AMINO-ACID-SEQUENCES AND STRUCTURES OF CHICKEN AND TURKEY BETA2-MICROGLOBULIN

The complete amino acid sequences of chicken and turkey beta-2-microglobulins have been determined by analyses of tryptic, V8-proteolytic and cyanogen bromide fragments, and by N-terminal sequencing. Mass spectrometric analysis of chicken beta-2-microglobulin supports the sequence-derived M(r) of 11,048. The higher apparent M(r) obtained for the avian beta-2-microglobulins as compared to human beta-2-microglobulin by SDS-PAGE is not understood. Chicken and turkey beta-2-microglobulin consist of 98 residues and deviate at seven positions: 60, 66, 74-76, 78 and 82. The chicken and turkey sequences are identical to human beta-2-microglobulin at 46 and 47 positions, respectively, and to bovine beta-2-microglobulin at 47 positions, i.e. there is about 47% identity between avian and mammalian beta-2-microglobulins. The known X-ray crystallographic structures of bovine beta-2-microglobulin and human HLA-A2 complex suggest that the seven chicken to turkey differences are exposed to solvent in the avian MHC class I complex. The key residues of beta-2-microglobulin invovled in alpha-chain contacts within the MHC class I molecule are highly conserved between chicken and man. This explains that heterologous human beta-2-microglobulin can substitute the chicken beta-2-microglobulin in exchange studies with B-F (chicken MHC class I molecule), and suggests that the MHC class I structure is conserved over long evolutionary distances.