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 intracellular environment and present them to CD8+ cytotoxic T lymphocytes. To understand the molecular details of the assembly (and disassembly) 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 model system human beta2M binding to mouse class I was used. The assay is strictly biochemical using purified reagents which interact in solution and complex formation is determined by size separation. It is specific 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 very fast (the ka is around 5 x 10(4)/M/s) whereas the dissociation is slow (the k(d) is around 8 x 10(-6)/s); the ratio of dissociation to association yields a calculated K(D) close to the observed value. At 37-degrees-C almost all of the purified class I participates in binding of the exogenously offered beta2M showing that a considerable exchange of the endogenous beta2M occurs. Finally, it was demonstrated that exogenous beta2m enhances binding to MHC class-I of short perfectly-matching peptides as well as longer peptides.