Secondary structure and 3D homology modeling of grass carp (Ctenopharyngodon idellus) major histocompatibility complex class I molecules

No information to date is available on the structure of fish major histocompatibility complex (MHC) class I and β2-microglobulin (β2m) proteins. In the present study, grass carp (Ctenopharyngodon idellus) MHC class I (Ctid-MHC I) and β2-microglobulin (Ctid-β2m) genes were expressed as soluble maltose binding protein (MBP)-proteins and purified in a pMAL-p2X/Escherichia coli TB1 system. The expressed proteins were purified on amylase affinity columns followed by DEAE–Sepharose. The purified products were identified by Western blotting with anti-MBP polyclonal antibodies. The MBP-Ctid-MHC I and MBP-Ctid-β2m were cleaved separately with Factor Xa, mixed together and purified on DEAE–Sepharose. The secondary structures were analyzed by circular dichroism (CD) spectrophotometry. The three-dimensional (3D) structure of their peptide-binding domain (PBD) was modeled based sequence homology. The sequence lengths of the α-helix, β-sheet, turn, and random coil in the Ctid-MHC I protein were 79 aa, 75 aa, 20 aa, and 99 aa, respectively. In the 97 aa of Ctid-β2m, the contents of the α-helix, β-sheet, turn, and random coil were 0 aa, 41 aa, 12 aa, and 44 aa, respectively. The Ctid-β2m protein displayed a typical β-sheet. Homology modeling of the Ctid-MHC I and Ctid-β2m proteins demonstrated similarities with the structure of human MHC class I proteins.