Function of chicken tapasin in MHC class I antigen presentation

The chicken tapasin gene was sequenced from seven MHC haplotypes. This revealed chicken tapasin to have a high level of allelic polymorphism and moderate sequence diversity, which contrasts with typical mammals, such as humans, where the tapasin, class I and TAP genes are not tightly linked, and where there are far fewer and less diverse alleles of tapasin. Analysis of the nature and distribution of polymorphic amino acids in chicken MHC-encoded proteins revealed that there are a few polymorphic amino acids in the domains of the chicken tapasin and TAP proteins that are likely to interact, which suggests that these proteins might associate irrespective of haplotype. However, with the majority of the polymorphic amino acids of chicken tapasin being located within the ER luminal domains, tapasin may participate in a “haplotype-specific” interaction with the BF2 class I protein, a possibility that is supported by a phylogenetic analysis of these proteins. A series of transfectants were produced in order to look for such an interaction, guided by the presumed sequence of events that led to the generation and evolution of a natural recombinant haplotype. This approach identified naturally polymorphic residues of class I molecules which appeared to influence the interaction with tapasin, and suggested that, following the recombinational event which generated the recombinant haplotype, tapasin and class I alleles have co-evolved in order to achieve optimal peptide-loading. When tapasin and class I alleles were mismatched, peptide-loading was shown to occur less efficiently.