Stable peptide binding to MHC class II molecule is rapid and is determinedby a receptive conformation shaped by prior association with low affinity peptides

Formation of stable class II MHC/peptide complex involves conformational ch anges and proceeds via an intermediate, Although this intermediate complex forms and dissociates in minutes, its conversion to a stable complex Is a v ery slow process, taking up to a few days to reach completion. Here, we inv estigate the different steps of this binding and demonstrate that the confo rmational changes necessary to generate a receptive molecule is the rate de termining slow step in the process, while formation of the stable MHC/pepti de complex is very rapid, With HLA-DR1 as our model class II molecule, we f irst used low affinity variants of hemagglutinin peptide (HA(306-318)), whi ch lack the principal anchor, to shape the conformation of the MHC and then studied the kinetics of stable binding of HA(306-318) to such an induced c onformation, me found that the apparent association rate of HA(306-318) is equivalent to the dissociation rate of the low affinity peptide. A 4- to 18 -fold enhancement in the binding rates of HA,,,,,, was observed depending o n the dissociation rates of the low affinity peptides, These results establ ish that 1) formation of stable MHC/peptide complexes is very rapid and 2) prior binding of low affinity peptide induces a receptive conformation in M HC for efficient stable peptide binding, Furthermore, in the absence of any free peptide, this receptive molecule rapidly reverts to slow binding beha vior toward the subsequently offered peptide. These results have important implications for the roles of low affinity MHC/peptide complexes in Ag pres entation.