Beta-2-microglobulin of rat major histocompatibility complex class I antigens

Detection of the beta 2-microglobulin (beta 2m) component of the rat MHC class I antigens has been difficult. In the present report, we have addressed this issue by a systematic study of rat class I antigens from red blood cells or from lymphocytes that were freshly isolated or cultured in the presence of autologous or heterologous sera and surface-labeled with 125I or intrinsically labeled with radioactive amino acids. First, specific radioiodination of rat beta 2m in association with the antigen heavy chain on red blood cells or lymphocytes is minimal, resulting in its poor identification by SDS-PAGE. Second, labeling with radioactive methionine or lysine gives a more intense beta 2m band with respect to the heavy chain than labeling with arginine or tyrosine. Third, the beta 2m component shows a large increase in intensity compared to the heavy chain when the antigens are isolated from lymphocytes that are cultured in the presence of fetal bovine serum prior to 125I-labeling. This increase is due to exchange of endogenous rat beta 2m with bovine beta 2m and to a much higher level of radioiodination of the latter. Fourth, rat red blood cells and lymphocytes contain free surface beta 2m molecules in addition to those associated with the antigen heavy chains. The free molecules show a much higher level of radioiodination than those associated with the heavy chains, and there is little exchange between the antigen-bound and the free beta 2m after radioiodination.(ABSTRACT TRUNCATED AT 250 WORDS)