4ozh | pdb_00004ozh
S16 protein complex
Structural highlights
Function[DQA1_HUMAN] Binds peptides derived from antigens that access the endocytic route of antigen presenting cells (APC) and presents them on the cell surface for recognition by the CD4 T-cells. The peptide binding cleft accommodates peptides of 10-30 residues. The peptides presented by MHC class II molecules are generated mostly by degradation of proteins that access the endocytic route, where they are processed by lysosomal proteases and other hydrolases. Exogenous antigens that have been endocytosed by the APC are thus readily available for presentation via MHC II molecules, and for this reason this antigen presentation pathway is usually referred to as exogenous. As membrane proteins on their way to degradation in lysosomes as part of their normal turn-over are also contained in the endosomal/lysosomal compartments, exogenous antigens must compete with those derived from endogenous components. Autophagy is also a source of endogenous peptides, autophagosomes constitutively fuse with MHC class II loading compartments. In addition to APCs, other cells of the gastrointestinal tract, such as epithelial cells, express MHC class II molecules and CD74 and act as APCs, which is an unusual trait of the GI tract. To produce a MHC class II molecule that presents an antigen, three MHC class II molecules (heterodimers of an alpha and a beta chain) associate with a CD74 trimer in the ER to form a heterononamer. Soon after the entry of this complex into the endosomal/lysosomal system where antigen processing occurs, CD74 undergoes a sequential degradation by various proteases, including CTSS and CTSL, leaving a small fragment termed CLIP (class-II-associated invariant chain peptide). The removal of CLIP is facilitated by HLA-DM via direct binding to the alpha-beta-CLIP complex so that CLIP is released. HLA-DM stabilizes MHC class II molecules until primary high affinity antigenic peptides are bound. The MHC II molecule bound to a peptide is then transported to the cell membrane surface. In B-cells, the interaction between HLA-DM and MHC class II molecules is regulated by HLA-DO. Primary dendritic cells (DCs) also to express HLA-DO. Lysosomal miroenvironment has been implicated in the regulation of antigen loading into MHC II molecules, increased acidification produces increased proteolysis and efficient peptide loading. Publication Abstract from PubMedCeliac disease is a T cell-mediated disease induced by dietary gluten, a component of which is gliadin. 95% of individuals with celiac disease carry the HLA (human leukocyte antigen)-DQ2 locus. Here we determined the T-cell receptor (TCR) usage and fine specificity of patient-derived T-cell clones specific for two epitopes from wheat gliadin, DQ2.5-glia-alpha1a and DQ2.5-glia-alpha2. We determined the ternary structures of four distinct biased TCRs specific for those epitopes. All three TCRs specific for DQ2.5-glia-alpha2 docked centrally above HLA-DQ2, which together with mutagenesis and affinity measurements provided a basis for the biased TCR usage. A non-germline encoded arginine residue within the CDR3beta loop acted as the lynchpin within this common docking footprint. Although the TCRs specific for DQ2.5-glia-alpha1a and DQ2.5-glia-alpha2 docked similarly, their interactions with the respective gliadin determinants differed markedly, thereby providing a basis for epitope specificity. T-cell receptor recognition of HLA-DQ2-gliadin complexes associated with celiac disease.,Petersen J, Montserrat V, Mujico JR, Loh KL, Beringer DX, van Lummel M, Thompson A, Mearin ML, Schweizer J, Kooy-Winkelaar Y, van Bergen J, Drijfhout JW, Kan WT, La Gruta NL, Anderson RP, Reid HH, Koning F, Rossjohn J Nat Struct Mol Biol. 2014 May;21(5):480-8. doi: 10.1038/nsmb.2817. Epub 2014 Apr , 28. PMID:24777060[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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