THE INTERACTION BETWEEN BETA-2-MICROGLOBULIN (BETA(2)M) AND PURIFIED CLASS-I MAJOR HISTOCOMPATIBILITY (MHC) ANTIGEN

The function of MHC class-I molecules is to sample peptides from the i ntracellular environment and present them to CD8+ cytotoxic T lymphocy tes. To understand the molecular details of the assembly (and disassem bly) of peptide-beta2m-class-I complexes a biochemical peptide-class-I binding assay has been generated recently and this paper reports on a similar assay for the interaction between beta2m and class I. As a mo del system human beta2M binding to mouse class I was used. The assay i s strictly biochemical using purified reagents which interact in solut ion and complex formation is determined by size separation. It is spec ific and highly sensitive. The observed affinity of the interaction, K (D), is close to 0.4 nm. The rate of association at 37-degrees-C is ve ry fast (the ka is around 5 x 10(4)/M/s) whereas the dissociation is s low (the k(d) is around 8 x 10(-6)/s); the ratio of dissociation to as sociation yields a calculated K(D) close to the observed value. At 37- degrees-C almost all of the purified class I participates in binding o f the exogenously offered beta2M showing that a considerable exchange of the endogenous beta2M occurs. Finally, it was demonstrated that exo genous beta2m enhances binding to MHC class-I of short perfectly-match ing peptides as well as longer peptides.