Isolation and characterization of chicken and turkey beta 2-microglobulin

Chicken and turkey beta 2-m were isolated from citrated plasma in sequential use of three chromatographic steps: affinity chromatography, gel filtration chromatography and anion-exchange chromatography. The purified protein was identified as beta 2-m by reaction with a beta 2-m specific monoclonal antibody and by the ability to recombine with the chicken MHC class I heavy chain. The purity was estimated by SDS-PAGE and IEF. The pI was between 5.1 and 5.3 for chicken beta 2-m and 4.7 and 4.8 for turkey beta 2-m, which fact is reflected in their different electrophoretic mobilities in agarose gel (turkey migrates in the alpha and chicken migrates in the beta region). The mol. wt of both chicken and turkey beta 2-m was 14,500 estimated by SDS-PAGE whereas calculations based on the amino acid compositions gave mol. wts of 11,000. EM280 was 15.9 for chicken beta 2-m and 16.4 for turkey beta 2-m. The amino acid compositions and sequences of the two avian beta 2-m molecules have been compared with earlier data from the literature. The sequence of the 23 N-terminal amino acids was found to be identical in our preparations from both chicken and turkey, namely DLTPKVQVYSRFPASAGTKNVLN, and is incompatible with a previously published sequence also thought to be from turkey beta 2-m. Reasons for our opinion that the molecules isolated and sequenced in this paper are the correct ones are given.