Studies on the molecular evolution of MHC class I genes and their polymorplusin

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We report the isolation and extensive analyses of highly polymorphic MHC class I genes from sharks (Triakis scyllia), that belong to the most primitive vertebrate group with jaws, the cartilaginous fish. Predicted complete peptide-binding domains showed retention of the critical amino acid residues that would interact with antigenic peptide termini and revealed extensive allelic polymorphism that can be comparable to those of human classical MHC class I molecules. Mosaic structures are apparent in these domains suggesting recombinational mechanisms to create allelic diversity. The present study demonstrated establishment of the basic strategy for antigen-presentation employed by MHC class I molecules and complete divergence of two polymorphic MHC classes at a phylogenetically primitive stage of vertebrate evolution.The elucidation of the complete peptide-binding domains of the highly polymorphic shark MHC class I genes offered us an opportunity to examine the characteristics of their predicted protein products in the light of the latest advance in the structural studies on the MHC class I molecules. The results suggest that the fundamental characteristics in the T cell recognition of the MHC class I molecule/peptide complex are expected to have been established at the early stage of the vertebrate evolution. The elucidation of the typical classical class I molecules from fishes and also of some MHC class I-related molecules may help us to explore the common denominator of the ancient class I molecules.