Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:3.4.25.1 (proteasome)
28,817 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The primary structures of two proteins that comprise PA28, an activator of the 20S proteasome, have been determined by cDNA cloning and sequencing. These protein subunits, termed PA28 alpha and PA28 beta, are about 50% identical to one another and are highly conserved between rat and human. PA28 alpha and PA28 beta are homologous to a previously described protein, Ki antigen, whose function is unknown. PA28 alpha, but neither PA28 beta nor Ki antigen, contains a 'KEKE motif', which has been postulated to promote the binding of proteins having this structural feature. PA28 alpha and PA28 beta were coordinately regulated by gamma-interferon, which greatly induced mRNA levels of both proteins in cultured cells. The mRNA level of the Ki antigen also increased in response to gamma-interferon treatment, but the magnitude of the increase was less than that for the PA28s, and the effect was transient. These results demonstrate the existence of a new protein family, at least two of whose members are involved in proteasome activation. They also provide the basis for future structure/function studies of PA28 subunits and the determination of their relative physiological roles in the regulation of proteasome activity.
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PMID:Primary structures of two homologous subunits of PA28, a gamma-interferon-inducible protein activator of the 20S proteasome. 778 12

Using reverse transcription polymerase chain reaction, we cloned and sequenced full-length mouse cDNAs for the two homologous subunits of the proteasome activator PA28 (PA28alpha and PA28beta), as well as for the related protein Ki. These proteins are highly conserved among species. Northern blot analysis of PA28a, PA28b, and Ki mRNA demonstrated broad tissue distribution. Although single transcripts were detected for PA28a and PA28b, two different sized transcripts were detected for mouse Ki, suggesting either alternative splicing or alternate polyadenylation sites. The levels of these transcripts increased in response to interferon-gamma (IFN-gamma) treatment in mouse H6 hepatoma cells, although PA28a and PA28b were induced to a greater extent than Ki, and the effect of IFN-gamma stimulation on Ki expression was transient. Southern blot analysis suggests that both PA28a and PA28b are multiple-copy genes, while Ki is a single-copy gene.
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PMID:Sequence and expression of mouse proteasome activator PA28 and the related autoantigen Ki. 916 94

The 20S proteasome is a multi-subunit protease responsible for the production of peptides presented by major histocompatibility complex (MHC) class I molecules. Recent evidence indicates that an interferon-gamma (IFN-gamma)-inducible PA28 activator complex enhances the generation of class I binding peptides by altering the cleavage pattern of the proteasome. In the present study, we determined the primary structures of the mouse PA28 alpha- and beta-subunits. The deduced amino acid sequences of the alpha- and beta-subunits were 49% identical. We also determined the primary structure of the mouse PA28 gamma-subunit (Ki antigen), a protein of unknown function structurally related to the alpha- and beta-subunits. The amino acid sequence identity of the gamma-subunit to the alpha- and beta-subunits was 40% and 32%, respectively. Interspecific backcross mapping showed that the mouse genes coding for the alpha- and beta-subunits (designated Psme1 and Psme2, respectively) are tightly linked and map close to the Atp5g1 locus on chromosome 14. Thus, unlike the LMP2 and LMP7 subunits, the IFN-gamma-inducible subunits of PA28 are encoded outside the MHC. The gene coding for the gamma-subunit (designated Psme3) was mapped to the vicinity of the Brca1 locus on chromosome 11. A computer search of the DNA databases identified a gamma-subunit-like protein in ticks and Caenorhabditis elegans, the organisms with no adaptive immune system. It appears that the IFN-gamma-inducible alpha- and beta-subunits emerged by gene duplication from a gamma-subunit-like precursor.
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PMID:PA28 subunits of the mouse proteasome: primary structures and chromosomal localization of the genes. 921 37

Rapamycin (RAPA) is a potent immunosuppressive drug, and certain of its direct or indirect targets might be of vital importance to the regulation of an immune response. In this study, we used differential hybridization to search for human genes whose expression was sensitive to RAPA. Seven RAPA-sensitive genes were found and one of them encoded a protein with high homology to the alpha subunit of a proteasome activator (PA28 alpha). This gene was later found to code for the beta subunit of the proteasome activator (PA28 beta). Activated T and B cells had up-regulated PA28 beta expression at the mRNA level. Such up-regulation could be suppressed by RAPA, FK506, and cyclosporin A. RAPA and FK506 also repressed the up-regulated PA28 alpha messages in phytohemagglutinin (PHA)-stimulated T cells. At the protein level, RAPA inhibited PA28 alpha and PA28 beta in the activated T cells according to immunoblotting and confocal microscopy. Probably as a consequence, there was a fourfold increase of proteasome activities in the peripheral blood mononuclear cell lysate after the PHA activation. RAPA could inhibit the enhanced part of the proteasome activity. Considering the critical role played by the proteasome in degrading regulatory proteins, our data suggest that the proteasome activator is a relevant and important downstream target of rapamycin, and that the immune response could be modulated through the activity of the proteasome.
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PMID:Rapamycin inhibits proteasome activator expression and proteasome activity. 939 99

Two activators, named PA700 and PA28, are known to bind to 20 S proteasomes, forming two different complexes. The PA700-proteasome complex, also known as the 26 S proteasome, can degrade intact proteins, whereas complexes with PA28 can degrade only peptides. Monoclonal antibodies to 20 S proteasomes or the p45 ATPase subunit (Trip1, Sug1) of PA700 precipitated the same set of proteins from HeLa extracts, including six different ATPase subunits of PA700. This shows that p45 is not present in other protein complexes and suggests that all 26 S proteasome particles contain the same set of ATPase subunits. Interferons alpha and gamma had no effect on the composition of the 26 S proteasome, except for the replacement of subunits delta, MB1 and Z with Lmp2, Lmp7 and MECL1 respectively. Surprisingly, antibodies to PA28 precipitated p42, a component of PA700. Conversely, anti-p45 antibodies precipitated not only 26 S proteasomes but also PA28 alpha, beta and gamma, indicating that 20 S proteasomes can simultaneously bind both PA700 and PA28. PA28 alpha beta is known to be involved in antigen presentation. Conceivably, intact substrate proteins are recognized by PA700 and fed into proteasomes whose cleavage specificity is optimized for antigen presentation on MHC class I by PA28 and three interferon inducible proteasome subunits.
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PMID:Simultaneous binding of PA28 and PA700 activators to 20 S proteasomes. 962 Aug 78

The activated 20 S proteasome, which has been found only in mammalian cells, is composed of two heptamer rings of an activator protein on each end of the 20 S proteasome and is inducible by interferon-gamma. A 20 S proteasome has been recently identified in a protozoan pathogen Trypanosoma brucei, but there has been no experimental evidence yet for the presence of a 26 S proteasome. Instead, an activated form of 20 S proteasome was isolated from this organism, which has significantly enhanced peptidase activities. It consists of an additional activator protein with an estimated molecular mass of 26 kDa (PA26) (To, W. Y., and Wang, C. C. (1997) FEBS Lett. 404, 253-262). The profile and sequences of tryptic peptides from PA26 were determined by mass spectrometry; no matches were found in the data base. The peptide sequences were used in reverse transcriptase-polymerase chain reaction to isolate a full-length cDNA clone encoding PA26. The protein sequence thus derived from it indicates little sequence identity with those of mammalian activator proteins PA28 alpha, beta, or gamma. There is only a single copy of PA26 gene in T. brucei. Purified recombinant PA26 polymerizes spontaneously to form heptamer ring with an outer diameter of 8.5 nm. The ring binds and activates 20 S proteasomes from T. brucei as well as rat, whereas human PA28alpha can neither bind nor activate T. brucei 20 S proteasome. The former is thus apparently more ubiquitous than PA28 in its capability of binding to and activating 20 S proteasomes. Its presence in T. brucei may also suggest a more ancient origin of proteasome activator proteins and a much wider involvement in protein degradation among other eukaryotic organisms than was originally envisaged.
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PMID:Structural and functional characterizations of the proteasome-activating protein PA26 from Trypanosoma brucei. 1056 54

In vitro PA28 binds and activates proteasomes. It is shown here that mice with a disrupted PA28b gene lack PA28a and PA28b polypeptides, demonstrating that PA28 functions as a hetero-oligomer in vivo. Processing of antigenic epitopes derived from exogenous or endogenous antigens is altered in PA28-/- mice. Cytotoxic T lymphocyte responses are impaired, and assembly of immunoproteasomes is greatly inhibited in mice lacking PA28. These results show that PA28 is necessary for immunoproteasome assembly and is required for efficient antigen processing, thus demonstrating the importance of PA28-mediated proteasome function in immune responses.
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PMID:Impaired immunoproteasome assembly and immune responses in PA28-/- mice. 1059 49

Proteasomes play a major role in non-lysosomal proteolysis and also in the processing of proteins for presentation by the MHC class I pathway. In animal cells they exist in several distinct molecular forms which contribute to the different functions. 26S proteasomes contain the core 20S proteasome together with two 19S regulatory complexes. Alternatively, PA28 complexes can bind to the ends of the 20S proteasome to form PA28-proteasome complexes and PA28-proteasome-19S hybrid complexes have also been described. Immunoproteasome subunits occur in 26S proteasomes as well as in PA28-proteasome complexes. We have found differences in the subcellular distribution of the different forms of proteasomes. The gamma-interferon inducible PA28 alpha and beta subunits are predominantly located in the cytoplasm, while 19S regulatory complexes (present at significant levels only in 26S complexes) are present in the nucleus as well as in the cytoplasm. Immunoproteasomes are greatly enriched at the endoplasmic reticulum (ER) where they may facilitate the generation of peptides for transport into the lumen of the ER. We have also investigated the effects of gamma-interferon on the levels and subcellular distribution of inducible subunits and regulator subunits. In each case gamma-interferon was found to increase the level but not to alter the distribution. Several subunits of proteasomes are phosphorylated including alpha subunits C8 (alpha7) and C9 (alpha3), and ATPase subunit S4 (rpt2). Our studies have shown that gamma-interferon treatment decreases the level of phosphorylation of proteasomes. We have investigated the role of phosphorylation of C8 by casein kinase II by site directed mutagenesis. The results demonstrate that phosphorylation at either one of the two sites is essential for the association of 19S regulatory complexes and that the ability to undergo phosphorylation at both sites gives the most efficient incorporation of C8 into the 26S proteasome.
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PMID:Regulation of proteasome complexes by gamma-interferon and phosphorylation. 1129 98

The proteasome activator PA28 is an interferon-gamma-inducible complex made up of two related subunits, named PA28 alpha and PA28 beta, with approximately 50% amino acid sequence identity. Accumulated evidence indicates that binding of this complex to the 20S proteasome enhances the generation of class I-binding peptides. Previously, we showed that the genes coding for PA28 alpha and PA28 beta, designated Psme1 and Psme2, respectively, are located approximately 6 kb apart with their 3' ends pointing toward each other on mouse Chromosome 14. In the present study, we sequenced the regions adjacent to Psme1 and Psme2. In a contiguous stretch of approximately 35 kb, we identified six genes arranged in the following order: Cg10671-like (a gene similar to Drosophila CG10671)-Psme1-Cgi112 (a ubiquitously expressed gene with no known function)-Psme2-Flj10111 (a gene coding for a protein with two RING finger domains)-Isgf3g (an interferon-gamma-inducible gene coding for an interferon-dependent, positive-acting transcription factor 3 gamma). Interestingly, the 3' untranslated region of Psme1 overlaps with that of Cgi112 by 7 bp. Database analysis indicates that the corresponding human genes also overlap by up to 7 bp in their 3' untranslated regions. The 5' end of the mouse, but not the human, gene coding for PA28 beta undergoes alternative splicing that is predicted to alter the N-terminal amino acid sequence. Comparison of the mouse sequence with a human draft sequence deposited in the NCBI database revealed that the overall organization of the region coding for the interferon-gamma-inducible proteasome activator is conserved between human and mouse.
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PMID:Nucleotide sequence analysis of the approximately 35-kb segment containing interferon-gamma-inducible mouse proteasome activator genes. 1134 88

The proteasome catalytic beta subunits LMP2, LMP7, and MECL-1 and two proteasome activator proteins, PA28 alpha and beta, are induced following exposure to IFN-gamma in vitro. Induction of these immunosubunits and the PA28 alpha/beta hetero-oligomer alters proteasome catalytic functions and specificity and enhances production of certain MHC class I epitopes. We sought to determine whether and to what extent proteasome subunit composition is regulated in vivo and to elucidate the mechanisms of such regulation. We analyzed basal expression levels of these inducible genes in normal, IFN-gamma-deficient, and Stat-1-deficient mice. Mice of all three genotypes display constitutive expression of the immunosubunits and PA28, demonstrating that basal expression in vivo is independent of endogenous IFN-gamma production. However, basal expression levels are reduced in Stat-1(-/-) mice, demonstrating a role for Stat-1 independent of IFN-gamma signaling. To demonstrate that IFN-gamma can induce these genes in vivo, mice were infected with Histoplasma capsulatum. Elevated expression of these genes followed the same time course as IFN-gamma expression in infected mice. IFN-gamma-deficient mice did not display elevated protein expression following infection, suggesting that other inflammatory cytokines produced in infected mice are unable to influence proteasome expression. Cytokines other than IFN-gamma also failed to influence proteasome gene expression in vitro in cell lines that had no basal expression of LMP2, LMP7, or MECL-1. Thus, both in vitro and in vivo data demonstrate that IFN-gamma is essential for up-regulation, but not constitutive expression, of immunoproteasome subunits in mice.
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PMID:Regulation of immunoproteasome subunit expression in vivo following pathogenic fungal infection. 1221 20


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