Nuclear multicatalytic proteinase alpha subunit RRC3: differential size, tyrosine phosphorylation, and susceptibility to antisense oligonucleotide treatment

The activation of the chymotrypsin-like activity of the proteasome is regulated by soluble adenyl cyclase/cAMP/protein kinase A pathway and required for human sperm capacitation

One of the first events of mammalian sperm capacitation is the activation of the soluble adenyl cyclase/cAMP/protein kinase A (SACY/cAMP/PKA) pathway. Here, we evaluated whether the increase in PKA activity at the onset of human sperm capacitation is responsible for the activation of the sperm proteasome and whether this activation is required for capacitation progress. Viable human sperm were incubated with inhibitors of the SACY/cAMP/PKA pathway. The chymotrypsin-like activity of the sperm proteasome was evaluated using a fluorogenic substrate. Sperm capacitation status was evaluated using the chlortetracycline assay and tyrosine phosphorylation. To determine whether proteasomal subunits were phosphorylated by PKA, the proteasome was immunoprecipitated and tested on a western blot using an antibody against phosphorylated PKA substrates. Immunofluorescence microscopy analysis and co-immunoprecipitation (IPP) were used to investigate an association between the catalytic subunit alpha of PKA (PKA-Cα) and the proteasome. The chymotrypsin-like activity of the sperm proteasome significantly increased after 5 min of capacitation (P < 0.001) and remained high for the remaining incubation time. Treatment with H89, KT5720 or KH7 significantly decreased the chymotrypsin-like activity of the proteasome (P < 0.001). IPP experiments indicated that PKA inhibition significantly modified phosphorylation of proteasome subunits. In addition, PKA-Cα colocalized with the proteasome in the equatorial segment and in the connecting piece, and co-immunoprecipitated with the proteasome. This is the first demonstration of sperm proteasome activity being directly regulated by SACY/PKA-Cα. This novel discovery extends our current knowledge of sperm physiology and may be used to manage sperm capacitation during assisted reproductive technology procedures.

Mammalian proteasome subtypes: Their diversity in structure and function

The 20S proteasome is a multicatalytic proteinase catalysing the degradation of the majority of intracellular proteins. Thereby it is involved in almost all basic cellular processes, which is facilitated by its association with various regulator complexes so that it appears in different disguises like 26S proteasome, hybrid-proteasome and others. The 20S proteasome has a cylindrical structure built up by four stacked rings composed of α- and β-subunits. Since the three active site-containing β-subunits can all or in part be replaced by immuno-subunits, three main subpopulations exist, namely standard-, immuno- and intermediate-proteasomes. Due to posttranslational modifications or/and genetic variations all α- and β-subunits occur in multiple iso- or proteoforms. This leads to the fact that each of the three subpopulations is composed of a variety of 20S proteasome subtypes. This review summarizes the knowledge of proteasome subtypes in mammalian cells and tissues and their possible biological and medical relevancy.

Post-translational modification of cardiac proteasomes: functional delineation enabled by proteomics

Proteasomes are ubiquitously expressed multicatalytic complexes that serve as key regulators of protein homeostasis. There are several lines of evidence indicating that proteasomes exist in heterogeneous subpopulations in cardiac muscle, differentiated, in part, by post-translational modifications (PTMs). PTMs regulate numerous facets of proteasome function, including catalytic activities, complex assembly, interactions with associating partners, subcellular localization, substrate preference, and complex turnover. Classical technologies used to identify PTMs on proteasomes have lacked the ability to determine site specificity, quantify stoichiometry, and perform large-scale, multi-PTM analysis. Recent advancements in proteomic technologies have largely overcome these limitations. We present here a discussion on the importance of PTMs in modulating proteasome function in cardiac physiology and pathophysiology, followed by the presentation of a state-of-the-art proteomic workflow for identifying and quantifying PTMs of cardiac proteasomes.

Proteasome system of protein degradation and processing

In eukaryotic cells, degradation of most intracellular proteins is realized by proteasomes. The substrates for proteolysis are selected by the fact that the gate to the proteolytic chamber of the proteasome is usually closed, and only proteins carrying a special "label" can get into it. A polyubiquitin chain plays the role of the "label": degradation affects proteins conjugated with a ubiquitin (Ub) chain that consists at minimum of four molecules. Upon entering the proteasome channel, the polypeptide chain of the protein unfolds and stretches along it, being hydrolyzed to short peptides. Ubiquitin per se does not get into the proteasome, but, after destruction of the "labeled" molecule, it is released and labels another molecule. This process has been named "Ub-dependent protein degradation". In this review we systematize current data on the Ub-proteasome system, describe in detail proteasome structure, the ubiquitination system, and the classical ATP/Ub-dependent mechanism of protein degradation, as well as try to focus readers' attention on the existence of alternative mechanisms of proteasomal degradation and processing of proteins. Data on damages of the proteasome system that lead to the development of different diseases are given separately.

Characterization and differential expression of a newly identified phosphorylated isoform of the human 20S proteasome beta7 subunit in tumor vs. normal cell lines

The search of new pharmacological targets with original mechanism of action within the ubiquitin-proteasome pathway is still a goal to be reached in oncopharmacology. Modification by phosphorylation/dephosphorylation has been found to be involved in cancer and to regulate functional activity of proteasome. Until now, phosphorylated forms of alpha subunits of the 20S human proteasome have been mostly reported. Here, we have rationally designed a polyclonal antibody specifically directed against a phosphorylated peptide sequence bearing the beta7 subunit Ser249 residue of the human 20S proteasome. This anti-beta7 phosphoSer249 antibody appeared to be a probe of choice to detect the presence of a phosphorylated isoform of the beta7 subunit of the human 20S proteasome using mono or two-dimensional gel electrophoresis. PhosphoSer249 was sensitive to acid phosphatase treatment of native 20S proteasome. Dephosphorylation affected the peptidylglutamyl-peptide hydrolyzing activity whereas the chymotrypsin-like and trypsin-like activities remained unchanged. A comparative analysis between human normal and tumor cells showed a differential expression of the phosphoSer249 beta7 isoform with a significantly lower detection in the proteasome isolated from tumor cells, suggesting its possible use as a biomarker.

Role of proteasomes in cellular regulation

The 26S proteasome is the key enzyme of the ubiquitin-dependent pathway of protein degradation. This energy-dependent nanomachine is composed of a 20S catalytic core and associated regulatory complexes. The eukaryotic 20S proteasomes demonstrate besides several kinds of peptidase activities, the endoribonuclease, protein-chaperone and DNA-helicase activities. Ubiquitin-proteasome pathway controls the levels of the key regulatory proteins in the cell and thus is essential for life and is involved in regulation of crucial cellular processes. Proteasome population in the cell is structurally and functionally heterogeneous. These complexes are subjected to tightly organized regulation, particularly, to a variety of posttranslational modifications. In this review we will summarize the current state of knowledge regarding proteasome participation in the control of cell cycle, apoptosis, differentiation, modulation of immune responses, reprogramming of these particles during these processes, their heterogeneity and involvement in the main levels of gene expression.

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

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

Proteasomes from Structure to Function: Perspectives from Archaea

Insight into the world of proteolysis has expanded considerably over the past decade. Energy-dependent proteases, such as the proteasome, are no longer viewed as nonspecific degradative enzymes associated solely with protein catabolism but are intimately involved in controlling biological processes that span life to death. The proteasome maintains this exquisite control by catalyzing the precisely timed and rapid turnover of key regulatory proteins. Proteasomes also interplay with chaperones to ensure protein quality and to readjust the composition of the proteome following stress. Archaea encode proteasomes that are highly related to those of eukaryotes in basic structure and function. Investigations of archaeal proteasomes coupled with those of eukaryotes has greatly facilitated our understanding of the molecular mechanisms that govern regulated protein degradation by this elaborate nanocompartmentalized machine.

Immunoproteasome expression in a nonimmune tissue, the ocular lens

Interferon gamma (IFN gamma) induces the expression of three catalytic subunits of the 20S proteasome that can replace their constitutive homologues to form the "immunoproteasome," named to reflect its antigen presentation function. However, immunoproteasome levels and their modulation in nonimmune tissues remain unknown. A disrupted lens differentiation program observed in transgenic mice that constitutively express IFN gamma in the immune-privileged lens tissue suggests a role for this cytokine in differentiation. We have developed a competitive RT-PCR assay that demonstrates substantially increased levels of immuno subunits and unchanged levels of constitutive subunits in transgenic compared to wild-type lenses. Similar results were observed with IFN gamma treated alpha TN4-1 lens epithelial cells. A comparison of these subunits in different immune and nonimmune mouse tissues revealed unique expression patterns. The presence of immuno subunits in nonimmune tissues such as lens suggests that the immunoproteasome may also have nonimmune functions, such as that in lens differentiation.

Mapping and structural dissection of human 20 S proteasome using proteomic approaches

The proteasome, a proteolytic complex present in all eukaryotic cells, is part of the ATP-dependent ubiquitin/proteasome pathway. It plays a critical role in the regulation of many physiological processes. The 20 S proteasome, the catalytic core of the 26 S proteasome, is made of four stacked rings of seven subunits each (alpha7beta7beta7alpha7). Here we studied the human 20 S proteasome using proteomics. This led to the establishment of a fine subunit reference map and to the identification of post-translational modifications. We found that the human 20 S proteasome, purified from erythrocytes, exhibited a high degree of structural heterogeneity, characterized by the presence of multiple isoforms for most of the alpha and beta subunits, including the catalytic ones, resulting in a total of at least 32 visible spots after Coomassie Blue staining. The different isoforms of a given subunit displayed shifted pI values, suggesting that they likely resulted from post-translational modifications. We then took advantage of the efficiency of complementary mass spectrometric approaches to investigate further these protein modifications at the structural level. In particular, we focused our efforts on the alpha7 subunit and characterized its N-acetylation and its phosphorylation site localized on Ser(250).

Electrophoretic analysis of phosphorylation of the yeast 20S proteasome

The 26S proteasome complex, consisting of two multisubunit complexes, a 20S proteasome and a pair of 19S regulatory particles, plays a major role in the nonlysosomal degradation of intracellular proteins. The 20S proteasome was purified from yeast and separated by two-dimensional gel electrophoresis (2-DE). A total of 18 spots separated by 2-DE were identified as the 20S proteasome subunits by peptide mass fingerprinting with matrix assisted laser desorption/ionization-time of flight-mass spectrometry (MALDI-TOF-MS). The alpha2-, alpha4- and alpha7-subunits gave multiple spots, which converged into one spot for each subunit when treated with alkaline phosphatase. The difference of pI between phosphorylated and dephosphorylated spots and their reaction against anti-phosphotyrosine antibody suggested that the alpha2- and alpha4-subunits are phosphorylated either at Ser or at Thr residue, and the alpha7-subunit is phosphorylated at Tyr residue(s). These phosphorylated subunits were analyzed by electrospray ionization-quadrupole time of flight-tandem MS (ESI-QTOF-MS/MS) to deduce the phosphorylation sites. The 20S proteasome has three different protease activities: chymotrypsin-like, trypsin-like and peptidylglutamyl peptide-hydrolyzing activities. The phosphatase treatment increased K(m) value for chymotrypsin-like activity of the 20S proteasome, indicating that phosphorylation may play an important role in regulating the proteasome activity.

Different proteasome subtypes in a single tissue exhibit different enzymatic properties

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

Purification and characterization of Candida albicans 20S proteasome: identification of four proteasomal subunits

The 20S proteasome from yeast cells of Candida albicans was purified by successive chromatographic steps to apparent homogeneity, as judged by nondenaturing and denaturing polyacrylamide gel electrophoresis. Its molecular mass was estimated to be 640 kDa by gel filtration. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate gave at least 10 bands in the range 20-32 kDa. Two-dimensional electrophoresis revealed the presence of at least 14 polypeptides. By electron microscopy after negative staining, the proteasome preparation appeared as typical symmetrical barrel-shaped particles. The enzyme cleaved the peptidyl-arylamide bonds in the model synthetic substrates Cbz-G-G-L-p-nitroanilide, Cbz-G-G-R-beta-naphthylamide, and Cbz-L-L-E-beta-naphthylamide (chymotrypsin-like, trypsin-like, and peptidylglutamyl-peptide-hydrolyzing activities). The differential sensitivity of these activities to aldehyde peptides and sodium dodecyl sulfate supported the multicatalytic nature of this enzyme. Three proteasomal subunits were identified as alpha6/Pre5, alpha3/Y13, and alpha5/Pup2 by internal sequencing of tryptic fragments. Their sequences perfectly matched the corresponding deduced amino acid sequences of the C. albicans genes. A fourth subunit was identified as alpha7/Prs1 by immunorecognition with a monoclonal antibody specific for C8, the human proteasome subunit homologue. Treatment of the intact isolated 20S proteasome with acid phosphatase and Western blot analysis of the separated components indicated that the alpha7/Prs1 subunit is obtained as a multiply phosphorylated protein.

Growth inhibition by a triple ribozyme targeted to repetitive B2 transcripts

The B2 family represents a group of short repetitive sequences that are found throughout the rodent genome and are analogous to the human Alu sequences. Certain B2 subfamilies are transcribed by RNA polymerase III (pol III), and this transcription is in part controlled by the retinoblastoma protein. In addition to their putative role in retrotranspositional events, these actively transcribed B2 RNAs show a predicted highly stable secondary structure. Although B2 transcripts are normally confined to the nucleus, they demonstrate altered compartmentation after carcinogen treatment, in cancers, and in immortalized and/or transformed cell lines, the significance of which is unclear. Because modulation of B2 transcripts did not seem feasible with an antisense approach, we designed a triple ribozyme (TRz) construct to down-regulate B2 transcripts. The B2-targeted TRz undergoes efficient self-cleavage, resulting in liberation of the internal hammerhead Rz, which we targeted to a single-stranded region of the consensus B2 sequence. The liberated internal targeted Rz was 20 times more active than the corresponding double-G mutant construct that could not undergo self-cleavage, and 5 times more active than the same Rz flanked by nonspecific vector sequences. The B2-targeted TRz was used to develop stable transfectant clones from an SV40-immortalized hepatocyte cell line. These transfectant clones all showed variably reduced growth rates, accompanied by significant reductions in both cytoplasmic and nuclear B2 RNA levels: linear regression analyses showed that their growth rates were directly related to residual cytoplasmic B2 levels. Reverse-transcription polymerase chain reaction (RT-PCR) analyses documented efficient self-liberation of the internal targeted Rz in vivo, and showed that the relative cytoplasmic expression levels generally paralleled the magnitude of the decrease in B2 transcripts. The RT-PCR analyses further demonstrated that up to 20% of the Rz was located in the nucleus, which presumably reflects competition between autocatalytic processing and nucleocytoplasmic transport of the initial TRz transcript.

Intracellular proteinases of invertebrates: calcium-dependent and proteasome/ubiquitin-dependent systems

Cytosolic proteinases carry out a variety of regulatory functions by controlling protein levels and/or activities within cells. Calcium-dependent and ubiquitin/proteasome-dependent pathways are common to all eukaryotes. The former pathway consists of a diverse group of Ca(2+)-dependent cysteine proteinases (CDPs; calpains in vertebrate tissues). The latter pathway is highly conserved and consists of ubiquitin, ubiquitin-conjugating enzymes, deubiquitinases, and the proteasome. This review summarizes the biochemical properties and genetics of invertebrate CDPs and proteasomes and their roles in programmed cell death, stress responses (heat shock and anoxia), skeletal muscle atrophy, gametogenesis and fertilization, development and pattern formation, cell-cell recognition, signal transduction and learning, and photoreceptor light adaptation. These pathways carry out bulk protein degradation in the programmed death of the intersegmental and flight muscles of insects and of individuals in a colonial ascidian; molt-induced atrophy of crustacean claw muscle; and responses of brine shrimp, mussels, and insects to environmental stress. Selective proteolysis occurs in response to specific signals, such as in modulating protein kinase A activity in sea hare and fruit fly associated with learning; gametogenesis, differentiation, and development in sponge, echinoderms, nematode, ascidian, and insects; and in light adaptation of photoreceptors in the eyes of squid, insects, and crustaceans. Proteolytic activities and specificities are regulated through proteinase gene expression (CDP isozymes and proteasomal subunits), allosteric regulators, and posttranslational modifications, as well as through specific targeting of protein substrates by a diverse assemblage of ubiquitin-conjugases and deubiquitinases. Thus, the regulation of intracellular proteolysis approaches the complexity and versatility of transcriptional and translational mechanisms.

Studies on rLMP7, a beta-subunit of the multicatalytic proteinase

Subunit rLMP7 of the multicatalytic proteinase (MCP), which has been associated with chymotrypsin-like proteinase activity, was examined in rat liver and hepatocyte-derived cell lines. rLMP7 was detected in both nucleus and cytosol in liver by immunohistochemistry and immunoblotting, using a peptide-specific anti-rLMP7 antibody. A M(r) 30,000 precursor protein was present only in cytosol, as was a minor component of M(r) 25,000. Mature rLMP7 (M(r) 23,000) was present in MCP in both nucleus and cytosol, although it was not detectable in the nuclear scaffold. Two rLMP7 cDNAs (designated rLMP7.1 and rLMP7.s) were identified by rapid amplification of 5' ends using RT/PCR, a result which was confirmed by Northern blot analysis and RNase protection assays. rLMP7.1 is 3-4x more abundant than rLMP7.s; it is 50 nt longer than the previously reported cDNA sequence and includes an upstream in-frame ATG within a consensus translation initiation sequence, which encodes the M(r) 30,000 rLMP7 precursor protein identified in vivo. rLMP7.s is 100 nt shorter than rLMP7.1 and does not contain the most 5' ATG. Transient transfection analyses with rLMP7.1 and rLMP7.s constructs coupled to green fluorescent protein showed that both transcripts were efficiently expressed in vivo. In vitro expression of these two rLMP7 cDNAs showed that rLMP7.1 produces the M(r) 30,000 precursor protein, whereas rLMP7.s produces two smaller peptides of M(r) 25,000 and 23,000. Purified 20S MCP preparations were able to proteolytically process the M(r) 30,000 precursor to the M(r) 25,000 product but not to the mature rLMP7 form. However, incorporation of this processed M(r) 25,000 product (or of either M(r) form produced from rLMP7.s) did not occur in vitro. In vitro processing and pulse-chase experiments suggested that the mature M(r) 23,000 subunit is derived, at least in part, from the M(r) 30,000 precursor. The M(r) 25,000 form may be a stable product produced directly from rLMP7.s.

Nuclear multicatalytic proteinase subunit RRC3 is important for growth regulation in hepatocytes

Multicatalytic proteinases (MCPs) are macromolecular structures involved in the intracellular degradation of many types of proteins. MCPs are composed of a 20S "core" of both structural (alpha) and presumed catalytic (beta) subunits, in association with regulatory proteins. They are characteristically found in both the nucleus and cytoplasm of cells, although mechanisms governing the subcellular distribution of MCPs are not known. RRC3, an alpha subunit of rat MCPs, contains both a putative nuclear localization signal (NLS) and a potential tyrosine phosphorylation site which could play a role in nuclear import, and the nuclear form of RRC3 appears to be involved in the regulation of cell growth. Here we have generated a variety of RRC3 expression constructs to study features of RRC3 important in nuclear localization and cell growth. PCR was utilized to develop constructs containing point mutations in either the putative NLS (K51 mutated to A) or at a potential tyrosine phosphorylation site (Y121 mutated to F), and an epitope from influenza hemagglutinin (HA) was added in triplicate to the C-terminus of the constructs as a means of identification. RRC3 constructs were then made in which the nucleotide sequence near the translation initiation site of RRC3 was modified in such a way that the amino acid sequence of the protein translated from the constructs is unchanged from that of normal RRC3, thus allowing differential modulation of endogenous RRC3 with antisense oligonucleotide treatment. These N-terminally modified constructs are designated mC3, mC3NLS, and mC3y. In vitro transcription/translation reactions with these constructs produced the expected products, which were immunoprecipitated with a mouse monoclonal anti-HA antibody. Immunohistochemical studies with hepatocyte cell lines transiently transfected with either mC3NLS or mC3y showed only cytoplasmic staining, whereas cells transfected with mC3 had a staining pattern typical of endogenous RRC3 (both cytoplasmic and nuclear) with strong staining of the nuclear perimeter. Immunoblot analyses of subcellular fractions from stably transfected CWSV1 cells showed mC3 product in both the cytosol and nucleus of cells, whereas mC3NLS or mC3y products were restricted to the cytosol. CWSV1 cells stably transfected with the pTet-Splice vector containing no insert (as a control) were markedly inhibited (80%) in cell growth and showed altered morphology when treated with antisense oligonucleotides targeted to endogenous RRC3, reproducing previous studies. Similarly, CWSV1 cells stably transfected with either mC3NLS or mC3y constructs showed analogous growth inhibition and morphologic alteration upon antisense treatment. In contrast, CWSV1 cells stably transfected with the mC3 construct showed normal growth and morphology following antisense oligonucleotide treatment, demonstrating that replenishment of nuclear RRC3 was necessary and sufficient to relieve growth inhibition. In 32P-metabolic labeling studies, mC3 was tyrosine-phosphorylated in cytosol as the full-length protein (M(r) 36000). mC3NLS was also phosphorylated in cytosol, whereas mC3y was not. Nuclear mC3 showed phosphorylation of a M(r) 27000 processed form while neither mC3NLS nor mC3y showed any phosphorylated nuclear products. Our results show that nuclear RRC3 is important in control of cell growth and that both the NLS and Y121 are important in nuclear localization of RRC3. Control of nuclear import by tyrosine phosphorylation may represent a novel regulatory mechanism, and our results further suggest that RRC3 may travel as a maverick subunit.

Subpopulations of proteasomes in rat liver nuclei, microsomes and cytosol

Mammalian proteasomes are composed of 14-17 different types of subunits, some of which, including major-histocompatibility-complex-encoded subunits LMP2 and LMP7, are non-essential and present in variable amounts. We have investigated the distribution of total proteasomes and some individual subunits in rat liver by quantitative immunoblot analysis of purified subcellular fractions (nuclei, mitochondria, microsomes and cytosol). Proteasomes were mainly found in the cytosol but were also present in the purified nuclear and microsomal fractions. In the nuclei, proteasomes were soluble or loosely attached to the chromatin, since they could be easily extracted by treatment with nucleases or high concentrations of salt. In the microsomes, proteasomes were on the outside of the membranes. Further subfractionation of the microsomes showed that the proteasomes in this fraction were associated with the smooth endoplasmic reticulum and with the cis-Golgi but were practically absent from the rough endoplasmic reticulum. Using monospecific antibodies for some proteasomal subunits (C8, C9, LMP2 and Z), the composition of proteasomes in nuclei, microsomes and cytosol was investigated. Although there appear not to be differences in proteasome composition in the alpha subunits (C8 and C9) in the different locations, the relative amounts of some beta subunits varied. Subunit Z was enriched in nuclear proteasomes but low in microsome-associated proteasomes, whereas LMP2, which was relatively low in nuclei, showed a small enrichment in the microsomes. These differences in subunit composition of proteasomes probably reflect differences in the function of proteasomes in distinct cell compartments.

Phosphorylation of proteasomes in mammalian cells. Identification of two phosphorylated subunits and the effect of phosphorylation on activity

The proteasome, a multimeric protease, plays an important role in nonlysosomal pathways of intracellular protein degradation. This study was undertaken to determine which subunits of mammalian proteasomes are phosphorylated and to investigate the possible role of phosphorylation in regulating proteasome activity and the association with regulatory components. Rat-1 fibroblasts were grown in the presence of [32P]phosphate and proteasomes were immunoprecipitated from cell lysates with proteasome-specific polyclonal antibodies. Subsequent analysis by two-dimensional polyacrylamide gel electrophoresis showed two radiolabeled proteasome subunits which were identified using monoclonal antibodies as C8 and C9. Treatment of human embryonic lung cells (L-132), under identical conditions, also showed the same two phosphorylated subunits. Phosphoamino acid analysis revealed phosphoserine to be present in both C8 and C9. Examination of the sequence of C9 showed a potential cGMP-dependent phosphorylation site (-Arg3-Arg-Tyr-Asp-Ser-Arg8-), whilst C8 contains several potential casein kinase II phosphorylation sites. Following immunoprecipitation by a monoclonal antibody and dephosphorylation by acid phosphatase, proteasomes were observed to have significantly lower activities when compared to phosphorylated proteasomes, implying that phosphorylation may be an important mechanism of regulating proteasome function. Free proteasomes were separated by gel-filtration from those complexed with regulatory complexes to form the 26S proteinase. The ratio of phosphorylation of C8 and C9 was found to be very similar in the two complexes but the level of phosphorylation was higher in the 26S proteinase than in free proteasomes.

Protease inhibitors and carcinogenesis: a review

This brief review article deals with the subject of anticarcinogenic activity of protease inhibitors (PI). Three basic premises are made: (1) Although PI are prevalent constituents of dietary staples such as soy products, which have been epidemiologically associated with reduced cancer incidences at multiple target sites, they are unlikely to be the active anticarcinogenic entities. Cooked soy products, which are devoid of PI activity, are equally as effective at reducing cancer development as raw soy products. Isoflavones are likely to represent major chemopreventive agents in soy, although other constituents may well contribute. (2) Although supplementation of diets with PI (natural or synthetic), or direct topical administration, results in lower cancer incidences in many experimental models in vivo, this effect appears to be indirect. Dietary PI are, in general, poorly absorbed from the GI tract, and never reach target organs in any measurable quantity. The most attractive hypothesis is that dietary PI could induce synthesis and distribution of endogenous PI (acute-phase reactants), which have widespread effects on cell growth and behavior. Effects of topical administration of PI also encompass prominent anti-inflammatory effects. (3) A spectrum of PI inhibit in vitro transformation induced by a variety of carcinogenic agents. Their effects can be grouped into three basic categories, affecting: (a) signal transduction pathways; (b) DNA repair processes; and (c) nuclear proteases. I suggest that the nuclear multicatalytic protease activity, in particular the chymotrypsin-like activity, represents an important cellular target for which considerable anecdotal support can be garnered.