The human transporter associated with antigen processing: molecular models to describe peptide binding competent states

Principles of peptide selection by the transporter associated with antigen processing

Our ability to fight pathogens relies on major histocompatibility complex class I (MHC-I) molecules presenting diverse antigens on the surface of diseased cells. The transporter associated with antigen processing (TAP) transports nearly the entire repertoire of antigenic peptides into the endoplasmic reticulum for MHC-I loading. How TAP transports peptides specific for MHC-I is unclear. In this study, we used cryo-EM to determine a series of structures of human TAP, both in the absence and presence of peptides with various sequences and lengths. The structures revealed that peptides of eight or nine residues in length bind in a similarly extended conformation, despite having little sequence overlap. We also identified two peptide-anchoring pockets on either side of the transmembrane cavity, each engaging one end of a peptide with primarily main chain atoms. Occupation of both pockets results in a global conformational change in TAP, bringing the two halves of the transporter closer together to prime it for isomerization and ATP hydrolysis. Shorter peptides are able to bind to each pocket separately but are not long enough to bridge the cavity to bind to both simultaneously. Mutations that disrupt hydrogen bonds with the N and C termini of peptides almost abolish MHC-I surface expression. Our findings reveal that TAP functions as a molecular caliper that selects peptides according to length rather than sequence, providing antigen diversity for MHC-I presentation.

The lysosomal transporter TAPL has a dual role as peptide translocator and phosphatidylserine floppase

TAPL is a lysosomal ATP-binding cassette transporter that translocates a broad spectrum of polypeptides from the cytoplasm into the lysosomal lumen. Here we report that, in addition to its well-known role as a peptide translocator, TAPL exhibits an ATP-dependent phosphatidylserine floppase activity that is the possible cause of its high basal ATPase activity and of the lack of coupling between ATP hydrolysis and peptide efflux. We also present the cryo-EM structures of mouse TAPL complexed with (i) phospholipid, (ii) cholesteryl hemisuccinate (CHS) and 9-mer peptide, and (iii) ADP·BeF₃. The inward-facing structure reveals that F449 protrudes into the cylindrical transport pathway and divides it into a large hydrophilic central cavity and a sizable hydrophobic upper cavity. In the structure, the peptide binds to TAPL in horizontally-stretched fashion within the central cavity, while lipid molecules plug vertically into the upper cavity. Together, our results suggest that TAPL uses different mechanisms to function as a peptide translocase and a phosphatidylserine floppase.

Physicochemistry shapes bioactivity landscape of pan-ABC transporter modulators: Anchor point for innovative Alzheimer's disease therapeutics

Alzheimer's disease (AD) is a devastating neurological disorder characterized by the pathological accumulation of macromolecular Aβ and tau leading to neuronal death. Drugs approved to treat AD may ameliorate disease symptoms, however, no curative treatment exists. Aβ peptides were discovered to be substrates of adenosine triphosphate-(ATP)-binding cassette (ABC) transporters. Activators of these membrane-bound efflux proteins that promote binding and/or translocation of Aβ could revolutionize AD medicine. The knowledge about ABC transporter activators is very scarce, however, the few molecules that were reported contain substructural features of multitarget (pan-)ABC transporter inhibitors. A cutting-edge strategy to obtain new drug candidates is to explore and potentially exploit the recently proposed multitarget binding site of pan-ABC transporter inhibitors as anchor point for the development of innovative activators to promote Aβ clearance from the brain. Molecular associations between functional bioactivities and physicochemical properties of small-molecules are key to understand these processes. This study provides an analysis of a recently reported unique multitarget dataset for the correlation between multitarget bioactivity and physicochemistry. Six novel pan-ABC transporter inhibitors were validated containing substructural features of ABC transporter activators, which underpins the relevance of the multitarget binding site for the targeted development of novel AD diagnostics and therapeutics.

Physiological and transcriptomic analyses reveal the toxicological mechanism and risk assessment of environmentally-relevant waterborne tetracycline exposure on the gills of tilapia (Oreochromis niloticus)

With the increasing application of tetracycline (TC) in medical treatment, animal husbandry and aquaculture in recent decades, high quantities of TC have been frequently detected in the aquatic environment, and accordingly TC-related toxicity and environmental pollution have become a global concern. The present study was performed to explore the toxicological influences of TC exposure at its environmentally relevant concentrations on the gills of tilapia Oreochromis niloticus, based on the alteration in histopathology, oxidative stress, inflammatory response, cell cycle, mitochondrial function, apoptosis, and transcriptomic analysis. Our findings revealed that TC exposure damaged the structure and function, induced oxidative stress, affected inflammatory responses, and reduced Na⁺/K⁺-ATPase (NKA) activity in the gills. TC also caused the inhibition in cell cycle, resulted in mitochondrial dysfunction and activated apoptosis. Further transcriptomic analysis indicated the extensive influences of TC exposure on the gill function, and immune system was the main target to waterborne TC exposure. These results elucidated that environmental TC had more complex toxicological effects on gills of fish than previously assessed, and provided novel insight into molecular toxicology of TC on fish and good basis for assessing the environmental risk of TC.

Binding mode analysis of ABCA7 for the prediction of novel Alzheimer's disease therapeutics

The adenosine-triphosphate-(ATP)-binding cassette (ABC) transporter ABCA7 is a genetic risk factor for Alzheimer's disease (AD). Defective ABCA7 promotes AD development and/or progression. Unfortunately, ABCA7 belongs to the group of 'under-studied' ABC transporters that cannot be addressed by small-molecules. However, such small-molecules would allow for the exploration of ABCA7 as pharmacological target for the development of new AD diagnostics and therapeutics. Pan-ABC transporter modulators inherit the potential to explore under-studied ABC transporters as novel pharmacological targets by potentially binding to the proposed 'multitarget binding site'. Using the recently reported cryogenic-electron microscopy (cryo-EM) structures of ABCA1 and ABCA4, a homology model of ABCA7 has been generated. A set of novel, diverse, and potent pan-ABC transporter inhibitors has been docked to this ABCA7 homology model for the discovery of the multitarget binding site. Subsequently, application of pharmacophore modelling identified the essential pharmacophore features of these compounds that may support the rational drug design of innovative diagnostics and therapeutics against AD.

Investigation of the Effects of Primary Structure Modifications within the RRE Motif on the Conformation of Synthetic Bovine Herpesvirus 1-Encoded UL49.5 Protein Fragments

Herpesviruses are the most prevalent viruses that infect the human and animal body. They can escape a host immune response in numerous ways. One way is to block the TAP complex so that viral peptides, originating from proteasomal degradation, cannot be transported to the endoplasmic reticulum. As a result, a reduced number of MHC class I molecules appear on the surface of infected cells and, thus, the immune system is not efficiently activated. BoHV-1-encoded UL49.5 protein is one such TAP transporter inhibitor. This protein binds to TAP in such a way that its N-terminal fragment interacts with the loops of the TAP complex, and the C-terminus stimulates proteasomal degradation of TAP. Previous studies have indicated certain amino acid residues, especially the RRE(9-11) motif, within the helical structure of the UL49.5 N-terminal fragment, as being crucial to the protein's activity. In this work, we investigated the effects of modifications within the RRE region on the spatial structure of the UL49.5 N-terminal fragment. The introduced RRE(9-11) variations were designed to abolish or stabilize the structure of the α-helix and, consequently, to increase or decrease protein activity compared to the wild type. The terminal structure of the peptides was established using circular dichroism (CD), 2D nuclear magnetic resonance (NMR), and molecular dynamics (MD) in membrane-mimetic or membrane-model environments. Our structural results show that in the RRE(9-11)AAA and E11G peptides the helical structure has been stabilized, whereas for the RRE(9-11)GGG peptide, as expected, the helix structure has partially unfolded compared to the native structure. These RRE modifications, in the context of the entire UL49.5 proteins, slightly altered their biological activity in human cells.

Compact Bidirectional Promoters for Dual-Gene Expression in a Sleeping Beauty Transposon

Promoter choice is an essential consideration for transgene expression in gene therapy. The expression of multiple genes requires ribosomal entry or skip sites, or the use of multiple promoters. Promoter systems comprised of two separate, divergent promoters may significantly increase the size of genetic cassettes intended for use in gene therapy. However, an alternative approach is to use a single, compact, bidirectional promoter. We identified strong and stable bidirectional activity of the RPBSA synthetic promoter comprised of a fragment of the human Rpl13a promoter, together with additional intron/exon structures. The Rpl13a-based promoter drove long-term bidirectional activity of fluorescent proteins. Similar results were obtained for the EF1-α and LMP2/TAP1 promoters. However, in a lentiviral vector, the divergent bidirectional systems failed to produce sufficient titres to translate into an expression system for dual chimeric antigen receptor (CAR) expression. Although bidirectional promoters show excellent applicability to drive short RNA in Sleeping Beauty transposon systems, their possible use in the lentiviral applications requiring longer and more complex RNA, such as dual-CAR cassettes, is limited.

Structure determination of UL49.5 transmembrane protein from bovine herpesvirus 1 by NMR spectroscopy and molecular dynamics

The transporter associated with antigen processing (TAP) directly participates in the immune response as a key component of the cytosolic peptide to major histocompatibility complex (MHC) class I protein loading machinery. This makes TAP an important target for viruses avoiding recognition by CD8+ T lymphocytes. Its activity can be suppressed by the UL49.5 protein produced by bovine herpesvirus 1, although the mechanism of this inhibition has not been understood so far.

Therefore, the main goal of our study was to investigate the 3D structure of bovine herpesvirus 1 - encoded UL49.5 protein. The final structure of the inhibitor was established using circular dichroism (CD), 2D nuclear magnetic resonance (NMR), and molecular dynamics (MD) in membrane mimetic environments. In NMR studies, UL49.5 was represented by two fragments: the extracellular region (residues 1–35) and the transmembrane-intracellular fragment (residues 36–75), displaying various functions during viral invasion. After the empirical structure determination, a molecular docking procedure was used to predict the complex of UL49.5 with the TAP heterodimer.

Our results revealed that UL49.5 adopted a highly flexible membrane-proximal helical structure in the extracellular part. In the transmembrane region, we observed two short α-helices. Furthermore, the cytoplasmic part had an unordered structure. Finally, we propose three different orientations of UL49.5 in the complex with TAP. Our studies provide, for the first time, the experimental structural information on UL49.5 and structure-based insight in its mechanism of action which might be helpful in designing new drugs against viral infections.

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The UL49.5 viral protein forms a helical structure in the biological membrane

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Our NMR-based 3D structure of UL49.5 differs from the theoretical predictions

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Apart from the protruding N-terminal helix the structure is buried in the membrane

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Attention should be paid to the turns in the external and transmembrane domains

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Molecular docking proposes three possible structures of the UL49.5/TAP complexes

Moving the Cellular Peptidome by Transporters

Living matter is defined by metastability, implying a tightly balanced synthesis and turnover of cellular components. The first step of eukaryotic protein degradation via the ubiquitin-proteasome system (UPS) leads to peptides, which are subsequently degraded to single amino acids by an armada of proteases. A small fraction of peptides, however, escapes further cytosolic destruction and is transported by ATP-binding cassette (ABC) transporters into the endoplasmic reticulum (ER) and lysosomes. The ER-resident heterodimeric transporter associated with antigen processing (TAP) is a crucial component in adaptive immunity for the transport and loading of peptides onto major histocompatibility complex class I (MHC I) molecules. Although the function of the lysosomal resident homodimeric TAPL-like (TAPL) remains, until today, only loosely defined, an involvement in immune defense is anticipated since it is highly expressed in dendritic cells and macrophages. Here, we compare the gene organization and the function of single domains of both peptide transporters. We highlight the structural organization, the modes of substrate binding and translocation as well as physiological functions of both organellar transporters.

Structure and Dynamics of Antigenic Peptides in Complex with TAP

The transporter associated with antigen processing (TAP) selectively translocates antigenic peptides into the endoplasmic reticulum. Loading onto major histocompatibility complex class I molecules and proofreading of these bound epitopes are orchestrated within the macromolecular peptide-loading complex, which assembles on TAP. This heterodimeric ABC-binding cassette (ABC) transport complex is therefore a major component in the adaptive immune response against virally or malignantly transformed cells. Its pivotal role predestines TAP as a target for infectious diseases and malignant disorders. The development of therapies or drugs therefore requires a detailed comprehension of structure and function of this ABC transporter, but our knowledge about various aspects is still insufficient. This review highlights recent achievements on the structure and dynamics of antigenic peptides in complex with TAP. Understanding the binding mode of antigenic peptides in the TAP complex will crucially impact rational design of inhibitors, drug development, or vaccination strategies.

Generating hESCs with reduced immunogenicity by disrupting TAP1 or TAPBP

Human embryonic stem cells (hESCs) are thought to be a promising resource for cell therapy, while it has to face the major problem of graft immunological rejection. Major histocompatibility complex (MHC) class I expressed on the cell surface is the major cause of graft rejection. Transporter associated with antigen presentation 1 (TAP1) and TAP-associated glycoprotein (TAPBP) play important roles in regulating MHC class I expression. In this study, we generated TAP1- and TAPBP-deficient hESC lines, respectively, using transcription activator-like effector nucleases technique. These cells showed deficient expression of MHC class I on the cell surface and reduced immunogenicity compared with wild types, but maintained normal pluripotency, karyotypes, and differentiation ability. Thus, our findings are instrumental in developing a universal cell resource with both pluripotency and hypo-immunogenicity for transplantation therapy in the future.

A mechanism of viral immune evasion revealed by cryo-EM analysis of the TAP transporter

Cellular immunity against viral infection and tumor cells depends on antigen presentation by the major histocompatibility complex class 1 molecules (MHC I). Intracellular antigenic peptides are transported into the endoplasmic reticulum (ER) by the transporter associated with antigen processing (TAP) and then loaded onto the nascent MHC I, which are exported to the cell surface and present peptides to the immune system¹. Cytotoxic T lymphocytes recognize non-self peptides and program the infected or malignant cells for apoptosis. Defects in TAP account for immunodeficiency and tumor development. To escape immune surveillance, some viruses have evolved strategies to either down-regulate TAP expression or directly inhibit TAP activity. To date neither the architecture of TAP nor the mechanism of viral inhibition has been elucidated at the structural level. In this study we describe the cryo-electron microscopy (cryo-EM) structure of human TAP in complex with its inhibitor ICP47, a small protein produced by the herpes simplex virus I. We show that the twelve transmembrane helices and two cytosolic nucleotide-binding domains (NBDs) of the transporter adopt an inward-facing conformation with the two NBDs separated. The viral inhibitor ICP47 forms a long helical hairpin, which plugs the translocation pathway of TAP from the cytoplasmic side. Association of ICP47 precludes substrate binding and also prevents NBD closure necessary for ATP hydrolysis. This work illustrates a striking example of immune evasion by persistent viruses. By blocking viral antigens from entering the ER, herpes simplex virus is hidden from cytotoxic T lymphocytes, which may contribute to establishing a lifelong infection in the host.

Conformational Changes of the Antibacterial Peptide ATP Binding Cassette Transporter McjD Revealed by Molecular Dynamics Simulations

The ATP binding cassette (ABC) transporters form one of the largest protein superfamilies. They use the energy of ATP hydrolysis to transport chemically diverse ligands across membranes. An alternating access mechanism in which the transporter switches between inward- and outward-facing conformations has been proposed to describe the translocation process. One of the main open questions in this process is the degree of opening of the transporter at different stages of the transport cycle, as crystal structures and biochemical data have suggested a wide range of distances between nucleotide binding domains. Recently, the crystal structure of McjD, an antibacterial peptide ABC transporter from Escherichia coli, revealed a new occluded intermediate state of the transport cycle. The transmembrane domain is closed on both sides of the membrane, forming a cavity that can accommodate its ligand, MccJ25, a lasso peptide of 21 amino acids. In this work, we investigate the degree of opening of the transmembrane cavity required for ligand translocation. By means of steered molecular dynamics simulations, the ligand was pulled from the internal cavity to the extracellular side. This resulted in an outward-facing state. Comparison with existing outward-facing crystal structures shows a smaller degree of opening in the simulations, suggesting that the large conformational changes in some crystal structures may not be necessary even for a large substrate like MccJ25.

Use of Functional Polymorphisms To Elucidate the Peptide Binding Site of TAP Complexes

TAP1/TAP2 complexes translocate peptides from the cytosol to the endoplasmic reticulum lumen to enable immune surveillance by CD8(+) T cells. Peptide transport is preceded by peptide binding to a cytosol-accessible surface of TAP1/TAP2 complexes, but the location of the TAP peptide-binding pocket remains unknown. Guided by the known contributions of polymorphic TAP variants to peptide selection, we combined homology modeling of TAP with experimental measurements to identify several TAP residues that interact with peptides. Models for peptide-TAP complexes were generated, which indicate bent conformation for peptides. The peptide binding site of TAP is located at the hydrophobic boundary of the cytosolic membrane leaflet, with striking parallels to the glutathione binding site of NaAtm1, a transporter that functions in bacterial heavy metal detoxification. These studies illustrate the conservation of the ligand recognition modes of bacterial and mammalians transporters involved in peptide-guided cellular surveillance.

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR): CLOSED AND OPEN STATE CHANNEL MODELS

The cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the ATP-binding cassette (ABC) transporter superfamily. CFTR controls the flow of anions through the apical membrane of epithelia. Dysfunctional CFTR causes the common lethal genetic disease cystic fibrosis. Transitions between open and closed states of CFTR are regulated by ATP binding and hydrolysis on the cytosolic nucleotide binding domains, which are coupled with the transmembrane (TM) domains forming the pathway for anion permeation. Lack of structural data hampers a global understanding of CFTR and thus the development of "rational" approaches directly targeting defective CFTR. In this work, we explored possible conformational states of the CFTR gating cycle by means of homology modeling. As templates, we used structures of homologous ABC transporters, namely TM(287-288), ABC-B10, McjD, and Sav1866. In the light of published experimental results, structural analysis of the transmembrane cavity suggests that the TM(287-288)-based CFTR model could correspond to a commonly occupied closed state, whereas the McjD-based model could represent an open state. The models capture the important role played by Phe-337 as a filter/gating residue and provide structural information on the conformational transition from closed to open channel.

Viral inhibition of the transporter associated with antigen processing (TAP): a striking example of functional convergent evolution

Herpesviruses are large DNA viruses that are highly abundant within their host populations. Even in the presence of a healthy immune system, these viruses manage to cause lifelong infections. This persistence is partially mediated by the virus entering latency, a phase of infection characterized by limited viral protein expression. Moreover, herpesviruses have devoted a significant part of their coding capacity to immune evasion strategies. It is believed that the close coexistence of herpesviruses and their hosts has resulted in the evolution of viral proteins that specifically attack multiple arms of the host immune system. Cytotoxic T lymphocytes (CTLs) play an important role in antiviral immunity. CTLs recognize their target through viral peptides presented in the context of MHC molecules at the cell surface. Every herpesvirus studied to date encodes multiple immune evasion molecules that effectively interfere with specific steps of the MHC class I antigen presentation pathway. The transporter associated with antigen processing (TAP) plays a key role in the loading of viral peptides onto MHC class I molecules. This is reflected by the numerous ways herpesviruses have developed to block TAP function. In this review, we describe the characteristics and mechanisms of action of all known virus-encoded TAP inhibitors. Orthologs of these proteins encoded by related viruses are identified, and the conservation of TAP inhibition is discussed. A phylogenetic analysis of members of the family Herpesviridae is included to study the origin of these molecules. In addition, we discuss the characteristics of the first TAP inhibitor identified outside the herpesvirus family, namely, in cowpox virus. The strategies of TAP inhibition employed by viruses are very distinct and are likely to have been acquired independently during evolution. These findings and the recent discovery of a non-herpesvirus TAP inhibitor represent a striking example of functional convergent evolution.

Computational characterization of structural dynamics underlying function in active membrane transporters

Active transport of materials across the cellular membrane is one the most fundamental processes in biology. In order to accomplish this task, membrane transporters rely on a wide range of conformational changes spanning multiple time and size scales. These molecular events govern key functional aspects in membrane transporters, namely, coordinated gating motions underlying the alternating access mode of operation, and coupling of uphill transport of substrate to various sources of energy, for example, transmembrane electrochemical gradients and ATP binding and hydrolysis. Computational techniques such as molecular dynamics simulations and free energy calculations have equipped us with a powerful repertoire of biophysical tools offering unparalleled spatial and temporal resolutions that can effectively complement experimental methodologies, and therefore help fill the gap of knowledge in understanding the molecular basis of function in membrane transporters.

ABC transporters in adaptive immunity

BACKGROUND

ABC transporters ubiquitously found in all kingdoms of life move a broad range of solutes across membranes. Crystal structures of four distinct types of ABC transport systems have been solved, shedding light on different conformational states within the transport process. Briefly, ATP-dependent flipping between inward- and outward-facing conformations allows directional transport of various solutes.

SCOPE OF REVIEW

The heterodimeric transporter associated with antigen processing TAP1/2 (ABCB2/3) is a crucial element of the adaptive immune system. The ABC transport complex shuttles proteasomal degradation products into the endoplasmic reticulum. These antigenic peptides are loaded onto major histocompatibility complex class I molecules and presented on the cell surface. We detail the functional modules of TAP, its ATPase and transport cycle, and its interaction with and modulation by other cellular components. In particular, we emphasize how viral factors inhibit TAP activity and thereby prevent detection of the infected host cell by cytotoxic T-cells.

MAJOR CONCLUSIONS

Merging functional details on TAP with structural insights from related ABC transporters refines the understanding of solute transport. Although human ABC transporters are extremely diverse, they still may employ conceptually related transport mechanisms. Appropriately, we delineate a working model of the transport cycle and how viral factors arrest TAP in distinct conformations.

GENERAL SIGNIFICANCE

Deciphering the transport cycle of human ABC proteins is the major issue in the field. The defined peptidic substrate, various inhibitory viral factors, and its role in adaptive immunity provide unique tools for the investigation of TAP, making it an ideal model system for ABC transporters in general. This article is part of a Special Issue entitled Structural biochemistry and biophysics of membrane proteins.

Three tapasin docking sites in TAP cooperate to facilitate transporter stabilization and heterodimerization

The TAP translocates peptide Ags into the lumen of the endoplasmic reticulum for loading onto MHC class I molecules. MHC class I acquires its peptide cargo in the peptide loading complex, an oligomeric complex that the chaperone tapasin organizes by bridging TAP to MHC class I and recruiting accessory molecules such as ERp57 and calreticulin. Three tapasin binding sites on TAP have been described, two of which are located in the N-terminal domains of TAP1 and TAP2. The third binding site is present in the core transmembrane (TM) domain of TAP1 and is used only by the unassembled subunits. Tapasin is required to promote TAP stability, but through which binding site(s) it is acting is unknown. In particular, the role of tapasin binding to the core TM domain of TAP1 single chains is mysterious because this interaction is lost upon TAP2 association. In this study, we map the respective binding site in TAP1 to the polar face of the amphipathic TM helix TM9 and identify key residues that are essential to establish the interaction. We find that this interaction is dispensable for the peptide transport function but essential to achieve full stability of human TAP1. The interaction is also required for proper heterodimerization of the transporter. Based on similar results obtained using TAP mutants that lack tapasin binding to either N-terminal domain, we conclude that all three tapasin-binding sites in TAP cooperate to achieve high transporter stability and efficient heterodimerization.

Analyses of conformational states of the transporter associated with antigen processing (TAP) protein in a native cellular membrane environment

The transporter associated with antigen processing (TAP) plays a critical role in the MHC class I antigen presentation pathway. TAP translocates cellular peptides across the endoplasmic reticulum membrane in an ATP hydrolysis-dependent manner. We used FRET spectroscopy in permeabilized cells to delineate different conformational states of TAP in a native subcellular membrane environment. For these studies, we tagged the TAP1 and TAP2 subunits with enhanced cyan fluorescent protein and enhanced yellow fluorescent protein, respectively, C-terminally to their nucleotide binding domains (NBDs), and measured FRET efficiencies under different conditions. Our data indicate that both ATP and ADP enhance the FRET efficiencies but that neither induces a maximally closed NBD conformation. Additionally, peptide binding induces a large and significant increase in NBD proximity with a concentration dependence that is reflective of individual peptide affinities for TAP, revealing the underlying mechanism of peptide-stimulated ATPase activity of TAP. Maximal NBD closure is induced by the combination of peptide and non-hydrolysable ATP analogs. Thus, TAP1-TAP2 NBD dimers are not fully stabilized by nucleotides alone, and substrate binding plays a key role in inducing the transition state conformations of the NBD. Taken together, these findings show that at least three steps are involved in the transport of peptides across the endoplasmic reticulum membrane for antigen presentation, corresponding to three dynamically and structurally distinct conformational states of TAP. Our studies elucidate structural changes in the TAP NBD in response to nucleotides and substrate, providing new insights into the mechanism of ATP-binding cassette transporter function.

[Expression of PEPT2 mRNA in lung tissue of rats with pulmonary fibrosis]

背景与目的

肺纤维化是肺癌放化疗后的常见病理改变，是阻碍药物转运到肺部的关键因素之一，肽转运载体已经成为合理设计肽和肽类药物的靶标，本研究旨在探讨肽转运载体2（peptide transporter 2, PEPT2）mRNA在肺纤维化大鼠肺组织中的表达。

方法

健康SD大鼠50只，随机分为5组。博莱霉素（bleomycin, BLM）7 d、14 d、28 d组：气管内一次性滴入博莱霉素溶液复制肺纤维化大鼠模型，分别于给药后7 d、14 d和28 d放血处死；生理盐水组滴入等量生理盐水，于14 d放血处死；正常组不做任何处理。各组取肺组织，光镜观察组织病理变化；检测样本羟脯氨酸含量；半定量RT-PCR检测肺组织PEPT2 mRNA表达。

结果

BLM 7 d组大鼠肺组织呈急性炎症性改变，无纤维增生；BLM 14 d组和28 d组大鼠肺组织均有纤维化改变，以28 d组最为明显。BLM 7 d组肺组织羟脯氨酸含量与正常对照组和生理盐水组相比无统计学差异（P > 0.05）；14 d组和28 d组大鼠肺组织羟脯氨酸含量均高于正常对照组和生理盐水组（P < 0.05）。各组肺组织PEPT2 mRNA的相对表达量无统计学差异（P > 0.05）。

结论

PEPT2 mRNA在博莱霉素致肺纤维化大鼠肺组织表达水平无明显变化，PEPT2可能是设计肺纤维化的新型肽类药物靶标之一。

The MHC I loading complex: a multitasking machinery in adaptive immunity

Recognition and elimination of virally or malignantly transformed cells are pivotal tasks of the adaptive immune system. For efficient immune detection, snapshots of the cellular proteome are presented as epitopes on major histocompatibility complex class I (MHC I) molecules for recognition by cytotoxic T cells. Knowledge about the track from the equivocal protein to the presentation of antigenic peptides has greatly expanded, leading to an astonishingly elaborate understanding of the MHC I peptide loading pathway. Here, we summarize the current view on this complex process, which involves ABC transporters, proteases, chaperones, and endoplasmic reticulum (ER) quality control. The contribution of individual proteins and subcomplexes is discussed, with a focus on the architecture and dynamics of the key player in the pathway, the peptide-loading complex (PLC).