ANTIGENIC PEPTIDE BINDING TO THE MOUSE MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II PROTEIN-I-E(K) - PEPTIDE STABILIZATION OF THE QUARTERNARY STRUCTURE OF I-E(K)

Major histocompatibility complex (MHC) class II MHC proteins are noncovalently associated heterodimeric (alpha-beta) molecules that bind and display antigenic peptides to CD4+ T cells. In the present work, three reactions were investigated: (i) the binding of a fluoresceinated peptide (representing residues 89-104 of pigeon cytochrome c, FpCytc) to the purified mouse MHC class II protein I-E(k); (ii) heterodimer disassembly (alpha-beta --> alpha + beta); and (iii) the dissociation of FpCytc from preformed complexes (I-E(k)-FpCytc). At pH 5.2, in the detergent octyl beta-D-glucoside, the cleavage of heterodimeric molecules into the individual alpha and beta-subunits is inhibited by excess FpCytc but not by the nonbinding peptide representing residues 323-339 of chicken ovalbumin (Ova). The kinetic results are consistent with a reaction mechanism whereby a first-order reaction generates a reactive intermediate which then undergoes competing first- and second-order reactions. The parallel cleavage and binding reactions have the same rate when [FpCytc] approximately 30-mu-M. The results demonstrate that a class II-peptide complex is significantly less prone to cleavage than the reactive heterodimeric intermediate; thus, peptide stabilizes the quarternary structure of MHC class II molecules.