THE INTERACTION OF BETA-2-MICROGLOBULIN (BETA(2)M) WITH MOUSE CLASS-I MAJOR HISTOCOMPATIBILITY ANTIGENS AND ITS ABILITY TO SUPPORT PEPTIDE BINDING - A COMPARISON OF HUMAN AND MOUSE BETA(2)M

The function of major histocompatibility complex (MHC) class I molecules is to sample peptides derived from intracellular proteins and to present these peptides to CD8(+) cytotoxic T lymphocytes. In this paper, biochemical assays addressing MHC class I binding of both peptide and beta(2)-microglobulin (beta(2)m) have been used to examine the assembly of the trimolecular MHC class I/beta(2)m/peptide complex. Recombinant human beta(2)m and mouse beta(2)m(a) have been generated to compare the binding of the two beta(2)m to mouse class I. It is frequently assumed that human beta(2)m binds to mouse class I heavy chain with a much higher affinity than mouse beta(2)m itself. We find that human beta(2)m only binds to mouse class I heavy chain with slightly (about 3-fold) higher affinity than mouse beta(2)m. In addition, we compared the effect of the two Barn upon peptide binding to mouse class I. The ability of human beta(2)m to support peptide binding correlated well with its ability to saturate mouse class I heavy chains. Surprisingly, mouse beta(2)m only facilitated peptide binding when mouse beta(2)m was used in excess (about 20-fold) of what was needed to saturate the class I heavy chains. The inefficiency of mouse beta(2)m to support peptide binding could not be attributed to a reduced affinity of mouse beta(2)m/MHC class I complexes for peptides or to a reduction in the fraction of mouse beta(2)m/MHC class I molecules participating in peptide binding. We have previously shown that only a minor fraction of class I molecules are involved in peptide binding, whereas most of class I molecules are involved in beta(2)m binding. We propose that mouse beta(2)m interacts with the minor peptide binding (i.e. the ''empty'') fraction with a lower affinity than human beta(2)m does, whereas mouse and human beta(2)m interact with the major peptide-occupied fraction with almost similar affinities. This would explain why mouse beta(2)m is less efficient than human beta(2)m in generating the peptide binding moiety, and identifies the empty MHC class I heavy chain as the molecule that binds human beta(2)m preferentially.