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

Proteasome dynamics between proliferation and quiescence stages of Saccharomyces cerevisiae

The ubiquitin-proteasome system (UPS) plays a critical role in cellular protein homeostasis and is required for the turnover of short-lived and unwanted proteins, which are targeted by poly-ubiquitination for degradation. Proteasome is the key protease of UPS and consists of multiple subunits, which are organized into a catalytic core particle (CP) and a regulatory particle (RP). In Saccharomyces cerevisiae, proteasome holo-enzymes are engaged in degrading poly-ubiquitinated substrates and are mostly localized in the nucleus during cell proliferation. While in quiescence, the RP and CP are sequestered into motile and reversible storage granules in the cytoplasm, called proteasome storage granules (PSGs). The reversible nature of PSGs allows the proteasomes to be transported back into the nucleus upon exit from quiescence. Nuclear import of RP and CP through nuclear pores occurs via the canonical pathway that includes the importin-αβ heterodimer and takes advantage of the Ran-GTP gradient across the nuclear membrane. Dependent on the growth stage, either inactive precursor complexes or mature holo-enzymes are imported into the nucleus. The present review discusses the dynamics of proteasomes including their assembly, nucleo-cytoplasmic transport during proliferation and the sequestration of proteasomes into PSGs during quiescence. [Formula: see text].

Importance of the ATP-ubiquitin-proteasome pathway in the degradation of soluble and myofibrillar proteins in rabbit muscle extracts

Recent studies have suggested that activation of the ubiquitin-proteasome pathway is primarily responsible for the rapid loss of muscle proteins in various types of atrophy. The present studies were undertaken to test if different classes of muscle proteins are degraded by this pathway. In extracts of rabbit psoas muscle, the complete degradation of soluble proteins to amino acids was stimulated up to 6-fold by ATP. Peptide aldehyde inhibitors of the proteasome or the removal of proteasomes markedly inhibited only the ATP-dependent process. Addition of purified myosin, actin, troponin, or tropomyosin to these extracts showed that these proteins served as substrates for the ubiquitin-proteasome pathway. By contrast, degradation of myoglobin did not require ATP, proteasomes, or any known proteases in muscles. When myosin, actin, and troponin were added as actomyosin complexes or as intact myofibrils to these extracts, they were not hydrolyzed at a significant rate, probably because in these multicomponent complexes, these proteins are protected from degradation. Accordingly, actin (but not albumin or troponin) inhibited the degradation of 125I-myosin, and actin was found to selectively inhibit ubiquitin conjugation to 125I-myosin. Also, the presence of tropomyosin inhibited the degradation of 125I-troponin. However, neither actin nor tropomyosin inhibited the degradation of 125I-lysozyme or soluble muscle proteins. Thus, specific interactions between the myofibrillar proteins appear to protect them from ubiquitin-dependent degradation, and the rate-limiting step in their degradation is probably their dissociation from the myofibril.

Mammalian Sug1 and c-Fos in the nuclear 26S proteasome

In a search for regulatory proteins that interact with the leucine zipper motif of c-Fos in the yeast two-hybrid screen, we have identified a protein (FZA-B) that has extensive sequence similarity to SUG1 of Saccharomyces cerevisiae. Here we show that FZA-B can functionally substitute for SUG1 in yeast and that FZA-B interacts with Fos proteins in vitro through their leucine zippers. In rat liver and in HeLa cells, FZA-B is present in the 26S proteasome complex, as is c-Fos. Immobilized antibody raised against an FZA-B-specific peptide depleted peptidase activity, proteasomal proteins, FZA-B, and c-Fos from a 26S proteasome preparation. FZA-B is found predominantly in the nuclear fraction of COS cells expressing an FZA-B transgene and in the nuclear 26S proteasome of HeLa cells. We conclude that FZA-B is the mammalian homolog of SUG1 (mSug1) and that it is present in the nuclear 26S proteasome of cells. Our results suggest that mSug1 may be involved in the degradation of c-Fos and other transcription factors.

Post-translational processing of a major histocompatibility complex-encoded proteasome subunit, LMP-2

Proteasomes are abundant, multisubunit protein complexes found in the cytoplasm and nucleus of eukaryotic cells that catalyze both ubiquitin-dependent and ubiquitin-independent protein degradation. In addition to their role in normal protein turnover, proteasomes are believed to be involved in the production of most antigenic peptides presented to T cells by major histocompatibility complex (MHC) class I molecules. A distinct subset of mouse proteasomes contain a subunit called LMP-2, which is encoded within the MHC. Here we demonstrate that a previously isolated proteasome cDNA clone encodes the LMP-2 subunit, and that two distinct forms of this subunit may be found in the proteasome complex. One form probably corresponds to the primary translation product, whereas the second form appears to be post-translationally processed by removal of the amino-terminal 20 amino acids. Determination of the location of intron/exon boundaries in the Lmp-2 gene indicated that these residues correspond precisely to the first exon of the gene.

The ubiquitin-mediated proteolytic pathway

Ubiquitin modification of a variety of protein targets within the cell plays important roles in many cellular processes. Among these are regulation of gene expression, regulation of cell cycle and division, involvement in the cellular stress response, modification of cell surface receptors, DNA repair, and biogenesis of mitochondria and ribosomes. The best studied modification occurs in the ubiquitin-dependent proteolytic pathway. Degradation of a protein by the ubiquitin system involves two discrete steps. Initially, multiple ubiquitin molecules are covalently linked in an ATP-dependent mode to the protein substrate. The protein moiety of the conjugate is then degraded by a specific protease into free amino acids with the release of free and reutilizable ubiquitin. This process also requires energy. In addition, stable mono-ubiquitin adducts are also found intracellularly, for example, those involving nucleosomal histones. Despite the considerable progress that has been made in elucidating the mode of action and roles of the ubiquitin system, many problems remain unsolved. For example, very little is known about the cellular substrates of the system and the signals that target them for conjugation and degradation. The scope of this review is to summarize briefly what is currently known on the role of the ubiquitin system in protein turnover, and to discuss in detail the mechanisms involved in selection of substrates for conjugation and in degradation of ubiquitin-conjugated proteins.

Purification and characterization of a latent form of multicatalytic proteinase from fish muscle

1. A latent form of multicatalytic proteinase (MCP) was purified to apparent homogeneity from white croaker muscle by DEAE-Sephacel, Mono-Q, Sephacryl S-300 and second Mono-Q chromatographies. 2. The enzyme preparation was electrophoretically and immunologically similar to MCP purified from the same source by a different method (Folco et al., 1988b, Archs Biochem. Biophys. 267, 599-605) but showed much lower chymotrypsin- and trypsin-like activities. 3. These activities responded to sodium dodecyl sulphate (SDS), urea and heat treatments in different ways: SDS stimulated both activities, urea stimulated the former and inhibited the latter and heating stimulated the former and did not affect the latter. 4. The stimulation of chymotrypsin-like activity by the three treatments was irreversible. 5. Exposure of MCP to SDS or urea in the absence of substrate rapidly inactivated it, whereas heat activation took place irrespective of the presence of substrate. 6. The stimulating effect of SDS on chymotrypsin-like activity was lost in the presence of urea. 7. These results suggest that the enzyme may be activated by different mechanisms.

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.

Molecular cloning of an essential yeast gene encoding a proteasomal subunit

We present the cloning and sequence of a Saccharomyces cerevisiae gene, PUP2, which encodes for a proteasomal subunit. The PUP2 protein is similar to other proteasomal components from yeast, as well as from Drosophila and rat. Although not-properly-folded proteins have been implicated to constitute substrates of proteasomes, we show that the accumulation of such proteins does not induce expression of the PUP2 gene. Finally, gene disruption experiments demonstrate that PUP2 belongs to the class of yeast proteasomal subunits that are essential for cell viability.

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.

Molecular interaction of the proteasome (multicatalytic proteinase). Evidence that the proteasome is not a constituent of the '26 S' multienzyme complex

On the basis of recent reports that suggested that proteasomes, via an ATP-dependent process, become integral components of a '26 S' complex possessing 3-carboxypropionyl-Leu-Leu-Val-Tyr 4-methylcoumarin-7-ylamide-hydrolysing activity, we have investigated the molecular interaction of proteasomes in ATP-stabilized fraction II (proteins absorbed on DEAE-matrix and eluted with 0.5 M-KCl) of rabbit reticulocytes and mouse liver. Analysis of the various extracts by (NH4)2SO4 fractionation, velocity-gradient centrifugation, non-denaturing PAGE and SDS/PAGE and immunoblotting with proteasome-specific antisera failed to identify the proteasome as part of a higher-molecular-mass '26 S' multienzyme complex. In all instances proteasomes are identified in their 'free' 650 kDa '20 S' form. In addition to the proteasome and independent of the presence of MgATP, we isolated a high-molecular-mass proteinase whose electrophoretic migration behaviour and sedimentation rate correspond to that of the previously described '26 S' proteinase. This '26 S' proteinase possesses a strong 3-carboxypropionyl-Leu-Leu-Val-Tyr 4-methylcoumarin-7-ylamide-hydrolysing activity and is composed of several non-identical polypeptides in the molecular-mass range 20-150 kDa. Despite its similarity to proteasomal enzyme activity, protein analysis and immunoblotting experiments demonstrate that neither the intact proteasome nor subunits thereof are components of the '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.

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.

The ubiquitin-mediated system for intracellular protein degradation

Degradation of proteins by the ubiquitin system involves several discrete steps. Initially, multiple molecules of ubiquitin are covalently conjugated to the target substrate in an energy-requiring reaction. The protein thus marked is degraded by a specific ATP-dependent protease, and free and reutilizable ubiquitin is released. In this review we discuss the mechanisms involved in ubiquitin activation, selection of substrates for conjugation, and subsequent degradation of ubiquitin-conjugated proteins in the cell-free system. In addition, we summarize briefly what is currently known of the physiological roles of the ubiquitin system in vivo.

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.

The human multicatalytic proteinase: affinity purification using a monoclonal antibody

A monoclonal antibody, coupled to Sepharose CL-4B, was used for the rapid purification of the human multicatalytic proteinase in a single chromatographic step under mild conditions. The enzyme was homogeneous as judged by nondenaturing polyacrylamide gel electrophoresis. Electrophoresis under dissociating and reducing conditions revealed at least ten components with molecular masses in the range 22-34 kDa. Affinity-purified enzyme was identical to conventionally purified enzyme with respect to enzymatic properties, molecular mass and subunit composition.

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.

Ubiquitinated protein conjugates are specifically enriched in the lysosomal system of fibroblasts

Ubiquitin-protein conjugates are found by immunogold electron microscopy to be enriched (12-fold) in the lysosomal compartment of 3T3-L1 fibroblasts. Treatment of fibroblasts with the cysteine protease inhibitor E-64 leads to an expansion of the lysosomal compartment and as a result an increase in the cellular content of ubiquitin-protein conjugates. There is no change in the specific enrichment of ubiquitin-protein conjugates in the lysosomal compartment following E-64 treatment. The results suggest that some ubiquitin-protein conjugates may normally be degraded lysosomally following sequestration by microautophagy and imply that protein ubiquitination may be one of the signals for protein uptake into lysosomes.

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.

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.

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.