A first-order reaction controls the binding of antigenic peptides to major histocompatibility complex class II molecules.

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Abstract
Major histocompatibility complex class II molecules have been reported to bind antigenic peptides very slowly in vitro. To investigate the molecular events that govern the slow binding reaction, we have determined the dependence of complex formation and dissociation on peptide concentration. The complex between the purified major histocompatibility complex class II protein I-Ek and a fluoresceinated peptide representing amino acids 89-104 of pigeon cytochrome c (FpCytc) was studied. Two important results emerge from this study. (i) At pH 5.4, the half-time for I-Ek-FpCytc complex formation is equal to approximately 7 hr for peptide concentrations that vary over a range of three orders of magnitude. There is in fact a small but significant decrease in the half-time for complex formation at low peptide concentrations. The small decrease in half-time is related to the release of endogenous peptides. (ii) At large ratios of peptide to protein [( FpCytc]/[I-Ek] greater than 40), the half-times for I-Ek-FpCytc complex formation and dissociation are equal to one another to within a factor of two between pH 7.5 and 4.5. The percent results demonstrate that a slow, first-order reaction precedes complex formation between I-Ek and FpCytc. This first-order reaction may involve a protein conformational change in addition to the release of endogenous peptides.