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

PAC1-PAC2 proteasome assembly chaperone retains the core α4-α7 assembly intermediates in the cytoplasm

The proteasome core particle (CP) is a cytoplasmic and nuclear protease complex and is comprised of two α-rings and two β-rings stacked in order of αββα. The assembly of CP proceeds by ordered recruitment of β-subunits on an α-ring with help of assembly chaperones PAC1-PAC2, PAC3-PAC4, and UMP1. However, the mechanism of α-ring formation remains unsolved. Here, we show that α4, α5, α6, and α7 form a core intermediate as the initial process of α-ring assembly, which requires PAC3-PAC4. α1 and α3 can be incorporated independently into the core α4-α7 intermediate, whereas α2 incorporation is dependent on preceding incorporation of α1. Through these processes, PAC1-PAC2 prevents nonproductive dimerization of α-ring assembly intermediates. We also found that PAC1-PAC2 overrides the effect of nuclear localization signals of α-subunits and retains α-ring assembly intermediates in the cytoplasm. Our results first show a detailed assembly pathway of proteasomal α-ring and explain the mechanism by which CP assembly occurs in the cytoplasm.

Autophagic clearance of proteasomes in yeast requires the conserved sorting nexin Snx4

Turnover of the 26S proteasome by autophagy is an evolutionarily conserved process that governs cellular proteolytic capacity and eliminates inactive particles. In most organisms, proteasomes are located in both the nucleus and cytoplasm. However, the specific autophagy routes for nuclear and cytoplasmic proteasomes are unclear. Here, we investigate the spatial control of autophagic proteasome turnover in budding yeast (Saccharomyces cerevisiae). We found that nitrogen starvation-induced proteasome autophagy is independent of known nucleophagy pathways but is compromised when nuclear protein export is blocked. Furthermore, via pharmacological tethering of proteasomes to chromatin or the plasma membrane, we provide evidence that nuclear proteasomes at least partially disassemble before autophagic turnover, whereas cytoplasmic proteasomes remain largely intact. A targeted screen of autophagy genes identified a requirement for the conserved sorting nexin Snx4 in the autophagic turnover of proteasomes and several other large multisubunit complexes. We demonstrate that Snx4 cooperates with sorting nexins Snx41 and Snx42 to mediate proteasome turnover and is required for the formation of cytoplasmic proteasome puncta that accumulate when autophagosome formation is blocked. Together, our results support distinct mechanistic paths in the turnover of nuclear versus cytoplasmic proteasomes and point to a critical role for Snx4 in cytoplasmic agglomeration of proteasomes en route to autophagic destruction.

Immunohistochemical analysis reveals variations in proteasome tissue expression in C. elegans

The ubiquitin-proteasome system (UPS) plays a crucial part in normal cell function by mediating intracellular protein clearance. We have previously shown that UPS-mediated protein degradation varies in a cell type-specific manner in C. elegans. Here, we use formalin-fixed, paraffin-embedded C. elegans sections to enable studies on endogenous proteasome tissue expression. We show that the proteasome immunoreactivity pattern differs between cell types and within subcellular compartments in adult wild-type (N2) C. elegans. Interestingly, widespread knockdown of proteasome subunits by RNAi results in tissue-specific changes in proteasome expression instead of a uniform response. In addition, long-lived daf-2(e1370) mutants with impaired insulin/IGF-1 signaling (IIS) display similar proteasome tissue expression as aged-matched wild-type animals. Our study emphasizes the importance of alternate approaches to the commonly used whole animal lysate-based methods to detect changes in proteasome expression occurring at the sub-cellular, cell or tissue resolution level in a multicellular organism.

Involvement of ALAD-20S Proteasome Complexes in Ubiquitination and Acetylation of Proteasomal α2 Subunits

The ubiquitin-proteasome pathway has gained attention as a potential chemotherapeutic target, owing to its importance in the maintenance of protein homeostasis and the observation that cancer cells are more dependent on this pathway than normal cells. Additionally, inhibition of histone deacetylases (HDACs) by their inhibitors like Vorinostat (SAHA) has also proven a useful strategy in cancer therapy and the concomitant use of proteasome and HDAC inhibitors has been shown to be superior to either treatment alone. It has also been reported that delta-aminolevulinic acid dehydratase (ALAD) is a proteasome-associated protein, and may function as an endogenous proteasome inhibitor. While the role of ALAD in the heme biosynthetic pathway is well characterized, little is known about its interaction with, and the mechanism by which it inhibits, the proteasome. In the present study, this ALAD-proteasome complex was further characterized in cultured prostate cancer cells and the effects of SAHA treatment on the regulation of ALAD were investigated. ALAD interacts with the 20S proteasomal core, but not the 19S regulatory cap. Some ubiquitinated species were detected in ALAD immunoprecipitates that have similar molecular weights to ubiquitinated proteasomal α2 subunits, suggesting preferred binding of ALAD to ubiquitinated α2. Additionally, SAHA treatment increases levels of ALAD protein and an acetylated protein with a molecular weight similar to the ubiquitinated α2 subunit. Thus, the results of this study suggest that ALAD may play a regulatory role in a previously unreported post-translational modification of proteasomal α subunits.

Molecular alterations in proteasomes of rat liver during aging result in altered proteolytic activities

Aging induces alterations of tissue protein homoeostasis. To investigate one of the major systems catalysing intracellular protein degradation we have purified 20S proteasomes from rat liver of young (2 months) and aged (23 months) animals and separated them into three subpopulations containing different types of intermediate proteasomes with standard- and immuno-subunits. The smallest subpopulation ΙΙΙ and the major subpopulation Ι comprised proteasomes containing immuno-subunits β1i and β5i beside small amounts of standard-subunits, whereas proteasomes of subpopulation ΙΙ contained only β5i beside standard-subunits. In favour of a relative increase of the major subpopulation Ι, subpopulation ΙΙ and ΙΙΙ were reduced for about 55 % and 80 %, respectively, in aged rats. Furthermore, in all three 20S proteasome subpopulations from aged animals standard-active site subunits were replaced by immuno-subunits. Overall, this transformation resulted in a relative increase of immuno-subunit-containing proteasomes, paralleled by reduced activity towards short fluorogenic peptide substrates. However, depending on the substrate their hydrolysing activity of long polypeptide substrates was significantly higher or unchanged. Furthermore, our data revealed an altered MHC class I antigen-processing efficiency of 20S proteasomes from liver of aged rats. We therefore suggest that the age-related intramolecular alteration of hepatic proteasomes modifies its cleavage preferences without a general decrease of its activity. Such modifications could have implications on protein homeostasis as well as on MHC class I antigen presentation as part of the immunosenescence process.

BSc2118 is a novel proteasome inhibitor with activity against multiple myeloma

OBJECTIVES

The ubiquitin-proteasome system emerged as a new therapeutic target in cancer treatment. The purpose of this study was to elucidate the effects of the novel proteasome inhibitor BSc2118 on t(4;14) positive and negative multiple myeloma (MM) cells and normal peripheral blood mononuclear cells (PBMNC).

METHODS

Human MM cell lines OPM-2, RPMI-8226, and U266 and primary MM cells from bone marrow aspirates were exposed to BSc2118. Cytotoxicity levels were evaluated using the MTT-test. BSc2118-induced apoptosis was analyzed by annexin-V assay. Further methods used included proteasomal activity determination, cell cycle analysis, western blot, and transcription factor assays.

RESULTS

In OPM-2, RPMI-8226, U266 cell lines and primary MM cells, BSc2118 caused dose-dependent growth inhibitory effects. After 48 h, dose-dependent apoptosis occurred both in cell lines and primary myeloma cells irrespective of t(4;14). A significant G2-M cell cycle arrest occurred after 24 h. Furthermore, we observed a marked inhibition of intracellular proteasome activity, an increase in intracellular p21 levels, and an inhibition of NF-kappaB activation. The toxicity against PBMNC remained low, suggesting a broad therapeutic range of this agent.

CONCLUSION

Taken together, BSc2118 shows significant antimyeloma activity and may be considered as a promising agent in cancer drug development.

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.

Distinct uptake mechanisms but similar intracellular processing of two different toll-like receptor ligand-peptide conjugates in dendritic cells

Covalent conjugation of Toll-like receptor ligands (TLR-L) to synthetic antigenic peptides strongly improves antigen presentation in vitro and T lymphocyte priming in vivo. These molecularly well defined TLR-L-peptide conjugates, constitute an attractive vaccination modality, sharing the peptide antigen and a defined adjuvant in one single molecule. We have analyzed the intracellular trafficking and processing of two TLR-L conjugates in dendritic cells (DCs). Long synthetic peptides containing an ovalbumin cytotoxic T-cell epitope were chemically conjugated to two different TLR-Ls the TLR2 ligand, Pam(3)CysSK(4) (Pam) or the TLR9 ligand CpG. Rapid and enhanced uptake of both types of TLR-L-conjugated peptide occurred in DCs. Moreover, TLR-L conjugation greatly enhanced antigen presentation, a process that was dependent on endosomal acidification, proteasomal cleavage, and TAP translocation. The uptake of the CpG approximately conjugate was independent of endosomally-expressed TLR9 as reported previously. Unexpectedly, we found that Pam approximately conjugated peptides were likewise internalized independently of the expression of cell surface-expressed TLR2. Further characterization of the uptake mechanisms revealed that TLR2-L employed a different uptake route than TLR9-L. Inhibition of clathrin- or caveolin-dependent endocytosis greatly reduced uptake and antigen presentation of the Pam-conjugate. In contrast, internalization and antigen presentation of CpG approximately conjugates was independent of clathrin-coated pits but partly dependent on caveolae formation. Importantly, in contrast to the TLR-independent uptake of the conjugates, TLR expression and downstream TLR signaling was required for dendritic cell maturation and for priming of naïve CD8(+) T-cells. Together, our data show that targeting to two distinct TLRs requires distinct uptake mechanism but follows similar trafficking and intracellular processing pathways leading to optimal antigen presentation and T-cell priming.

The murine cardiac 26S proteasome: an organelle awaiting exploration

Multiprotein complexes have been increasingly recognized as essential functional units for a variety of cellular processes, including the protein degradation system. Selective degradation of proteins in eukaryotes is primarily conducted by the ubiquitin proteasome system. The current knowledge base, pertaining to the proteasome complexes in mammalian cells, relies largely upon information gained in the yeast system, where the 26S proteasome is hypothesized to contain a 20S multiprotein core complex and one or two 19S regulatory complexes. To date, the molecular structure of the proteasome system, the proteomic composition of the entire 26S multiprotein complexes, and the specific designated function of individual components within this essential protein degradation system in the heart remain virtually unknown. A functional proteomic approach, employing multidimensional chromatography purification combined with liquid chromatography tandem mass spectrometry and protein chemistry, was utilized to explore the murine cardiac 26S proteasome system. This article presents an overview on the subject of protein degradation in mammalian cells. In addition, this review shares the limited information that has been garnered thus far pertaining to the molecular composition, function, and regulation of this important organelle in the cardiac cells.

Regulation of nuclear proteasome by Rhp6/Ubc2 through ubiquitination and destruction of the sensor and anchor Cut8

While proteasome is central to the degradation of cellular ubiquitinated proteins, the control of its nuclear function is barely understood. Here we show that the fission yeast ubiquitin-conjugating Rhp6/Ubc2/Rad6 and ligating enzymes Ubr1 are responsible for nuclear enrichment of proteasome through the function of Cut8, a nuclear envelope protein. Cut8 is an Rhp6 substrate that physically interacts with and tethers proteasome. Nonubiquitinatable K-all-R Cut8 weakly interacts with proteasome and fails to enrich nuclear proteasome. Consistently, the nuclear enrichment of proteasome also fails in rhp6 and ubr1 null mutants. Further, cut8 null and cut8 K-all-R mutants are hypersensitive to DNA damage, probably due to the paucity of nuclear proteasome. Thus, Rhp6 enhances the retention of nuclear proteasome through regulating Cut8. The short-lived nature of Cut8 is crucial for feedback enrichment of the proteasome within the nucleus. This is likely to be a conserved mechanism as we describe a Cut8 homolog in flies.

Large- and small-scale purification of mammalian 26S proteasomes

The 26S proteasome is an ATP-dependent protease known to collaborate with ubiquitin, whose polymerization acts as a marker for regulated and enforced destruction of unnecessary proteins in eukaryotic cells. It is an unusually large multi-subunit protein complex, consisting of a central catalytic machine (called the 20S proteasome or CP/core particle) and two terminal regulatory subcomplexes, termed PA700 or RP/regulatory particle, that are attached to both ends of the central portion in opposite orientations to form an enzymatically active proteasome. To date, proteolysis driven by the ubiquitin-proteasome system has been shown to be involved in a diverse array of biologically important processes, such as the cell cycle, immune response, signaling cascades, and developmental programs; and the field continues to expand rapidly. Whereas the proteasome complex has been highly conserved during evolution because of its fundamental roles in cells, it has also acquired considerable diversity in multicellular organisms, particularly in mammals, such as immunoproteasomes, PA28, S5b, and various alternative splicing forms of S5a (Rpm 10). However, the details of the ultimate pathophysiological roles of mammalian proteasomes have remained elusive. This article focuses on methods for assay and purification of 26S proteasomes from mammalian cells and tissues.

Intracellular localization of proteasomes

Proteasomes are present in the cytoplasm and in the nuclei of all eukaryotic cells, however their relative abundance within those compartments is highly variable. In the cytoplasm, proteasomes associate with the centrosomes, cytoskeletal networks and the outer surface of the endoplasmic reticulum (ER). In the nucleus, proteasomes are present throughout the nucleoplasm but are void from the nucleoli. Sometimes they associate with discrete subnuclear domains called the PML nuclear bodies (POD domains). PML bodies in the nucleus, and the pericentrosomal area of the cytoplasm may function as proteolytic centers of the cell, since they are enriched in components of the proteasome system. Under conditions of impaired proteolysis proteasomes and ubiquitinated proteins further accumulate at these locations, forming organized aggregates. In case of the pericentrosomal area those aggregates have been termed "aggresomes". Once formed, aggresomes can impair the function of the proteasome system, which may promote apoptosis. Under favorable conditions they can be cleared, probably by autophagy.

Cytoplasmic trafficking of minute virus of mice: low-pH requirement, routing to late endosomes, and proteasome interaction

The cytoplasmic trafficking of the prototype strain of minute virus of mice (MVMp) was investigated by analyzing and quantifying the effect of drugs that reduce or abolish specific cellular functions on the accumulation of viral macromolecules. With this strategy, it was found that a low endosomal pH is required for the infection, since bafilomycin A(1) and chloroquine, two pH-interfering drugs, were similarly active against MVMp. Disruption of the endosomal network by brefeldin A interfered with MVMp infection, indicating that viral particles are routed farther than the early endocytic compartment. Pulse experiments with endosome-interfering drugs showed that the bulk of MVMp particles remained in the endosomal compartment for several hours before its release to the cytosol. Drugs that block the activity of the proteasome by different mechanisms, such as MG132, lactacystin, and epoxomicin, all strongly blocked MVMp infection. Pulse experiments with the proteasome inhibitor MG132 indicated that MVMp interacts with cellular proteasomes after endosomal escape. The chymotrypsin-like but not the trypsin-like activity of the proteasome is required for the infection, since the chymotrypsin inhibitors N-tosyl-L-phenylalanine chloromethyl ketone and aclarubicin were both effective in blocking MVMp infection. However, the trypsin inhibitor Nalpha-p-tosyl-L-lysine chloromethyl ketone had no effect. These results suggest that the ubiquitin-proteasome pathway plays an essential role in the MVMp life cycle, probably assisting at the stages of capsid disassembly and/or nuclear translocation.

Identification of the goldfish 20S proteasome beta6 subunit bound to nuclear matrix

Proteasomes are large, multisubunit particles that act as the proteolytic machinery for most of the regulated intracellular protein breakdown in eukaryotic cells. Proteasomes are present in both the nucleus and cytoplasm. When we analyzed the molecular composition of protein constituents of the nuclear matrix preparation of goldfish oocytes by two-dimensional polyacrylamide gel electrophoresis followed by sequence analysis, we found a 26 kDa spot identical in amino acid sequence to the beta6 subunits of the 20S proteasome. No spot of other subunits of 20S proteasome was detected. Here we describe the cloning, sequencing and expression analysis of Carassius auratus, beta6_ca, which encodes one of the proteasome beta subunits from goldfish ovary. From the screening of an ovarian cDNA library, two types of cDNA were obtained, one 941 bp and the other 884 bp long. The deduced amino acid sequences comprise 239 and 238 residues, respectively. These deduced amino acid sequences are highly homologous to those of beta6 subunits of other vertebrates. Immunoblot analysis of nuclear matrix using anti-proteasome antibodies showed only a spot of beta6_ca. These results suggest that the beta6 subunit of the goldfish 20S proteasome, beta6_ca, is responsible for anchoring proteasomes in the nucleus.

The import pathway of human and Thermoplasma 20S proteasomes into HeLa cell nuclei is different from that of classical NLS-bearing proteins

Wild-type proteasomes of human erythrocytes and the archaeon Thermoplasma acidophilum compete with each other for transport into nuclei of digitonin-permeabilized HeLa cells in the presence of an energy-regenerating system and rabbit reticulocyte lysate. 'NLS'-mutated Thermoplasma proteasomes were also able to compete with human proteasomes in the same assay, although with lower efficiency. Furthermore, in contrast to the other archaeal and bacterial cell lysates tested, the Thermoplasma cytosol efficiently supported nuclear import of human and Thermoplasma proteasomes. However, the same lysate could barely direct the nuclear transport of BSA-NLSsv40 peptide conjugates or the classical NLS-bearing protein, nucleoplasmin. Finally, additional importin alpha/beta significantly decreased the import efficiency of both human and Thermoplasma proteasomes. Taken together, these results suggest that nuclear import of proteasomes may use a novel pathway that is different from that of classical NLS-bearing proteins.

Disappearance of a novel protein component of the 26S proteasome during Xenopus oocyte maturation

We have prepared polyclonal antibodies against Xenopus 20S proteasomes. The antibodies cross-react with several proteins that are common to 20S and 26S proteasomes and with at least two proteins that are unique to 26S proteasomes. The antibodies were used to analyze changes in the components of proteasomes during oocyte maturation and early development of Xenopus laevis. A novel protein with a molecular weight of 48 kDa, p48, was clearly detected in immature oocytes, but was found at very low levels in mature oocytes and ovulated eggs. p48 was reduced to low levels during oocyte maturation, after maturation-promoting factor was activated. The amount of p48 in eggs remained low during early embryonic development, but increased again after the midblastula transition. These results show that at least one component of 26S proteasomes changes during oocyte maturation and early development and suggest that alterations in proteasome function may be important for the regulation of developmental events, such as the rapid cell cycles, of the early embryo.

Nature and role of proteasomes in maturation of fish oocytes

The proteasome is an essential component of the proteolytic pathway in eukaryotic cells and is responsible for the degradation of most cellular proteins. Proteasomes are sorted into two types, 20S and 26S. The 20S proteasome forms the catalytic core of the 26S proteasome. The 26S proteasome is involved in the ubiquitin-dependent protein degradation pathway. Cyclins and cdk inhibitors or c-mos products, proteins critical to the regulation of the cell cycle, are known to be degraded by the ubiquitin pathway. Thus the 26S proteasome is thought to be involved in the regulation of cell cycle events. This review focuses on advances in the study of the biochemical properties and functions of the 20S and 26S proteasomes in the fish meiotic cell cycle.

The DUG gene of Drosophila melanogaster encodes a structural and functional homolog of the S. cerevisiae SUG1 predicted ATPase associated with the 26S proteasome

The DUG gene of Drosophila encodes a putative ATPase that is a structural and functional homolog of the yeast SUG1 product. When introduced into S. cerevisiae, the Drosophila DUG gene rescued the lethality associated with a SUG1 mutant. Anti-DUG antibodies recognized a protein that migrated in high molecular weight complexes, along with components of the 26S proteasome, and also immunoprecipitated components of the 26S proteasome from embryonic extracts. Proteins recognized by the affinity-purified antibody raised against DUG were localized in either a punctate cytoplasmic distribution or in the nucleus, depending on the cell type, consistent with the subcellular localization of the 26S proteasome in various cell types.

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.

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.

Regulatory features of multicatalytic and 26S proteases

It should be clear from the foregoing accounts that our understanding of MCP and 26S regulation is still rudimentary. Moreover, we have only recently identified about a dozen natural substrates of these two proteases. Those outside the field may view the situation with some dismay. Those who study the MCP and 26S enzymes are provided with rich opportunities to address fundamental questions of protein catabolism and metabolic regulation.

Puromycin-sensitive aminopeptidase. Sequence analysis, expression, and functional characterization

Among the molecular mechanisms that control the cell division cycle, proteolysis has emerged as a key regulatory process enabling cells to pass critical check points. Such proteolysis involves a cascade of enzymes including a multisubunit complex termed 26S proteasome. Here we report on the analysis of a novel mouse cDNA encoding the puromycin-sensitive aminopeptidase (PSA) and on its expression in COS cells and 3T3 fibroblasts. PSA is 27-40% homologous to several known Zn(2+)-binding aminopeptidases including aminopeptidase N. Immunohistochemical analysis revealed that PSA is localized to the cytoplasm and to the nucleus and associates with microtubules of the spindle apparatus during mitosis. Furthermore, puromycin and bestatin both arrested the cell cycle, leading to an accumulation of cells in G2/M phase, and ultimately induced cells to undergo apoptosis at concentrations that inhibit PSA. Control experiments including cycloheximide further suggested that the induction of apoptosis by puromycin was not attributable to inhibition of protein synthesis. Taken together, these data favor the novel idea that PSA participates in proteolytic events essential for cell growth and viability.

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

Multicatalytic proteinases (MCPs) are macromolecular structures involved in intracellular degradation of many types of proteins. MCPs are composed of a 20S "core" which consists of both structural (alpha) and presumed catalytic (beta) subunits in association with complexes of accessory proteins. Immunohistochemical studies have shown MCP subunits to be largely cytoplasmic, although nuclear localization is also observed. Reverse transcription/polymerase chain reaction amplifications were performed with redundant primers to conserved regions within known subunits, in an attempt both to identify potential new subunits and to define the repertoire of subunits expressed in hepatocytes. No new subunits were identified, and we found that RRC3, an alpha subunit of MCPs which contains a putative nuclear localization signal (NLS), was the predominant alpha subunit expressed in hepatocytes and hepatocyte-derived cell lines. Antibodies were developed against a unique C-terminal peptide region of RRC3. Immunohistochemical studies using affinity-purified antibodies showed that RRC3 has both cytoplasmic and nuclear localizations. Immunoprecipitation/immunoblot analyses showed that a significant proportion of nuclear RRC3 was associated with the nuclear scaffold (NS). NS RRC3 showed a significantly smaller M(r) (24,000) than the cytoplasmic form (M(r) 28,000), and only the nuclear form contained phosphotyrosine. In metabolic labeling experiments with [32P]orthophosphate, the major nuclear and NS form observed showed an M(r) of 24,000, whereas no labeling of cytosolic RRC3 was observed. A minor 32P-labeled band of M(r) 28,000 was also observed in nuclei, and this M(r) 28,000 form was found in the soluble nuclear extract within MCP complexes. These results suggest that tyrosine phosphorylation of the cytosolic form (M(r) 28,000) rapidly triggers nuclear import, which is in turn quickly followed by conversion to the major M(r) 24,000 form associated with NS. Treatment with antisense oligonucleotides targeted to the initiation site of RRC3 reduced the growth of a hepatocyte-derived cell line by 95% and produced a marked morphological change (in the absence of overt toxicity). Under these treatment conditions, RRC3 mRNA was dramatically reduced. RRC3 protein was also dramatically reduced in the NS, but showed only a small reduction in cytosol, suggesting that the nuclear RRC3 may be important in cell growth and differentiation.

Degradation of proteasomes by lysosomes in rat liver

Proteasomes are high-molecular-mass multisubunit complexes which are believed, either by themselves or as a part of the 26S proteinase complex, to play a central role in extralysosomal pathways of intracellular protein breakdown. We have addressed the degradation of proteasomes in rat liver, investigating the possible role of lysosomes. Affinity-purified antibodies against rat liver proteasomes were used for immunoblot analysis of isolated lysosomes. Although proteasomes are not found in lysosomes from normally fed rats, they were found to accumulate in lysosomes of rats treated with leupeptin (an inhibitor of lysosomal proteases) and could also be detected in lysosomes isolated from livers of starved (24 h) rats. Proteinase-K treatment of these fractions, as well as immunogold procedures, show that a proportion of the proteasomes are inside lysosomes. Comparison of the amount of proteasomes found in lysosomes by immunoblotting with their experimentally determined half life (8.3 days) is consistent with an important role of these organelles in the degradation of rat liver proteasomes. Nevertheless, these data do not exclude the possibility that some nonlysosomal degradation of proteasome components also occurs. Since proteasomes were localized in autophagic vacuoles, it is likely that they are taken up mainly by nonselective autophagy. However, using an in vitro system, it was found that, under conditions of starvation, proteasomes may also be taken up into lysosomes and degraded via the heat-shock cognate protein of 73 kDa (hsc73)-mediated transport.

Roles of proteasomes in cell growth

Proteasomes are large, unique protein complexes catalyzing energy- and ubiquitin-dependent proteolysis. Recent studies have revealed that these complexes are involved in two important cellular functions. One is to make antigen fragments for major histo-compatibility complex (MHC) class I-restricted antigen presentation and the other is to regulate the cell cycle by proteolysis. Here we review only the latter function of proteasomes. Proteasomes are widely distributed in eukaryotic cells, but their levels have been shown to be particularly high in various immature cells, such as cancerous, fetal and lymphoblastic cells, and agents including cell differentiation were found to suppress their expression. These conditions also regulate the expression of ubiquitin genes in a similar way, suggesting that proteasomes act ubiquitin-dependently in their 26S form in immature cells. High levels of proteasomes were found immunochemically in the nuclei of rapidly growing cells, indicating that proteasomes are important for eukaryotic cell growth. Indeed, gene disruptions of most subunits of proteasomes in yeast resulted in total suppression of cell growth and cell death. Short-lived regulatory factors of the cell cycle, such as Fos, p53, Mos, and cyclins are degraded by the proteasome-ubiquitin pathway under phosphorylated or dephosphorylated conditions. Ornithine decarboxylase, which is also a short-lived enzyme and is involved in the early phase of cell growth, is quickly degraded by proteasomes with antizyme, but without ubiquitination. Recently, we found that one of the regulatory factors of 26S proteasomes, p31, is a homologue of Nin1p, whose mutation caused inhibition of the cell cycle in yeast. These results indicate that proteasomes play important roles in regulation of the cell cycle in eukaryotes.

Cytolocation of prosome antigens on intermediate filament subnetworks of cytokeratin, vimentin and desmin type

Analysis by double-label indirect immunofluorescence of PtK1 and HeLa cells had previously demonstrated that prosome* antigens form networks that superimpose on those of the intermediate filaments of the cytokeratin type. We show here that in PtK1 cells various prosomal antigens also reside to a variable extent on intermediate filaments subnetworks of the vimentin type. In proliferating human fibroblasts the prosome and vimentin networks were found to coincide, while in proliferating myoblasts of the C2.7 mouse myogenic cell line the prosomal antigens seem to superimpose on the intermediate filaments of the desmin type. Thus, the prosomes, which are RNP particles of variable composition and subcomplexes of untranslated mRNP, and carry a multicatalytic proteinase activity, seem to co-localize with the specific kind of cytoplasmic intermediate filament in relation to the cell type. These results, which generalize the previous data, are discussed in view of possible role(s) for prosomes in mRNA metabolism and/or intermediate filaments remodelling.

Molecular cloning of cDNAs for two Xenopus proteasome subunits and their expression in adult tissues

Proteasome, a large protein complex with ATP-dependent protease activities, is composed of non-identical but closely related multi-subunits. Using cDNAs for rat proteasome subunits as probes, we obtained three cDNA clones for two Xenopus proteasome subunits from ovary cDNA library. The primary structures of the three cDNAs showed high homology to the corresponding proteasome subunits of other mammalian species (above 90%) and also considerable homology to those of Drosophila and yeast. These results indicate that the sequences of proteasome subunits are well conserved during evolution. Northern blot hybridization revealed that RNAs for the newly isolated subunits (XC8 and XC9) and the previously isolated subunit (XC3) occur at very high levels in testis and ovary, at moderately high levels in lung, skin kidney and spleen, and at low levels in liver, stomach and muscle. It was also shown that relative amounts of the mRNAs for the three subunits are similar in all the adult tissues examined. From these results, we concluded that the expression of the genes for the three subunits (XC3 XC8 and XC9-1) takes place in a roughly coordinated manner in different adult tissues.

The major-histocompatibility-complex-encoded beta-type proteasome subunits LMP2 and LMP7. Evidence that LMP2 and LMP7 are synthesized as proproteins and that cellular levels of both mRNA and LMP-containing 20S proteasomes are differentially regulated

The proteasome (high-molecular-mass multicatalytic proteinase complex) is composed of a large number of non-identical protein subunits of the alpha and beta types. The mouse beta-type subunits LMP2 and LMP7 (LMP, low-molecular-mass protein) are encoded within the mouse major histocompatibility complex (MHC II) region, and are thought to connect the proteasome to the MHC class-I antigen-processing pathway. In the present communication, we have analysed the two proteasome subunits with regard to their identity within the proteasome complex, their protein levels, their amounts of mRNA in different mouse tissues and cell lines, and have investigated the intracellular localization of LMP2 and LMP7 subunits in thymus and liver by immunocytology. Our experiments indicate that LMP2 and LMP7 subunits are synthesized as precursor proteins of 24 kDa and 30 kDa, respectively, and that only the processed 21-kDa and 23-kDa subunits are part of the 20S proteasome complex. The proportion of LMP2-subunit-containing and LMP7-subunit-containing proteasome complexes, as well as LMP2 and LMP7 mRNA levels, vary strongly and are shown to be dependent on the tissues or cell lines analysed. Furthermore, high LMP2 and LMP7 mRNA levels do not always correlate with high protein levels, suggesting a specific translational mechanism which controls proteasome subunit synthesis. Generally, mRNA levels appear to be particularly high in those tissues which are known to be involved in MHC class-I antigen presentation. Immunocytological analysis shows a strong nuclear localization of the subunits in cells of the thymus, while in the liver they appear to be evenly distributed between the two cellular compartments. Our data support the idea that both LMP2 and LMP7 proteins are non-essential proteasome subunits which are probably involved in the regulation of proteasome activities. The function of the two subunits, however, may not be restricted to the proposed role of proteasomes in antigen presentation.

Peptide sequencing identifies MSS1, a modulator of HIV Tat-mediated transactivation, as subunit 7 of the 26 S protease

Subunit 7 is an integral component of the human erythrocyte 26 S protease. Peptide sequence analysis reveals that 22 amino acids from the N-terminus of subunit 7 correspond exactly to the N-terminus of MSS1, a modulator of HIV gene expression. Additional internal peptides from subunit 7 obtained by CNBr cleavage also match 100% with the deduced amino acid sequence of MSS1. Based on the fact that directly sequenced peptides from subunit 7 are identical to more than 12% of the hypothetical translation product of MSS1, and the fact that the molecular weight of subunit 7 (49 kDa) corresponds to the predicted molecular weight of MSS1 (48,633 Da), we conclude that subunit 7 is MSS1.

Proteasomes: multicatalytic proteinase complexes

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Changes in intracellular localization of proteasomes in immortalized ovarian granulosa cells during mitosis associated with a role in cell cycle control

We describe the isolation and characterization of proteasomes from recently established immortalized ovarian granulosa cell lines and their intracellular distribution during mitosis and during cAMP-induced differentiation, as revealed by immunofluorescence microscopy. In interphase, proteasomes were localized in small clusters throughout the cytoplasm and the nuclear matrix. In prophase, a substantial increase in proteasomal staining was observed in the perichromosomal area. A dramatic increase occurred in metaphase and in early anaphase; the chromosomes remained unstained. In late anaphase, intensive staining remained associated mainly with the spindle fibers. In telophase and early interphase of the daughter cells, intensive staining of proteasomes persisted in the nuclei. In contrast, in cells stimulated to differentiate by forskolin, which substantially elevates intracellular cAMP in these cell lines, only a weak staining of proteasomes was revealed in both the nucleus and the cytoplasm. Double staining of nondividing cells with antibodies to proteasomes and to tubulin did not show colocalization of proteasomes and microtubules. In contrast, dividing cells show a preferential concentration of proteasomes around spindle microtubules during metaphase and anaphase. The observed spatial and temporal distribution pattern of proteasomes during mitosis is highly reminiscent of the behavior of cyclins [Pines, J. & Hunter, T. (1991) J. Cell Biol. 115, 1-17]. Since proteasome accumulation appears to coincide with disappearance of cyclins A and B1 from the spindle apparatus, it is suggested that proteasomes may play a role in termination of mitosis by degrading the cyclins, which act as regulatory elements.

Localization of a sequence motif complementary to the nuclear localization signal in proteasomes from Thermoplasma acidophilum by immunoelectron microscopy

A sequence motif complementary to the nuclear localization signal (NLS) has been localized in proteasomes from Thermoplasma acidophilum by immunoelectron microscopy using sequence-specific antibodies. The antibodies were generated in two different ways: by immunization with a carrier-coupled peptide and by isolation of the sequence-specific antibody from an immune serum against native proteasomes using a peptide-affinity column. The sequence specificity of the isolated antibody was confirmed by a PEPSCAN-ELISA performed on overlapping nonapeptides deduced from the sequence of the alpha-subunit of the Thermoplasma proteasome. Compared to the antibody induced by the carrier-coupled peptide this antibody fraction showed a much higher affinity for native proteasomes. The attachment site of the Fab portion of the antibody to the proteasome was mapped by electron microscopy in conjunction with image processing. The antibody was found to bind to the periphery of the two outer "disks" of the proteasome complex formed by the alpha-subunits.

Immunocytochemical localization of the multicatalytic proteinase (proteasome) in crustacean striated muscles

Multicatalytic proteinase (MCP) is thought to play a central role in the processing and turnover of intracellular proteins in eukaryotic cells. Immunocytochemistry was used to determine the intracellular distribution of the MCP in the claw muscles of the land crab, Gecarcinus lateralis, and the claw and abdominal muscles of the American lobster, Homarus americanus. Cryosections were stained with an affinity-purified polyclonal antibody to lobster MCP that cross-reacted with the land crab enzyme. Two types of staining were observed: a diffuse cytoplasmic staining, and a dense aggregate staining primarily associated with invaginations of the cell membrane. The cytoplasmic staining appeared reticulated in favorable transverse sections due to a preferential localization of MCP to the intermyofibrillar space. The aggregate staining was associated with neither nuclei nor mitochondria, since stains specific for these organelles (Hoechst stain and nicotinamide adenine dinucleotide diaphorase histochemistry, respectively) did not colocalize with the aggregates. Trypsinlike peptidase activities of isolated microsomal and postmicrosomal fractions indicated that less than 1% of the total MCP was associated with the microsomal fraction. Immunoprecipitation of the same fractions confirmed the presence of MCP in the microsomes as well as in the cytosol. These results suggest that the MCP is primarily associated with cytoplasmic components; the aggregate staining may result from the association of the MCP with cellular membrane systems.

Inhibition of DNA polymerases by tripeptide derivative protease inhibitors

Benzyloxycarbonyl(Z)-Leu-Leu-Leu-al and dansyl(Dns)-Leu-Leu-Leu-CH2Cl, well known as protease inhibitors, effectively inhibit the activities of DNA polymerases alpha, beta and gamma from rat liver and pol I from Escherichia coli, but the ability of these inhibitors to inhibit terminal deoxynucleotidyl transferase (TdT) is weak. The mode of inhibition by these tripeptide analogues is non-competitive with dNTP. The Ki values for Z-Leu-Leu-Leu-al and Dns-Leu-Leu-Leu-CH2Cl are 6.25 x 10(-5) M and 6.56 x 10(-5) M, respectively.

Proteolysis, proteasomes and antigen presentation

Proteins presented to the immune system must first be cleaved to small peptides by intracellular proteinases. Proteasomes are proteolytic complexes that degrade cytosolic and nuclear proteins. These particles have been implicated in ATP-ubiquitin-dependent proteolysis and in the processing of intracellular antigens for cytolytic immune responses.

Comparison of the multicatalytic proteinases isolated from the nucleus and cytoplasm of chicken red blood cells

1. Two chromatographically distinct multicatalytic proteinases (MCP's) were isolated from the cytoplasm of chicken red blood cells and one MCP was purified from the nuclei. 2. The nuclear and the majority (97-99%) of the cytoplasmic multicatalytic proteolytic activity were chromatographically similar and differed from the minor cytoplasmic activity in their elution from hydroxylapatite, number of subunits on 2D-SDS-PAGE, and in their sensitivity to proteinase inhibitors. 3. Dichloroisocoumarin, a serine proteinase inhibitor, inhibited the hydrolysis of fluorogenic peptides but stimulated the degradation of casein by the multicatalytic proteinases suggesting that this enzyme has distinct active sites for protein and peptide hydrolysis.

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.

Cell cycle-dependent change of proteasome distribution during embryonic development of the ascidian Halocynthia roretzi

The proteasome is a multicatalytic proteinase complex composed of nonidentical subunits. By immunocytochemical analysis using monoclonal antibody raised against the egg proteasome, we demonstrate that the proteasome undergoes changes in its subcellular distribution, depending on the cell division cycle during embryonic development of the ascidian Halocynthia roretzi. During interphase, the proteasome is localized in the nucleus, i.e., in the nucleoplasm and along the nuclear membrane. The proteasome disappears from the nucleoplasm in prophase and from the nuclear envelope in prometaphase. During early metaphase, the proteasome is detectable in the chromosomes and, at late stages of metaphase, the immunoreactivity also occurs in the peripheral region of each spindle pole and at the mitotic spindle. In anaphase, however, the staining disappears in the mitotic apparatus. In telophase, the proteasome is again localized in the newly formed nucleus. In addition to the localization in the nucleus and around the mitotic apparatus, the proteasome shows cytoplasmic localization throughout the cell division cycle. Such a change of subcellular distribution of the proteasome is clearly demonstrated in the synchronously dividing blastomeres and also is believed to occur in the postcleavage embryos. These observations suggest that the proteasome may play a key role in the progression of cell division cycle.

Primary structure of the Thermoplasma proteasome and its implications for the structure, function, and evolution of the multicatalytic proteinase

The proteasome or multicatalytic proteinase is a high molecular mass multisubunit complex ubiquitous in eukaryotes but also found in the archaebacterial proteasome is made of two different subunits only, and yet the complexes are almost identical in size and shape. Cloning and sequencing the gene encoding the small (beta) subunit of the T. acidophilum complex completes the primary structure of the archaebacterial proteasome. The similarity of the derived amino acid sequences of 233 (alpha) and 211 (beta) residues, respectively, indicates that they arose from a common ancestral gene. All the sequences of proteasomal subunits from eukaryotes available to date can be related to either the alpha-subunit or beta-subunit of the T. acidophilum "Urproteasome", and they can be distinguished by means of a highly conserved N-terminal extension, which is characteristic for alpha-type subunits. On the basis of circumstantial evidence we suggest that the alpha-subunits have regulatory and targeting functions, while the beta-subunits carry the active sites.

Molecular cloning and functional analysis of three subunits of yeast proteasome

The genes encoding three subunits of Saccharomyces cerevisiae proteasome were cloned and sequenced. The deduced amino acid sequences were homologous not only to each other (30 to 40% identity) but also to those of rat and Drosophila proteasomes (25 to 65% identity). However, none of these sequences showed any similarity to any other known sequences, including various proteases, suggesting that these proteasome subunits may constitute a unique gene family. Gene disruption analyses revealed that two of the three subunits (subunits Y7 and Y8) are essential for growth, indicating that the proteasome and its individual subunits play an indispensable role in fundamental biological processes. On the other hand, subunit Y13 is not essential; haploid cells with a disrupted Y13 gene can proliferate, although the doubling time is longer than that of cells with nondisrupted genes. In addition, biochemical analysis revealed that proteasome prepared from the Y13 disrupted cells contains tryptic and chymotryptic activities equivalent to those of nondisrupted cells, indicating that the Y13 subunit is not essential for tryptic or chymotryptic activity. However, the chymotryptic activity of the Y13 disrupted cells is not dependent on sodium dodecyl sulfate (SDS), an activator of proteasome, since nearly full activity was observed in the absence of SDS. Thus, the activity in proteasome of the Y13 disrupted cells might result in unregulated intracellular proteolysis, thus leading to the prolonged cell cycle. These results indicate that cloned proteasome subunits having similar sequences to the yeast Y13 subunit are structural, but not catalytic, components of proteasome. It is also suggested that two subunits (Y7 and Y8) might occupy positions essential to proteasome structure or activity, whereas subunit Y13 is in a nonessential but important position.

Molecular cloning and functional analysis of three subunits of yeast proteasome

The genes encoding three subunits of Saccharomyces cerevisiae proteasome were cloned and sequenced. The deduced amino acid sequences were homologous not only to each other (30 to 40% identity) but also to those of rat and Drosophila proteasomes (25 to 65% identity). However, none of these sequences showed any similarity to any other known sequences, including various proteases, suggesting that these proteasome subunits may constitute a unique gene family. Gene disruption analyses revealed that two of the three subunits (subunits Y7 and Y8) are essential for growth, indicating that the proteasome and its individual subunits play an indispensable role in fundamental biological processes. On the other hand, subunit Y13 is not essential; haploid cells with a disrupted Y13 gene can proliferate, although the doubling time is longer than that of cells with nondisrupted genes. In addition, biochemical analysis revealed that proteasome prepared from the Y13 disrupted cells contains tryptic and chymotryptic activities equivalent to those of nondisrupted cells, indicating that the Y13 subunit is not essential for tryptic or chymotryptic activity. However, the chymotryptic activity of the Y13 disrupted cells is not dependent on sodium dodecyl sulfate (SDS), an activator of proteasome, since nearly full activity was observed in the absence of SDS. Thus, the activity in proteasome of the Y13 disrupted cells might result in unregulated intracellular proteolysis, thus leading to the prolonged cell cycle. These results indicate that cloned proteasome subunits having similar sequences to the yeast Y13 subunit are structural, but not catalytic, components of proteasome. It is also suggested that two subunits (Y7 and Y8) might occupy positions essential to proteasome structure or activity, whereas subunit Y13 is in a nonessential but important position.

Molecular cloning of cDNAs for two subunits of rat multicatalytic proteinase. Existence of N-terminal conserved and C-terminal diverged sequences among subunits

cDNA clones for two subunits (designated subunits K and L) of rat liver multicatalytic proteinase (MCP) were isolated using oligonucleotide probes synthesized according to their partial amino acid sequences. The encoded polypeptides of subunits K and L consisted of 255 and 261 amino acid residues with calculated molecular mass of 28.3 kDa and 29.5 kDa, respectively. Northern blot analysis revealed that subunits K and L were expressed in all tissues examined and their expression patterns were almost identical. The deduced amino acid sequences showed no similarities to known protein sequences other than the recently reported sequences of rat and Drosophila MCP subunits. Sequence comparison of MCP subunits of rat and Drosophila revealed that the N-terminal two-thirds of the sequence (especially the N-terminal approximately 20 residues) is conserved, but the C-terminal third of the sequence shows no similarity, suggesting functional and structural roles for both regions. Implications for the structural and functional aspects of MCP subunits are discussed based on the sequence similarity.

Possible mechanism of nuclear translocation of proteasomes

Proteasomes (multicatalytic proteinase complexes), which are identical to the ubiquitous eukaryotic 20S particles, are localized in both the cytoplasm and the nucleus, but the mechanism of their co-localization in the two compartments is unknown. On examination of the primary structures of subunits of proteasomes, a consensus sequence for nuclear translocation of proteins, X-X-K-K(R)-X-K(R) (where X is any residue), was found to be present in some subunits and to be highly conserved in the subunits of a wide range of eukaryotes. In addition, proteasomal subunits were found to bear a cluster of acidic amino acid residues and also a potential tyrosine phosphorylation site that was located in the same polypeptide chain as the nuclear location signal. These structural properties suggest that two sets of clusters with positive and negative charges serve to regulate the translocation of proteasomes from the cytoplasm to the nucleus, and that phosphorylation of tyrosine in certain subunits may play an additional role in transfer of proteasomes into the nucleus.