The conformation of Ld induced by beta 2-microglobulin is fixed during de novo synthesis and irreversible by exchange or dissociation

Abstract

Data are presented that support the hypothesis that beta 2m controls the folding of the ligand binding site of newly synthesized class I molecules. This conclusion was indicated by comparisons of two antigenic forms of the Ld molecule separated by sequential immunoprecipitation. Whereas, mAb 30-5-7+ Ld molecules were found to exist either as free H chains or associated with beta 2m, 30-5-7- Ld molecules showed no beta 2m association. Chemical comparisons showed 30-5-7- Ld molecules to be highly sensitive to proteolysis relative to 30-5-7+ Ld molecules. Experiments employing a construct with the Ld gene juxtaposed to the inducible metallothionein promoter indicated that the ratio of the antigenic forms of Ld was determined by the relative synthesis of beta 2m vs class I proteins. Pulse-chase experiments demonstrated that the two antigenic forms of Ld do not share a precursor-product relationship, but do display disparate rates of intracellular transport. beta 2m dissociation or exchange at the cell surface was found not to affect the ratio of the two antigenic forms of Ld. In contrast to these findings with Ld, the Dd and Ddml molecules were not detected in alternative conformations, thus mapping this property to the N-terminus of the class I molecule. These findings support the notion that beta 2m induces conformation on the alpha 1/alpha 2 domains of Ld molecules during de novo synthesis and once beta 2m-conformed, the class I structure is fixed and irreversible.