Genetic Characterization of the TAPBP and Its Haplotypic Association with BF2 in the Chicken Major Histocompatibility Complex

TAPBP is a key chaperone of the peptide-loading complex that facilitates peptide loading onto major histocompatibility complex class I (MHC I) molecules. This study characterized TAPBP alleles in Korean Native Chickens (KNCs), identified novel variants, and evaluated haplotypic associations with BF2. Thirty-six samples representing six KNC lines were genotyped using LEI0258 and the MHC-B SNP panel, and individuals homozygous at both markers were classified into 16 groups. The same samples were subjected to Sanger sequencing of TAPBP exons 3-8. Sequences were assembled and aligned against MHC-B reference haplotypes and the Red Junglefowl reference. Additional comparisons with "tapasin allele" datasets enabled the identification of novel variants. Six novel nucleotide variants were detected across exons 3-6, including one nonsynonymous substitution in exon 4 (D251H). This residue corresponds to position Q265 in human TAPBP and lies adjacent to residues involved in MHC I interaction, suggesting potential functional relevance. Furthermore, TAPBP exhibited high haplotype diversity (Hd = 0.93) and moderate nucleotide diversity ({pi} = 0.00892), with exon 5 showing the highest diversity ({pi} = 0.01). B9 was the most frequent haplotype at the nucleotide level, whereas B6/B24 predominated at the amino acid level. Comparison with BF2 data revealed haplotype-dependent pairing patterns: BF2-B9 consistently matched TAPBP-B9, whereas BF2-B6 was associated with distinct TAPBP nucleotide variants, indicating allelic diversification within a shared haplotypic background. Homozygosity at LEI0258 and the SNP panel corresponded with TAPBP homozygosity, supporting marker-based prediction. These findings highlight potential BF2-TAPBP associations and provide a foundation for understanding variation in MHC I peptide loading.