeta and iota in RNA degradation

We have identified two distinct subunits of 20 S proteasomes that are associated with RNase activity. Proteasome subunits zeta and iota, eluted from two-dimensional Western blots, hydrolysed tobacco mosaic virus RNA, whereas none of the other subunits degraded this substrate under the same conditions. Additionally, proteasomes were dissociated by 6 M urea, and subunit zeta, containing the highest RNase activity, was isolated by anion-exchange chromatography and gel filtration. Purified subunit zeta migrated as a single spot on two-dimensional PAGE with a molecular mass of approx. 28 kDa. Addition of anti-(subunit zeta) antibodies led to the co-precipitation of this proteasome subunit and nuclease activity. This is the first evidence that proteasomal alpha-type subunits are associated with an enzymic activity, and our results provide further evidence that proteasomes may be involved in cellular RNA metabolism.

Subunit arrangement in the human 20S proteasome

In human 20S proteasomes two copies of each of seven different alpha-type and seven different beta-type subunits are assembled to form a stack of four seven-membered rings, giving the general structure alpha(1-7), beta(1-7), beta(1-7), alpha(1-7). By means of immunoelectron microscopy and chemical crosslinking of neighboring subunits, we have determined the positions of the individual subunits in the proteasome. The topography shows that for the trypsin-like, the chymotrypsin-like, and the postglutamyl cleaving activities, the pairs of beta type subunits, which are thought to form active sites, are nearest neighbors.

Screening for molecules interacting with proteasomes in Thermoplasma acidophilum

Thermoplasma acidophilum cell extracts were fractionated by gel filtration. Proteasomes were eluted as two major peaks. The first one (molecular mass(r) about 2 MDa) contained proteasomes associated with DNA/protein complexes. Proteasomes eluted in the other peak were partially resolved into three subpeaks and based on their preferential hydrolysis of casein, Z-GGL-MCA, and suc-LLVY-MCA, were designated C, L and Y, respectively. Further purification of proteasomes from peak Y resulted in a homogenous enzyme preparation, whereas proteasomes purified from peak C contained a homomultimeric protein composed of 20 kDa subunits. Thus, association of proteasomes with this protein seems to be responsible for the observed increase in molecular mass and for inhibition of caseinolytic activity by Ca2+-ions.

Structural and functional properties of proteasome activator PA28

The proteasome activator PA28 or 11S regulator is a protein complex composed of two different but homologous polypeptides, termed PA28alpha and PA28beta. The purified activator protein (approximately 200 kDa) is a ring-shaped heteromultimer containing the two polypeptides, possibly with an (alpha3beta3 stoichiometry. The activator, which by itself shows no hydrolytic activity elicits activation of the proteasome's multiple peptidase activities by binding to the terminal rings of the proteinase. In vitro, active PA28 can be reconstituted from isolated alpha and beta subunits, yielding two different oligomers: with the single alpha subunit, PA28alpha homomultimers with moderate stimulatory activity toward 20S proteasomes are obtained whereas isolated beta-subunits are unable to form oligomers and are devoid of stimulatory activity. However, in the presence of both subunits, alphabeta heteromultimers form, concomitant with restoration of full stimulatory activity. The recent finding that PA28 modulates the proteasome-catalyzed production of antigenic peptides presented to the immune system on MHC class I molecules indicates a cellular function of the activator in antigen processing.

Reconstitution of proteasome activator PA28 from isolated subunits: optimal activity is associated with an alpha,beta-heteromultimer

PA28, a 200 kDa activator of 20S proteasomes, was purified from human placenta and was gel electrophoretically resolved into two different subunits, alpha and beta. In reconstitution experiments, alpha-subunits alone were found to re-associate forming homooligomers with an M(r) of about 200 kDa, which elicit a stimulatory effect on proteasomal peptide-hydrolyzing activity, albeit at a moderate level. Under the same conditions, isolated beta-subunits were neither found to associate nor did they display stimulatory activity. Significantly, when both alpha- and beta-subunits were present in the reconstitution assay, heteromultimers formed, concomitant with a marked increase in stimulatory activity when compared with that of alpha-homooligomers. The reconstituted PA28alpha,beta protein is indistinguishable from purified PA28 by several criteria: it displays the same molecular mass, shows the same abundance of alpha- and beta-subunits and has a similar stimulatory activity toward 20S proteasomes. These results indicate that optimal PA28 activity is associated with a heteromultimeric structure which contains the alpha- and beta-subunits in fixed stoichiometry, most likely as an alpha3beta3-heterohexamer.

Phosphoamino acids in proteasome subunits

Proteasomes, the major catalysts of the non-lysosomal proteolytic pathway in eukaryotic cells, were analyzed for their content of phosphoamino acids using polyacrylamide gel electrophoresis and subsequent detection on Western blots by phosphoamino acid antibodies. No specific binding to proteasome subunits was observed with phosphoserine or phosphothreonine antibodies, whereas phosphotyrosine antibodies were bound by a single proteasome subunit, which was identified in rat as well as in human proteasomes as subunit C7-1. Since dephosphorylation of the subunit by phosphatases was not possible, analysis of phosphoamino acid content of all proteasome subunits was performed using another method. All proteasome subunits were isolated from 2D-polyacrylamide gels and subjected to partial acid hydrolysis. Phosphoamino acids were subsequently detected by capillary electrophoresis after their derivatization with phenylisothiocyanate. This analysis revealed no phosphorylated amino acid in subunit C7-1, however, subunit C3 contained phosphotyrosine and phosphothreonine, and phosphoserine was detected in subunits zeta, C5, C8 and C9.

Proteasome activator PA28 and its interaction with 20 S proteasomes

An activator of the 20 S proteasome has been purified to apparent homogeneity from rabbit erythrocytes, liver, and skeletal muscle. The activator displays an M(r) of about 200,000 upon sizing chromatography and, as judged by gel electrophoresis under denaturing conditions, is composed of two species of subunit of about equal abundance and with M(r) of 31 and 29 kDa. Upon isoelectric focusing, the activator is resolved into two major bands with pI values in the range of pH 5.1 and 5.5, corresponding to the two subunits. Limited proteolytic cleavage with trypsin results, for each subunit, in a distinct fragmentation pattern, indicating that in the rabbit, the native activator molecule occurs either as two homomultimers or as heteromultimers. The activator shows no hydrolytic activity by itself. However, when combined with proteasomes, it enhances, in a dose-related manner, the distinct peptidase activities of the proteinase. The activation process requires binding of the activator protein to the proteinase. This association, however, is reversible with recovery of active proteinase and activator protein. In vitro experiments suggest that, in vivo, the activator is bound to 20 S proteasomes rather than occurring as the free molecule.

Studies on the activation by ATP of the 26 S proteasome complex from rat skeletal muscle

The 26 S proteasome complex is thought to catalyse the breakdown of ubiquitinated proteins within eukaryotic cells. In addition it has been found that the complex also degrades short-lived proteins such as ornithine decarboxylase in a ubiquitin-independent manner. Both proteolytic processes are paralleled by the hydrolysis of ATP. Here we show that ATP also affects the hydrolytic activity towards fluorigenic peptide substrates by the 26 S proteasome complex from rat skeletal muscle tissue. Low concentrations of ATP (about 25 microM) optimally activate the so-called chymotryptic and tryptic activity by increasing the rate of peptide hydrolysis but not peptidylglutamylpeptide hydrolysis. Activation of the enzyme by ATP is transient but this effect can be enhanced and prolonged by including in the assay an ATP-regenerating system, indicating that ATP is hydrolysed by the 26 S proteasome complex. Although ATP cannot be substituted for by adenosine 5'-[beta,gamma-methylene]triphosphate or AMP, hydrolysis of the phosphoanhydride bond of ATP seems not to be necessary for the activation process of the proteasome complex, a conclusion drawn from the findings that ATP analogues such as adenosine 5'-[beta,gamma-imido]triphosphate, adenosine 5'-O-[gamma-thio]triphosphate, adenosine 5'-O-[beta-thio]-diphosphate and adenosine 5'-[alpha,beta-methylene]triphosphate give the same effect as ATP, and vanadate does not prevent ATP activation. These effects are independent of the presence of Mg2+. Thus, ATP and other nucleotides may act as allosteric activators of peptide-hydrolysing activities of the 26 S proteasome complex as has also been found with the lon protease from Escherichia coli.

The human proteasome subunit HsN3 is located in the inner rings of the complex dimer

Subunit HsN3 of the human proteasome is a beta-type subunit homologous to PRE4 from yeast, X1 beta from Xenopus and RN3 from the rat. Using electron microscopy, the binding sites of a monoclonal antibody with specificity for subunit HsN3 have been located in the two juxtaposed inner rings of the human proteasome. Subunit HsN3 was present in two copies, one in each ring, in accordance with our concept of two identical halves making up the complete human proteasome. The subunit is involved in the trypsin-like as well as the peptidylglutamyl-peptide cleavage activities.

The 20S/26S proteasomal pathway of protein degradation in muscle tissue

Similar to all other eukaryotic cells and tissues muscle tissue contains the proteolytic system of 20S/26S proteasomes with the 20S proteasome existing predominantly in a latent state. Unlike with the mammalian enzyme in vitro transition from the latent to the activated state of the 20S proteasomes isolated from muscle of several fish species and from lobster can be achieved by heat shock. It is very likely that the activated state of the 20S proteasome corresponds to the physiologically active form of the enzyme since only that one is able to attack sarcoplasmic and myofibrillar proteins to any significant extent. As perfusion of rat hindquarters with presumptive low molecular mass activators like free fatty acids does not result in an activation of the muscle proteasome other--possibly protein activators--may serve this purpose in vivo. The 26S proteasome complex may be regarded as such a proteasome/activator complex. The 26S proteasome complex has the ability to degrade protein (-ubiquitin-conjugates) by an ATP-consuming reaction. Since increased amounts of ubiquitinated proteins as well as an enhanced activity of the ATP (-ubiquitin)-dependent proteolytic system have been measured in rat muscle tissue during various catabolic conditions, it is not unlikely that this pathway is responsible for catalysis of muscle protein breakdown.

Evidence indicating that the human proteasome is a complex dimer

Using monoclonal antibodies and electron microscopy, the relative positions of two subunit species (32 kDa, pI 6.8; 28.4 kDa, pI 7.9) have been determined on the proteasome (multicatalytic proteinase). From the epitope occupancy, it appears that both subunits occur twice in a proteasome: once in each of the two terminal disks that close off the barrel-like particle. The result favors a model of a complex dimer, and is discussed in the light of the architectural concept that has emerged from our recent immunoelectron microscopic studies of the less complex archaebacterial proteasome.

In vitro activation of the 20S proteasome

The effect of chemical compounds like sodium dodecyl sulfate (SDS), fatty acid esters of glycerol, carnitine and coenzyme A, phospholipids, histones, polylysines as well as homobifunctional chemical cross-linkers on the various proteolytic activities of mammalian proteasomes have been tested. Most of the reagents enhance these activities, and some, e.g. fatty acid CoA esters, histones and the chemical cross-linkers, exert dual effects, i.e. activation and inhibition at the same time, depending on the activity measured. With optimally activating concentrations of SDS, no structural changes in proteasomes can be detected by electron microscopy. Formation of micelles at supra-optimal detergent concentrations may be a reason for irreversible denaturation of the proteasome.

Biochemical properties of the proteasome from Thermoplasma acidophilum

We have purified proteasomes to apparent homogeneity from the archaebacterium Thermoplasma acidophilum. This proteinase has a molecular mass of about 650 kDa and an isoelectric point of 5.6. The proteasome hydrolyses peptide substrates containing an aromatic residue adjacent to the reporter group, as well as [14C]methylated casein optimally at pH 8.5 and 90 degrees C. The enzyme activity is enhanced severalfold by Mg2+ and Ca2+ at 25-500 mM. This increase in activity results primarily from a change in Km. The serine-proteinase inhibitors diisopropylfluorophosphate and 3,4-dichloroisocoumarin irreversibly inhibit the enzyme, obviously by modification of both the alpha and beta subunits in the proteasome. The inhibition of proteasomal activity by the peptidylchloromethanes, Cbz-Leu-Leu-CH2Cl and Cbz-Ala-Ala-Phe-CH2Cl (Cbz, benzyloxycarbonyl), is reversible and predominantly of a competitive type. The enzyme is not activated by any of the compounds that typically stimulate the activities of the eukaryotic proteasome.

Localization of a sequence motif complementary to the nuclear localization signal in proteasomes from Thermoplasma acidophilum by immunoelectron microscopy

A sequence motif complementary to the nuclear localization signal (NLS) has been localized in proteasomes from Thermoplasma acidophilum by immunoelectron microscopy using sequence-specific antibodies. The antibodies were generated in two different ways: by immunization with a carrier-coupled peptide and by isolation of the sequence-specific antibody from an immune serum against native proteasomes using a peptide-affinity column. The sequence specificity of the isolated antibody was confirmed by a PEPSCAN-ELISA performed on overlapping nonapeptides deduced from the sequence of the alpha-subunit of the Thermoplasma proteasome. Compared to the antibody induced by the carrier-coupled peptide this antibody fraction showed a much higher affinity for native proteasomes. The attachment site of the Fab portion of the antibody to the proteasome was mapped by electron microscopy in conjunction with image processing. The antibody was found to bind to the periphery of the two outer "disks" of the proteasome complex formed by the alpha-subunits.

Evidence indicating that the multicatalytic proteinase of rabbit reticulocytes is not incorporated as a core enzyme into a 26 S proteinase complex

We have reinvestigated the recent proposal that the multicatalytic proteinase, together with other components of reticulocyte lysate, may become incorporated into a very large, "26 S" proteinase complex via an ATP-dependent process. Different from these published results, we consistently isolate the multicatalytic proteinase as a 650,000 Da "20 S" multisubunit proteinase. Analysis on nondenaturing polyacrylamide gels of reticulocyte fractions containing the putative complexed form of the multicatalytic proteinase reveal that activity against succinyl-Leu-Leu-Val-Tyr-7-amino-4-methylcoumarin is associated with two groups of protein of different molecular mass. One migrates like multicatalytic proteinase purified to homogeneity, displays, on sodium dodecyl sulfate gels, a set of protein species in the range of 23,000-32,000 Da, characteristic of the multicatalytic proteinase, and is recognized by a monospecific antibody to the enzyme. In contrast, the activity associated with the higher molecular mass (26 S) proteinase complex lacks the typical multicatalytic proteinase subunits and is devoid of antigenic material, when tested with the antibody. These results confirm and extend our recent findings in mouse liver by showing that the multicatalytic proteinase is not a constituent of a 26 S proteinase complex.

Primary structure of the Thermoplasma proteasome and its implications for the structure, function, and evolution of the multicatalytic proteinase

The proteasome or multicatalytic proteinase is a high molecular mass multisubunit complex ubiquitous in eukaryotes but also found in the archaebacterial proteasome is made of two different subunits only, and yet the complexes are almost identical in size and shape. Cloning and sequencing the gene encoding the small (beta) subunit of the T. acidophilum complex completes the primary structure of the archaebacterial proteasome. The similarity of the derived amino acid sequences of 233 (alpha) and 211 (beta) residues, respectively, indicates that they arose from a common ancestral gene. All the sequences of proteasomal subunits from eukaryotes available to date can be related to either the alpha-subunit or beta-subunit of the T. acidophilum "Urproteasome", and they can be distinguished by means of a highly conserved N-terminal extension, which is characteristic for alpha-type subunits. On the basis of circumstantial evidence we suggest that the alpha-subunits have regulatory and targeting functions, while the beta-subunits carry the active sites.