Affinage

Showing TAPBPL — TAPBPR is a alias.

TAPBPL

Tapasin-related protein · UniProt Q9BX59

Length
468 aa
Mass
50.2 kDa
Annotated
2026-06-10
35 papers in source corpus 18 papers cited in narrative 18 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 7/7 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

TAPBPL (TAPBPR) is an MHC class I-dedicated chaperone and peptide-exchange catalyst that operates in the ER/Golgi to shape the displayed immunopeptidome (PMID:23401559, PMID:26439010). It binds peptide-receptive MHC-I/β2-microglobulin complexes, with β2m essential for the association, and engages the same MHC-I face used by tapasin (requiring MHC-I residue T134) in a mutually exclusive manner, acting outside the peptide-loading complex (PMID:23401559, PMID:24163410). Crystal and cryo-EM structures show that TAPBPR remodels the MHC-I groove—relaxing it, reshaping the A and F pockets, and inserting a 'scoop loop' that stabilizes the empty groove and acts as an internal peptide surrogate to impede peptide rebinding—thereby releasing low-affinity peptides and capturing a peptide-receptive state (PMID:29025991, PMID:29025996, PMID:32167472, PMID:39786927). Catalytically, a 'leucine lever' (L30 within the K22-D35 loop) drives peptide dissociation in a manner dictated by the MHC-I F pocket, while the loop hovers over the groove to lower the affinity threshold for peptide capture, biasing presentation toward high-affinity ligands and restricting repertoire diversity (PMID:26439010, PMID:30484775, PMID:34039964). TAPBPR couples editing to quality control by recruiting UGGT1 (via its conserved Cys94) to reglucosylate peptide-free MHC-I, after which calreticulin recognizes the monoglucosylated glycan and transfers MHC-I back to the tapasin/PLC (PMID:28425917, PMID:37345806). Beyond MHC-I, TAPBPR chaperones the non-classical molecule MR1 in a ligand-independent manner and catalyzes exchange of small-molecule ligands in the MR1 groove (PMID:35725941). Functionally, both membrane-anchored and soluble luminal TAPBPR edit peptides on surface MHC-I to enable T cell activation and killing, and engineered high-fidelity variants extend this exchange across multiple HLA allomorphs (PMID:30213851, PMID:39786927).

Mechanistic history

Synthesis pass · year-by-year structured walk · 15 steps
  1. 2002 Medium

    Established that TAPBPR exists as a tapasin-like protein localized primarily to the ER with cell-surface expression, framing it as a candidate MHC-I pathway component distinct from tapasin.

    Evidence Subcellular fractionation and cell-surface biotinylation with sequence/structural analysis

    PMID:11920573

    Open questions at the time
    • No demonstrated binding partner or function
    • Lacks an obvious ER retention signal, mechanism of localization unresolved
  2. 2013 High

    Defined TAPBPR as a tapasin homolog that binds MHC-I/β2m independently of the PLC and traffics through the Golgi, distinguishing it mechanistically from the PLC-resident chaperone tapasin and identifying its competitive, mutually exclusive binding to the same MHC-I face.

    Evidence Co-IP, fractionation, pulse-chase maturation assays, knockdown/overexpression, and mutagenesis of TAPBPR/MHC-I (T134) in tapasin- or TAPBPR-deficient cells

    PMID:23401559 PMID:24163410

    Open questions at the time
    • Catalytic activity not yet demonstrated
    • Structural basis of the shared binding interface unresolved at this stage
  3. 2014 Medium

    Showed that domain architecture governs function, with the membrane-proximal IgC domain required for MHC-I binding and cytoplasmic-tail variation tuning surface MHC-I downregulation.

    Evidence RT-PCR isoform identification, Co-IP, and cell-surface MHC-I expression assays

    PMID:24444341

    Open questions at the time
    • Functional relevance of isoforms in vivo unclear
    • Single lab
  4. 2015 High

    Demonstrated that TAPBPR is a bona fide peptide-exchange catalyst that dissociates and reloads peptides and discriminates by affinity, establishing it as an active editor that restricts the cellular peptide repertoire rather than a passive binder.

    Evidence In vitro fluorescence polarization dissociation/loading assays and immunopeptidomics of TAPBPR-depleted cells

    PMID:26439010

    Open questions at the time
    • Structural mechanism of groove remodeling not yet defined
    • Catalytic residues unidentified
  5. 2016 High

    Showed TAPBPR selectively engages peptide-free or low-affinity MHC-I and is displaced by high-affinity peptide, and that it is structurally homologous to tapasin, establishing the conformational basis of affinity-based editing.

    Evidence Recombinant binding assays, fluorescence polarization, SAXS, and mutagenesis

    PMID:26869717

    Open questions at the time
    • Atomic-resolution interface not yet resolved
  6. 2017 High

    Resolved the atomic mechanism of editing: TAPBPR relaxes the MHC-I groove, remodels the A and F pockets, and inserts a scoop loop into the empty groove that interferes with peptide binding, directly explaining how low-affinity peptides are released and a receptive state is captured.

    Evidence Two independent X-ray crystal structures of TAPBPR–MHC-I complexes

    PMID:29025991 PMID:29025996

    Open questions at the time
    • Dynamics of peptide capture not captured by static structures
    • Catalytic residue contribution not quantified
  7. 2017 High

    Connected editing to glycan-based quality control by showing TAPBPR recruits UGGT1 (via Cys94) to reglucosylate peptide-free MHC-I, enhancing calreticulin recognition and re-entry into the PLC, establishing a recycling circuit.

    Evidence Co-IP/MS interactome, C94 mutagenesis, glycan analysis, and functional loading assays

    PMID:28425917

    Open questions at the time
    • In-cell stoichiometry of the multimeric complex unresolved
    • Calreticulin hand-off step not yet reconstituted
  8. 2018 High

    Identified the catalytic 'leucine lever' (L30 in the K22-D35 loop) that drives peptide dissociation in an F-pocket-dependent manner, assigning a specific residue to the editing reaction.

    Evidence Site-directed mutagenesis, cell-based peptide exchange assays, immunopeptidomics

    PMID:30484775

    Open questions at the time
    • Energetics of the lever mechanism not fully defined
  9. 2018 High

    Demonstrated TAPBPR functions at the cell surface, with membrane-anchored and exogenously added soluble luminal TAPBPR editing surface MHC-I to load immunogenic peptides and trigger T cell responses, expanding its functional compartment and revealing immunotherapeutic potential.

    Evidence Plasma membrane-targeted and soluble TAPBPR constructs with T cell IFN-γ and cytotoxicity assays

    PMID:30213851

    Open questions at the time
    • Efficiency across diverse HLA allomorphs not established at this stage
  10. 2021 High

    Separated the chaperoning and editing functions by mapping stabilization of nascent empty MHC-I to a broad scaffolding surface while showing the scoop loop hovers above the groove to promote peptide capture, refining the model of how the loop acts as a selectivity filter and peptide surrogate.

    Evidence Deep mutagenesis, solution NMR, ITC, FP, and in vitro reconstitution of loop variants

    PMID:32167472 PMID:34039964

    Open questions at the time
    • Dynamic intermediate of peptide capture not visualized
  11. 2021 Medium

    Reported a distinct immunological role for TAPBPL as a T cell co-inhibitory ligand acting through a putative receptor on activated T cells, separate from its MHC-I chaperone function.

    Evidence Recombinant TAPBPL-Ig fusion, in vitro T cell proliferation/cytokine assays, in vivo EAE model, and blocking antibody

    PMID:33938620

    Open questions at the time
    • Receptor identity not established
    • Mechanism of receptor engagement uncharacterized
    • Single lab
  12. 2022 High

    Extended TAPBPR's chaperone scope to the non-classical molecule MR1, showing ligand-independent engagement and catalysis of small-molecule ligand exchange, indicating a broader role in non-classical antigen presentation.

    Evidence In vitro assays, paramagnetic and 19F-NMR relaxation dispersion, and restrained molecular dynamics

    PMID:35725941

    Open questions at the time
    • Cellular relevance of MR1 chaperoning not demonstrated
    • Single lab
  13. 2023 High

    Confirmed the UGGT1 reglucosylation step in a fully reconstituted system with purified human proteins, establishing the minimal components sufficient for TAPBPR-promoted glycan modification of peptide-free MHC-I.

    Evidence Reconstituted in vitro system, glycoengineering, LC-MS glycan analysis

    PMID:37345806

    Open questions at the time
    • Downstream calreticulin transfer not addressed here
  14. 2025 High

    Visualized an antigen-proofreading intermediate at 3.0 Å showing transient P2/P3 anchor contacts with the nascent groove and engineered a high-fidelity TAPBPR variant for robust surface exchange across multiple HLA allomorphs, advancing both mechanistic and translational understanding.

    Evidence Cryo-EM structure of a peptide-decoy-bound MHC-I/TAPBPR complex and cell-surface exchange assays with an engineered variant

    PMID:39786927

    Open questions at the time
    • Kinetics of the proofreading transition not fully resolved
  15. 2025 Medium

    Reconstituted the hand-off step, showing calreticulin transfers peptide-receptive MHC-I from TAPBPR back to tapasin dependent on the UGGT1-generated monoglucosylated glycan, with distinct calreticulin domains required for disengagement versus docking, closing the editing-to-PLC recycling loop.

    Evidence In vitro reconstitution with purified components and calreticulin domain mutagenesis (preprint)

    PMID:bio_10.1101_2025.11.25.690393

    Open questions at the time
    • Preprint not yet peer-reviewed
    • In-cell validation of the transfer mechanism pending

Open questions

Synthesis pass · forward-looking unresolved questions
  • The identity of the TAPBPL co-inhibitory receptor on T cells and the molecular basis of that signaling pathway remain undefined.
  • Receptor unidentified
  • Relationship between chaperone and co-inhibitory functions unknown

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0140096 catalytic activity, acting on a protein 4 GO:0044183 protein folding chaperone 3 GO:0098772 molecular function regulator activity 3 GO:0060089 molecular transducer activity 1
Localization
GO:0005783 endoplasmic reticulum 2 GO:0005886 plasma membrane 2 GO:0005794 Golgi apparatus 1
Pathway
R-HSA-168256 Immune System 3 R-HSA-392499 Metabolism of proteins 2
Partners
B2MCALRMR1TAPBPUGGT1
Complex memberships
TAPBPR–MHC-I/β2m complexTAPBPR–UGGT1 complex

Evidence

Reading pass · 18 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2002 TAPBPR (TAPBP-R) is localized primarily in the ER with some cell-surface expression, lacks an obvious ER retention signal, and contains structural motifs similar to tapasin (V-C1 IgSF member); subcellular fractionation and biotinylation experiments established this localization. Subcellular localization by fractionation and cell-surface biotinylation; sequence/structural analysis European journal of immunology Medium 11920573
2013 TAPBPR binds MHC class I/β2-microglobulin complexes in the ER, does not bind ERp57 or calreticulin, is not an integral component of the peptide-loading complex (PLC), and the TAPBPR:MHC-I complex trafficks through the Golgi apparatus—demonstrating a function beyond the ER/cis-Golgi. β2-microglobulin is essential for TAPBPR–MHC-I association. TAPBPR expression decreases the rate of MHC-I maturation and prolongs PLC association. Co-immunoprecipitation, subcellular fractionation, pulse-chase/maturation assays, knockdown/overexpression cell-based assays Proceedings of the National Academy of Sciences of the United States of America High 23401559
2013 Residues in the N-terminal and C-terminal domains of TAPBPR are required for MHC-I association, and MHC-I residue T134 (critical for tapasin binding) is also essential for TAPBPR binding; TAPBPR and tapasin bind mutually exclusively to the same face of MHC-I. In the absence of tapasin, TAPBPR–MHC-I association increases, but TAPBPR loss does not increase tapasin–MHC-I interaction. Mutagenesis of TAPBPR and MHC-I residues, Co-immunoprecipitation, competitive binding assays in cells lacking tapasin or TAPBPR Journal of immunology High 24163410
2014 Alternative splicing of TAPBPL generates isoforms with distinct MHC-I binding properties: loss of exon 5 (removing the membrane-proximal IgC domain) abolishes MHC-I binding, while a longer cytoplasmic tail isoform binds MHC-I but has attenuated ability to downregulate surface MHC-I expression. RT-PCR identification of splice isoforms, protein expression and Co-immunoprecipitation, cell-surface MHC-I expression assays Immunology Medium 24444341
2015 TAPBPR functions as a peptide exchange catalyst: it catalyses dissociation of peptides from peptide-MHC I complexes, enhances loading of peptide-receptive MHC-I molecules, and discriminates between peptides based on affinity in vitro. In cells, TAPBPR depletion increases the diversity of peptides presented on MHC-I, indicating TAPBPR restricts the peptide repertoire. In vitro peptide dissociation and loading assays (fluorescence polarization), immunopeptidomics of TAPBPR-depleted cells eLife High 26439010
2016 TAPBPR binds HLA-A*02:01 and other MHC-I molecules that are peptide-free or loaded with low-affinity peptides; the interaction is reversed by high-affinity peptides in an affinity-dependent manner. SAXS confirms structural similarity of TAPBPR to tapasin. TAPBPR stabilizes peptide-receptive conformations of MHC-I to permit peptide editing. Recombinant protein binding assays, fluorescence polarization, small-angle X-ray scattering (SAXS), mutagenesis Proceedings of the National Academy of Sciences of the United States of America High 26869717
2017 Crystal structure of TAPBPR in complex with MHC-I reveals that TAPBPR remodels the peptide-binding groove of MHC-I: it causes groove relaxation, modifies key binding pockets (including the A and F pockets), and induces domain adjustments, resulting in release of low-affinity peptide and capture of a peptide-receptive MHC-I state. X-ray crystallography of TAPBPR–MHC-I complex Science High 29025991
2017 A second crystal structure of the TAPBPR–MHC-I complex independently confirms that TAPBPR functions as a peptide selector by remodeling the MHC-I α2-1-helix region, stabilizing the empty groove, and inserting a loop ('scoop loop') into the groove that interferes with peptide binding. X-ray crystallography of TAPBPR–MHC-I complex Science High 29025996
2017 TAPBPR interacts with UDP-glucose:glycoprotein glucosyltransferase 1 (UGT1/UGGT1), forming a multimeric complex dependent on a conserved cysteine at position 94 in TAPBPR. TAPBPR promotes UGT1-mediated reglucosylation of the glycan on peptide-receptive MHC-I molecules, thereby enhancing MHC-I recognition by calreticulin and promoting re-entry into the peptide-loading complex. Co-immunoprecipitation/MS identification of TAPBPR-associated proteins, mutagenesis (C94), glycan analysis, functional peptide loading assays eLife High 28425917
2018 TAPBPR mediates peptide dissociation from MHC-I through a 'leucine lever' mechanism: leucine 30 (L30) within the K22-D35 loop of TAPBPR is critical for facilitating peptide dissociation. The molecular features of the MHC-I F pocket determine whether TAPBPR-mediated peptide dissociation occurs in a loop-dependent manner. Site-directed mutagenesis of TAPBPR loop residues, cell-based peptide exchange assays, immunopeptidomics eLife High 30484775
2018 Membrane-targeted TAPBPR at the plasma membrane retains peptide editor function and catalyzes peptide exchange on surface MHC-I. Soluble TAPBPR (luminal domain alone) added exogenously to intact cells also functions as a peptide editor on surface MHC-I, enabling loading of immunogenic peptides and subsequent T cell activation (IFN-γ secretion and cytotoxic killing). Plasma membrane-targeted TAPBPR constructs, exogenous soluble TAPBPR addition to cells, T cell functional assays (IFN-γ ELISA, cytotoxicity assay) Proceedings of the National Academy of Sciences of the United States of America High 30213851
2020 The scoop loop of TAPBPR acts as an internal peptide surrogate: it stabilizes empty MHC-I molecules by directly occupying the peptide-binding groove and impedes peptide rebinding, functioning as an additional selectivity filter for the immunopeptidome. Reconstitution with defined components demonstrated the scoop loop's contribution to MHC-I–chaperone complex stability and peptide editing. Reconstituted in vitro system with purified TAPBPR variants (loop deletion/mutation), biophysical stability assays, peptide exchange assays eLife High 32167472
2021 Deep mutagenesis reveals that residues important for TAPBPR chaperoning activity (stabilizing nascent empty MHC-I) map to a large scaffolding surface excluding the scoop loop, while loop mutations specifically affect TAPBPR interactions with properly conformed MHC-I relevant to peptide editing. Solution NMR, ITC, and FP assays show the loop hovers above the MHC-I groove to promote capture of incoming peptides, lowering affinity requirements for peptide selection. Deep mutagenesis, solution NMR, isothermal titration calorimetry (ITC), fluorescence polarization (FP) assays Nature communications High 34039964
2021 TAPBPL acts as a T cell co-inhibitory molecule: a soluble TAPBPL-Ig fusion protein inhibits T cell proliferation, activation, and cytokine production in vitro; a putative TAPBPL receptor is expressed on activated CD4 and CD8 T cells; in vivo TAPBPL-Ig attenuates experimental autoimmune encephalomyelitis in mice. Recombinant TAPBPL-Ig fusion protein, in vitro T cell proliferation and cytokine assays, in vivo EAE mouse model, anti-TAPBPL blocking antibody EMBO molecular medicine Medium 33938620
2022 TAPBPR chaperones MR1 (MHC-related protein 1) in a ligand-independent manner, unlike its interaction with MHC-I. Paramagnetic NMR combined with restrained molecular dynamics shows TAPBPR engages conserved MR1 surfaces inducing similar structural adaptations as in MHC-I/TAPBPR structures. TAPBPR affects exchange kinetics of noncovalent metabolites (e.g., diclofenac) with the MR1 groove, acting as a catalyst. In vitro biochemical assays, paramagnetic NMR, 19F-NMR relaxation dispersion experiments, restrained molecular dynamics simulations Nature chemical biology High 35725941
2023 In a fully reconstituted in vitro system with purified human proteins, TAPBPR promotes reglucosylation of peptide-free MHC-I by UGGT1, confirmed by glycoengineering combined with LC-MS analysis of glycan composition. Reconstituted in vitro system with purified components, glycoengineering, liquid chromatography-mass spectrometry (LC-MS) glycan analysis eLife High 37345806
2025 CryoEM structure at 3.0 Å of an MHC-I/TAPBPR complex bound to a peptide decoy reveals that antigen proofreading is mediated by transient P2/P3 anchor interactions with the nascent peptide-binding groove, where conserved MHC-I residues stabilize incoming peptides through backbone-focused contacts. A high-fidelity TAPBPR variant enables robust cell-surface peptide exchange across multiple HLA allomorphs. Cryo-EM structure determination (3.0 Å), functional peptide exchange assays on cell surface with engineered TAPBPR variant Proceedings of the National Academy of Sciences of the United States of America High 39786927
2025 In a reconstituted system with isolated components, calreticulin mediates transfer of peptide-receptive MHC-I from TAPBPR back to tapasin (PLC), dependent on recognition of the monoglucosylated MHC-I glycan generated by UGGT1. The C-terminal acidic helix of calreticulin is dispensable for disengaging reglucosylated MHC-I from TAPBPR but essential for docking MHC-I onto tapasin. In vitro reconstitution with purified isolated components, calreticulin domain mutagenesis, functional transfer assays bioRxiv (preprint)preprint Medium bio_10.1101_2025.11.25.690393

Source papers

Stage 0 corpus · 35 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2017 Crystal structure of a TAPBPR-MHC I complex reveals the mechanism of peptide editing in antigen presentation. Science (New York, N.Y.) 107 29025991
2017 Structure of the TAPBPR-MHC I complex defines the mechanism of peptide loading and editing. Science (New York, N.Y.) 105 29025996
2013 Tapasin-related protein TAPBPR is an additional component of the MHC class I presentation pathway. Proceedings of the National Academy of Sciences of the United States of America 103 23401559
2015 TAPBPR alters MHC class I peptide presentation by functioning as a peptide exchange catalyst. eLife 82 26439010
2016 Interaction of TAPBPR, a tapasin homolog, with MHC-I molecules promotes peptide editing. Proceedings of the National Academy of Sciences of the United States of America 80 26869717
2017 TAPBPR bridges UDP-glucose:glycoprotein glucosyltransferase 1 onto MHC class I to provide quality control in the antigen presentation pathway. eLife 58 28425917
2013 The binding of TAPBPR and Tapasin to MHC class I is mutually exclusive. Journal of immunology (Baltimore, Md. : 1950) 50 24163410
2021 TAPBPR promotes antigen loading on MHC-I molecules using a peptide trap. Nature communications 49 34039964
2002 A human TAPBP (TAPASIN)-related gene, TAPBP-R. European journal of immunology 49 11920573
2018 TAPBPR mediates peptide dissociation from MHC class I using a leucine lever. eLife 42 30484775
2020 A loop structure allows TAPBPR to exert its dual function as MHC I chaperone and peptide editor. eLife 38 32167472
2018 Utilizing TAPBPR to promote exogenous peptide loading onto cell surface MHC I molecules. Proceedings of the National Academy of Sciences of the United States of America 37 30213851
2017 Recent advances in Major Histocompatibility Complex (MHC) class I antigen presentation: Plastic MHC molecules and TAPBPR-mediated quality control. F1000Research 33 28299193
2017 Properties of the tapasin homologue TAPBPR. Current opinion in immunology 32 28528220
2020 Structural and dynamic studies of TAPBPR and Tapasin reveal the mechanism of peptide loading of MHC-I molecules. Current opinion in immunology 28 32402827
2015 TAPBPR: a new player in the MHC class I presentation pathway. Tissue antigens 23 25720504
2021 The role of MHC I protein dynamics in tapasin and TAPBPR-assisted immunopeptidome editing. Current opinion in immunology 20 34265495
2022 TAPBPR employs a ligand-independent docking mechanism to chaperone MR1 molecules. Nature chemical biology 18 35725941
2021 Identification of TAPBPL as a novel negative regulator of T-cell function. EMBO molecular medicine 17 33938620
2023 Chaperone function in antigen presentation by MHC class I molecules-tapasin in the PLC and TAPBPR beyond. Frontiers in immunology 13 37398669
2019 Production of soluble pMHC-I molecules in mammalian cells using the molecular chaperone TAPBPR. Protein engineering, design & selection : PEDS 11 32725167
2025 CryoEM structure of an MHC-I/TAPBPR peptide-bound intermediate reveals the mechanism of antigen proofreading. Proceedings of the National Academy of Sciences of the United States of America 10 39786927
2021 Why TAPBPR? Implications of an additional player in MHC class I peptide presentation. Current opinion in immunology 10 34052734
2023 The ER folding sensor UGGT1 acts on TAPBPR-chaperoned peptide-free MHC I. eLife 8 37345806
2022 Dynamics of peptide loading into major histocompatibility complex class I molecules chaperoned by TAPBPR. Physical chemistry chemical physics : PCCP 8 35575131
2020 The Ins and Outs of TAPBPR. Current opinion in immunology 8 32814254
2014 TAPBPR isoforms exhibit altered association with MHC class I. Immunology 8 24444341
2023 Visualising tapasin- and TAPBPR-assisted editing of major histocompatibility complex class-I immunopeptidomes. Current opinion in immunology 7 37245412
2023 Get into the groove! The influence of TAPBPR on cargo selection. Current opinion in immunology 4 37295041
2025 Reanalysis of Immunopeptidomics Datasets Provides Mechanistic Insight into TAPBPR-Mediated Peptide Editing on HLA-A, -B and -C Molecules. Wellcome open research 2 38800518
2025 TAPBPR promotes editing of the HLA-B44 peptide repertoire, increasing the presentation of peptides containing a C-terminal tryptophan. Frontiers in immunology 2 41246309
2023 Administration of Recombinant TAPBPL Protein Ameliorates Collagen-Induced Arthritis in Mice. International journal of molecular sciences 1 37762076
2026 Utilizing TAPBPR for Peptide Loading, Dissociation, and Exchange on Plasma Membrane-Expressed MHC-I. Methods in molecular biology (Clifton, N.J.) 0 41479006
2026 Mouse TAPBPR shows functional similarity to human TAPBPR in shaping the MHC-I immunopeptidome. Frontiers in immunology 0 42064070
2024 CryoEM structure of an MHC-I/TAPBPR peptide bound intermediate reveals the mechanism of antigen proofreading. bioRxiv : the preprint server for biology 0 39211162

Missed literature

Know a paper Affinage missed for TAPBPL? Flag it for the maintainers and the community.

No submissions yet.