KINETIC INTERMEDIATES IN THE REACTIONS BETWEEN PEPTIDES AND PROTEINS OF MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II

The kinetics of the reactions between fluorescently labeled sperm whal e myoglobin-(110-121) peptide and the murine major histocompatibility complex class II protein I-E(d) have been analyzed. The presence in so lution of both short-and long-lived protein-peptide complexes is demon strated by the biphasic dissociation of the myoglobin peptide from I-E (d). The formation of the long-lived terminal complex is preceded by a characteristic induction phase. It is shown that the initially formed complex of the myoglobin peptide and I-E(d) is a kinetic intermediate that undergoes a unimolecular reaction to form the terminal complex. Reactions between peptides and the class II proteins thus involve an i ntermediate structurally distinct from the terminal complex. The termi nal complex presumably has a structure that is biologically active and similar to the published class II protein-peptide crystal structure.