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<title>Peptide-loading complex</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Peptide-loading complex</span></span>

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<p>The <b>peptide-loading complex</b> (PLC) is a short-lived, multisubunit <a href="Membrane_protein" title="Membrane protein">membrane protein</a> complex that is located in the <a href="Endoplasmic_reticulum" title="Endoplasmic reticulum">endoplasmic reticulum</a> (ER). It orchestrates <a href="Peptide" title="Peptide">peptide</a> translocation and selection by <a href="MHC_class_I" title="MHC class I">major histocompatibility complex class I (MHC-I)</a> molecules. Stable peptide-MHC I complexes are released to the cell surface to promote <a href="T-cell" class="mw-redirect" title="T-cell">T-cell</a> response against malignant or infected cells. In turn, T-cells recognize the activated peptides, which could be <a href="Immunogenicity" title="Immunogenicity"> immunogenic or non-immunogenic</a>.

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<div class="mw-heading mw-heading2"><h2 id="Overview">Overview</h2></div>

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<p>A PLC assembly consists of seven subunits, including <a href="Transporter_associated_with_antigen_processing" title="Transporter associated with antigen processing">the transporters</a> associated with <a href="Antigen_processing" title="Antigen processing">antigen processing</a> (TAP1 and TAP2 – jointly referred to as TAP), the oxidoreductase <a href="ERp57" class="mw-redirect" title="ERp57">ERp57</a>, the MHC-I heterodimer, and the <a href="Chaperone_(protein)" title="Chaperone (protein)">chaperones</a> <a href="Tapasin" title="Tapasin">tapasin</a> and <a href="Calreticulin" title="Calreticulin">calreticulin</a>. TAP transports proteasomal degradation products from the <a href="Cytosol" title="Cytosol">cytosol</a> into the <a href="Lumen_(anatomy)" title="Lumen (anatomy)"> lumen</a> of the ER, where they are loaded onto MHC-I molecules. The peptide-MHC-I complexes then move via a secretory pathway to the cell surface, presenting their antigenic load to cytotoxic T-cells.

</p><p>In general, preliminary MHC-I heavy chains are chaperoned by the calnexin–calreticulin system in the ER. Together with β2-microglobulin (β2m), MHC-I heavy chains form assemblies of heterodimers that act as receptors for antigenic peptides. Empty MHC-I heterodimers are recruited by calreticulin and form short-lived macromolecular PLC where the chaperone tapasin further provides stabilization in the MHC-I molecules. Furthermore, ERp57 and tapasin form <a href="Disulfide" title="Disulfide">disulfide</a>-linked conjugates, and tapasin is crucial for maintaining the structural stability of the PLC as well as facilitating optimal peptide loading. After final quality control, during which MHC-I heterodimers undergo peptide editing, stable peptide–MHC-I complexes are released to the cell surface for T-cell recognition. The PLC can serve a large variety of MHC-I <a href="Allomorph" title="Allomorph">allomorphs</a>, thus playing a central role in the differentiation and priming of T lymphocytes, and in controlling viral infections and tumour development.<sup id="cite_ref-Blees2017_1-0" class="reference"><a href="#cite_note-Blees2017-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="Structure">Structure</h2></div>

<p>The structure of the human PLC has been determined using single-particle electron cryo-microscopy (cryo-EM).<sup id="cite_ref-Blees_2017_2-0" class="reference"><a href="#cite_note-Blees_2017-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> The PLC, measuring 150 Å by 150 Å and with a total height of 240 Å, is organized around the Transporter associated with Antigen Processing (TAP). It includes molecules such as tapasin, calreticulin, ERp57, and Major Histocompatibility Complex class I (MHC-I), arranged in a pseudo-symmetric pattern.

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</style><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="Transporter_associated_with_antigen_processing" title="Transporter associated with antigen processing">Transporter associated with antigen processing</a></div>

<p>TAP is a heterodimeric complex, consisting of TAP1 (ABCB2) and TAP2 (ABCB3) members of the ABC transporter superfamily. The common feature of all ABC transporters is their organization: 1) into two transmembrane domains (TMDs) and 2) into two <a href="Nucleotide" title="Nucleotide">nucleotide</a>-binding domains (NBDs). Both intramolecular domains are coupled to each other and when <a href="Adenosine_triphosphate" title="Adenosine triphosphate">ATP</a> binding is in progress, <a href="Protein_conformation" class="mw-redirect" title="Protein conformation">conformational</a> changes in the TMDs allow proteasomal degradation products to move across the membrane. TAP recognizes and transports the antigen peptides produced in the cytosol straight into the ER, while tapasin recognizes the kind of peptides that have the ability to form stable complexes with MHC-I. This process is known as peptide proofreading or editing. Peptides selected through proofreading<sup id="cite_ref-Thomas<<-2017_3-0" class="reference"><a href="#cite_note-Thomas<<-2017-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> improve MHC-I stability; tapasin also contributes to the editing of immunogenic peptide epitopes. However, only lately it was proven via biochemical, biophysical, and structural studies that a key function in adaptive immunity, the <a href="Catalytic" class="mw-redirect" title="Catalytic">catalytic</a> mechanism of peptide proofreading, is performed by tapasin and TAPBPR (TAP-binding protein-related, a tapasin homologue).<sup id="cite_ref-Thomas_2017_4-0" class="reference"><a href="#cite_note-Thomas_2017-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading3"><h3 id="Tapasin">Tapasin</h3></div>

<div role="note" class="hatnote navigation-not-searchable">Main article: <a href="Tapasin" title="Tapasin">Tapasin</a></div>

<p>Cresswell and co-workers first discovered tapasin (TAP-associated glycoprotein) as a 48 kDa protein in complexes isolated with TAP1 antibodies from digitonin lysates of human B lymphoblastoid cells.<sup id="cite_ref-Ortmann_1994_5-0" class="reference"><a href="#cite_note-Ortmann_1994-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> Tapasin binds HC/β2m along with ER chaperones to the peptide transporter.<sup id="cite_ref-Sadasivan_1996_6-0" class="reference"><a href="#cite_note-Sadasivan_1996-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> It is located in the ER and its function comprises holding together class I molecules jointly with the chaperone calreticulin and the ERp57 to TAP. Studies of a tapasin-deficient cell line and from mice bearing a disrupted tapasin gene, the short-lived complex of class I molecules.

</p><p>Tapasin and TAP are very important for the stabilization of the class I molecules and also for the optimization of the peptide presented to cytotoxic T cells.<sup id="cite_ref-Momburg_2002_7-0" class="reference"><a href="#cite_note-Momburg_2002-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> A PLC-independent tapasin homologue protein named TAPBPR<sup id="cite_ref-Thomas_2017_4-1" class="reference"><a href="#cite_note-Thomas_2017-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> was found that has the ability to act as a second MHC-I specific peptide proofreader or editor, but does not possess a transmembrane domain.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> Tapasin and TAPBPR<sup id="cite_ref-Thomas_2017_4-2" class="reference"><a href="#cite_note-Thomas_2017-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> share similar binding interfaces on MHC-I, as shown with the <a href="X-ray_structure" class="mw-redirect" title="X-ray structure">X-ray structure</a> of TAPBPR with MHC-I (heavy chain and β2 microglobulin). The use of a photo-cleavable high-affinity peptide allowed researchers to form a stable (bound) MHC-I molecules and afterwards to form a stable TAPBPR<sup id="cite_ref-Thomas_2017_4-3" class="reference"><a href="#cite_note-Thomas_2017-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> and MHC-I complex with cleavage by UV light of the photoinduced peptide.

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<div role="note" class="hatnote navigation-not-searchable">Main article: <a href="PDIA3" title="PDIA3">PDIA3</a></div>

<p>ERp57 is an enzyme of the <a href="Thiol_oxidoreductase" title="Thiol oxidoreductase">thiol oxidoreductase</a> family located in the ER.<sup id="cite_ref-Frickel<<-2004_9-0" class="reference"><a href="#cite_note-Frickel<<-2004-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> It is attached to substrates in an indirect fashion through association with the molecular chaperone calreticulin of the peptide-loading complex,<sup id="cite_ref-Oliver_1999_10-0" class="reference"><a href="#cite_note-Oliver_1999-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-pmid16567808_11-0" class="reference"><a href="#cite_note-pmid16567808-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> In early stages of generation of MHC-I molecules, ERp57 is associated with free MHC-I heavy chains. As a result, its function is determined by the formation of disulfide bonds in heavy chains, by oxidative folding of the heavy chain, and finally by the fact that ERp57 is loading the peptides onto MHC-I molecules.

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<p><a href="MHC-I" class="mw-redirect" title="MHC-I">MHC-I</a> heavy chains may work as chaperones with the aid of the <a href="Calnexin" title="Calnexin">calnexin</a>-<a href="Calreticulin" title="Calreticulin">calreticulin</a> complex in the ER. In addition to this, β2-microglobulin (β2m) is attached to the heavy chains of the heterodimers and as a whole they act as receptors for antigenic peptides. When MHC-I chains are empty, they are recruited by calreticulin and form a transient PLC. </p><p> Tapasin regularly plays a role in the stabilization of MHC-I. Only after MHC-I heterodimers are deployed for peptide proofreading or editing, stable pMHC-I (peptide-MHC-I) complexes are released to the cell surface for recognition and destruction of virus-infected or malignantly <a href="Neoplasm" title="Neoplasm"> neoplastic</a> cells. In general, each individual organism owns a collection of six MHC-I molecules (three from each parent). Thus, in <a href="Autoimmunity" title="Autoimmunity"> autoimmune</a> emergencies, compatible donors are relatives who own a similar collection of MHC-I molecules, apart from those of the recipient.

</p><div class="mw-heading mw-heading3"><h3 id="Calreticulin">Calreticulin</h3></div>

<p><a href="Calreticulin" title="Calreticulin">Calreticulin</a> – especially its lectin-like domain – interacts with MHC-I. The P domain faces the MHC-I peptide-binding site towards ERp57. This orientation makes it possible for tapasin to attach and secure MHC-I. This translocation of TAP facilitates its opening out into an ER luminal cavity, edged by standard membrane entry points such as those for tapasin and MHC-I. These two entry points facilitate the recruitment of MHC-I with optimal peptide loading and eventual release of MHC-I in T-cell surfaces for recognition.

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<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFBleesJanulieneHofmannKoller2017" class="citation journal cs1">Blees, A.; Januliene, D.; Hofmann, T.; Koller, N.; Blanchet, C.; Rupp, B.; Ellenrieder, L.; Dalpke, A.; Zimmermann, R.; Heck, A.; Hillenbrand, M.; Diederichs, K.; Than, M. (2017). <a rel="nofollow" class="external text" href="https://doi.org/10.1016%2Fj.tibs.2017.05.005">"The MHC I peptide loading complex: a multitasking machinery in adaptive immunity"</a>. <i>Trends in Biochemical Sciences</i>. <b>42</b> (8): <span class="nowrap">648–</span>665. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://doi.org/10.1016%2Fj.tibs.2017.05.005">10.1016/j.tibs.2017.05.005</a></span>. <a href="PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/28579196">28579196</a>.</cite></span>

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