EVIDENCE FOR A CONFORMATIONAL CHANGE IN A CLASS-II MAJOR HISTOCOMPATIBILITY COMPLEX MOLECULE OCCURRING IN THE SAME PH RANGE WHERE ANTIGEN-BINDING IS ENHANCED

Many class II histocompatibility complex molecules bind antigenic pept ides optimally at low pH, consistent with their exposure to antigen in acidic endosomal compartments. While it has been suggested that a par tially unfolded state serves as an intermediate involved in peptide bi nding, very little evidence for such a state has been obtained. In thi s report, we show that the murine class II molecule IE(k) becomes incr easingly less stable to sodium dodecyl sulfate-induced dissociation si nce the pH is decreased in the same range that enhances antigenic pept ide binding. Furthermore, at mildly acidic pH levels, IE(k) binds the fluorescent dye 1-anilino-naphthalene-8-sulfonic acid (ANS), a probe f or exposed nonpolar sites in proteins, suggesting that protonation pro duces a molten globule-like state. The association of IE(k) with a sin gle high-affinity peptide had only a small effect in these two assays, indicating that the changes that occur are distal to the peptide-bind ing groove. Circular dichroism analysis shows that a pH shift from neu tral to mildly acidic pH causes subtle changes in the environment of a romatic residues but does not grossly disrupt the secondary structure of IE(k). We propose a model in which perturbations in interdomain con tacts outside the peptide-binding domain of IE(k) occur at acidic pH, producing a partially unfolded state that facilitates optimal antigen binding.