Crystal Structures Of Ctid-UBA And The Mechanism Of Presenting Antigenic Peptides

The major histocompatibility complex (MHC) class Ⅰ molecules play an important role in cytotoxic T lymphocyte (CTL) immune response. Although MHC class Ⅰ genes appeared suddenly in jawed fish, the class I structure and the possibility of restricted peptide presentation remain undetermined. The first grass carp MHC I crystal structures, Ctid-UBA complexed with peptides derived from SVCV, are solved in this study. The pCtid-UBA is found to be the canonical pMHC I structures but with three specific species characteristics. First, the great shift of α3domain and a key residue substitution indicate that the fish MHC I and CD8interaction is different form higher vertebrates. Second, an extremely large interface area are discovered between Ctid-UBA and Ctid-β2m in pCtid-UBA. Moreover, either the Ctid-β2m-Ⅰ or P2m-Ⅱ molecule, as expressed from2p2ms loci in the selfsame fish, could assemble into a termolecular complex in the same way. Third, the structure of pCtid-UBA presents the canonical6pockets observed in other species, but their amino acid compositions are unique and the D pocket mainly determines the peptide binding motif. With the help of water molecules, the peptide adopts a typical M conformation which is suitable for TCR docking. Furthermore, the vital residues of pCtid-UBA for epitope-peptide presentation are highly conserved and extended from fish to mammalian MHC class I alleles.40Ctid-UBA sequences are cloned from grass carp spleen and the polymorphism analysis of the cloning sequences together with other uploaded sequences show that they can be divided into3branches. The high variation sites which composed PBG are labeled on the crystal structures of Ctid-UBA to illustate their influences of the bias of peptide presentation.The crystal structures of grass carp β2m-Ⅱ and zebrafish β2m-Ⅰ(Dare-β2m-Ⅰ) are also determined. By comparing with other species, their overall structures are similar. Besides the highly conserved disulfide bond formed by residues Cys25and Cys80between strands B and F, there is an additional hydrogen bond between strands C and E composed by Ile37and Lys66which is highly conserved and only found in teleosts, and the hydrophobic pocket around the center of proteins indicate a more stable interaction between the "large and small β-sheets" than that in other species. Both the Ctid-β2m-Ⅰ/Ⅱ molecule could assemble into a similar termolecular complex with the heavy chain. The structures of Dare-β2m-Ⅰ and Dare-β2m-Ⅱ show the same conformation and only4different amino acids between them distribute on the interface with the heavy chain.In conclusion, the crystal structures of Ctid-UBA, Ctid-β2m-Ⅱ and Dare-β2m-Ⅰ are first solved, and illustrated the mechanism of presenting antigenic peptides. The data are helpful to illustrate the mechanism of CTL responses activated by MHC Ⅰ in fish, and make up the essential information to track the evolution of this key molecule in the AIS.