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 20S proteasome is a protease complex of functional importance for antigen processing. Two of the 14 proteasome subunits, delta and MB1, can be replaced by the major histocompatibility complex (MHC)-encoded and interferon-gamma (IFN-gamma)-inducible subunits LMP2 and LMP7, respectively. LMP2 and LMP7 alter the cleavage site specificity of the 20S proteasome and are required for the efficient generation of T cell epitopes from a number of viral proteins and for optimal MHC class I cell surface expression. We compared the 20S proteasome subunit pattern from IFN-gamma-induced and non-induced mouse fibroblasts on two-dimensional gels and identified a third subunit exchange by microsequencing: the non-MHC-encoded subunit MECL-1 is induced by IFN-gamma and replaces a sofar barely characterized beta subunit designated 'MC14'. In analogy to LMP2 and LMP7, MECL-1 may be functional in MHC class I-restricted antigen presentation.
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PMID:A third interferon-gamma-induced subunit exchange in the 20S proteasome. 862 80

Proteasomes are the multisubunit proteases thought to be involved in the generation of peptides presented by MHC class I molecules. When cells are stimulated with IFN-gamma, two MHC encoded subunits, LMP2 and LMP7, are incorporated into the proteasomal complex, presumably by displacing the housekeeping subunits, designated Y and X, respectively. These changes in the subunit composition appear to facilitate class I-mediated Ag presentation, presumably bu altering the cleavage specificities of the proteasome. Here we show that the cartilaginous fish, the most primitive class of vertebrates in which the MHC has been identified, have both LMP7 and X genes. Interestingly, nurse sharks, a member of the cartilaginous fish, appear to have two LMP7 genes, one encoding a typical LMP7 subunit and the other encoding a less typical one. In contrast, only cDNA clones with residues characteristic of X were identified in hagfishes and lampreys, the two extant members of the jawless fish in which no MHC has been identified. Pairwise amino acid sequence comparison and phylogenetic tree analysis showed that the subunits encoded by these clones were nearly equidistant from LMP7 and X, suggesting that the LMP7 gene might have emerged after the appearance of the jawless fish. Sequence comparison of the LMP7 and X/X-like subunits isolated from various vertebrate species showed that, unlike the X/X-like subunit, the LMP7 subunit displays a striking interspecies sequence variability in the vicinity of its catalytic site.
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PMID:Isolation of low molecular mass polypeptide complementary DNA clones from primitive vertebrates. Implications for the origin of MHC class I-restricted antigen presentation. 866 94

Accumulating evidence has implicated the proteasome in the processing of protein along the major histocompatibility complex (MHC) class I presentation pathway. The availability of potent proteasome inhibitors provides an opportunity to examine the role of proteasome function in antigen presentation by MHC class I molecules to CD8+ cytotoxic T lymphocytes (CTLs). We have investigated the processing and presenting of antigenic epitopes from influenza hemagglutinin in target cells treated with the inhibitor of proteasome activity MG132. In the absence of proteasome activity, the processing and presentation of the full-length hemagglutinin was abolished, suggesting the requirement for proteasome function in the processing and presentation of the hemagglutinin glycoprotein. Epitope-containing translation products as short as 21 amino acids when expressed in target cells required proteasome activity for processing and presentation of the hemagglutin epitope to CTLs. However, when endogenous peptides of 17 amino acids or shorter were expressed in target cells, the processing and presentation of epitopes contained in these peptides were insensitive to the proteasome inhibitor. Our results support the hypothesis that proteasome activity is required for the generation of peptides presented by MHC class I molecules and that the requirement for proteasome activity is dependent on the size of the translation product expressed in the target cell. The implications of these findings are discussed.
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PMID:The requirement for proteasome activity class I major histocompatibility complex antigen presentation is dictated by the length of preprocessed antigen. 866 12

Elimination of foreign pathogens requires detection of the presence of such microorganisms somewhere in the body. This task relies on specialized cells, among which specific lymphocytes permanently circulate throughout the body searching for signals indicative of the presence of invasive microorganisms. In contrast to B lymphocytes, T lymphocytes are unable to recognize bacteria or viruses in their native form. The structure of their antigen receptor only allows them to bind to small peptidic fragments that have to be stably presented by specific molecules at the surface of specific cells. These professional "antigen presenting cells" capture antigens and alert the immune system by expressing at their surface molecular complexes formed by their own major histocompatibility molecules (MHC) and fragments of the infectious agent. Extracellular microorganisms are captured by phagocytosis and digested into small peptides in the endosomal compartment of antigen presenting cells. The peptides able to bind to MHC class II molecules are transported to the cell surface. These antigen-MHC complexes are recognized by antigen specific CD4+ T lymphocytes, thus leading to the enhancement of antibody formation and of inflammatory responses which eliminate extracellular bacterial. In contrast, viruses or bacteria able to survive within the cytoplasm of the antigen presenting cells are digested by a specific multicatalytic enzymatic complex (the proteasome). The antigenic peptides released into the cytosol will be transported into the endoplasmic reticulum by an active peptide pump. The peptides able to bind to the groove of MHC class I molecules are transported to the cell surface. Their recognition by specific cytotoxic CD8+ lymphocytes leads to the destruction of the cells identified as infected. Thus, the mechanisms of antigen processing and presentation are able to generate a wide variety of antigenic fragments. Depending on the initial extra- or intracellular localization of the microorganism, some antigenic peptides will appear on the surface of antigen presenting cells on either MHC class I or class II molecules which are specifically recognized by either CD4+ or CD8+ lymphocytes. Only the antigenic peptides that are generated by this process and able to bind to MHC molecules of antigen presenting cells will be recognized by circulating lymphocytes and thus induce antigen specific immune response. Their identification therefore forms the basis of the defense mechanisms against infectious diseases and of novel immunization strategies.
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PMID:[Molecular basis for detection of infectious agents]. 872 Mar 22

Effective MHC class I peptide loading requires the proteolytic degradation of cytosolic proteins and the TAP-mediated translocation of peptides across the membrane of the endoplasmic reticulum. The proteasome is emerging as the main cytosolic protease generating class I binding peptides. The recent elucidation of the proteasome crystal structure, together with the use of functional inhibitors, has enhanced our understanding of proteasome function. Genetic analysis of a novel mutant cell line emphasizes the importance of the TAP-class I interaction in the assembly of mature class I heterotrimers, and suggests that additional MHC-encoded components are required.
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PMID:Processing and delivery of peptides presented by MHC class I molecules. 872 47

Class I presentation of microinjected native OVA by a temperature-sensitive ubiquitin conjugation mutant, ts85, but not wild-type murine cells, was markedly inhibited following incubation at a nonpermissive temperature. In contrast, the nonpermissive temperature did not affect class I presentation of a minimal OVA peptide expressed in the cytosol. Therefore, these results provide a second example in which a temperature sensitive mutation in the ubiquitin conjugation pathway inhibits MHC class I presentation of native OVA. Surprisingly, incubation at the nonpermissive temperature did not inhibit class I presentation of chemically denatured and alkylated OVA microinjected into the cytosol of mutant cells. Similarly, the presentation of endogenously synthesized OVA (which is expressed from a recombinant vaccinia virus and, presumably, is misfolded in the cytosol) was also not inhibited in both mutant cell lines. Methylation of the lysine groups in denatured OVA, which blocks ubiquitin conjugation, reduced but did not eliminate the presentation of denatured OVA, providing evidence for both ubiquitin-dependent and ubiquitin-independent pathways for class I presentation. In contrast, a proteasome inhibitor blocked class I presentation of all forms of OVA, while a control peptide aldehyde was not inhibitory. These results indicate that modification of the structure of a protein can influence its requirements for ubiquitin conjugation for efficient class I presentation, with the key alteration possibly being the loss of proper conformation. However, regardless of the form of the Ag, the proteasome appears to be required for generating peptides from both endogenously synthesized and microinjected OVA for class I presentation.
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PMID:Chemical denaturation and modification of ovalbumin alters its dependence on ubiquitin conjugation for class I antigen presentation. 875 9

We show that six proteasome-associated proteins are induced by IFN-gamma, corresponding to three proteasome beta-type subunits and their precursors: the MHC-linked subunits (LMP-2 and LMP-7) and LMP-10. Concurrently, incorporation of LMP-9, LMP-17, and LMP-19 into proteasomes is reduced. LMP-10 appears to be the product of a previously cloned proteasome subunit gene, MECL-1. MECL-1 transcription is increased in the presence of IFN-gamma, whereas the transcription of two other proteasome genes, Lmp-15 and Lmp-3, is not affected. The three IFN-gamma-inducible subunits and their constitutively expressed counterparts contain most or all of the catalytic sites of the proteasome. Independent assortment of LMP-2, LMP-7, and LMP-10 into different proteasome complexes may thus generate up to 36 unique proteasome subsets. This may increase the repertoire of potentially antigenic peptides for presentation by MHC class I.
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PMID:Identification of MECL-1 (LMP-10) as the third IFN-gamma-inducible proteasome subunit. 878 91

The human cytomegalovirus genome encodes proteins that trigger destruction of newly synthesized major histocompatibility complex (MHC) class I molecules. The human cytomegalovirus gene US2 specifies a product capable of dislocating MHC class I molecules from the endoplasmic reticulum to the cytosol and delivering them to the proteasome. This process involves the Sec61 complex, in what appears to be a reversal of the reaction by which it translocates nascent chains into the endoplasmic reticulum.
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PMID:Sec61-mediated transfer of a membrane protein from the endoplasmic reticulum to the proteasome for destruction. 894 60

MHC class I molecules present antigenic peptides that are mostly derived from endogenous cytosolic proteins. Recent studies addressing the function of the proteasome and its activator complexes have advanced our understanding of the cytosolic processing of peptides. Transporters associated with antigen processing (TAPs) translocate these peptides to the endoplasmic reticulum where MHC class I molecules, which are retained in transient complexes with chaperones and TAPs, await them for binding. The sequence specificity and the peptide length preference of TAPs roughly meet the requirements of class I molecules in a range of different species, suggesting evolutionary shaping of the specificity of TAPs.
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PMID:Generation, intracellular transport and loading of peptides associated with MHC class I molecules. 903 71

N-acetyl-L-leucyl-L-leucyl-L-norleucinal (LLnL), which reversibly inhibits the proteasome in addition to other proteases, and a more specific irreversible inhibitor of the proteasome, lactacystin, were found to cause the accumulation of major histocompatibility complex (MHC) class I heavy chains in the cytosol of the beta2-microglobulin-deficient cell line Daudi and the TAP-deficient cell line .174. These cell lines, which are severely impaired in their ability to fold MHC class I heavy chain, showed an accumulation of soluble class I heavy chains at different rates over a period of hours in the presence of LLnL. The accumulation of soluble class I heavy chains in the presence of either LLnL or lactacystin was easily revealed in Daudi and .174 but almost undetectable in a Daudi transfectant expressing beta2-microglobulin and in 45.1, the wild-type parent of .174. The soluble class I heavy chain was also found to be devoid of its N-linked glycan and to be located in the cytosol. When the gene for ICP47, a herpes simplex virus protein that blocks the translocation of peptides into the endoplasmic reticulum, was transfected into 45.1, a similar accumulation of soluble MHC class I heavy chain was detectable. These data suggest that in cells where the MHC class I molecule is unable to assemble properly, the misfolded heavy chain is removed from the endoplasmic reticulum to the cytosol, deglycosylated, and degraded by the proteasome.
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PMID:Misfolded major histocompatibility complex class I heavy chains are translocated into the cytoplasm and degraded by the proteasome. 905 Aug 76


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