Formation and dissociation of short-lived class II MHC-peptide complexes

The incubation of a detergent-solubilized class II MHC protein with excess peptide at 37 degrees C leads to the formation of long-lived protein-peptide complexes (alpha beta P*), which have reported dissociation half-times at 37 degrees C from 30 to 100 h (alpha beta P*-->alpha beta + P*). Here we report an unexpected temperature effect on the reaction between class II MHC and added peptide. When the detergent-solubilized mouse class II MHC protein I-Ad is incubated with excess labeled peptide at 4 degrees C, a large fraction of the resultant complexes are relatively short-lived, with dissociation half-times at 40 degrees C from 2 to 0.2 h. Short-lived complexes formation and dissociation are both characterized by nonexponential kinetics. Short-lived I-A(d)-peptide complexes may contain two peptides, where the second, added fluorescent peptide is prevented from utilizing all the potential intermolecular interactions in the binding site due to the prior partial occupation of the binding site by a prebound peptide.