Chemical modification of the bovine pituitary multicatalytic proteinase complex by N-acetylimidazole. Reversible activation of casein hydrolysis

Bowman-Birk proteinase inhibitor from Cajanus cajan seeds: purification, characterization, and insecticidal properties

A red gram proteinase inhibitor (RgPI) was purified from red gram ( Cajanus cajan ) seeds by using ammonium sulfate precipitation and ion-exchange, affinity, and gel filtration chromatography. SDS-PAGE under nonreducing condition revealed two protein bands with molecular masses of approximately 8.5 and approximately 16.5 kDa corresponding to monomeric and dimeric forms of RgPI, respectively. Similarly, matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry also confirmed the presence of dimer as well as other oligomeric forms: trimer, tetramer, and pentamer. Reduction of RgPI with dithiothreitol (DTT) led to the dissociation of the dimeric and oligomeric forms. Native-PAGE and two-dimensional gel electrophoresis indicated the existence of isoinhibitors with pI values of 5.95, 6.25, 6.50, 6.90, and 7.15, respectively. The MALDI-TOF-TOF mass spectrum and N-terminal sequence 'DQHHSSKACC' suggested that the isolated RgPI is a member of the Bowman-Birk inhibitor family. RgPI exhibited noncompetitive type inhibitory activity against bovine pancreatic trypsin and chymotrypsin, with inhibition constants of 292 and 2265 nM, respectively. It was stable up to a temperature of 80 degrees C and was active over a wide pH range between 2 and 12. However, reduction with DTT or 2-mercaptoethanol resulted in loss of inhibitory activity against trypsin and chymotrypsin. It also decreased the activity of larval midgut trypsin-like proteinases in Manduca sexta . Its insecticidal property was further confirmed by reduction in the growth and development of these larvae, when supplemented in the diet.

Purification and characterization of a Bowman-Birk proteinase inhibitor from the seeds of black gram (Vigna mungo)

A proteinase inhibitor (BgPI) was purified from black gram, Vigna mungo (cv. TAU-1) seeds by using ammonium sulfate fractionation, followed by ion-exchange, affinity and gel-filtration chromatography. BgPI showed a single band in SDS-PAGE under non-reducing condition with an apparent molecular mass of approximately 8kDa correlating to the peak 8041.5Da in matrix assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrum. BgPI existed in different isoinhibitor forms with pI values ranging from 4.3 to 6.0. The internal sequence "SIPPQCHCADIR" of a peak 1453.7 m/z, obtained from MALDI-TOF-TOF showed 100% similarity with Bowman-Birk inhibitor (BBI) family. BgPI exhibited non-competitive-type inhibitory activity against both bovine pancreatic trypsin (K(i) of 309.8nM) and chymotrypsin (K(i) of 10.7muM), however, with a molar ratio of 1:2 with trypsin. BgPI was stable up to a temperature of 80 degrees C and active over a wide pH range between 2 and 12. The temperature-induced conformational changes in secondary structure are reversed when BgPI was cooled from 90 to 25 degrees C. Further, upon reduction with dithiothreitol, BgPI lost both its inhibitory activity as well as secondary structural conformation. Lysine residue(s) present in the reactive site of BgPI play an important role in inhibiting the bovine trypsin activity. The present study provides detailed biochemical characteristic features of a BBI type serine proteinase inhibitor isolated from V. mungo.

Redox modulation of cellular metabolism through targeted degradation of signaling proteins by the proteasome

Under conditions of oxidative stress, the 20S proteasome plays a critical role in maintaining cellular homeostasis through the selective degradation of oxidized and damaged proteins. This adaptive stress response is distinct from ubiquitin-dependent pathways in that oxidized proteins are recognized and degraded in an ATP-independent mechanism, which can involve the molecular chaperone Hsp90. Like the regulatory complexes 19S and 11S REG, Hsp90 tightly associates with the 20S proteasome to mediate the recognition of aberrant proteins for degradation. In the case of the calcium signaling protein calmodulin, proteasomal degradation results from the oxidation of a single surface exposed methionine (i.e., Met145); oxidation of the other eight methionines has a minimal effect on the recognition and degradation of calmodulin by the proteasome. Since cellular concentrations of calmodulin are limiting, the targeted degradation of this critical signaling protein under conditions of oxidative stress will result in the downregulation of cellular metabolism, serving as a feedback regulation to diminish the generation of reactive oxygen species. The targeted degradation of critical signaling proteins, such as calmodulin, can function as sensors of oxidative stress to downregulate global rates of metabolism and enhance cellular survival.

Selective degradation of oxidized calmodulin by the 20 S proteasome

We have investigated the mechanisms that target oxidized calmodulin for degradation by the proteasome. After methionine oxidation within calmodulin, rates of degradation by the 20 S proteasome are substantially enhanced. Mass spectrometry was used to identify the time course of the proteolytic fragments released from the proteasome. Oxidized calmodulin is initially degraded into large proteolytic fragments that are released from the proteasome and subsequently degraded into small peptides that vary in size from 6 to 12 amino acids. To investigate the molecular determinants that result in the selective degradation of oxidized calmodulin, we used circular dichroism and fluorescence spectroscopy to assess oxidant-induced structural changes. There is a linear correlation between decreases in secondary structure and the rate of degradation. Calcium binding or the repair of oxidized calmodulin by methionine sulfoxide reductase induces comparable changes in alpha-helical content and rates of degradation. In contrast, alterations in the surface hydrophobicity of oxidized calmodulin do not alter the rate of degradation by the proteasome, indicating that changes in surface hydrophobicity do not necessarily lead to enhanced proteolytic susceptibility. These results suggest that decreases in secondary structure expose proteolytically sensitive sites in oxidized calmodulin that are cleaved by the proteasome in a nonprocessive manner.

Catalytic activities of the 20 S proteasome, a multicatalytic proteinase complex

The proteasome, a multisubunit, multicatalytic proteinase complex, is attracting growing attention as the main intracellular, extralysosomal, proteolytic system involved in ubiquitin-(Ub) dependent and Ub-independent intracellular proteolysis. Its involvement in the mitotic cycle, and control of the half-life of most cellular proteins, functions absolutely necessary for cell growth and viability, make it an attractive target for researchers of intracellular metabolism and an important target for pharmacological intervention. The proteasome belongs to a new mechanistic class of proteases, the N-terminal nucleophile hydrolases, where the N-terminal threonine residue functions as the nucleophile. This minireview focuses on the three classical catalytic activities of the proteasome, designated chymotrypsin-like, trypsin-like, and peptidyl-glutamyl-peptide hydrolyzing in eukaryotes and also the activities of the more simple Archaebacteria and Eubacteria proteasomes. Other catalytic activities of the proteasome and their possible origin are also examined. The specificity of the catalytic components toward synthetic substrates, natural peptides, and proteins and their relationship to the catalytic centers are reviewed. Some unanswered questions and future research directions are suggested.

Effect of high hydrostatic pressures on 20S proteasome activity

The 20S proteasome is the catalytic core of the ubiquitin proteolytic pathway, which is implicated in many cellular processes. The cylindrical structure of this complex consists of four stacked rings of seven subunits each. The central cavity is formed by two beta catalytic subunit rings in which protein substrates are progressively degraded. The 20S proteasome is isolated in a latent form which can be activated in vitro by various chemical and physical treatments. In this study, the effects of high hydrostatic pressures on 20S proteasome enzymatic activity were investigated. When proteasomes were subjected to increasing hydrostatic pressures, a progressive loss of peptidase activities was observed between 75 and 150 MPa. The inactivation also occurred when proteasomes were pressurized in the presence of synthetic peptide substrates; this may be the result of the dissociation of the 20S particle into its subunits under pressure, as was shown by PAGE. Pressurized proteasomes also lost their caseinolytic activity. In contrast, in the presence of casein, the pressure-induced inactivation and the dissociation of the 20S particles were prevented. In addition, in comparison to that observed at atmospheric pressure, their caseinolytic activity was increased under pressure. Following depressurization, the caseinolytic activity returned to basal levels but was further enhanced following an additional pressurization treatment. Thus, the structure of the 20S particle exhibits a certain degree of plasticity. This pressure-induced activation of the 20S proteasome is discussed in relation to its hollow structure, its currently accepted proteolytic mechanism and the general effect of high pressures on the biochemical reactions and structures of biopolymers.

Activation of the 20S proteasome of Xenopus oocytes by cardiolipin: blockage of the activation of trypsin-like activity by the substrate

The effects of an activator, cardiolipin, on the three peptidase activities of the 20S proteasome of Xenopus oocytes were examined. The trypsin-like activity was activated when the enzyme was treated with cardiolipin before the addition of the substrate, but there was no appreciable activation when cardiolipin was added concomitantly with the substrate. On the other hand, the chymotrypsin-like peptidase and peptidylglutamylpeptide hydrolase (PGPH) were activated regardless of the sequence of addition. When very low concentrations of the substrate (e.g. 0.1-0.5 microM; about 1/100 of the K(m)) were used, cardiolipin strongly activated trypsin-like peptidase by the simultaneous addition but not after substrate addition. These results suggest that the trypsin-type substrate produces a conformational change in the enzyme in a concentration-dependent manner which makes the activator sites inaccessible to cardiolipin.

Characterization of a multicatalytic proteinase complex (20S proteasome) from Trypanosoma brucei brucei

African trypanosomes are tsetse-transmitted protozoan parasites that cause sleeping sickness in humans and 'Nagana' in animals. A high relative molecular mass multicatalytic proteinase complex (MCP) was purified and biochemically characterized from the cytosolic fraction of Trypanosoma brucei brucei. The isolation procedure consisted of fractionation of the lysate by high speed centrifugation, chromatography on Q-sepharose molecular sieve filtration on Sephacryl S-300, chromatography on HA-Ultrogel and glycerol density gradient centrifugation (10-40%). The final enzyme preparation yielded a single protein band corresponding to a relative molecular mass of 630 kDa on a non-denaturing polyacrylamide gel. The enzyme hydrolyses a wide range of peptide substrates characteristic of chymotrypsin-like, trypsin-like, peptidylglutamylpeptide-hydrolysing activities determined by fluorogenic peptides, Z-Gly-Gly-Leu-NHMec, Z-Arg-Arg-NHMec and Z-Leu-Leu-Glu-beta NA, respectively. The enzyme was found to have a wide variation in pH optimal activity profile, with optimum activity against Z-Gly-Gly-Leu-NHMec at 7.8, Z-Arg-Arg-NHMec at pH 10.5 and Z-Leu-Leu-Glu-beta NA at pH 8.0, showing that the different activities are distinct. The enzyme hydrolysed oxidized proteins. In addition, the chymotryptic and trypsin-like activities were susceptible to inhibition by peptide aldehyde inhibitors with variable inhibition effects. The study demonstrates the presence of a non-lysosomal proteasome pathway of intracellular protein degradation in the bloodstream form of T. b. brucei. Further, the ability of the enzyme to hydrolyse most oxidized proteins, and the high immunogenicity exhibited suggests a possible involvement of the enzyme in pathogenesis of the disease.

The antitumor drug aclacinomycin A, which inhibits the degradation of ubiquitinated proteins, shows selectivity for the chymotrypsin-like activity of the bovine pituitary 20 S proteasome

The antitumor drug aclacinomycin A was previously shown to inhibit the degradation of ubiquitinated proteins in rabbit reticulocyte lysates with an IC50 of 52 microM (Isoe, T., Naito, M., Shirai, A., Hirai, R., and Tsuruo, T.(1992) Biochim. Biophys. Acta 1117, 131-135). We report here that from all the catalytic activities of the 20 S proteasome tested, the chymotrypsin-like activity was the only one affected by the antitumor drug. An important requirement for inhibition of the chymotrypsin-like activity seemed to be the presence of hydrophobic nonpolar residues in positions P1 to P3. Degradation of Z-E(OtBu)AL-pNA and Z-LLL-AMC at pH 7.5 was dramatically (87-98%) inhibited by 50 microM of the drug, while that of Z-GGL-pNA (containing uncharged polar residues in positions P2 and P3) and succinyl-LLVY-AMC (containing an uncharged polar residue in the P1 position) was inhibited only 11 and 24%, respectively. Aclacinomycin A had no effect on cathepsin B, stimulated trypsin, and inhibited chymotrypsin and, to a lesser extent, calpain. The aglycone and sugar moieties of the cytotoxic drug are essential for inhibition. The results presented here support a major role for the chymotrypsin-like activity in the degradation of ubiquitinated proteins. Aclacinomycin A is the first described non-peptidic inhibitor showing discrete selectivity for the chymotrypsin-like activity of the 20 S proteasome.

Effects of major-histocompatibility-complex-encoded subunits on the peptidase and proteolytic activities of human 20S proteasomes. Cleavage of proteins and antigenic peptides

The proteasome is responsible for the non-lysosomal degradation of misfolded, transient, or ubiquitin-tagged proteins. This fact and the identification of two major-histocompatibility-complex-(MHC)-encoded proteasomal subunits, LMP2/7, suggest an important role of the proteasome in antigen processing. Using purified 20S proteasomes from a wild-type and a LMP2/7-deletion T lymphoblastoid cell line, we analyzed the effect of LMP2/7 on the peptidase and proteolytic activities of the complex in the context of various purification and activation methods. The incorporation of LMP2/7 alters the peptidase activity against fluorogenic substrates, but these effects are not reflected in the time-dependent degradation pattern of oxidized insulin B chain or of peptide epitopes of an antigenic protein. No effect of LMP2/7 on the degradation pattern of these substrates was observed by either reverse-phase chromatography, pool sequencing, or mass spectrometry. The 20S proteasome can cleave insulin B chain at nearly every position, showing that the P1 position alone does not determine the cleavage sites. The maximum of the length distribution of the end products, makes these ideal candidates for MHC display; yet we find that a natural epitope derived from human histone H3 is further degraded by 20S proteasomes. Alanine scans and substitutions with related amino acids of this epitope indicate that, as in insulin B chain, the cleavage sites are not determined by the P1 position alone.

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

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

Comparison of the effect of calpain inhibitors on two extralysosomal proteinases: the multicatalytic proteinase complex and m-calpain

The potencies of three peptide aldehyde inhibitors of calpain (calpain inhibitors 1 and 2 and calpeptin) as inhibitors of four catalytic activities of the multicatalytic proteinase complex (MPC) were compared with their potencies as inhibitors of m-calpain. The chymotrypsinlike activity (cleavage after hydrophobic amino acids) and the caseinolytic activity (degradation of beta-casein) of MPC were strongly inhibited by calpain inhibitors 1 and 2 (IC50 values in the low micromolar range). Cleavage by MPC after acidic amino acids (peptidylglutamyl-peptide bond hydrolyzing activity) and basic amino acids (trypsinlike activity) was inhibited less effectively, declining moderately with increasing concentrations of calpain inhibitors 1 and 2. Calpeptin only weakly inhibited the four MPC activities, yet was the most potent inhibitor of m-calpain. These results indicate that caution must be exercised when calpain inhibitors 1 and 2 are used to infer calpain function. Calpeptin may be a better choice for such studies, although its effect on other cysteine or serine proteinases remains to be determined.

Monoclonal antibodies to the human multicatalytic proteinase (proteasome)

Multicatalytic proteinase is an intracellular enzyme composed of at least 12 different subunits. Seven murine hybridoma cell lines secreting antibodies to human multicatalytic proteinase (MCP) were established. The antibodies reacted with 4 different subunits of the oligomeric protein. Three of the antibodies bound to identical or closely spaced epitopes on the largest subunit, as shown by binding competition. Some of the antibodies cross-reacted with MCP from rat or rabbit, but none with lobster MCP. Glycoprotein components could not be detected in human MCP. The monoclonal antibodies and two polyclonal rabbit antibodies did not specifically inhibit the enzymatic activity of human MCP. Electrophoretic analysis of MCP immunoprecipitated from human placenta, liver, kidney, or HeLa cell extracts with antibodies to 3 different subunits suggested that the subunit compositions are very similar or identical.

Enzymatic changes of the bovine pituitary multicatalytic proteinase complex, induced by magnesium ions

The effect of magnesium ions on the catalytic activities of the bovine pituitary multicatalytic proteinase complex (MPC) was studied. Mg2+ markedly stimulated the breakdown of dephosphorylated beta-casein (caseinolytic activity) and the hydrolysis of Cbz-Leu-Leu-Glu-2-naphthylamide (peptidylglutamyl peptide bond hydrolyzing activity) by a 1700-fold purified preparation of MPC. Cleavage of Cbz-D-Ala-Leu-Arg-2-naphthylamide (trypsin-like activity) was strongly inhibited and cleavage of Cbz-Gly-Gly-Leu-p-nitroanilide (chymotrypsin-like activity) was weakly inhibited. Similar results were produced when enzymatic activities in the absence of Mg2+ were measured at 52 degrees C rather than at 37 degrees C. Trace protein impurities were removed by phenyl-Sepharose chromatography. This additional chromatographic step, while not changing the specific activities of hydrolysis of the three synthetic chromogenic substrates, led to a marked activation of the breakdown of dephosphorylated beta-casein. Mg2+ was not able to further stimulate the caseinolytic activities of either the phenyl-Sepharose-treated preparation or the preparation measured at 52 degrees C. Mg2+ therefore converts a "repressed" form of MPC to an "activated" form, possibly by promoting dissociation of a protein inhibitor, and may serve as a physiological regulator of this enzyme complex.