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Before metamorphosis, tadpoles do not efficiently express the single classical MHC class I (MHC-I) molecule Xela-UAA, but after metamorphosis, adults express this molecule in abundance. To elucidate the Ag-presenting mechanism of Xela-UAA, in this study, the Xela-UAA structure complex (pXela-UAAg) bound with a peptide from a synthetic random peptide library was determined. The amino acid homology between the Xela-UAA and MHC-I sequences of different species is <45%, and these differences are fully reflected in the three-dimensional structure of pXela-UAAg. Because of polymorphisms and interspecific differences in amino acid sequences, pXela-UAAg forms a distinct peptide-binding groove and presents a unique peptide profile. The most important feature of pXela-UAAg is the two-amino acid insertion in the \u03b12-helical region, which forms a protrusion of \u223c3.8 \u00c5 that is involved in TCR docking. Comparison of peptide-MHC-I complex (pMHC-I) structures showed that only four amino acids in \u03b22-microglobulin that were bound to MHC-I are conserved in almost all jawed vertebrates, and the most unique feature in nonmammalian pMHC-I molecules is that the AB loop bound \u03b22-microglobulin. Additionally, the binding distance between pMHC-I and CD8 molecules in nonmammals is different from that in mammals. These unique features of pXela-UAAg provide enhanced knowledge of T cell immunity and bridge the knowledge gap regarding the coevolutionary progression of the MHC-I complex from aquatic to terrestrial species.","bibjson":{"author":[{"initials":"L","lastname":"Ma","name":"Ma L"},{"initials":"N","lastname":"Zhang","name":"Zhang N"},{"initials":"Z","lastname":"Qu","name":"Qu Z"},{"initials":"R","lastname":"Liang","name":"Liang R"},{"initials":"L","lastname":"Zhang","name":"Zhang L"},{"initials":"B","lastname":"Zhang","name":"Zhang B"},{"initials":"G","lastname":"Meng","name":"Meng G"},{"initials":"JM","lastname":"Dijkstra","name":"Dijkstra JM"},{"initials":"S","lastname":"Li","name":"Li S"},{"initials":"MC","lastname":"Xia","name":"Xia MC"}],"identifier":[{"id":"10.4049/jimmunol.1900865","type":"doi"},{"id":"31776204","type":"pubmed"}],"issue":[null],"journal":{"iso_abbreviation":"J. Immunol.","name":""},"pages":[null],"title":"A Glimpse of the Peptide Profile Presentation by <i>Xenopus laevis</i> MHC Class I: Crystal Structure of p<i>Xela</i>-UAA Reveals a Distinct Peptide-Binding Groove.","type":"article","url":"http://www.jimmunol.org/lookup/doi/10.4049/jimmunol.1900865","volume":[null],"year":[2019]},"in_pmc":"N","in_pmce":"Y","open_access":"N"},"resolution":"2.80","same_as":{"pdbe":{"url":"https://www.ebi.ac.uk/pdbe/entry/pdb/6a2b"},"rcsb":{"url":"https://www.rcsb.org/structure/6a2b"}},"species":{"common_name":"African clawed frog","match_type":"histo:assign_species","scientific_name":"Xenopus laevis","slug":"xenopus_laevis"},"tcr":null,"title":"Xela-UAAg binding \"YMMPRHWPI\" at 2.80&#8491; resolution","unique_chain_count":3}}