No information to date is available on the structure of fish major histocompatibility complex (MHC) class I and beta 2-microglobulin (beta 2m) proteins. In the present study, grass carp (Ctenopharyngodon idellus) MHC class I (Ctid-MHC1) and beta(2)-microglobulin (Ctid-beta 2m) genes were expressed as soluble maltose binding protein (MBP)-proteins and purified in a pMAL-p2X/Escherichia coli TB I 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-MHCI and MBP-Oid-beta 2m 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 alpha-helix, beta-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-beta 2m, the contents of the alpha-helix, beta-sheet, turn, and random coil were 0 aa, 41 aa, 12 aa, and 44 aa, respectively. The Ctid-beta 2m protein displayed a typical beta-sheet. Homology modeling of the Ctid-MHC I and Ctid-beta 2m proteins demonstrated similarities with the structure of human MHC class I proteins. (c) 2006 Elsevier Inc. All rights reserved.