MHC class II function preserved by low-affinity peptide interactions preceding stable binding

MAJOR histocompatibility complex class II molecules and their peptide ligands show unusual interaction kinetics, with slow association and dissociation rates that yield an apparent equilibrium constant of ∼10 ^-6-10^-8 M (refs 1-5). However, there is evidence for a specific, rapidly formed, short-lived complex⁶. The altered migration on SDS-polyacrylamide gel electrophoresis of class II molecules upon stable peptide binding^7-9 has led to the hypothesis that the two kinetically distinguishable types of class II-peptide complexes correspond to different structures. In accord with this model, we demonstrate here that insect cell-derived HLA-DR1 class II molecules show fast, almost stoichiometric occupancy with rapidly dissociating peptide while remaining sensitive to SDS-induced chain dissociation. The same DR1 molecules slowly and quantitatively form long-lived complexes resistant to SDS-induced denaturation. Surprisingly, low-affinity interaction with peptide protects class II from denaturation at physiological temperature, a finding that has implications for understanding the role of invariant chain in the intracellular behaviour of class II molecules.