The latent form of macropain (high molecular weight multicatalytic protease) restores ATP-dependent proteolysis to soluble extracts of BHK fibroblasts pretreated with anti-macropain antibodies

Ηigh-resolution structure of mammalian PI31-20S proteasome complex reveals mechanism of proteasome inhibition

Proteasome-catalyzed protein degradation mediates and regulates critical aspects of many cellular functions and is an important element of proteostasis in health and disease. Proteasome function is determined in part by the types of proteasome holoenzymes formed between the 20S core particle that catalyzes peptide bond hydrolysis and any of multiple regulatory proteins to which it binds. One of these regulators, PI31, was previously identified as an in vitro 20S proteasome inhibitor, but neither the molecular mechanism nor the possible physiologic significance of PI31-mediated proteasome inhibition has been clear. Here we report a high-resolution cryo-EM structure of the mammalian 20S proteasome in complex with PI31. The structure shows that two copies of the intrinsically disordered carboxyl terminus of PI31 are present in the central cavity of the closed-gate conformation of the proteasome and interact with proteasome catalytic sites in a manner that blocks proteolysis of substrates but resists their own degradation. The two inhibitory polypeptide chains appear to originate from PI31 monomers that enter the catalytic chamber from opposite ends of the 20S cylinder. We present evidence that PI31 can inhibit proteasome activity in mammalian cells and may serve regulatory functions for the control of cellular proteostasis.

Structural Insights into Substrate Recognition and Processing by the 20S Proteasome

Four decades of proteasome research have yielded extensive information on ubiquitin-dependent proteolysis. The archetype of proteasomes is a 20S barrel-shaped complex that does not rely on ubiquitin as a degradation signal but can degrade substrates with a considerable unstructured stretch. Since roughly half of all proteasomes in most eukaryotic cells are free 20S complexes, ubiquitin-independent protein degradation may coexist with ubiquitin-dependent degradation by the highly regulated 26S proteasome. This article reviews recent advances in our understanding of the biochemical and structural features that underlie the proteolytic mechanism of 20S proteasomes. The two outer α-rings of 20S proteasomes provide a number of potential docking sites for loosely folded polypeptides. The binding of a substrate can induce asymmetric conformational changes, trigger gate opening, and initiate its own degradation through a protease-driven translocation mechanism. Consequently, the substrate translocates through two additional narrow apertures augmented by the β-catalytic active sites. The overall pulling force through the two annuli results in a protease-like unfolding of the substrate and subsequent proteolysis in the catalytic chamber. Although both proteasomes contain identical β-catalytic active sites, the differential translocation mechanisms yield distinct peptide products. Nonoverlapping substrate repertoires and product outcomes rationalize cohabitation of both proteasome complexes in cells.

Distinct specificities of Mycobacterium tuberculosis and mammalian proteasomes for N-acetyl tripeptide substrates

The proteasome of Mycobacterium tuberculosis (Mtb) is a validated and drug-treatable target for therapeutics. To lay ground-work for developing peptide-based inhibitors with a useful degree of selectivity for the Mtb proteasome over those of the host, we used a library of 5,920 N-acetyl tripeptide-aminomethylcoumarins to contrast the substrate preferences of the recombinant Mtb proteasome wild type and open gate mutant, the Rhodococcus erythropolis proteasome, and the bovine proteasome with activator PA28. The Mtb proteasome was distinctive in strictly preferring P1 = tryptophan, particularly in combination with P3 = glycine, proline, lysine or arginine. Screening results were validated with Michalis-Menten kinetic analyses of 21 oligopeptide aminomethyl-coumarin substrates. Bortezomib, a proteasome inhibitor in clinical use, and 17 analogs varying only at P1 were used to examine the differential impact of inhibitors on human and Mtb proteasomes. The results with the inhibitor panel confirmed those with the substrate panel in demonstrating differential preferences of Mtb and mammalian proteasomes at the P1 amino acid. Changing P1 in bortezomib from Leu to m-CF(3)-Phe led to a 220-fold increase in IC(50) against the human proteasome, whereas changing a P1 Ala to m-F-Phe decreased the IC(50) 400-fold against the Mtb proteasome. The change of a P1 Ala to m-Cl-Phe led to an 8000-fold shift in inhibitory potency in favor of the Mtb proteasome, resulting in 8-fold selectivity. Combinations of preferred amino acids at different sites may thus improve the species selectivity of peptide-based inhibitors that target the Mtb proteasome.

A precipitating role for truncated alpha-synuclein and the proteasome in alpha-synuclein aggregation: implications for pathogenesis of Parkinson disease

Parkinson disease and other alpha-synucleinopathies are characterized by the deposition of intraneuronal alpha-synuclein (alphaSyn) inclusions. A significant fraction (about 15%) of alphaSyn in these pathological structures are truncated forms that have a much higher propensity than the full-length alphaSyn to form aggregates in vitro. However, little is known about the role of truncated alphaSyn species in pathogenesis or the means by which they are generated. Here, we have provided an in vitro mechanistic study demonstrating that truncated alphaSyns induce rapid aggregation of full-length protein at substoichiometric ratios. Co-overexpression of truncated alphaSyn with full-length protein increases cell vulnerability to oxidative stress in dopaminergic SH-SY5Y cells. These results suggest a precipitating role for truncated alphaSyn in the pathogenesis of diseases involving alphaSyn aggregation. In this regard, the A53T mutation found in some cases of familial Parkinson disease exacerbates the accumulation of insoluble alphaSyns that correlates with the onset of pathology in transgenic mice expressing human alphaSyn-A53T mutant. The caspase-like activity of the 20 S proteasome produces truncated fragments similar to those found in patients and animal models from degradation of unstructured alphaSyn. We propose a model in which incomplete degradation of alphaSyn, especially under overloaded proteasome capacity, produces highly amyloidogenic fragments that rapidly induce the aggregation of full-length protein. These aggregates in turn reduce proteasome activity, leading to further accumulation of fragmented and full-length alphaSyns, creating a vicious cycle of cytotoxicity. This model has parallels in other neurodegenerative diseases, such as Huntington disease, where coaggregation of poly(Q) fragments with full-length protein has been observed.

Effect of temperature and pressure on the proteolytic specificity of the recombinant 20S proteasome from Methanococcus jannaschii

The hydrolytic specificity of the recombinant 20S proteasome from the deep-sea thermophile Methanococcus jannaschii was evaluated toward oxidized insulin B-chain across a range of temperatures (35 degrees, 55 degrees, 75 degrees, and 90 degrees C) and hydrostatic pressures (1, 250, 500, and 1,000 atm). Of the four temperatures considered, the same maximum overall hydrolysis rate was observed at both 55 degrees and 75 degrees C, which are much lower than the T(opt) of 116 degrees C previously observed for a small amide substrate (Michels and Clark 1997). At 35 degrees C the rates of cleavage were highest at the carboxyl side of glutamine and leucine, whereas at the three higher temperatures, the most rapid cleavages occurred after leucine and glutamic acid residues. The distribution of proteolytic fragments and the cleavage sequence also varied between the lowest and higher temperatures. Application of hydrostatic pressure did not increase proteasome activity, as observed previously for the amide substrate (Michels and Clark 1997), but instead significantly reduced the overall conversion of the polypeptide substrate. Overall cleavage patterns observed for the recombinant M. jannaschii proteasome were similar to those reported previously for Thermoplasma acidophilum (Akopian et al. 1997) and human proteasomes (Dick et al. 1991), indicating that proteasome specificity has been conserved despite significant environmental diversity.

Structural and functional effects of PA700 and modulator protein on proteasomes

Control and targeting of the proteolytic activity of the major intracellular protease, the proteasome, is accomplished by various regulatory protein complexes that may form higher-order assemblies with the proteasome. An activator of proteolytic activity, PA700, has been shown to have an ATP-dependent stimulatory effect on the peptidase activities of the proteasome, and another protein factor, the modulator, further enhances the effect of PA700. Here we show that the addition of PA700 endows the proteasome with the ability to cleave ubiquitinated proteins, a property associated with the previously isolated 26 S form of the proteasome. The modulator further stimulates this specific activity, without having any such effect on the proteasome alone. Using electron microscopy, we show that addition of PA700 causes the appearance of protein "caps" at one or both ends of proteasomes, forming structures that are indistinguishable from 26 S proteasomes. Quantitation of the numbers of uncapped, singly capped and doubly capped complexes indicates cooperativity in the association of PA700 with the two ends of the proteasome. Addition of modulator protein makes no further structural modification that is detectable by electron microscopy, but does cause an increase in the number of capped complexes visible at subsaturating concentrations of PA700. Hence PA700 converts the proteasome both functionally and structurally to the 26 S form, and the modulator promotes this transformation, apparently without stable association with the resulting complex.

Identification, purification, and characterization of a PA700-dependent activator of the proteasome

The activity of the intracellular protease, the proteasome, is modulated by a number of specific regulatory proteins. One such regulator, PA700, is a 700,000-Da multisubunit protein that activates hydrolytic activities of the proteasome via a mechanism that involves the ATP-dependent formation of a proteasome-PA700 complex. Four subunits of PA700 have been shown previously to be members of a protein family that contains a consensus sequence for ATP binding, and purified PA700 expresses ATPase activity. We report here the identification, purification, and initial characterization of a new modulator of the proteasome. The modulator has no direct effect on the activity of the proteasome, but enhances PA700 activation of the proteasome by up to 8-fold. This activation is associated with the formation of a proteasome/PA700-containing complex that is significantly larger than that formed in its absence. The modulator has a native Mr of approximately 300,000, as determined by gel filtration chromatography, and is composed of three electrophoretically distinct subunits with Mr values of 50,000, 42,000, and 27,000 (p50, p42, and p27, respectively). Amino acid sequence analysis of the subunits shows that p50 and p42 are members of the same ATP-binding protein family found in PA700. The p50 subunit is identical to TBP1, a protein previously reported to interact with human immunodeficiency virus Tat protein (Nelbock, P., Dillion, P. J., Perkins, A., and Rosen, C. A. (1990) Science 248, 1650-1653), while the p42 subunit seems to be a new member of the family. The p27 subunit has no significant sequence similarity to any previously described protein. Both p50 and p42, but not p27, were also identified as components of PA700, increasing the number of ATP-binding protein family members in this complex to six. Thus, p50 and p42 are subunits common to two protein complexes that regulate the proteasome. The PA700-dependent proteasome activator represents a new member of a growing list of proteins that regulate proteasome activity.

Metabolic adaptations supporting anoxia tolerance in reptiles: recent advances

Animal survival during severe hypoxia and/or anoxia is enhanced by a variety of biochemical adaptations including adaptations of fermentative pathways of energy production and, most importantly, the ability to sharply reduce metabolic rate by 5-20 fold and enter a hypometabolic state. The biochemical regulation of metabolic arrest is proving to have common molecular principles that extend across phylogenetic lines and that are conserved in different types of arrested states (not only anaerobiosis but also estivation, hibernation, etc.). Our new studies with anoxia-tolerant vertebrates have identified a variety of regulatory mechanisms involved in both metabolic rate depression and in the aerobic recovery process using as models the freshwater turtle Trachemys scripta elegans and garter snakes Thamnophis sirtalis parietalis. Mechanisms include: 1) post-translational modification of cellular and functional proteins by reversible phosphorylation and changes in protein kinase (PKA, PKC) and/or phosphatase activities to regulate this, 2) reversible enzyme binding associations with subcellular structural elements, 3) differential gene expression and/or mRNA translation producing new mRNA variants and new protein products, 4) changes in protease activity, particularly the multicatalytic proteinase complex, and 5) both constitutive and anoxia-induced modifications to cellular antioxidant systems to deal with oxidative stress during the anoxic-aerobic transition of recovery.

Involvement of the 20S proteasome in the degradation of ornithine decarboxylase

Eukaryotic cells have been shown to contain two high-molecular-mass proteases of 700 kDa and 1400 kDa (20S and 26S proteases, respectively). It has been suggested that the 20S protease, also known as proteasome, may constitute the catalytic core of the 26S protease. While the role of the free 20S protease in intracellular protein degradation is unclear, the 26S protease is implicated in the degradation of ubiquinated proteins. We have recently demonstrated, that ornithine decarboxylase (ODC), one of the most labile proteins in mammalian cells, is degraded via an ATP-dependent but ubiquitin-independent proteolytic pathway. Here we extend these observations by demonstrating that in reticulocyte lysate ODC degradation is inhibited by antibodies raised against the C9 subunit of rat proteasome. Partial fractionation of the lysate demonstrated preferential degradation of ODC in the fraction of the lysate proteins that are precipitated by 38% ammonium sulfate. Since it was demonstrated that the 26S protease precipitates at this concentration of ammonium sulfate while the 20S proteasome remains soluble, our results suggest that the 26S protease is the one degrading ODC.

Regulation by proteolysis: energy-dependent proteases and their targets

A number of critical regulatory proteins in both prokaryotic and eukaryotic cells are subject to rapid, energy-dependent proteolysis. Rapid degradation combined with control over biosynthesis provides a mechanism by which the availability of a protein can be limited both temporally and spatially. Highly unstable regulatory proteins are involved in numerous biological functions, particularly at the commitment steps in developmental pathways and in emergency responses. The proteases involved in energy-dependent proteolysis are large proteins with the ability to use ATP to scan for appropriate targets and degrade complete proteins in a processive manner. These cytoplasmic proteases are also able to degrade many abnormal proteins in the cell.

Demonstration that a human 26S proteolytic complex consists of a proteasome and multiple associated protein components and hydrolyzes ATP and ubiquitin-ligated proteins by closely linked mechanisms

It is known that two types of high-molecular-mass protease complexes are present in the cytosol of mammalian cells; a 20S latent multicatalytic proteinase named the proteasome, and a large proteolytic complex with an apparent sedimentation coefficient of 26S that catalyzes ATP-dependent breakdown of proteins conjugated with ubiquitin. In this work, we first demonstrated that a low concentration of SDS was required for activation of the latent proteasome, whereas the 26S complex degraded substrates for proteasomes in the absence of SDS. Moreover, the 26S complex was greatly stabilized in the presence of 2 mM ATP and 20% glycerol. Based on these characteristics, we next devised a novel procedure for purification of the 26S proteolytic complexes from human kidney. In this procedure, the proteolytic complexes were precipitated from cytoplasmic extracts by ultracentrifugation for 5 h at 105000 x g, and the large 26S complexes were clearly separated from the 20S proteasomes by molecular-sieve chromatography on a Biogel A-1.5 m column. The 26S enzyme was then purified to apparent homogeneity by successive chromatographies on hydroxyapatite and Q Sepharose, then by glycerol density-gradient centrifugation. Electrophoretic and immunochemical analyses showed that the purified human 26S complex consisted of multiple subunits of proteasomes with molecular masses of 21-31 kDa and 13-15 protein components ranging in molecular mass over 35-110 kDa, which were directly associated with the proteasome. The purified 26S proteolytic complex degraded 125I-labeled lysozyme-ubiquitin conjugates in an ATP-dependent manner. The 26S enzyme also showed high ATPase activity, which was copurified with the complex. Vanadate and hemin strongly inhibited not only ATP cleavage, but also ATP-dependent breakdown of ubiquitinligated proteins, suggesting that the 26S complex hydrolyzes ATP and ubiquitinated proteins by closely linked mechanisms. These findings indicate that the 26S complex consists of a proteasome with proteolytic function and multiple other components including an ATPase that regulates energy-dependent, ubiquitin-mediated protein degradation.

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.

Purification and characterization of a protein inhibitor of the 20S proteasome (macropain)

An inhibitory protein for the 20S proteasome (also known as macropain, the multicatalytic proteinase complex and 20S proteinase) has been purified from bovine red blood cells. The inhibitor has an apparent molecular weight of 31,000 on SDS-PAGE and appears to form multimers under nondenaturing conditions. This protein inhibited all three of the putatively distinct catalytic activities of proteasome A (the active form of the proteinase) characterized by the hydrolysis of synthetic peptides such as Z-VLR-MNA, Z-GGL-AMC or Suc-LLVY-AMC and Z-LLE-beta NA. The inhibitor also prevented the hydrolysis of large protein substrates such as casein, lysozyme and bovine serum albumin. Proteasome L (the latent form of the proteinase) does not degrade these large protein substrates, but does hydrolyze the three synthetic peptides at rates similar to those by proteasome A. The inhibitor inhibited only two of these peptidase activities of proteasome L (hydrolysis of Z-GGL-AMC and of Z-LLE-beta NA or Suc-LLVY-AMC); it had no effect on the hydrolysis of Z-VLR-MNA. The inhibitor was specific for inhibition of the proteasome and had no effect on the activity of any other proteinase tested including trypsin, chymotrypsin, papain, subtilisin and both isoforms of calpain. Kinetic analysis indicates that the inhibitor interacted with the proteasome by a mechanism involving tight-binding. Because the proteasome appears to be a key component of the ATP/ubiquitin-dependent pathway of intracellular protein degradation, the inhibitor may represent an important regulatory protein of this pathway.

Isolation and characterization of a novel endogenous inhibitor of the proteasome

A novel endogenous inhibitor of the proteasome (high molecular weight multicatalytic protease) has been isolated and characterized from human erythrocytes. After purification by ion-exchange and sizing chromatography, the inhibitor displayed a native molecular mass of approximately 200 kDa and contained a single subunit of 50 kDa with an isoelectric point of 6.9. Although the inhibitor noncompetitively blocks proteolysis of [methyl-14C]-alpha-casein (Ki = 7.1 x 10(-8) M) and inhibits hydrolysis of Suc-Leu-Leu-Val-Tyr-AMC, it did not affect hydrolysis of other peptide substrates, such as MeOSuc-Phe-Leu-Phe-MNA and Z-Ala-Arg-Arg-MNA. To further characterize the 50-kDa inhibitor, a monoclonal antibody (MI-8) was generated that showed specific binding upon Western blot analysis of both native PAGE and SDS-PAGE. Immunoprecipitation with MI-8 specifically removed inhibitor activity against the proteasome. The 50-kDa inhibitor is distinct from a previously described 40-kDa inhibitor of the proteasome (Murakami, K., & Etlinger, J.D. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 7588-7592) on the basis of lack of cross-reactivity with MI-8 and dissimilar peptide digest patterns. It is suggested that these endogenous inhibitors may have a role in ATP/ubiquitin-dependent proteolysis and/or other cellular functions involving this protease.

Cloning and sequence analysis of pituitary cDNA encoding the beta-subunit of Xenopus proteasome

The proteasome is a multicatalytic proteinase composed of a number of non-identical subunits. A Xenopus pituitary cDNA was isolated and found to code for the beta-subunit of proteasome. The amino acid sequence deduced from the open reading frame consisted of 215 amino acid residues with a calculated molecular weight of 23,979. A comparative structural analysis indicated that the proteasome subunits can be divided into two groups with the same evolutionary origin. One group consists of subunits with an N-terminally blocked residue and includes components C2, C3, C8 and C9, while the second group of non-blocked proteins includes component C5 and the beta-subunit.

The primary structures of four subunits of the human, high-molecular-weight proteinase, macropain (proteasome), are distinct but homologous

Macropain (proteasome) is a high-molecular-weight proteinase complex composed of at least 13 electrophoretically distinct subunits. Previous work, including peptide mapping and limited amino acid sequencing, suggested that most of the subunits belong to an evolutionarily related group of different gene products (Lee et al. (1990) Biochim. Biophys. Acta. 1037, 178-185). In order to define the extent and pattern of subunit relatedness, and to determine the structural basis for possible similarities and differences in subunit functions, we are deducing the primary structures of macropain subunits by cDNA cloning and DNA sequence analysis. We report here the primary structures of four subunits. The data clearly demonstrate that the proteins represent different, but homologous gene products. Surprisingly, no evidence for homology with any other protein, including proteinases, was obtained. These results suggest that macropain is comprised of a previously unidentified family of evolutionarily related polypeptides. Because biochemical data indicate that macropain contains several different proteinase activities, the current results raise the possibility that the macropain complex is composed of a group of novel proteinases, distinct from those of other structurally identifiable proteinase families.

Sodium dodecyl sulfate and heat induce two distinct forms of lobster muscle multicatalytic proteinase: the heat-activated form degrades myofibrillar proteins

A multicatalytic proteinase (MCP) purified from lobster claw and abdominal muscles degrades a variety of peptide and protein substrates. The enzyme is activated by low concentrations (0.03%) of sodium dodecyl sulfate (SDS) and brief (1 min) heating at 60 degrees C. The lobster MCP can assume three stable and functionally distinct states in vitro; these are classified as the basal, heat-activated, and SDS-activated forms. The basal MCP possessed high trypsin-like peptidase activity and low chymotrypsin-like peptidase, peptidylglutamyl-peptide hydrolase, and caseinolytic activities; incubation of the basal form with SDS stimulated the peptidylglutamyl-hydrolase activity about 30-fold and inhibited the other three activities 80% to 100%. Heating the basal form stimulated caseinolytic activity about 6-fold with little effect on the peptidase activities. The heat-activated enzyme also degraded myosin, tropomyosin, troponin, and actin depolymerizing factor; alpha-actinin was resistant to proteolysis. Incubation of the heat-activated MCP with SDS inhibited the trypsin-like, chymotrypsin-like, and proteinase activities 95 to 100% and stimulated the peptidylglutamyl-hydrolase activity about 16-fold. Incubation of myosin with either the basal or the heat-activated forms in the presence of SDS generated identical proteolytic fragments of the myosin heavy chain, suggesting that SDS induced a third form that can be produced from either the basal or the heat-activated forms. The heat-activated form produced proteolytic fragments of myosin heavy chain different from those generated by either basal or heat-activated enzymes in the presence of SDS. Furthermore, 100 mM KCl stimulated the caseinolytic activity of the heat-activated form 24% and inhibited the trypsin-like and peptidylglutamyl-hydrolase activities 56 and 20%, respectively. These results, though indirect, suggest that heating induced a proteinase activity that was distinct from the three peptidase activities. Activation of the basal form with SDS was reversible, since precipitation of dodecyl sulfate with 100 mM KCl restored trypsin-like activity and inhibited peptidylglutamyl-hydrolase activity. In contrast, removal of dodecyl sulfate from the SDS-activated form that was derived from the heat-activated MCP induced its conversion to the basal form. Thus, although heat-activation was irreversible, the heat-activated form was converted back to the basal form via the SDS-activated form.

Molecular cloning and sequence analysis of cDNAs for five major subunits of human proteasomes (multi-catalytic proteinase complexes)

Proteasomes are multicatalytic proteinase complexes consisting of a set of non-identical polypeptide components. Of these multiple components, the nucleotide sequences of five major subunits (named HC2, HC3, HC5, HC8 and HC9) of human proteasomes have been determined from recombinant cDNA clones by screening a human HepG2 hepatoblastoma cell cDNA library with rat proteasome cDNAs isolated previously as probes. The polypeptides deduced from their nucleotide sequences consisted of 263, 234, 241, 255 and 261 amino acid residues with calculated molecular weights of 29,554, 25,897, 26,487, 28,431 and 29,482, respectively, which are encoded by single independent genes. The primary structures of these subunits of human proteasomes closely resemble those of their rat counterparts and show considerably high inter-subunit homology, although the homology of HC5 is relatively low. These findings, together with the structural similarities of other eukaryotic proteasomes including those of Drosophila and yeast (Saccharomyces cerevisiae) support and extend the previously proposed concept that eukaryotic proteasome genes form a multi-gene family with the same evolutionary origin.

ATP-stimulated degradation of endogenous proteins in cell-free extracts of BHK 21/C13 fibroblasts. A key role for the proteinase, macropain, in the ubiquitin-dependent degradation of short-lived proteins

Baby hamster kidney (BHK) 21/C13 cell proteins, labeled with [35S]methionine, [14C]leucine or [3H]leucine in intact cells, were degraded in soluble, cell-free extracts by an ATP-stimulated process. The stimulatory effect of ATP appeared to require ATP hydrolysis and was mediated to a large extent by ubiquitin. Although the cell extracts contained endogenous ubiquitin, supplementation with exogenous ubiquitin increased ATP-dependent proteolysis by up to 2-fold. Furthermore, antibodies against the E1 ubiquitin conjugating enzyme specifically inhibited both conjugation of [125I]ubiquitin to endogenous proteins and ATP/ubiquitin-dependent proteolysis. Addition of purified E1 to antibody-treated extracts restored conjugation and proteolysis. Proteins containing the amino acid analogues canavanine and azatryptophan were also degraded in vitro by an ATP/ubiquitin-dependent process but at a rate up to 2-fold faster than normal proteins. These results indicate that soluble, cell-free extracts of BHK cells can selectively degrade proteins whose rates of degradation are increased in intact cells. Treatment of cell-free extracts with antibodies against the high molecular weight proteinase, macropain, also greatly inhibited the ATP/ubiquitin-dependent degradation of endogenous proteins. Proteolysis was specifically restored when purified macropain L was added to the antibody-treated extracts. Treatment of cell extracts with both anti-macropain and anti-E1 antibodies reduced ATP/ubiquitin-dependent proteolysis to the same extent as treatment with either antibody alone. Furthermore, proteolysis could be restored to the double antibody treated extracts only after addition of both purified E1 and macropain. These results provide strong evidence for an important role for macropain in the ATP/ubiquitin-dependent degradation of endogenous proteins in BHK cell extracts.

Interaction of human erythrocyte multicatalytic proteinase with polycations

The multicatalytic proteinase from human erythrocytes (macropain, proteasome) is a large enzyme composed of at least six distinct subunits ranging in molecular masses from 20 to 30 kDa. As its name implies, this proteinase appears to contain multiple catalytic sites with differing specificities toward peptide substrates. Several polycationic substances, including polylysines, polyarginine, protamine and histone H1 markedly stimulated caseinolytic activity of the proteinase. Activation was instantaneous, and involved increasing the Vmax of the proteinase for casein. Prolonged preincubation with polylysine at 37 degrees C resulted in autolytic inactivation of the proteinase. The polylysine concentrations required for half-maximal activation or autolytic inactivation were the same. A 23 kDa subunit of the proteinase disappeared at the same rate as loss of catalytic activity, and with the same pH dependence and polylysine concentration dependence. These results suggest that polylysine perturbs the structure of the multicatalytic proteinase, resulting in increased catalytic activity toward substrates; and, with prolonged exposure, allowing autoproteolytic inactivation to occur. The 23 kDa subunit appeared to be required for expression of caseinolytic activity, and may therefore be a catalytic subunit of the complex having activity against casein.

Relationships among the subunits of the high molecular weight proteinase, macropain (proteasome)

An analysis of the subunits of the high molecular weight proteinase, macropain (multicatalytic proteinase or proteasome) from human erythrocytes has been conducted using N-terminal amino acid sequencing, gel electrophoresis and reverse-phase peptide mapping. This analysis provided evidence for the existence of 13 subunits of different primary structure. Five subunits were susceptible to the Edman degradation and yielded unique N-terminal sequences. Similarities among these sequences, however, indicated that the subunits are homologous. Two-dimensional gel electrophoresis discriminated 10 major components, which included two of the subunits for which N-terminal sequences had been determined and eight N-terminally blocked subunits. Tryptic peptide mapping indicated that all 10 of these components have a different amino acid sequence. Tryptic peptides from some of the subunits were subjected to amino acid sequence analysis, and the data indicated that all the subunits tested in this way are related by common ancestry. The data suggest that at least nine of the total of 13 subunits are encoded by members of the same gene family; the remaining four subunits have not yet been investigated in sufficient detail to establish their relationships. No evidence for a close relationship with any previously investigated proteinase family has been found. Finally, through a comparison of the 'latent' and 'active' forms of macropain, the study established a close similarity in the subunit composition of these catalytically very different species, although proteolytic degradation of selected subunits appears in the active form of the enzyme.