MHC class I/peptide interactions: binding specificity and kinetics

Recent developments in the preparation of soluble analogues of the major histocompatibility complex (MHC) class I molecules as well as in the application of real time biosensor technology have permitted the direct analysis of the binding of MHC class I molecules to antigenic peptides. Using synthetic peptide analogues with cysteine substitutions at appropriate positions, peptides can be immobilized on a dextran-modified gold biosensor surface with a specific spatial orientation. A full set of such substituted peptides (known as 'pepsicles', as they are peptides on a stick) representing antigenic or self peptides can be used in the functional mapping of the MHC class I peptide binding site. Scans of sets of peptide analogues reveal that some amino acid side chains of the peptide are critical to stable binding to the MHC molecule, while others are not. This is consistent with functional experiments using substituted peptides and three-dimensional molecular models of MHC/peptide complexes. Detailed analysis of the kinetic dissociation rates (kd) of the MHC molecules from the specifically coupled solid phase peptides reveals that the stability of the complex is a function of the particular peptide, its coupling position, and the MHC molecule. Measured kd values for antigenic peptide/class I interactions at 25 degrees C are in the range of ca 10(-4)-10(-6)/s. Biosensor methodology for the analysis of the binding of MHC class I molecules to solid-phase peptides using real time surface plasmon resonance offers a rational approach to the general analysis of protein/peptide interactions.