ATP-stimulated proteolysis in soluble extracts of BHK 21/C13 cells. Evidence for multiple pathways and a role for an enzyme related to the high-molecular-weight protease, macropain

PI31 is a positive regulator of 20S immunoproteasome assembly

PI31 (Proteasome Inhibitor of 31,000 Da) is a 20S proteasome-binding protein originally identified as an inhibitor of in vitro 20S proteasome activity. Although recent studies have provided a detailed structural basis for this activity, the physiologic significance of PI31-mediated proteasome inhibition remains uncertain and alternative cellular roles for PI31 have been described. Here we report a role for PI31 as a positive regulator for the assembly of the 20S immuno-proteasome (20Si), a compositionally and functionally distinct isoform of the proteasome that is poorly inhibited by PI31. Genetic ablation of PI31 in mammalian cells had no effect on the cellular content or activity of constitutively expressed proteasomes but reduced the cellular content and activity of interferon-γ-induced immuno-proteasomes. This selective effect is a consequence of defective 20Si assembly, as indicated by the accumulation of 20Si assembly intermediates. Our results highlight a distinction in the assembly pathways of constitutive and immuno-proteasomes and indicate that PI31 plays a chaperone-like role for the selective assembly of 20S immunoproteasomes.

The role of proteasomes in tumorigenesis

Protein homeostasis is the basis of normal life activities, and the proteasome family plays an extremely important function in this process. The proteasome 20S is a concentric circle structure with two α rings and two β rings overlapped. The proteasome 20S can perform both ATP-dependent and non-ATP-dependent ubiquitination proteasome degradation by binding to various subunits (such as 19S, 11S, and 200 PA), which is performed by its active subunit β1, β2, and β5. The proteasome can degrade misfolded, excess proteins to maintain homeostasis. At the same time, it can be utilized by tumors to degrade over-proliferate and unwanted proteins to support their growth. Proteasomes can affect the development of tumors from several aspects including tumor signaling pathways such as NF-κB and p53, cell cycle, immune regulation, and drug resistance. Proteasome-encoding genes have been found to be overexpressed in a variety of tumors, providing a potential novel target for cancer therapy. In addition, proteasome inhibitors such as bortezomib, carfilzomib, and ixazomib have been put into clinical application as the first-line treatment of multiple myeloma. More and more studies have shown that it also has different therapeutic effects in other tumors such as hepatocellular carcinoma, non-small cell lung cancer, glioblastoma, and neuroblastoma. However, proteasome inhibitors are not much effective due to their tolerance and singleness in other tumors. Therefore, further studies on their mechanisms of action and drug interactions are needed to investigate their therapeutic potential.

Ubiquitin-mediated fluorescence complementation reveals that Jun ubiquitinated by Itch/AIP4 is localized to lysosomes

Ubiquitin family peptide modifications regulate the functions and stabilities of many proteins. We have developed an approach for the visualization of ubiquitinated proteins in living cells designated ubiquitin-mediated fluorescence complementation (UbFC). This approach is based on complementation among fragments of fluorescent proteins when they are brought together by the covalent conjugation of ubiquitin fused to one fragment to a substrate protein fused to a complementary fragment. The UbFC strategy enables simultaneous visualization of proteins modified by different ubiquitin family peptides and comparison of their effects on protein localization. Visualization of ubiquitinated Jun revealed that it was localized predominantly to cytoplasmic structures. In contrast, Jun conjugated to small ubiquitin-related modifier 1 (SUMO1) was localized to subnuclear foci. Comparison of the distribution of ubiquitinated Jun with markers for various cytoplasmic compartments revealed that ubiquitinated Jun was localized to lysosomal vesicles. Fractionation of cell lysates confirmed that the majority of ubiquitinated Jun partitioned to the cytoplasmic fraction, and density gradient centrifugation analysis demonstrated that it cosedimented with lysosomal beta-hexosaminidase activity. Mutation of a recognition sequence for the E3 ligase Itch/AIP4 prevented Jun ubiquitination and stabilized it in cells. Inhibition of lysosomal protein degradation by bafilomycin or chloroquine stabilized Jun but had no effect on the stability of mutated Jun that was not ubiquitinated by Itch/AIP4. The visualization of ubiquitinated Jun in living cells has uncovered a lysosomal pathway for Jun degradation that involves ubiquitination by Itch/AIP4.

Ubiquitylation of the transducin betagamma subunit complex. Regulation by phosducin

G proteins (Galphabetagamma) are essential signaling molecules, which dissociate into Galpha and Gbetagamma upon activation by heptahelical membrane receptors. We have identified the betagamma subunit complex of the photoreceptor-specific G protein, transducin (T), as a target of the ubiquitin-proteasome pathway. Ubiquitylated species of the transducin gamma-subunit (Tgamma) but not the alpha- or beta-subunits were assembled de novo in bovine photoreceptor preparations. In addition, Tgamma was exclusively ubiquitylated when Tbetagamma was dissociated from Talpha. Ubiquitylation of Tbetagamma on Tgamma was selectively catalyzed by human ubiquitin-conjugating enzymes UbcH5 and UbcH7 and was coincident with degradation of the entire Tbetagamma subunit complex in vitro by a mechanism requiring ATP and the proteasome. We also show that Tbetagamma association with phosducin, a photoreceptor-specific protein of unknown physiological function, blocks Tbetagamma ubiquitylation and subsequent degradation. Phosphorylation of phosducin by Ca(2+)/calmodulin-dependent protein kinase II, which inhibits phosducin-Tbetagamma complex formation, completely restored Tbetagamma ubiquitylation and degradation. We conclude that Tbetagamma is a substrate of the ubiquitin-proteasome pathway and suggest that phosducin serves to protect Tbetagamma following the light-dependent dissociation of Talphabetagamma.

IGF-I has no effect on postexercise suppression of the ubiquitin-proteasome system in rat skeletal muscle

Both exercise and insulin-like growth factor I (IGF-I) are known to have major hypertrophic effects in skeletal muscle; however, the interactive effect of exogenous IGF-I and exercise on muscle protein turnover or the ubiquitin-proteasome pathway has not been reported. In the present study, we have examined the interaction between endurance exercise training and IGF-I treatment on muscle protein turnover and the ubiquitin-proteasome pathway in the postexercise period. Adult male rats (270-280 g) were randomized to receive 5 consecutive days of progressive treadmill exercise and/or IGF-I treatment (1 mg. kg body wt(-1). day(-1)). Twenty-four hours after the last bout of exercise, the rate of protein breakdown in incubated muscles was significantly reduced compared with that in unexercised rats. This was associated with a significant reduction in the chymotrypsin-like activity of the proteasome and the rate of ubiquitin-proteasome-dependent casein hydrolysis in muscle extracts from exercised compared with unexercised rats. In contrast, the muscle expression of the 20S proteasome subunit beta-1, ubiquitin, and the 14-kDa E2 ubiquitin-conjugating enzyme was not altered by exercise or IGF-I treatment 24 h postexercise. Exercise had no effect on the rates of total mixed muscle protein synthesis in incubated muscles 24 h postexercise. IGF-I treatment had no effect on muscle weights or the rates of protein turnover 24 h after endurance exercise. These results suggest that a suppression of the ubiquitin-proteasome proteolytic pathway after endurance exercise may contribute to the acute postexercise net protein gain.

Interferon gamma regulates accumulation of the proteasome activator PA28 and immunoproteasomes at nuclear PML bodies

PA28 is an interferon (gamma) (IFN(gamma)) inducible proteasome activator required for presentation of certain major histocompatibility (MHC) class I antigens. Under basal conditions in HeLa and Hep2 cells, a portion of nuclear PA28 is concentrated at promyelocytic leukemia oncoprotein (PML)-containing bodies also commonly known as PODs or ND10. IFN(gamma) treatment greatly increased the number and size of the PA28- and PML-containing bodies, and the effect was further enhanced in serum-deprived cells. PML bodies are disrupted in response to certain viral infections and in diseases such as acute promyelocytic leukemia (APL). Like PML, PA28 was delocalized from PML bodies by expression of the cytomegalovirus protein, IE1, and in NB4 cells, an APL model line. Moreover, retinoic acid treatment, which causes remission of APL in patients and reformation of PML-containing bodies in NB4 cells, relocalized PA28 to this site. In contrast, the proteasome, the functional target of PA28, was not detected at PML bodies under basal conditions in HeLa and Hep2 cells, but IFN(gamma) promoted accumulation of ‘immunoproteasomes’ at this site. These results establish PA28 as a novel component of nuclear PML bodies, and suggest that PA28 may assemble or activate immunoproteasomes at this site as part of its role in proteasome-dependent MHC class I antigen presentation.

Increased levels of oxidative stress markers detected in the brains of mice devoid of prion protein

Although minor abnormalities have been reported in prion protein (PrP) knock-out (Prnp-/-) mice, the normal physiological function of PrP, the causative agent implicated in transmissible spongiform encephalopathies (TSE), remains unresolved. Since there are increasing correlations between oxidative stress and amyloidoses, we decided to investigate whether PrP plays a role in oxidative modulation. We found higher levels of oxidative damage to proteins and lipids in the brain lysates of Prnp-/- as compared to wild-type (WT) mice of the same genetic background. These two indicators, protein oxidation and lipid peroxidation, are hallmarks of cellular oxidative damage. Elevated levels of ubiquitin-protein conjugates were also observed in Prnp-/- mice, a probable consequence of cellular attempts to remove the damaged proteins as indicated by increased proteasome activity. Taken together, these findings are indicative of a role for PrP in oxidative homeostasis in vivo.

Chronic contractile activity upregulates the proteasome system in rabbit skeletal muscle

Remodeling of skeletal muscle in response to altered patterns of contractile activity is achieved, in part, by the regulated degradation of cellular proteins. The ubiquitin-proteasome system is a dominant pathway for protein degradation in eukaryotic cells. To test the role of this pathway in contraction-induced remodeling of skeletal muscle, we used a well-established model of continuous motor nerve stimulation to activate tibialis anterior (TA) muscles of New Zealand White rabbits for periods up to 28 days. Western blot analysis revealed marked and coordinated increases in protein levels of the 20S proteasome and two of its regulatory proteins, PA700 and PA28. mRNA of a representative proteasome subunit also increased coordinately in contracting muscles. Chronic contractile activity of TA also increased total proteasome activity in extracts, as measured by the hydrolysis of a proteasome-specific peptide substrate, and the total capacity of the ubiquitin-proteasome pathway, as measured by the ATP-dependent hydrolysis of an exogenous protein substrate. These results support the potential role of the ubiquitin-proteasome pathway of protein degradation in the contraction-induced remodeling of skeletal muscle.

Activity and regulation of the centrosome-associated proteasome

Regulated proteolysis is important for maintaining appropriate cellular levels of many proteins. The bulk of intracellular protein degradation is catalyzed by the proteasome. Recently, the centrosome was identified as a novel site for concentration of the proteasome and associated regulatory proteins (Wigley, W. C., Fabunmi, R. P., Lee, M. G., Marino, C. R., Muallem, S., DeMartino, G. N., and Thomas, P. J. (1999) J. Cell Biol. 145, 481-490). Here we provide evidence that centrosomes contain the active 26 S proteasome that degrades ubiquitinated-protein and proteasome-specific peptide substrates. Moreover, the centrosomes contain an ubiquitin isopeptidase activity. The proteolytic activity is ATP-dependent and is inhibited by proteasome inhibitors. Notably, treatment of cells with inhibitors of proteasome activity promotes redistribution of the proteasome and associated regulatory proteins to the centrosome independent of an intact microtubule system. These data provide biochemical evidence for active proteasomal complexes at the centrosome, highlighting a novel function for this organizing structure.

Polyglutamine-expanded androgen receptors form aggregates that sequester heat shock proteins, proteasome components and SRC-1, and are suppressed by the HDJ-2 chaperone

Spinal bulbar muscular atrophy is a neurodegenerative disorder caused by a polyglutamine expansion in the androgen receptor (AR). We show in transiently transfected HeLa cells that an AR containing 48 glutamines (ARQ48) accumulates in a hormone-dependent manner in both cytoplasmic and nuclear aggregates. Electron microscopy reveals both types of aggregates to have a similar ultrastructure. ARQ48 aggregates sequester mitochondria and steroid receptor coactivator 1 and stain positively for NEDD8, Hsp70, Hsp90 and HDJ-2/HSDJ. Co-expression of HDJ-2/HSDJ significantly represses aggregate formation. ARQ48 aggregates also label with antibodies recognizing the PA700 proteasome caps but not 20S core particles. These results suggest that ARQ48 accumulates due to protein misfolding and a breakdown in proteolytic processing. Furthermore, the homeostatic disturbances associated with aggregate formation may affect normal cell function.

Proteolytic degradation of heme-modified hepatic cytochromes P450: A role for phosphorylation, ubiquitination, and the 26S proteasome?

The resident integral hepatic endoplasmic reticulum (ER) proteins, cytochromes P450 (P450s), turn over in vivo with widely varying half-lives. We and others (Correia et al., Arch. Biochem. Biophys. 297, 228, 1992; and Tierney et al., Arch. Biochem. Biophys. 293, 9, 1992) have previously shown that in intact animals, the hepatic P450s of the 3A and 2E1 subfamilies are first ubiquitinated and then proteolyzed after their drug-induced suicide inactivation. Our findings with intact rat hepatocytes and ER preparations containing native P450s and P450s inactivated via heme modification of the protein have revealed that the proteolytic degradation of heme-modified P450s requires a cytosolic ATP-dependent proteolytic system rather than lysosomal or ER proteases (Correia et al., Arch. Biochem. Biophys. 297, 228, 1992). Using purified cumene hydroperoxide-inactivated P450s (rat liver P450s 2B1 or 3A and/or a recombinant human liver P450 3A4) as models, we now document that these heme-modified enzymes are indeed ubiquitinated and then proteolyzed by the 26S proteasome, but not by its 20S proteolytic core. In addition, our studies indicate that the ubiquitination of these heme-modified P450s is preceded by their phosphorylation. It remains to be determined whether, in common with several other cellular proteins, such P450 phosphorylation is indeed required for their degradation. Nevertheless, these findings suggest that the membrane-anchored P450s are to be included in the growing class of ER proteins that undergo ubiquitin-dependent 26S proteasomal degradation.

Synthesis and decay of calmodulin-ubiquitin conjugates in cell-free extracts of various rabbit tissues

Calmodulin is the natural substrate for ubiquitin-ligation by the enzyme ubiquitin-calmodulin ligase (uCaM-synthetase; EC 6.3.2.21). The activity of this ligase is regulated by the binding of the second messenger Ca2+ to the substrate calmodulin, which increases the activity ca. 10-fold. Up till now, two components of the ligase could be identified: uCaM Syn-F1 and uCaM Syn-F2, the first of which binds to ubiquitin and the second which binds to calmodulin. Since the physiological role of this enzyme is still unclear, this study was designed to examine whether the activity of uCaM-Synthetase in 40,000 x g tissue supernatants correlates with the calmodulin content in the various tissues. In reticulocytes, spleen, erythrocytes, testis and brain, which are rich in uCaM synthetase, the tissue contents calculated on the basis of activity measurements were between 4-80-fold higher than in red and white skeletal muscle. These activities did not correlate with the respective calmodulin contents of the tissues indicating that other factors were determining these enzyme levels. A second aim was to gain information on the role of the ATP-ubiquitin-dependent proteolytic pathway in those tissues displaying uCaM synthetase activity. In the reticulocyte system which contains the classical ATP-ubiquitin-dependent proteolytic pathway as measured with 125I-BSA, no ubiquitin-dependent degradation of calmodulin could be detected. We therefore examined the other tissues of the rabbit with the substrate 125I-BSA and succeeded in finding a ubiquitin-independent ATP-dependent proteolytic activity in every case but no ubiquitin-dependent activity. The ubiquitin-independent activity was highest in smooth muscle and red skeletal muscle being ca. 3-4-fold higher than in lung and testis. In 50% of the tissue crude extracts the time curve of calmodulin ubiquitylation progressed through a maximum indicating a dynamic steady state based on conjugate synthesis and decay. If a ubiquitylation pulse of 30 min was followed in liver crude extracts by the addition of EGTA, which specifically inhibits ubiquityl-calmodulin synthesis, a half-life of calmodulin-conjugate decay of 15-20 min is observed. A similar conjugate half-life of ca. 30 min was observed after addition of EDTA excluding that conjugate decay is due to an ATP-dependent proteolytic process. Studying the decay of purified ubiquitin-125I-BH-calmodulin conjugates in cell-free reticulocyte extracts led to the discovery of an ATP-independent isopeptidase activity which splits ubiquitin-calmodulin conjugates without leading to detectable calmodulin fragments. The rapid decay of ubiquitin-calmodulin conjugates in tissue extracts can therefore be plausibly explained by a ubiquityl-calmodulin splitting isopeptidase activity.

Multicatalytic proteinase is associated with characteristic oval structures in cortical Lewy bodies: an immunocytochemical study with light and electron microscopy

The ATP-ubiquitin-dependent proteolytic pathway (ubiquitin pathway) is believed to be involved in the formation of various neuronal inclusion bodies including Lewy bodies (LBs), a pathological hallmark of Parkinson disease and diffuse Lewy body disease (DLBD). Since multicatalytic proteinase (MCP) is involved in the ubiquitin pathway, an investigation of whether MCP is involved in neuronal inclusion bodies would provide a clue to the mechanism underlying the formation of neuronal inclusion bodies as well as to the pathogenesis of degenerative neurological disorders. In this study, we investigated detailed immunolocalization of MCP in LBs in DLBD brains using light and electron microscopy. We raised three different monoclonal antibodies against purified human MCP. Each of them recognized different sets of MCP subunits on Western blotting. Immunohistochemically, anti-MCP antibodies recognized all ubiquitin-positive cortical LBs in situ as well as those isolated from frozen DLBD cortices, suggesting that MCP is present in LBs as a whole molecule exhibiting protease activity. In electron microscopy, MCP immunoreactivity (MCP-IR) was exclusively localized on a characteristic oval structure with an approximate diameter of 100 nm. This structure was distributed throughout the LBs and was devoid of ubiquitin immunoreactivity. Treatment of isolated LBs with 2% SDS, but not with 0.5% Triton X-100, removed this structure from LBs in which fibrous materials predominated. Ubiquitin immunoreactivity was also decreased in isolated LBs treated with 2% SDS, suggesting that the fibrous structures in LBs were not ubiquitinated in situ. Thus, it is suggested that LBs are subjected to a proteolytic process in which MCP plays a role via processing of specific components of LBs.

Skin proteasomes (high-molecular-weight protease): purification, enzymologic properties, gross structure, and tissue distribution

Proteasomes (high-molecular-weight protease) were purified from rat skin, and their enzymologic properties, gross structure, and tissue distribution were investigated. Skin proteasomes were purified by successive (NH4)2SO4 fractionation and by phenyl Sepharose CL-4B and HPLC gel filtration chromatography. The molecular weights of the proteasomes were estimated from gel filtration to be 750 kD. On sodium dodecylsulfate-polyacrylamide gel electrophoresis, the purified enzymes dissociated into several bands, the majority falling into the range of 36-20 kD. Two-dimensional electrophoretic analysis demonstrated approximately 10-15 separate protein spots with pl values varying between 3 and 10. As analyzed by electron microscopy, the gross structure of the enzymes showed an almost symmetrical ring-shaped particle with a small hole in the center. Succinyl-leucyl-leucyl-valyl-tyrosine-4-methylcoumaryl-7-amide, a fluorogenic substrate for serine proteinases, demonstrated the highest activity in terms of substrate specificity. Sodium dodecylsulfate, Ca++, and some free fatty acids activated enzyme activity. Activity was inhibited by diisopropylfluorophosphate, leupeptin, N-ethylmaleimide, iodoacetamide, and chymostatin. These results show that both serine and cysteine residues are related to the enzyme activity of proteasomes. Total and specific enzyme activities in the epidermis were, respectively, 10 and 20 times higher than in the dermis. Immunohistochemical studies utilizing the avidin-biotin complex method with monoclonal antibody revealed that the enzyme is distributed throughout the epidermis. These findings indicate the epidermal localization of proteasomes.

An ATP-stabilized inhibitor of the proteasome is a component of the 1500-kDa ubiquitin conjugate-degrading complex

Proteins conjugated to ubiquitin are degraded by a 26S (1500-kDa) proteolytic complex that, in reticulocyte extracts, can be formed by the association of three factors: CF-1, CF-2, and CF-3. One of these factors, CF-3, has been shown to be the proteasome, a 650-kDa multicatalytic protease complex. We have purified a 250-kDa inhibitor of the proteasome and shown that it corresponds to CF-2. In the presence or absence of ATP, this factor inhibited hydrolysis by the proteasome of both fluorogenic tetrapeptides and protein substrates. When the inhibitor, proteasome, and CF-1 were incubated together in the presence of ATP and Mg2+, degradation of ubiquitin-125I-lysozyme occurred. Both the inhibitory activity and the ability to reconstitute ubiquitin-125I-lysozyme degradation were very labile at 42 degrees C, but both activities were stabilized by ATP or a nonhydrolyzable ATP analog. SDS/PAGE indicated that the 250-kDa inhibitor fraction contained a major subunit of 40 kDa (plus some minor bands). The 125I-labeled inhibitor and purified proteasome formed a complex. When CF-1, ATP, and Mg2+ were also present, the 125I-labeled inhibitor along with the proteasome formed a complex of 1500 kDa. The inhibitor (CF-2) thus appears to be an ATP-binding component that regulates proteolysis within the 1500-kDa complex.

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.

26S multicatalytic proteinase complexes decrease during the differentiation of murine erythroleukemia cells

Changes in multicatalytic proteinase activity during differentiation were investigated using Me2SO-induced differentiation of murine erythroleukemia cells as a model. The apparent ATP-dependent multicatalytic proteinase activity decreased in the Me2SO-treated cells with ATP-dependent incorporation of [3H]diisopropyl fluorophosphate decreasing notably after Me2SO-treatment. This decrease in activity does not seem to arise from a cessation of cell-proliferation, because no significant changes in proteinase activity were observed under different culture conditions. Hydroxyapatite column chromatography was employed to analyze the form of multicatalytic proteinase. It was clearly demonstrated that the 26S form of the proteinase decrease in the differentiated cells relative to normal cells. Multicatalytic proteinase-associated proteins that bind to the proteinase in an ATP-dependent manner were purified on an anti-multicatalytic proteinase IgG conjugated column. Only a small amount of protein was recovered from the differentiated cells. These results suggest that the decrease in multicatalytic proteinase-associated proteins that occurs upon cell-differentiation abolishes the ATP-dependent activity of the proteinase.

Conserved N-terminal sequences in homologous subunits of the multicatalytic proteinase complex (proteasome)

The bovine lens multicatalytic proteinase complex (MPC) (MW 700 kDa) comprises at least twelve subunits in the molecular mass range 22-35 kDa. Three of the subunits, L1 (27 kDa), L2 (24 kDa) and L3 (29 kDa), were purified by reverse phase HPLC. Their amino acid composition and N-terminal sequences indicate that they are not identical. L1 and L2 subunits show very high (greater than 90%) sequence homology with specific subunits of rat liver and human reticulocyte MPC and these are considered to be homologous components of the MPC which are highly conserved in evolution.

Properties of subunits of the multicatalytic proteinase complex revealed by the use of subunit-specific antibodies

The multicatalytic proteinase (MCP) is a high-molecular-mass non-lysosomal proteinase that gives rise to a characteristic pattern of bands of molecular mass 22-34 kDa on SDS/PAGE gels. Isoelectric-focusing gels of the enzyme purified from rat liver show 16 bands with isoelectric points in the range of pH 5-8.5. Two-dimensional PAGE gels reveal that there are more than the previously reported 13 polypeptides associated with the MCP from rat liver and show a pattern of 15-20 major spots and several minor ones, similar to that of MCP isolated from some other sources. Possible relationships between the different polypeptides were investigated by immunoblot analysis of electrophoretically purified proteinase subunits with affinity-purified subunit-specific antibodies as well as antibodies raised against individual denatured subunits of the complex. The results demonstrate that many of the major polypeptide components of the MCP complex are antigenically distinct. Moreover comparison of immunoreactive material in crude cell extracts with that in purified MCP preparations has shown that the polypeptides are not derived from a smaller number of higher-molecular-mass subunits. Also, individual subunits have the same apparent molecular mass in a variety of rat tissues, suggesting close similarity between MCPs of different tissues. The highest concentrations of MCP subunits occur in liver and kidney. Gel-filtration analysis of crude extracts has demonstrated that MCP polypeptides are also associated with a higher-molecular-mass complex, which may be the 26 S proteinase that has been implicated in the degradation of ubiquitin-protein conjugates.

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.

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.

Evidence for ATP and ubiquitin dependent degradation of proteins in cultured bovine lens epithelial cells

Degradation of endogenous lens proteins has been difficult to show under physiological conditions using lens tissue preparations. In contrast, active proteolytic systems in cultured bovine lens epithelial (BLE) cells have been demonstrated previously. BLE cells also contain ubiquitin, a 76 amino-acid polypeptide which is conjugated to proteins in an ATP/Mg(2+)-dependent process prior to their cytosolic proteolysis. In this study, we show that histone H2A, alpha-crystallin, and actin are conjugated to ubiquitin, resulting in higher molecular mass species, which are detected by anti-ubiquitin antibodies. These proteins are also degraded in cell-free assays containing BLE cell supernatants under physiological conditions in an ATP/Mg(2+)-dependent manner. Observation of 125I-labeled proteolytic fragments was made after SDS polyacrylamide gel electrophoresis of the assays. Quantitation of trichloroacetic acid-soluble radiolabeled fragments generated in the presence of ATP/Mg2+ revealed that, with BLE cell supernatant, 25% of the histone H2A was degraded in 3 hr. Proteolysis of alpha-crystallin and actin amounted to 2.3% and 2.9%, respectively. The requirement of ATP/Mg2+ for proteolysis and the observation of ubiquitin conjugation to the same proteins is consistent with the presence of a ubiquitin-dependent proteolytic pathway in BLE cells. Additionally, in this study the BLE cell proteases were even more active on some substrates than the reticulocyte ubiquitin/ATP-dependent proteolytic system.

Abnormally high expression of proteasomes in human leukemic cells

Proteasomes are eukaryotic ring-shaped or cylindrical particles with multicatalytic protease activities. To clarify the involvement of proteasomes in tumorigenesis of human blood cells, we compared their expression in human hematopoietic malignant tumor cells with that in normal peripheral blood mononuclear cells. Immunohistochemical staining showed considerably increased concentrations of proteasomes in leukemic cells from the bone marrow of patients with various types of leukemia and the predominant localization of these proteasomes in the nuclei. Moreover, enzyme immunoassay and Northern blot analysis indicated that the concentrations of proteasomes and their mRNA levels were consistently much higher in a variety of malignant human hematopoietic cell lines than in resting peripheral lymphocytes and monocytes from healthy adults. Proteasome expression was also greatly increased in normal blood mononuclear cells during blastogenic transformation induced by phytohemagglutinin; their expression increased in parallel with induction of DNA synthesis and returned to the basal level with progress of the cell cycle. Thus, abnormally high expression of proteasomes may play an important role in transformation and proliferation of blood cells and in specific functions of hematopoietic tumor cells.

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.

Molecular and biochemical properties of the ATP-stimulated multicatalytic proteinase, ingensin, from rat liver

1. A high-molecular-mass multicatalytic proteinase, ingensin, has been purified from rat liver and biochemically characterized. Trypsinization in the presence of ATP prevented the degradation of ingensin subunits. 2. Glutaraldehyde, which copolymerizes proteins, increased the apparent molecular mass of the subunits on SDS-PAGE, indicating the occurrence of covalent crosslinking of subunits. ATP, in this case, lowered the extent of covalent crosslinking. These results suggest that ATP altered the conformation of ingensin subunits. 3. Urea-induced autodigestion experiments demonstrated that some low-molecular-weight subunits selectively disappeared without changes in the contents of other subunits. The chymotryptic activity of the proteinase was more resistant to autodigestion than its tryptic activity. Therefore, we conclude that separate subunits of the enzyme are responsible for the different peptide-hydrolyzing activities.

Lens proteasome shows enhanced rates of degradation of hydroxyl radical modified alpha-crystallin

Proteasome, a high molecular weight protease complex (HMP, approximately 600 kDa) was isolated from bovine eye lens epithelium tissue. In contrast with prior reports, lens proteasome degraded the major lens protein alpha-crystallin and S-carboxymethylated bovine serum albumin at 37 degrees C, mostly to trichloroacetic acid precipitable polypeptides. The proteasome, thus isolated, was labile at 55 degrees C. As indicated by the ability of p-chloromercuribenzoate and N-ethylmaleimide to block activity, a thiol group is required for activity. Alpha-crystallin was oxidized by exposure to 60Co-irradiation under an atmosphere of N2O (1-50 kilorads). This dose delivered 0.1-5.7 mol of hydroxyl radicals per mol of crystallin. Irradiation resulted in increased heterogeneity, aggregation, and fragmentation of the crystallin preparation. The proteolytic susceptibility of alpha-crystallin to the lens HMP was enhanced by the irradiation in a dose-dependent manner up to 20 kilorads (.OH concentration up to 2.3 mol per mol of alpha-crystallin). When 50 kilorads (5.7 mol .OH per mol of alpha-crystallin) was used, there was extensive aggregation and no enhancement in proteolysis over the unirradiated sample. The data indicate that the lens HMP can degrade mildly photooxidized lens proteins, but proteins which are extensively damaged are not degraded and may accumulate. This may be related to cataract formation.

Separation of yeast proteasome subunits. Immunoreactivity with antibodies against ATP-dependent protease Ti from Escherichia coli

On two-dimensional gel electrophoresis, proteasomes (multicatalytic proteinase complexes) from the yeast Saccharomyces cerevisiae were separated into a characteristic set of approximately 20 components with molecular weights of 21,000 to 31,000 and isoelectric points of 3.5 to 7.5. The main components were isolated by reverse-phase high performance liquid chromatography on a TSK gel phenyl-5PW RP column and named YC1 to YC11, in order of their elution. Immuno-blot analysis showed that two components (YC1-alpha and YC1-beta) with molecular weights of 30,800 and 28,300 strongly cross-reacted with antibody against the P-component of ATP-dependent protease Ti from Escherichia coli, but no components were found to react with antibodies against the A-component of protease Ti or another ATP-dependent protease La (the Ion gene product) of Escherichia coli. These results indicate a structural relationship between eukaryotic proteasomes and bacterial ATP-dependent protease Ti.

The presence of ATP + ubiquitin-dependent proteinase and multicatalytic proteinase complex in bovine brain

The presence of two distinct high-molecular-weight proteases with similar pH optima in the weakly alkaline region was shown in cytosol of the bovine brain cortex. They were separated by ammonium sulfate fractionation and each was further purified by DEAE-Sephacel, Sephacryl S-300, DEAE-Cibacron Blue 3GA-agarose, heparin-agarose, and Sepharose 6B chromatography. The larger enzyme (Mr 1,400 kDa), which precipitates at 0-38% ammonium sulfate saturation, seems to be active in ATP + ubiquitin (Ub)-dependent proteolysis; it has low basal caseinolytic activity that is stimulated 3-fold by ATP, and when Ub is present ATP causes a 4.5-fold stimulation. A second proteinase was also found to be present (Mr 700 kDa) that precipitates at 38-80% ammonium sulfate saturation, is composed of multiple subunits ranging in Mr from 18 to 30 kDa, and degrades both protein and peptide substrates, demonstrating trypsin-, chymotrypsin- and cucumisin-like activities. Catalytic, biochemical, and immunological characteristics of this proteinase indicate that it is a multicatalytic proteinase complex (MPC), whose enzyme activity, in contrast to that of MPC from bovine pituitaries (1-3), is stimulated 1.7-fold by addition of ATP in the absence of ubiquitin at the early steps of purification; this property is lost during the course of further purification. Both proteinases are present in the nerve cells, since the primary chicken embryonic telencephalon neuronal cell culture extracts contain both ATP + Ub-dependent proteinase and MPC activities.

Ubiquitin-protein conjugates accumulate in the lysosomal system of fibroblasts treated with cysteine proteinase inhibitors

Mouse fibroblasts (3T3-L1 cells) accumulate detergent- and salt-insoluble aggregates of proteins conjugated to ubiquitin when incubated in the presence of inhibitors of lysosomal cysteine cathepsins, including E-64. These ubiquitin-protein conjugates co-fractionate with lysosomes on density gradients and are found in multivesicular dense bodies which by electron microscopy appear to be engaged in microautophagy. Both E-64 and ammonium chloride increase the intracellular concentration of free ubiquitin, but only E-64 leads to the formation of insoluble lysosomal ubiquitin-protein conjugates. The results are discussed in relation to the possible intracellular roles of ubiquitin conjugation.

ATP-dependent incorporation of 20S protease into the 26S complex that degrades proteins conjugated to ubiquitin

Previous studies have indicated that the ATP-dependent 26S protease complex that degrades proteins conjugated to ubiquitin is formed by the assembly of three factors in an ATP-requiring process. We now identify one of the factors as the 20S "multicatalytic" protease, a complex of low molecular weight subunits widely distributed in eukaryotic cells. Comparison of the subunit compositions of purified 20S and 26S complexes indicates that the former is an integral part of the latter. By the use of detergent treatment to activate latent protease activity, we show that the 20S protease becomes incorporated into the 26S complex in the ATP-dependent assembly process. It thus seems that the 20S protease is the "catalytic core" of the 26S complex of the ubiquitin proteolytic pathway.

Purification and characterization of a multicatalytic proteinase from crustacean muscle: comparison of latent and heat-activated forms

A high-molecular-weight (Mr 740,000) multicatalytic proteinase (MCP) was purified over 3100-fold from soluble extracts of lobster claw and abdominal muscles. The enzyme was extracted from muscle in a latent state; brief (3 min) heating of an ammonium sulfate fraction (45-65% saturation) at 60 degrees C irreversibly activated the proteinase while denaturing about 55% of the protein. MCP was further purified by chromatography on two sequential arginine-Sepharose columns and a Mono Q column with a yield of 60%. About 1.12 mg MCP was obtained for every 100 g tissue. In addition to [14C]methylcasein, the MCP hydrolyzed synthetic peptide substrates of trypsin and chymotrypsin at pH 7.75. Serine protease inhibitors (diisopropyl fluorophosphate, phenylmethanesulfonyl fluoride, aprotinin, benzamidine, soybean trypsin inhibitor, chloromethyl ketones), leupeptin, antipain, hemin, sulfhydryl-blocking reagents (N-ethylmaleimide, mersalyl acid, p-chloromercurisulfonic acid, iodoacetamide) suppressed activity while Ep-475, a specific inhibitor of cysteine proteinases, had no effect, suggesting the MCP is a serine proteinase with one or more cysteine residues indirectly involved in catalysis. The latent MCP was purified using the same procedure as that for the active form, except that thermal activation was omitted. The elution characteristics of latent MCP from the arginine-Sepharose and Mono Q columns were identical to those of active MCP. Since the purified latent form could still be activated by heating, activation did not involve denaturation of an endogenous inhibitor or substrate. Subunit compositions of both forms were identical in two-dimensional polyacrylamide gels; each was composed of eight polypeptides with molecular weights between 25,000 and 32,500 and a ninth polypeptide with a molecular weight of 41,000. Electron microscopy of negatively stained material showed that each form was a cylinder-shaped particle (approximately 10 x 15 nm) consisting of a stack of four rings with a hollow center; no differences in shape, dimensions, or submolecular structure were observed. These results suggest that activation probably involved small conformational changes rather than covalent modifications or rearrangement of subunits within the complex.

RNA degrading activity is tightly associated with the multicatalytic proteinase, ingensin

The multicatalytic proteinase, ingensin, was purified to homogeneity from chicken liver. rRNA-degrading activity was co-eluted with the purified multicatalytic proteinase from a TSK-3000SW column. This RNA-degrading activity was inactivated by heat treatment and the addition of a low concentration of SDS. Therefore, the RNA-degrading activity co-eluted with the multicatalytic proteinase was not due to contamination by low-molecular-mass RNases. These results strongly suggest that this RNA-degrading activity was tightly associated with the multicatalytic proteinase, ingensin.

High-molecular-mass proteinases in rabbit reticulocytes: the multicatalytic proteinase is an ATP-independent enzyme and ATP-activated proteolysis is in part associated with a cysteine proteinase complexed to alpha 1-macroglobulin

We have investigated the proteolytic degradation of [14C]methylcasein and 125I-labeled bovine serum albumin at pH 7.8 and 37 degrees C by lysates of rabbit reticulocytes purified from rabbit blood by two different procedures. (I) Lysates obtained from reticulocytes after removal of plasma and buffy coat as well as after washing of cells, degraded casein and albumin, and released from the two substrates 1.3%/h and 0.4%/h, respectively, of acid-soluble radioactivity. The activity towards both substrates was stimulated about 4-fold by ATP/Mg2+. Chromatography of whole blood on a column of cellulose prior to washing and lysis of cells had profound but differential effects on these activities in that stimulation of casein-degradation by ATP/Mg2+ was almost completely lost, whereas degradation of albumin, albeit at a low rate, was measurable in the presence of ATP/Mg2+ only. (II) Degradation of casein by these lysates is largely inhibited by a monospecific antibody against rabbit multicatalytic proteinase, whereas digestion of albumin is not affected by the antibody, either in the presence or absence of ATP/Mg2+. The latter activity is partially inhibited by a specific antibody against rabbit alpha 1-macroglobulin. (III) The immunoreactive amount of multicatalytic proteinase is about 1.2 micrograms per mg of lysate protein and almost identical in the two lysates. In contrast, the immunologically detectable levels of alpha 1-macroglobulin vary and are much lower in reticulocyte-lysates following chromatography on cellulose than in lysates from washed reticulocytes. (IV) Caseinolytic activity of multicatalytic proteinase, purified from rabbit reticulocyte lysate, is not activated by ATP/Mg2+ and the enzyme is proteolytically inactive towards albumin. On the other hand, a complex consisting of the proteinase inhibitor alpha 1-macroglobulin and the cysteine proteinase, cathepsin B, does degrade both substrates at pH 7.8, in an ATP/Mg2+-activated fashion. From these results it is concluded that the multicatalytic proteinase is an ATP-independent enzyme and a cellular constituent of rabbit reticulocytes whereas the activity stimulated by ATP/Mg2+ appears to be associated, at least in part, with a cysteine proteinase complexed to alpha 1-macroglobulin.

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

Specific immunoadsorption of the high molecular weight multicatalytic protease, macropain, from postmicrosomal extracts of BHK fibroblasts inhibited ATP-dependent proteolysis of exogenous protein substrates. The immunoprecipitated macropain represented the latent (L) form of the protease because it had low protease activity but was activated by methods that activate purified macropain L. Reconstitution of the antibody-treated extracts with purified macropain L, but not macropain A, from bovine heart or human erythrocytes, completely restored ATP-dependent proteolysis, even though ATP did not directly activate either purified macropain L or the immunoprecipitated protease. Reconstituted ATP-dependent proteolysis was saturable with respect to added macropain and never exceeded the level of proteolysis present in the original extract. These results indicate that macropain L plays a key role in ATP-dependent proteolysis but suggest that the protease may require interaction with or modification by another cellular component to demonstrate this effect.

The high molecular weight multicatalytic proteinase, macropain, exists in a latent form in human erythrocytes

The high molecular weight multicatalytic proteinase, macropain, has been purified from human erythrocytes in two forms that differ in caseinolytic activity up to 100-fold. Each form has a native molecular weight of 600,000 and is composed of a number of subunits ranging in molecular weights from 35,000 to 21,000. Although the two proteinase forms share a number of electrophoretically indistinguishable subunits, there are also subunits unique to the respective forms. The less active proteinase represents a latent enzyme because it was fully activated by two procedures including dialysis against water and pretreatment with low concentrations of sodium dodecyl sulfate. These procedures caused differential changes in the caseinolytic and two peptidase activities of the proteinase. An Mr 35,000 subunit, characteristic of latent macropain, is immunologically related to at least one of the other components of active macropain and disappeared after proteinase activation by dialysis. Nevertheless, loss of this subunit was not the cause of the increased activity. These results suggest that the proteolytic activity of cells may be regulated by the activation of the latent form of macropain.

Involvement of the proteasome in various degradative processes in mammalian cells

Eukaryotic cells contain a 700-kDa proteolytic complex (the "proteasome" or multicatalytic endopeptidase complex), whose role in intracellular protein breakdown is unclear. It has been suggested that the proteasome functions in the rapid degradation of oxidant-damaged proteins and in the ATP-dependent proteolytic pathway. To test these possibilities, oxidant-damaged hemoglobin and albumin were produced by treating hemoglobin and albumin with phenylhydrazine, with hydroxyl radicals, or with both hydroxyl and superoxide radicals. After oxidant damage, these proteins were degraded more rapidly in erythrocyte extracts and also by the purified proteasome. However, complete removal of proteasomes from these extracts by immunoprecipitation (or inhibitors of its proteolytic activity) did not reduce the breakdown of oxidant-damaged hemoglobin and decreased degradation of hydroxyl- and superoxide-treated proteins by only 30-40%. Thus, erythrocytes must contain another proteolytic system for degradation of oxidant-damaged proteins. In contrast, immunoprecipitation of proteasomes with polyclonal or monoclonal antibodies prevented the ATP/ubiquitin-dependent degradation of lysozyme and also blocked the ATP-stimulated degradation of ubiquitin-conjugated lysozyme in reticulocyte and skeletal muscle extracts. These data indicate a critical role of the proteasome in the degradation of ubiquitin-conjugated proteins and suggest that the proteasome is associated with or is a component of the larger ubiquitin-conjugate-degrading enzyme complex.

Direct evidence for nuclear and cytoplasmic colocalization of proteasomes (multiprotease complexes) in liver

Subcellular localization of the large multicatalytic protease complexes called proteasomes, which have been found in soluble fractions of various cells, was examined by biochemical, immunological, and immunohistological methods. Rat liver nuclei, purified by two different procedures, showed high activities for degrading [3H]methylcasein and various fluorogenic oligopeptides with neutral and weakly alkaline pH optima. On gel filtration, all of these peptidase activities were recovered in a single peak with the unusually large molecular weight of about 600,000. Properties of the proteolytic activity in crude extracts of the nucleus and the cytoplasm were very similar. Immunoelectrophoretic blot analysis showed the presence of appreciable concentrations of proteasomes with similar immunoreactivity in isolated nuclear and cytosolic fractions. Moreover, immunohistochemical staining of human liver showed that proteasomes were predominantly localized in the nuclear matrix but also were present diffusely in the cytoplasm of hepatocytes. These findings indicate the nuclear and cytoplasmic colocalization of proteasomes.

Half-life of proteasomes (multiprotease complexes) in rat liver

Proteasomes (large multicatalytic proteinase complexes) are abundant in rat liver, constituting approximately 1.0% of the total soluble proteins. In the present study, the apparent half-life of the proteasomes was determined to be 12-15 days from the decay curve of isotopically labeled enzymes in vivo, suggesting their slow turnover. This finding together with the ubiquitous distribution of proteasomes in eukaryotic cells (Tanaka et al., 1988 J. Biol. Chem. 263, 16209-16217) indicates that proteasomes belong to a family of proteins with house-keeping function.

Autodegradation of rat liver proteasomes (large multicatalytic proteinase complexes)

Purified proteasomes (large multicatalytic proteinase complexes) were found to be very stable, showing no change in activities or structures during prolonged incubation in medium of pH 7.5 at 37 degrees C. However, on addition of urea they were degraded autocatalytically in a time- and dose-dependent manner, suggesting that destruction of the proteasomal complexes acts as a signal for their autolysis. ATP at a physiological concentration greatly stimulated the urea-dependent breakdown of proteasomes. The autolysis induced by urea was almost completely inhibited by hemin, but not by other protease inhibitors tested, such as leupeptin, chymostation and Ep-475. Thus, autolytic degradation of proteasomes appears to be important for the regulation of enzyme levels in eukaryotic cells.

Common epitopes of bovine lens multicatalytic-proteinase-complex subunits

Component polypeptides of both the bovine lens and pituitary multicatalytic proteinase complexes demonstrate different immunoreactivities with a polyclonal antiserum raised against the purified pituitary enzyme. Four (Mr 24000, 26000, 34000 and 38000) of eight bands that have been resolved by SDS/polyacrylamide-gel electrophoresis are stained in immunoblot experiments. Monospecific antibodies obtained from this antiserum by affinity purification from the 38000- and 34000-Mr bands of the lens enzyme bound equally well to either band, but showed little or no binding to the 26000- and 24000-Mr bands upon immunoblotting. Antibody affinity-purified from the 24000-Mr band showed comparable binding to the 24000-, 34000- or 38000-Mr band. One explanation of these results is that the 24000-Mr polypeptide is derived from the higher-Mr polypeptide(s) and has lost some of the common immunodeterminants.

The multicatalytic proteinase of mammalian cells

A high-molecular-weight nonlysosomal proteinase has recently been discovered in mammalian cells. It is a widely distributed and abundant enzyme which has attracted attention because of its complex multisubunit structure and its unusual catalytic properties. The 700-kDa proteinase is composed of many different types of low-molecular-weight subunits (Mr 21,000-34,000) arranged in a hollow cylindrical structure. This 20 S complex is very similar, if not identical, to the 19-20 S cylindrical particles, ring-type particles, or prosomes which have been isolated from several different types of eukaryotic cells. The proteinase appears to have at least two distinct catalytic sites and can cleave bonds on the carboxyl side of basic, hydrophobic, or acidic amino acid residues. Inhibition of proteinase activity by thiol reagents supports the suggestion that the enzyme is a cysteine proteinase but there is some evidence that it may be a serine proteinase and the catalytic mechanism is at present unknown. ATP has little effect on proteinase activity in most purified preparations but recently the proteinase has been implicated in ATP-dependent pathways of protein degradation. Ther is a second type of high-molecular-weight complex multisubunit proteinase, a 26 S particle, which catalyzes the ATP-dependent degradation of ubiquitin-protein conjugates. The precise function of these two complex proteinases in intracellular proteolysis remains to be elucidated.

An enzyme related to the high molecular weight multicatalytic proteinase, macropain, participates in a ubiquitin-mediated, ATP-stimulated proteolytic pathway in soluble extracts of BHK 21/C13 fibroblasts

Soluble, cell-free extracts of BHK 21/C13 fibroblasts degraded a variety of exogenous proteins to acid-soluble peptides at pH 8.0. ATP stimulated the rates of proteolysis. Both the absolute rate of proteolysis and the magnitude of the ATP effect depended on the specific substrate. For example, casein was degraded approximately 10-fold faster than lysozyme, but lysozyme degradation was more highly stimulated by ATP than was casein degradation. Ubiquitin enhanced the ATP-stimulated proteolysis of each substrate in both postmicrosomal extracts and DEAE-cellulose fractionated extracts. In each extract, ubiquitin enhanced the ATP-stimulated degradation of lysozyme to a greater degree than that of casein. These results suggested that lysozyme was degraded by a pathway that was more dependent upon ubiquitin than was casein. Further evidence for this conclusion was obtained in studies using substrates whose amino groups were blocked by extensive methylation or carbamoylation. The high molecular weight proteinase, macropain, appears to be involved in the ATP-stimulated degradation of both substrates. Specific immunoprecipitation of macropain with polyclonal antibodies resulted in the inhibition of ATP-stimulated proteinase activity both in the absence and presence of ubiquitin. These results indicate that macropain plays a role in both ubiquitin-mediated and ubiquitin-independent ATP-stimulated proteolysis in BHK cell extracts.