Subunit of the '20S' proteasome (multicatalytic proteinase) encoded by the major histocompatibility complex

Identification of human cancers deficient in antigen processing

Intracellular antigens must be processed before presentation to CD8+ T cells by major histocompatibility complex (MHC) class I molecules. Using a recombinant vaccinia virus (Vac) to transiently express the Kd molecule, we studied the antigen processing efficiency of 26 different human tumor lines. Three cell lines, all human small cell lung carcinoma, consistently failed to process endogenously synthesized proteins for presentation to Kd-restricted, Vac-specific T cells. Pulse-chase experiments showed that MHC class I molecules were not transported by these cell lines from the endoplasmic reticulum to the cell surface. This finding suggested that peptides were not available for binding to nascent MHC molecules in the endoplasmic reticulum. Northern blot analysis of these cells revealed low to nondetectable levels of mRNAs for MHC-encoded proteasome components LMP-7 and LMP-2, as well as the putative peptide transporters TAP-1 and TAP-2. Treatment of cells with interferon gamma enhanced expression of these mRNAs and reversed the observed functional and biochemical deficits. Our findings suggest that downregulation of antigen processing may be one of the strategies used by tumors to escape immune surveillance. Potential therapeutic applications of these findings include enhancing antigen processing at the level of the transcription of MHC-encoded proteasome and transporter genes.

Putting together an MHC class I molecule

Formation of MHC class I complexes involves proper folding of the subunits, their assembly and interaction with peptides. Several proteins contributing to this process have been described, but a number of questions remain, in particular those concerning early folding steps and interactions with peptide in the course of biosynthesis.

The prosomal RNA-binding protein p27K is a member of the alpha-type human prosomal gene family

Monoclonal antibodies demonstrated high conservation during evolution of a prosomal protein of M(r) 27,000 and differentiation--specific expression of the epitope. More than 90% of the reacting antigen was found as a p27K protein in the free messenger ribonucleoprotein (mRNP) fraction but another protein of M(r) 38,000, which shared protease fingerprint patterns with the p27K polypeptide, was also labelled in the nuclear and polyribosomal fractions. Sequencing of cDNA recombinant clones encoding the p27/38K protein and comparison with another prosomal protein, p30-33K, demonstrated the existence of a common characteristic sequence pattern containing three highly conserved segments. The genes Hs PROS-27 and Hs PROS-30 were mapped to chromosomes 14 (14q13) and 11 (11p15.1), respectively. The structure of the p27K protein shows multiple potential phosphorylation sites, an NTP-binding fold and an RNA-binding consensus sequence. The Hs PROS-27/beta-galactosidase fusion protein binds a single RNA of about 120 nucleotides from total HeLa cell RNA. Sequence comparisons show that the Hs PROS-27 and Hs PROS-30 genes belong to the gene family that encodes the prosome--MCP (multicatalytic proteinase)--proteasome proteins. Comparison with other members of the family from various species allowed us to show that the tripartite consensus sequence characteristic of the alpha-type sub-family is conserved from archeobacteria to man. The members of this gene family are characterised by very high evolutionary conservation of amino acid sequences of homologous genes and 20%-35% sequence similarity, between different family member within the same species and are clearly distinct from the beta-type family.

Cell biology of antigen presentation

MHC class I molecules present degradation products derived from intracellular proteins, whereas MHC class II molecules generally present peptides derived from extracellular or surface proteins. Recent insights into the cell biology of MHC class I and II molecules explain this difference.

Genomic polymorphism, recombination, and linkage disequilibrium in human major histocompatibility complex-encoded antigen-processing genes

Recently, two subunits of a large cytosolic protease and two putative peptide transporter proteins were found to be encoded by genes within the class II region of the major histocompatibility complex (MHC). These genes have been suggested to be involved in the processing of antigenic proteins for presentation by MHC class I molecules. Because of the high degree of polymorphism in MHC genes, and previous evidence for both functional and polypeptide sequence polymorphism in the proteins encoded by the antigen-processing genes, we tested DNA from 27 consanguineous human cell lines for genomic polymorphism by restriction fragment length polymorphism (RFLP) analysis. These studies demonstrate a strong linkage disequilibrium between TAP1 and LMP2 RFLPs. Moreover, RFLPs, as well as a polymorphic stop codon in the telomeric TAP2 gene, appear to be in linkage disequilibrium with HLA-DR alleles and RFLPs in the HLA-DO gene. A high rate of recombination, however, seems to occur in the center of the complex, between the TAP1 and TAP2 genes.

The ubiquitin-mediated proteolytic pathway

Ubiquitin modification of a variety of protein targets within the cell plays important roles in many cellular processes. Among these are regulation of gene expression, regulation of cell cycle and division, involvement in the cellular stress response, modification of cell surface receptors, DNA repair, and biogenesis of mitochondria and ribosomes. The best studied modification occurs in the ubiquitin-dependent proteolytic pathway. Degradation of a protein by the ubiquitin system involves two discrete steps. Initially, multiple ubiquitin molecules are covalently linked in an ATP-dependent mode to the protein substrate. The protein moiety of the conjugate is then degraded by a specific protease into free amino acids with the release of free and reutilizable ubiquitin. This process also requires energy. In addition, stable mono-ubiquitin adducts are also found intracellularly, for example, those involving nucleosomal histones. Despite the considerable progress that has been made in elucidating the mode of action and roles of the ubiquitin system, many problems remain unsolved. For example, very little is known about the cellular substrates of the system and the signals that target them for conjugation and degradation. The scope of this review is to summarize briefly what is currently known on the role of the ubiquitin system in protein turnover, and to discuss in detail the mechanisms involved in selection of substrates for conjugation and in degradation of ubiquitin-conjugated proteins.

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.

Ubiquitinated proteasome inhibitor is a component of the 26 S proteasome complex

Western blot analysis, using a polyclonal antibody to the 240-kDa endogenous inhibitor of the 20 S proteasome, revealed that the inhibitor is a component of the 26 S complex. Although isolated inhibitor displayed a single 40-kDa band on SDS-PAGE, the antibody detected a 55-kDa component in the 26 S proteasome complex. Ubiquitin polyclonal antibody recognized the same 55-kDa component but did not react with free 40-kDa inhibitor subunit. Addition of purified 40-kDa inhibitor to a ubiquitin ligating system also generated the 55-kDa species. In crude erythrocyte extracts, most of the inhibitor migrated at 55 kDa in the presence of ATP but shifted to 40 kDa in the absence of ATP, consistent with removal of ubiquitin. It is suggested that ubiquitination of the inhibitor may be involved in regulating assembly and/or activity of the 26 S proteasome complex.

A homolog of the proteasome-related RING10 gene is essential for yeast cell growth

Proteasomes are intracellular protein complexes displaying multiproteolytic activities. These complexes have been implicated in the antigen degradation process that generates peptides associated with the major histocompatibility complex (MHC) class-I molecule. RING10 and RING12 are genes encoded by the class-II region of the human MHC that have sequence homology to proteasome-encoding genes. We have identified a yeast gene, called PRG1, that encodes a protein predicted to contain 55.6% sequence identity to 80% of the RING10 gene product. Genomic disruption of PRG1 revealed that it is essential for yeast cell growth. These data strongly indicate that the antigen-processing system present in vertebrates evolved from a basic cellular process present in all organisms.

Proteasome subunits encoded by the major histocompatibility complex are not essential for antigen presentation

Major histocompatibility complex (MHC) class I molecules bind and deliver peptides derived from endogenously synthesized proteins to the cell surface for survey by cytotoxic T lymphocytes. It is believed that endogenous antigens are generally degraded in the cytosol, the resulting peptides being translocated into the endoplasmic reticulum where they bind to MHC class I molecules. Transporters containing an ATP-binding cassette encoded by the MHC class II region seem to be responsible for this transport. Genes coding for two subunits of the '20S' proteasome (a multicatalytic proteinase) have been found in the vicinity of the two transporter genes in the MHC class II region, indicating that the proteasome could be the unknown proteolytic entity in the cytosol involved in the generation of MHC class I-binding peptides. By introducing rat genes encoding the MHC-linked transporters into a human cell line lacking both transporter and proteasome subunit genes, we show here that the MHC-encoded proteasome subunit are not essential for stable MHC class I surface expression, or for processing and presentation of antigenic peptides from influenza virus and an intracellular protein.

CD8+ T cell recognition of an endogenously processed epitope is regulated primarily by residues within the epitope

Cytotoxic T lymphocytes (CTL) recognize short antigenic peptides associated with cell surface class I major histocompatibility complex (MHC) molecules. This association presumably occurs between newly synthesized class I MHC molecules and peptide fragments in a pre-Golgi compartment. Little is known about the factors that regulate the formation of these antigenic peptide fragments within the cell. To examine the role of residues within a core epitope and in the flanking sequences for the generation and presentation of the newly synthesized peptide fragment recognized by CD8+ CTL, we have mutagenized the coding sequence for the CTL epitope spanning residues 202-221 in the influenza A/Japan/57 hemagglutinin (HA). In this study over 60 substitution mutations in the epitope were tested for their effects on target cell sensitization using a cytoplasmic viral expression system. The HA202-221 site contains two overlapping subsites defined by CTL clones 11-1 and 40-2. Mutations in HA residues 204-213 or residues 210-219 often abolished target cell lysis by CTL clones 11-1 and 40-2, respectively. Although residues outside the core epitope did not usually affect the ability to be lysed by CTL clones, substitution of a Gly residue for Val-214 abolished lysis by clone 11-1. These data suggest that residues within a site that affect MHC binding and T cell receptor recognition appear to play the predominant role in dictating the formation of the antigenic complex recognized by CD8+ CTL, and therefore the antigenicity of the protein antigen presented to CD8+ T cells. Most alterations in residues flanking the endogenously expressed epitope do not appreciably affect the generation and recognition of the site.

Impaired intracellular transport and cell surface expression of nonpolymorphic HLA-E: evidence for inefficient peptide binding

The assembly of the classical, polymorphic major histocompatibility complex class I molecules in the endoplasmic reticulum requires the presence of peptide ligands and beta 2-microglobulin (beta 2m). Formation of this trimolecular complex is a prerequisite for efficient transport to the cell surface, where presented peptides are scanned by T lymphocytes. The function of the other class I molecules is in dispute. The human, nonclassical class I gene, HLA-E, was found to be ubiquitously transcribed, whereas cell surface expression was difficult to detect upon transfection. Pulse chase experiments revealed that the HLA-E heavy chain in transfectants, obtained with the murine myeloma cell line P3X63-Ag8.653 (X63), displays a significant reduction in oligosaccharide maturation and intracellular transport compared with HLA-B27 in corresponding transfectants. The accordingly low HLA-E cell surface expression could be significantly enhanced by either reducing the culture temperature or by supplementing the medium with human beta 2m, suggesting inefficient binding of endogenous peptides to HLA-E. To analyze whether HLA-E binds peptides and to identify the corresponding ligands, fractions of acid-extracted material from HLA-E/X63 transfectants were separated by reverse phase HPLC and were tested for their ability to enhance HLA-E cell surface expression. Two fractions specifically increased the HLA class I expression on the HLA-E transfectant clone.

Mutant HLA-A201 heavy chains with lowered affinity for beta 2m are transported after growth at reduced temperatures

Forty-five site-directed mutants bearing single amino acid substitutions in the alpha 3 domain of the class I molecule HLA-A201 were previously transfected into CIR cells and screened for surface expression by antibody binding. Eight mutants are expressed at significantly reduced levels relative to HLA-A201, including two with substitutions at residues contacting beta 2m. One of the latter mutations, position 242 gln > lys (242K), is now shown to block assembly with beta 2m and prevent intracellular transport at 37 degrees C. At temperatures ranging from 21 degrees C to 30 degrees C, 242K heavy chains and beta 2m form dimers that are exported to the cell surface. Surface expression at 26 degrees C is not blocked by cycloheximide pretreatment, but is completely inhibited by Brefeldin A, suggesting that at 37 degrees C preformed heavy chains accumulate in the ER. Glycans on the retained heavy chains are sensitive to digestion by Endo H, but become Endo H resistant after cells are grown at 26 degrees C. Preincubation of 242K cells with synthetic peptides shown previously to bind HLA-A201 does not increase reactivity with anti-HLA-A2 antibodies, suggesting that the defective phenotype is not due to instability of cell surface mutant class I dimers, but derives instead from impaired assembly of 242K heavy chains with beta 2m inside the cell. This contrasts with mutant cells such as .174, T2 and RMA-S, which exhibit defects in internal peptide transporters, but assemble and export "empty" dimers to their surfaces that can be stabilized subsequently by exogenous peptides. 242K mutants may therefore be suited uniquely for studying assembly and peptide binding to class I molecules in the ER.

The pathogenetic aspects of spondyloarthropathies from the point of view of HLA-B27

The association of HLA-B27 and seronegative spondyloarthropathies, especially ankylosing spondylitis has been known for almost two decades. The spontaneous development of spondyloarthropathy like joint disease in HLA-B27 transgenic rats verifies the suspicion that the HLA-B27 antigens are directly responsible for disease development. With the recently revealed crystal structure of HLA-B27 and understanding of the class I molecule in general, a new hypothesis can be formulated based on the assumption that the pathogenesis of these diseases is a subversion of the physiological function.

Synthetic analogues of chymostatin. Inhibition of chymotrypsin and Streptomyces griseus proteinase A

A series of analogues of chymostatin, including Z-Arg-Leu-Phe-aldehyde (Z-Arg-Leu-Phe-H), have been synthesized. Analysis of the inhibitory potential of these analogues permits identification of residues and interactions that are important for inhibitory activity. Moreover, the structure-function relationship for Z-Arg-Leu-Phe-H and chymostatin inhibition of chymotrypsin and Streptomyces griseus proteinase A (SGPA) was probed further with the aid of molecular mechanics. This analysis identified interactions that provide an explanation for the enhanced activity of the natural product, chymostatin, over the synthetic analogues in the inhibition of chymotrypsin but not SGPA.

Cytomegalovirus prevents antigen presentation by blocking the transport of peptide-loaded major histocompatibility complex class I molecules into the medial-Golgi compartment

Selective expression of murine cytomegalovirus (MCMV) immediate-early (IE) genes leads to the presentation by the major histocompatibility complex (MHC) class I molecule Ld of a peptide derived from MCMV IE protein pp89 (Reddehase, M.J., J. B. Rothbard, and U.H. Koszinowski. 1989. Nature (Lond.). 337:651). Characterization of endogenous antigenic peptides identified the pp89 peptide as the nonapeptide 168YPHFMPTNL176 (del Val, M., H.-J. Schlicht, T. Ruppert, M.J. Reddehase, and U.H. Koszinowski. 1991. Cell. 66:1145). Subsequent expression of MCMV early genes prevents presentation of pp89 (del Val, M., K. Münch, M.J. Reddehase, and U.H. Koszinowski. 1989. Cell. 58:305). We report on the mechanism by which MCMV early genes interfere with antigen presentation. Expression of the IE promoter-driven bacterial gene lacZ by recombinant MCMV subjected antigen presentation of beta-galactosidase to the same control and excluded antigen specificity. The Ld-dependent presence of naturally processed antigenic peptides also in nonpresenting cells located the inhibitory function subsequent to the step of antigen processing. The finding that during the E phase of MCMV gene expression the MHC class I heavy chain glycosylation remained in an Endo H-sensitive form suggested a block within the endoplasmic reticulum/cis-Golgi compartment. The failure to present antigenic peptides was explained by a general retention of nascent assembled trimolecular MHC class I complexes. Accordingly, at later stages of infection a significant decrease of surface MHC class I expression was seen, whereas other membrane glycoproteins remained unaffected. Thus, MCMV E genes endow this virus with an effective immune evasion potential. These results also indicate that the formation of the trimolecular complex of MHC class I heavy chain, beta 2-microglobulin, and the finally trimmed peptide is completed before entering the medial-Golgi compartment.

Antigen presentation in virus infection

Important recent advances have been made in our understanding of antigen processing of cytoplasmic antigens and presentation by class I molecules of the MHC. Peptide transporter-like molecules encoded within the MHC have been characterized and have, by transfection, corrected some of the presentation-mutant cell lines. The nature of peptide-MHC class I interactions has been clarified by further resolution of the HLA A2 and B27 crystals and elution of peptides. The differences between antigenicity and immunogenicity for viral antigens have been highlighted by studies in transgenic animals.

Peptide-induced stabilization and intracellular localization of empty HLA class I complexes

The human cell line T2 has been reported to be class I assembly deficient, and accordingly expresses reduced amounts of HLA-A2 and no HLA-B5 at the cell surface. By immunoblotting we observe the steady-state class I heavy chain levels of T2 to be near normal when compared with the identical class I alleles of the wild-type cell line T1. In pulse chase experiments, formation of heavy chain beta 2-microglobulin complexes is observed for both HLA-A2 and HLA-B5. Culture at reduced temperatures (26 or 20 degrees C) does not increase the amount of class I molecules transported, unlike what has been reported for the class I assembly-deficient mouse mutant cell line RMA-S. The HLA-B5 and the HLA-A2 complexes formed by T2 are thermolabile in cell lysates, albeit to different degrees. The thermolability of HLA-B5 can be overcome by addition of HLA-B5-presentable peptides, obtained by trifluoroacetic acid extraction from an HLA-B5-positive cell line, underlining the necessity of peptide for class I stability and indicating that T2-derived class I complexes are devoid of peptide. Cytoplast fusion of T2 cells with RMA-S cells shows the defect in class I assembly of RMA-S to be similar to that of T2. Localization of class I molecules observed by immuno-electron microscopy reveals the accumulation in the T2 cell line of both HLA-B5 and HLA-A2 in the endoplasmic reticulum (ER). Class I molecules are present in all the cisternae of the Golgi complex of T2, but the ratio of HLA-A and -B locus products in the Golgi area differs significantly from that at the cell surface. We conclude that the requirement for peptide in transport of class I molecules manifests itself at a stage beyond the ER, most likely the Golgi area.

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.

The MHC: relationship between linkage and function

It is intriguing that several genes with associated functions, including all of class I and class II genes, as well as some genes affecting antigen presentation of both class I and class II pathways, are linked in the MHC. Recent observations have led to speculation that there may be a functional explanation for keeping these related genes together.

Proteasomes are regulated by interferon gamma: implications for antigen processing

Class I major histocompatibility complex (MHC) molecules present antigenic peptides of cytoplasmic origin to T cells. As the lengths of these peptides seem restricted to eight or nine amino acids, an unusual proteolytic system must play a role in antigen processing. Proteasomes, a major extralysosomal proteolytic system, are responsible for the degradation of cytoplasmic proteins. We demonstrate that several proteasomal subunits, including MHC-encoded subunits, are regulated by interferon gamma. These data and the finding that MHC-encoded and other interferon gamma-regulated proteasomal subunits are uniquely associated with proteasomes strongly suggest that the immune system has recruited proteasomes for antigen processing.

Isolation and characterization of the MHC linked beta-type proteasome subunit MC13 cDNA

We have cloned and analysed the second mouse MHC-linked proteasome subunit, designated MC13, which appears to be homologous to the human RING10 proteasome protein. The isolated cDNA has an ORF encoding a protein of 276 amino acids with a molecular weight of ca. 30 kDa. Sequence alignment reveals that the subunit MC13 and several other mammalian proteasome subunits are encoded by a second proteasome gene family. This second gene family encodes subunits of the beta-type, reveals striking sequence similarities with the beta-subunit of archaebacterial proteasomes and is related to, but distinct from, the genes encoding the so-called alpha-type subunits.

Specificity of antigen processing for MHC class I restricted presentation is conserved between mouse and man

Alloreactive mouse cytotoxic T lymphocytes (CTL) specific for particular peptides presented by H-2Kb molecules on mouse cells were found to recognize human cells transfected with Kb. The CTL-recognized peptides (probably derived from conserved proteins) were extracted from Kb-expressing human or mouse cells, respectively, and compared biochemically by high resolution high performance liquid chromatography. The results strongly suggest identity of peptides processed by cells from both species and thus indicate that the specificity of the processing machinery used in the major histocompatibility complex (MHC) class I-restricted pathway, probably including enzymes, transport mechanisms, and chaperons, is highly conserved across species. The results are consistent with the notion that MHC molecules themselves have an instructive role in processing.

Molecular characterization of the genomic regions of the Drosophila alpha-type subunit proteasome genes PROS-Dm28.1 and PROS-Dm35

The proteasome (multicatalytic proteinase) consists of a large number of non-identical protein subunits which are encoded by the evolutionarily conserved PROS gene family. Using the PROS-Dm35 and PROS-Dm28.1 cDNAs as probes, we have isolated the corresponding genomic DNA clones of Drosophila melanogaster. In situ hybridization shows that the members of the PROS gene family are not organized in a single gene cluster and that, in contrast to the PROS-Dm35 gene, the PROS-Dm28.1 gene is localized on the X chromosome. Analysis of the genomic organization of the PROS-Dm28.1 and PROS-Dm35 genes reveals that both genes are interrupted by two small introns whereby the relative positions of the introns within the two coding regions are not conserved. Neither gene possesses a distinct transcriptional start site as shown by nuclease S1 analysis. Since the promoter regions also do not contain a TATA box, PROS genes appear to be typical house-keeping genes. A putative heat-shock element in the promoter region of the PROS-Dm35 gene was shown to be inactive on stress induction when fused to a reporter gene and tested in transient transfections assays. In addition, promoter deletion analysis demonstrates that the promoter region between positions -605 and -330 contains sequence elements important for PROS-Dm35 gene activity and that deletions beyond position -150 result in an almost complete inhibition of transcription.

A molecular model of MHC class-I-restricted antigen processing

Cells of higher vertebrates have evolved mechanisms that allow a sample of their intracellular contents to be available for surveillance by the immune system. This display of intracellular material is in the form of peptides bound to cell surface major histocompatibility complex (MHC) class I molecules. In this review, John Monaco presents a model of the mechanisms by which this takes place, based on the recent identification of a number of new genes in the MHC.

Allelic variants of the human putative peptide transporter involved in antigen processing

Antigen processing for presentation of peptide epitopes by major histocompatibility complex (MHC) class I molecules involves genes in the MHC class II region. Among these, PSF1 and PSF2 encode subunits of a transporter, which presumably delivers cytosolic peptides across the endoplasmic reticulum membrane to class I molecules. This close functional relationship of the transporter and class I heavy chain genes and their linkage within the MHC raise the question of whether PSF1 and PSF2, like most class I genes, are polymorphic. By single-strand conformation polymorphism analysis and DNA sequencing, a small number of amino acid sequence variants of both PSF1 and PSF2 was identified in a panel of cell lines. This limited polymorphism may contribute to a higher degree of variability at the level of the functional transporter, in which different alleles of the PSF1 and PSF2 subunits may be combined. A possible involvement of the PSF1 and PSF2 genes in susceptibility to MHC-associated diseases was examined in a preliminary assessment in patients with ankylosing spondylitis, insulin-dependent diabetes mellitus, or celiac disease.

cDNA cloning of rat proteasome subunit RC1, a homologue of RING10 located in the human MHC class II region

The nucleotide sequence of a cDNA that encodes a new subunit, named RCl, of rat proteasomes (multicatalytic proteinase complexes) has been determined. The polypeptide predicted from the open reading frame consisted of 208 amino acid residues with a calculated molecular mass of 23, 130, which is consistent with the size obtained by electrophoretic analysis of purified RCl. The partial amino acid sequences of several fragments of RCl, obtained by protein chemical analyses, were found to be in excellent accordance with those deduced from the cDNA sequence. Surprisingly, the overall structure of RCl was found to be almost identical to that of recently isolated RING10, whose gene is located in the class II region of the human MHC gene cluster. This finding suggests that RCl is a homologue of human RING10, supporting the proposal that proteasomes are involved in the antigen processing pathway.

HLA-A2 molecules in an antigen-processing mutant cell contain signal sequence-derived peptides

The mutant human cell line T2 is defective in antigen presentation in the context of class I major histocompatibility complex (MHC) molecules, and also in that transfected T2 cells show poor surface expression of exogenous human class I (HLA) alleles. Both defects are thought to lie in the transport of antigenic peptides derived from cytosolic proteins into the endoplasmic reticulum (ER), as peptide-deficient class I molecules might be expected to be either unstable or retained in the ER. The products of several mouse class I (H-2) genes, and the endogenous gene HLA-A2 do, however, reach the surface of T2 cells at reasonable levels although they are non-functional. We report here that, as expected, poorly surface-expressed HLA molecules do not significantly bind endogenous peptides. Surprisingly, H-2 molecules expressed in T2 also lack associated peptides, arguing that 'empty' complexes of mouse class I glycoproteins with human beta 2-microglobulin are neither retained in the ER nor unstable. HLA-A2 molecules, however, do bind high levels of a limited set of endogenous peptides. We have sequenced three of these peptides and find that two, a 9-mer and an 11-mer, are derived from a putative signal sequence (of IP-30, an interferon-gamma-inducible protein), whereas a third, a 13-mer, is of unknown origin. The unusual length of two of the peptides argues that the 9-mers normally associated with HLA-A2 molecules may be generated before their transport from the cytosol rather than in a pre-Golgi compartment. To our knowledge, this is the first report of the isolation of a fragment of a eukaryotic signal peptide generated in vivo.

The human class II MHC protein HLA-DR1 assembles as empty alpha beta heterodimers in the absence of antigenic peptide

We have produced the human class II histocompatibility protein, HLA-DR1, as a soluble, secreted glycoprotein in insect cells infected with baculoviruses carrying truncated alpha and beta subunit genes. The peptide-binding site is empty, and the empty molecules are fully competent to bind antigenic peptide. We used the empty molecules to measure an intrinsic rate for peptide association, and to investigate the role of peptide in stabilizing the class II structure. Peptide binding kinetics for the empty molecule are only 10-fold faster than for peptide exchange into an occupied site, suggesting that a conformational change may accompany peptide binding. The native alpha beta heterodimer assembles in the absence of antigenic peptide, but peptide binding stabilizes the empty heterodimer against aggregation and against SDS-induced denaturation.

New genes in the MHC that encode proteins for antigen processing

Most cells process proteins into short peptides that are displayed on the cell surface bound to class I or class II proteins encoded by the major histocompatibility complex (MHC). These protein-peptide complexes can then be recognized by the circulating lymphocytes of the immune system. Several genes found recently in the MHC encode proteins with possible roles in the supply of peptides to class I molecules. The results imply that the peptides are produced in the cytoplasm by proteasomes and are translocated into the endoplasmic reticulum by 'peptide transporters' related to the multidrug resistance proteins. While there is little biochemical evidence to validate these ideas, Robert DeMars and Thomas Spies discuss here the arguments supporting this view. New data indicate that there may also be factors for class II peptide-processing hidden in the MHC.

T cells and human autoimmune thyroid disease: emerging data show lack of need to invoke suppressor T cell problems

Human T cells recognize self and foreign antigens when such antigens are processed into small peptides and bound to molecules coded for by genes of the HLA region on chromosome 6. The part of the T-cell surface which is responsible for such recognition is a set of molecules coded for by a variety of genes and known as the T-cell-receptor complex. In animal models, T cells are able to transfer autoimmune thyroiditis and T cells have, therefore, long been implicated in the etiology of human autoimmune thyroid disease (AITD). Information gained from the study of intrathyroidal T cells and thyroid antigen-specific T-cell clones has shown that in patients with Graves' disease, mainly helper T-cell clones have been obtained, whereas in autoimmune (Hashimoto's) thyroiditis cytolytic T-cell clones may be predominant. Such thyroid antigen-specific T cells have now been shown to recognize one or other of the three major thyroid-specific antigens; thyroglobulin, thyroid peroxidase, or the TSH receptor and efforts are currently in progress to characterize the T-cell epitopes of these major thyroid autoantigens. Recent findings of restricted T-cell receptor V gene use amongst intrathyroidal T cells confirm the primary role of T cells in human thyroid autoimmune processes leading to AITD. However, the mechanisms whereby such autoreactive T cells escape deletion and anergy, and how they become activated, remain uncertain. There is compelling evidence that the thyroid cell itself, by expressing HLA molecules, and presenting antigen directly to the T cells, may initiate disease, perhaps after an external insult.