Kinetics and specificity of peptide-MHC class II complex displacement reactions

The peptide-induced acceleration of the dissociation of pre-formed complexes of the detergent-solubilized mouse class II molecules IEd and IEk with fluorescein-labeled peptides was investigated using high-performance size exclusion chromatography. While it is generally believed that functional complexes of MHC class II alpha beta heterodimers and peptides have a 1:1 stoichiometry, the data provide qualitative as well as quantitative kinetic evidence that the enhancement of the release of one peptide by a second peptide is due to a two-peptide intermediate. Different combinations of peptides were tested for their ability to accelerate each other's release from IEd. The importance of positive charge for the interaction with IEd was confirmed by the finding that not only dynorphin 1-13 but also poly-L-lysine (14-19 mer) and a peptide corresponding to a mitochondrial presequence (net charge +6) efficiently enhance the release of pre-bound peptides. SDS-PAGE analysis revealed that the efficiently displacing peptides do not stabilize the IEd alpha beta heterodimer at acidic pH, in contrast to the IEd-restricted antigenic peptide HEL 107-116. The data support a mechanism in which the second peptide binds specifically to the pre-formed class II-peptide complex, which, depending on the properties of the peptides involved, leads to the destabilization of the complex and the release of the first peptide.