Purification and characterization of mouse beta-2 microglobulin: allelic variants from two different strains

Beta-2 microglobulin (beta 2M) is a 12,000 dalton protein associated with membrane-bound cell surface antigens. Variants of beta 2M, beta 2MA and beta 2MB, were first detected by Michaelson et al. (Immunogenetics 11, 93-95, 1980). An improved method was used to purify beta 2MA and beta 2MB from BALB/c and C57BL/6 mouse livers, respectively. Reproducible yields of 10% were obtained. The purifications were accomplished by a 3 M sodium thiocyanate (NaSCN) extraction of a crude membrane fraction, an acid precipitation step, gel filtration on Sephadex G-75 and ion-exchange chromatography on DEAE-cellulose and CM-cellulose in that order. The elution profile of beta 2MA and beta 2MB on the ion-exchange columns was found to be different, indicating the presence of structural changes. beta 2MA was found to be more acidic (pI = 7.35) than beta 2MB (pI = 7.68) by isoelectric focusing in gels. Complete sequence analysis of beta 2MA and partial sequence analysis of beta 2MB (61 of 99 residues) were performed by automated Edman degradation of the intact chain and of the overlapping peptides obtained by: (a) tryptic cleavage at arginines after acetimidation of lysine side chains, (b) BNPS-skatole cleavage at tryptophan residues and (c) hydroxylamine cleavage at asparagine-glycine linkages. A comparison of the primary structure of beta 2MA to the partial amino acid sequence obtained for beta 2MB revealed a single amino acid substitution (aspartic acid for alanine at position 85) that accounts for the differences in biochemical properties observed.