RESUMEN
Background: Major histocompatibility class I (MHC-I, human leukocyte antigen [HLA]-I in humans) molecules present small fragments of the proteome on the cell surface for immunosurveillance, which is pivotal to control infected and malignant cells. Immunogenic peptides are generated and selected in the MHC-I antigen processing and presentation pathway. In this pathway, two homologous molecules, tapasin and TAPBPR, optimise the MHC-I peptide repertoire that is ultimately presented at the plasma membrane. Peptide exchange on HLA-I by human TAPBPR involves the flexible loop region K22-D35, with the leucine at position 30 (L30) involved in mediating peptide dissociation. However, our understanding of the exact molecular mechanisms governing TAPBPR-mediated peptide exchange on HLA-I allotypes remains incomplete. Methods: Here, in-depth re-analyses of published immunopeptidomics datasets was used to further examine TAPBPR peptide editing activity and mechanism of action on HLA-I. The role of the TAPBPR editing loop in opening the HLA-I peptide binding groove was assessed using a molecular dynamics simulation. Results: We show that TAPBPR shapes the peptide repertoire on HLA-A, -B and -C allotypes. The TAPBPR editing loop was not essential to allow HLA-I to adopt an open state. L30 in the TAPBPR editing loop was typically sufficient to mediate peptide repertoire restriction on the three HLA-I allotypes expressed by HeLa cells. TAPBPR was also able to load peptides onto HLA-I in a loop-dependent manner. Conclusions: These results unify the previously hypothesised scoop loop and peptide trap mechanisms of TAPBPR-mediated peptide exchange, with the former involved in peptide filtering and the latter in peptide loading.
Major histocompatibility complex (MHC) class I molecules play an essential role in alerting the immune system to infection and cellular changes. They do this by displaying small fragments of proteins (peptides) from pathogen-infected cells and tumours on the cell surface to immune cells. When activated, immune cells can then destroy the target cell. In 2015, we discovered that a novel accessory protein, called TAPBPR, assists in the selection of peptides displayed on MHC class I molecules for immune surveillance. A specific region in the TAPBPR protein the editing loop is known to be involved in removing peptides from MHC class I. However, our understanding of the process of peptide selection on MHC class I molecules remains incomplete. Here, we show that TAPBPR is not only involved in removing peptides from MHC class I molecules but also assists in peptide loading. Additionally, we demonstrate that the TAPBPR editing loop is involved in both removing and loading of peptides. Our results suggest that TAPBPR fine-tunes the peptide repertoire displayed on three different types of MHC class I molecules. Developing our understanding of the mechanisms of peptide selection on MHC class I molecules has important implications in disease and the development of new therapies.
RESUMEN
Since the discovery of Transporter associated with antigen processing-binding protein-related (TAPBPR) over two decades ago, extensive studies have explored its function in the context of the major histocompatibility complex class-I (MHC-I) antigen processing and presentation pathway. As a chaperone and peptide editor, TAPBPR was recently revealed to have overlapping structural features when resolved with peptide-receptive MHC-I molecules compared with the two newly solved tapasin:MHC-I structures. Despite this, the two chaperones seem to have a unique criteria for loading high-affinity peptides on MHC-I molecules. Yet, the mechanism of action of how TAPBPR creates its distinct filter in cargo selection for peptide-receptive MHC-I molecules continues to be a subject of debate.
Asunto(s)
Presentación de Antígeno , Proteínas Portadoras , Humanos , Antígenos de Histocompatibilidad Clase I , Inmunoglobulinas/metabolismo , Proteínas de la Membrana/metabolismo , PéptidosRESUMEN
Glycosylation plays a crucial role in the folding, structure, quality control and trafficking of glycoproteins. Here, we explored whether the glycosylation status of MHC class I (MHC-I) molecules impacts their affinity for the peptide editor, TAPBPR. We demonstrate that the interaction between TAPBPR and MHC-I is stronger when MHC-I lacks a glycan. Subsequently, TAPBPR can dissociate peptides, even those of high affinity, more easily from non-glycosylated MHC-I compared to their glycosylated counterparts. In addition, TAPBPR is more resistant to peptide-mediated allosteric release from non-glycosylated MHC-I compared to species with a glycan attached. Consequently, we find the glycosylation status of HLA-A*68:02, -A*02:01 and -B*27:05 influences their ability to undergo TAPBPR-mediated peptide exchange. The discovery that the glycan attached to MHC-I significantly influences the affinity of their interactions with TAPBPR has important implications, on both an experimental level and in a biological context.
Asunto(s)
Antígenos de Histocompatibilidad Clase I/metabolismo , Inmunoglobulinas/metabolismo , Proteínas de la Membrana/metabolismo , Presentación de Antígeno/fisiología , Glicosilación , Células HeLa , HumanosRESUMEN
MHC class I (MHC-I) molecules undergo an intricate folding process in order to pick up antigenic peptide to present to the immune system. In recent years, the discovery of a new peptide editor for MHC-I has added an extra level of complexity in our understanding of how peptide presentation is regulated. On top of this, the incredible diversity in MHC-I molecules leads to significant variation in the interaction between MHC-I and components of the antigen processing and presentation pathway. Here, we review our current understanding regarding how polymorphisms in human leukocyte antigen class I molecules influence their interactions with key components of the antigen processing and presentation pathway. A deeper understanding of this may offer new insights regarding how apparently subtle variation in MHC-I can have a significant impact on susceptibility to disease.
Asunto(s)
Presentación de Antígeno/genética , Antígenos/genética , Antígenos de Histocompatibilidad Clase I/genética , Péptidos/genética , Presentación de Antígeno/inmunología , Antígenos/inmunología , Antígenos de Histocompatibilidad Clase I/inmunología , Antígenos de Histocompatibilidad Clase II/genética , Antígenos de Histocompatibilidad Clase II/inmunología , Humanos , Péptidos/inmunología , Polimorfismo Genético/genéticaRESUMEN
The peptide editor TAPBPR is the newest member of the major histocompatibility complex class I (MHC-I) antigen processing and presentation pathway. Since 2013, studies have explored the functions and mechanisms of action of this tapasin homolog. Here, we review the key insights gained from structural studies of the TAPBPR:MHC-I complex and the involvement of the TAPBPR loop in peptide exchange. However, despite recent advances, the question still remains: why do we need TAPBPR? The recent appreciation that different MHC-I allotypes vary in their ability to interact with TAPBPR, together with a role for TAPBPR in alternative presentation pathways highlights that much remains unknown concerning the biological need for TAPBPR.
Asunto(s)
Presentación de Antígeno/inmunología , Antígenos de Histocompatibilidad Clase I/inmunología , Inmunoglobulinas/inmunología , Proteínas de la Membrana/inmunología , Péptidos/inmunología , HumanosRESUMEN
Peptide presentation on MHC class I molecules (MHC-I) is central to mounting effective antiviral and antitumoral immune responses. The tapasin-related protein TAPBPR is an MHC-I peptide editor which shapes the final peptide repertoire displayed on the cell surface. Here, we review recent findings which further elucidate the mechanisms by which TAPBPR performs peptide editing on a molecular level, and how glycosylation on MHC-I influences the interaction with TAPBPR and the peptide loading complex. We also explore how the function of TAPBPR can be utilized to promote exogenous peptide loading directly onto plasma-membrane expressed MHC-I. This has led to the development of new assays to investigate TAPBPR-mediated peptide editing and uncovered translational opportunities of utilizing TAPBPR to treat human disease.
Asunto(s)
Presentación de Antígeno , Inmunoglobulinas , Proteínas de la Membrana , Retículo Endoplásmico/metabolismo , Antígenos de Histocompatibilidad Clase I , Humanos , Inmunoglobulinas/metabolismo , Proteínas de la Membrana/metabolismoRESUMEN
Understanding how peptide selection is controlled on different major histocompatibility complex class I (MHC I) molecules is pivotal for determining how variations in these proteins influence our predisposition to infectious diseases, cancer, and autoinflammatory conditions. Although the intracellular chaperone TAPBPR edits MHC I peptides, it is unclear which allotypes are subjected to TAPBPR-mediated peptide editing. Here, we examine the ability of 97 different human leukocyte antigen (HLA) class I allotypes to interact with TAPBPR. We reveal a striking preference of TAPBPR for HLA-A, particularly for supertypes A2 and A24, over HLA-B and -C molecules. We demonstrate that the increased propensity of these HLA-A molecules to undergo TAPBPR-mediated peptide editing is determined by molecular features of the HLA-A F pocket, specifically residues H114 and Y116. This work reveals that specific polymorphisms in MHC I strongly influence their susceptibility to chaperone-mediated peptide editing, which may play a significant role in disease predisposition.
Asunto(s)
Antígenos HLA-A/química , Antígenos HLA-A/metabolismo , Antígenos de Histocompatibilidad Clase I/química , Antígenos de Histocompatibilidad Clase I/metabolismo , Inmunoglobulinas/metabolismo , Proteínas de la Membrana/metabolismo , Presentación de Antígeno , Células HEK293 , Antígeno HLA-A2/química , Antígeno HLA-A2/metabolismo , Antígeno HLA-A24/química , Antígeno HLA-A24/metabolismo , Antígenos HLA-B/genética , Antígenos HLA-B/metabolismo , Antígenos HLA-C/metabolismo , Células HeLa , Antígenos de Histocompatibilidad Clase I/genética , Humanos , Alotipos de Inmunoglobulinas , Inmunoglobulinas/genética , Proteínas de la Membrana/genética , Proteínas de Transporte de Membrana/genética , Proteínas de Transporte de Membrana/metabolismo , Chaperonas Moleculares/química , Chaperonas Moleculares/metabolismo , Péptidos/química , Péptidos/metabolismo , Polimorfismo Genético , Unión Proteica , Dominios Proteicos/genéticaRESUMEN
We recently discovered that TAPBPR promotes reglucosylation of the N-linked glycan on MHC class I molecules, a modification that restores their recognition by calreticulin and reincorporation into the peptide-loading complex. We wondered whether TAPBPR displayed some degree of glycan specificity, as is known to be the case for tapasin via its interaction with calreticulin & ERp57, or whether its interaction with MHC class I was glycan independent. Here, we explored this by comparing the ability of TAPBPR to bind to MHC class I containing either an intact or disrupted NxS/T glycosylation consensus sequence. In contrast to tapasin, TAPBPR bound strongly to MHC class I molecules that lacked N-linked glycosylation, suggesting that the TAPBPR:MHC class I interaction is glycan independent. Furthermore, we found that glycosylated HLA-A2 preferentially interacts with tapasin rather than TAPBPR, possibly explaining, in part, why MHC class I molecules bind efficiently to tapasin in the face of an alternative chaperone. The distinction in glycan specificity between the two peptide editors suggests that TAPBPR may bind to MHC class I molecules that are associated with a broader diversity of oligosaccharides attached compared with tapasin. This may explain, to some extent, the ability of TAPBPR to interact with MHC class I molecules outside of the ER.
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Genes MHC Clase I/fisiología , Antígenos de Histocompatibilidad Clase I/metabolismo , Inmunoglobulinas/metabolismo , Proteínas de la Membrana/metabolismo , Calreticulina/metabolismo , Línea Celular Tumoral , Retículo Endoplásmico/metabolismo , Glicosilación , Células HeLa , Humanos , Proteínas de Transporte de Membrana/metabolismo , Péptidos/metabolismo , Proteína Disulfuro Isomerasas/metabolismoRESUMEN
Tapasin and TAPBPR are known to perform peptide editing on major histocompatibility complex class I (MHC I) molecules; however, the precise molecular mechanism(s) involved in this process remain largely enigmatic. Here, using immunopeptidomics in combination with novel cell-based assays that assess TAPBPR-mediated peptide exchange, we reveal a critical role for the K22-D35 loop of TAPBPR in mediating peptide exchange on MHC I. We identify a specific leucine within this loop that enables TAPBPR to facilitate peptide dissociation from MHC I. Moreover, we delineate the molecular features of the MHC I F pocket required for TAPBPR to promote peptide dissociation in a loop-dependent manner. These data reveal that chaperone-mediated peptide editing on MHC I can occur by different mechanisms dependent on the C-terminal residue that the MHC I accommodates in its F pocket and provide novel insights that may inform the therapeutic potential of TAPBPR manipulation to increase tumour immunogenicity.
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Antígenos de Histocompatibilidad Clase I/inmunología , Inmunoglobulinas/inmunología , Proteínas de la Membrana/inmunología , Simulación del Acoplamiento Molecular , Péptidos/inmunología , Secuencia de Aminoácidos , Presentación de Antígeno/inmunología , Sitios de Unión/genética , Antígenos HLA-A/química , Antígenos HLA-A/inmunología , Antígenos HLA-A/metabolismo , Células HeLa , Antígenos de Histocompatibilidad Clase I/química , Antígenos de Histocompatibilidad Clase I/metabolismo , Humanos , Inmunoglobulinas/química , Inmunoglobulinas/metabolismo , Leucina/química , Leucina/inmunología , Leucina/metabolismo , Proteínas de la Membrana/química , Proteínas de la Membrana/metabolismo , Mutación , Péptidos/metabolismo , Unión Proteica , Dominios ProteicosRESUMEN
The repertoire of peptides displayed at the cell surface by MHC I molecules is shaped by two intracellular peptide editors, tapasin and TAPBPR. While cell-free assays have proven extremely useful in identifying the function of both of these proteins, here we explored whether a more physiological system could be developed to assess TAPBPR-mediated peptide editing on MHC I. We reveal that membrane-associated TAPBPR targeted to the plasma membrane retains its ability to function as a peptide editor and efficiently catalyzes peptide exchange on surface-expressed MHC I molecules. Additionally, we show that soluble TAPBPR, consisting of the luminal domain alone, added to intact cells, also functions as an effective peptide editor on surface MHC I molecules. Thus, we have established two systems in which TAPBPR-mediated peptide exchange on MHC class I can be interrogated. Furthermore, we could use both plasma membrane-targeted and exogenous soluble TAPBPR to display immunogenic peptides on surface MHC I molecules and consequently induce T cell receptor engagement, IFN-γ secretion, and T cell-mediated killing of target cells. Thus, we have developed an efficient way to by-pass the natural antigen presentation pathway of cells and load immunogenic peptides of choice onto cells. Our findings highlight a potential therapeutic use for TAPBPR in increasing the immunogenicity of tumors in the future.
Asunto(s)
Presentación de Antígeno , Antígenos de Histocompatibilidad Clase I/inmunología , Inmunoglobulinas/inmunología , Proteínas de la Membrana/inmunología , Péptidos/inmunología , Animales , Células HeLa , Antígenos de Histocompatibilidad Clase I/genética , Humanos , Inmunidad Celular , Inmunoglobulinas/genética , Interferón gamma/genética , Interferón gamma/inmunología , Células MCF-7 , Proteínas de la Membrana/genética , Ratones , Ratones Noqueados , Péptidos/genética , Receptores de Antígenos de Linfocitos T/genética , Receptores de Antígenos de Linfocitos T/inmunología , Linfocitos T/inmunologíaRESUMEN
The presentation of antigenic peptides by MHC class I molecules plays a vital role in generating T cell responses against infection and cancer. Over the last two decades the central role of tapasin as a peptide editor that influences the loading and optimisation of peptides onto MHC class I molecules has been extensively characterised. Recently, it has become evident that the tapasin-related protein, TAPBPR, functions as a second peptide editor which influences the peptides displayed by MHC class I molecules. Here, we review the discovery of TAPBPR and current understanding of this novel protein in relation to its closest homologue tapasin.
Asunto(s)
Inmunoglobulinas/metabolismo , Proteínas de la Membrana/metabolismo , Subgrupos de Linfocitos T/metabolismo , Animales , Presentación de Antígeno/inmunología , Microambiente Celular/inmunología , Epítopos/química , Epítopos/inmunología , Antígenos de Histocompatibilidad Clase I/inmunología , Antígenos de Histocompatibilidad Clase I/metabolismo , Humanos , Inmunoglobulinas/genética , Proteínas de la Membrana/genética , Proteínas de Transporte de Membrana/genética , Proteínas de Transporte de Membrana/metabolismo , Chaperonas Moleculares/genética , Chaperonas Moleculares/metabolismo , Péptidos/química , Péptidos/inmunología , Polimorfismo Genético , Unión Proteica , Isoformas de Proteínas , Transducción de Señal , Subgrupos de Linfocitos T/inmunologíaRESUMEN
Recently, we revealed that TAPBPR is a peptide exchange catalyst that is important for optimal peptide selection by MHC class I molecules. Here, we asked whether any other co-factors associate with TAPBPR, which would explain its effect on peptide selection. We identify an interaction between TAPBPR and UDP-glucose:glycoprotein glucosyltransferase 1 (UGT1), a folding sensor in the calnexin/calreticulin quality control cycle that is known to regenerate the Glc1Man9GlcNAc2 moiety on glycoproteins. Our results suggest the formation of a multimeric complex, dependent on a conserved cysteine at position 94 in TAPBPR, in which TAPBPR promotes the association of UGT1 with peptide-receptive MHC class I molecules. We reveal that the interaction between TAPBPR and UGT1 facilities the reglucosylation of the glycan on MHC class I molecules, promoting their recognition by calreticulin. Our results suggest that in addition to being a peptide editor, TAPBPR improves peptide optimisation by promoting peptide-receptive MHC class I molecules to associate with the peptide-loading complex.
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Presentación de Antígeno , Glucosiltransferasas/metabolismo , Antígenos de Histocompatibilidad Clase I/metabolismo , Inmunoglobulinas/metabolismo , Proteínas de la Membrana/metabolismo , Línea Celular , Humanos , Mapeo de Interacción de Proteínas , Multimerización de ProteínaRESUMEN
Our understanding of the antigen presentation pathway has recently been enhanced with the identification that the tapasin-related protein TAPBPR is a second major histocompatibility complex (MHC) class I-specific chaperone. We sought to determine whether, like tapasin, TAPBPR can also influence MHC class I peptide selection by functioning as a peptide exchange catalyst. We show that TAPBPR can catalyse the dissociation of peptides from peptide-MHC I complexes, enhance the loading of peptide-receptive MHC I molecules, and discriminate between peptides based on affinity in vitro. In cells, the depletion of TAPBPR increased the diversity of peptides presented on MHC I molecules, suggesting that TAPBPR is involved in restricting peptide presentation. Our results suggest TAPBPR binds to MHC I in a peptide-receptive state and, like tapasin, works to enhance peptide optimisation. It is now clear there are two MHC class I specific peptide editors, tapasin and TAPBPR, intimately involved in controlling peptide presentation to the immune system.
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Presentación de Antígeno , Antígenos de Histocompatibilidad Clase I/metabolismo , Inmunoglobulinas/metabolismo , Proteínas de la Membrana/metabolismo , Antígenos/metabolismo , Línea Celular , Humanos , Péptidos/metabolismo , Unión ProteicaRESUMEN
The human MHC class I protein HLA-B*27:05 is statistically associated with ankylosing spondylitis, unlike HLA-B*27:09, which differs in a single amino acid in the F pocket of the peptide-binding groove. To understand how this unique amino acid difference leads to a different behavior of the proteins in the cell, we have investigated the conformational stability of both proteins using a combination of in silico and experimental approaches. Here, we show that the binding site of B*27:05 is conformationally disordered in the absence of peptide due to a charge repulsion at the bottom of the F pocket. In agreement with this, B*27:05 requires the chaperone protein tapasin to a greater extent than the conformationally stable B*27:09 in order to remain structured and to bind peptide. Taken together, our data demonstrate a method to predict tapasin dependence and physiological behavior from the sequence and crystal structure of a particular class I allotype. Also watch the Video Abstract.
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Antígeno HLA-B27/química , Proteínas de Transporte de Membrana/metabolismo , Chaperonas Moleculares/metabolismo , Espondilitis Anquilosante/inmunología , Secuencia de Aminoácidos , Animales , Secuencia de Bases , Sitios de Unión/genética , Línea Celular , Antígeno HLA-B27/genética , Humanos , Ratones , Modelos Moleculares , Simulación de Dinámica Molecular , Unión Proteica , Conformación Proteica , Pliegue de Proteína , Análisis de Secuencia de ADN , Espondilitis Anquilosante/genéticaRESUMEN
The tapasin-related protein TAPBPR is a novel component of the antigen processing and presentation pathway, which binds to MHC class I coupled with ß2-microglobulin. We describe six alternatively spliced TAPBPR transcripts from the TAPBPL gene and investigate three of these at a protein level. TAPBPR transcripts lacking exon 5 result in loss of the membrane proximal IgC domain and loss of ability to bind to MHC class I. Alternative acceptor and donor splice sites in exon 4 of TAPBPR altered the reading frame in the IgV domain and produced a truncated TAPBPR product. An additional exon in the TAPBPL gene was identified that encodes extra residues in the cytoplasmic tail of TAPBPR. This longer TAPBPR protein interacted with MHC class I but was attenuated in its ability to down-regulate surface expression of MHC class I. The abundance of these alternative transcripts in peripheral blood mononuclear cells and dendritic cells suggests an important role of TAPBPR isoforms in vivo.
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Antígenos de Histocompatibilidad Clase I/inmunología , Inmunoglobulinas/inmunología , Proteínas de la Membrana/inmunología , Células Dendríticas/citología , Células Dendríticas/inmunología , Células HeLa , Humanos , Inmunoglobulinas/genética , Proteínas de la Membrana/genética , Isoformas de Proteínas/genética , Isoformas de Proteínas/inmunologíaRESUMEN
The loading of peptide Ags onto MHC class I molecules is a highly controlled process in which the MHC class I-dedicated chaperone tapasin is a key player. We recently identified a tapasin-related molecule, TAPBPR, as an additional component in the MHC class I Ag-presentation pathway. In this study, we show that the amino acid residues important for tapasin to interact with MHC class I are highly conserved on TAPBPR. We identify specific residues in the N-terminal and C-terminal domains of TAPBPR involved in associating with MHC class I. Furthermore, we demonstrate that residues on MHC class I crucial for its association with tapasin, such as T134, are also essential for its interaction with TAPBPR. Taken together, the data indicate that TAPBPR and tapasin bind in a similar orientation to the same face of MHC class I. In the absence of tapasin, the association of MHC class I with TAPBPR is increased. However, in the absence of TAPBPR, the interaction between MHC class I and tapasin does not increase. In light of our findings, previous data determining the function of tapasin in the MHC class I Ag-processing and presentation pathway must be re-evaluated.
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Antígeno HLA-A2/metabolismo , Inmunoglobulinas/metabolismo , Proteínas de la Membrana/metabolismo , Proteínas de Transporte de Membrana/metabolismo , Secuencia de Aminoácidos , Presentación de Antígeno , Células HEK293 , Antígeno HLA-A2/genética , Células HeLa , Humanos , Inmunoglobulinas/genética , Proteínas de la Membrana/genética , Proteínas de Transporte de Membrana/genética , Datos de Secuencia Molecular , Unión Proteica/genética , Conformación Proteica , Dominios y Motivos de Interacción de Proteínas/genética , ARN Interferente Pequeño/genéticaRESUMEN
Tapasin is an integral component of the peptide-loading complex (PLC) important for efficient peptide loading onto MHC class I molecules. We investigated the function of the tapasin-related protein, TAPBPR. Like tapasin, TAPBPR is widely expressed, IFN-γ-inducible, and binds to MHC class I coupled with ß2-microglobulin in the endoplasmic reticulum. In contrast to tapasin, TAPBPR does not bind ERp57 or calreticulin and is not an integral component of the PLC. ß2-microglobulin is essential for the association between TAPBPR and MHC class I. However, the association between TAPBPR and MHC class I occurs in the absence of a functional PLC, suggesting peptide is not required. Expression of TAPBPR decreases the rate of MHC class I maturation through the secretory pathway and prolongs the association of MHC class I on the PLC. The TAPBPR:MHC class I complex trafficks through the Golgi apparatus, demonstrating a function of TAPBPR beyond the endoplasmic reticulum/cis-Golgi. The identification of TAPBPR as an additional component of the MHC class I antigen-presentation pathway demonstrates that mechanisms controlling MHC class I expression remain incompletely understood.
Asunto(s)
Presentación de Antígeno/inmunología , Antígenos de Histocompatibilidad Clase I/inmunología , Inmunoglobulinas/metabolismo , Proteínas de la Membrana/metabolismo , Presentación de Antígeno/efectos de los fármacos , Calnexina/metabolismo , Calreticulina/metabolismo , Retículo Endoplásmico/efectos de los fármacos , Retículo Endoplásmico/metabolismo , Aparato de Golgi/efectos de los fármacos , Aparato de Golgi/metabolismo , Células HEK293 , Antígenos HLA-A/metabolismo , Células HeLa , Humanos , Interferón gamma/farmacología , Cinética , Proteínas de Transporte de Membrana/metabolismo , Péptidos/inmunología , Unión Proteica/efectos de los fármacos , Proteína Disulfuro Isomerasas/metabolismo , Multimerización de Proteína/efectos de los fármacos , Transporte de Proteínas/efectos de los fármacos , Microglobulina beta-2/metabolismoRESUMEN
Major histocompatibility complex (MHC) class I molecules present cell internally derived peptides at the plasma membrane for surveillance by cytotoxic T lymphocytes. The surface expression of most class I molecules at least partially depends on the endoplasmic reticulum protein, tapasin, which helps them to bind peptides of the right length and sequence. To determine what makes a class I molecule dependent on support by tapasin, we have conducted in silico molecular dynamics (MD) studies and laboratory experiments to assess the conformational state of tapasin-dependent and -independent class I molecules. We find that in the absence of peptide, the region around the F pocket of the peptide binding groove of the tapasin-dependent molecule HLA-B*44:02 is in a disordered conformational state and that it is converted to a conformationally stable state by tapasin. This novel chaperone function of tapasin has not been described previously. We demonstrate that the disordered state of class I is caused by the presence of two adjacent acidic residues in the bottom of the F pocket of class I, and we suggest that conformational disorder is a common feature of tapasin-dependent class I molecules, making them essentially unable to bind peptides on their own. MD simulations are a useful tool to predict such conformational disorder of class I molecules.
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Antígenos de Histocompatibilidad Clase I/química , Proteínas de Transporte de Membrana/farmacología , Conformación Proteica/efectos de los fármacos , Línea Celular , Antígeno HLA-B44/inmunología , Antígenos de Histocompatibilidad Clase I/efectos de los fármacos , Humanos , Simulación de Dinámica Molecular , Unión ProteicaRESUMEN
Leukocyte Ig-like receptors (LILRs) are a family of innate immune receptors predominantly expressed by myeloid cells that can alter the Ag presentation properties of macrophages and dendritic cells. Several LILRs bind HLA class I. Altered LILR recognition due to HLA allelic variation could be a contributing factor in disease. We comprehensively assessed LILR binding to >90 HLA class I alleles. The inhibitory receptors LILRB1 and LILRB2 varied in their level of binding to different HLA alleles, correlating in some cases with specific amino acid motifs. LILRB2 displayed the weakest binding to HLA-B*2705, an allele genetically associated with several autoimmune conditions and delayed progression of HIV infection. We also assessed the effect of HLA class I conformation on LILR binding. LILRB1 exclusively bound folded ß(2)-microglobulin-associated class I, whereas LILRB2 bound both folded and free H chain forms. In contrast, the activating receptor LILRA1 and the soluble LILRA3 protein displayed a preference for binding to HLA-C free H chain. To our knowledge, this is the first study to identify the ligand of LILRA3. These findings support the hypothesis that LILR-mediated detection of unfolded versus folded MHC modulates immune responses during infection or inflammation.