A STRUCTURAL TRANSITION IN CLASS-II MAJOR HISTOCOMPATIBILITY COMPLEX PROTEINS AT MILDLY ACIDIC PH

Peptide binding by class II major histocompatibility complex proteins is generally enhanced at low pH in the range of hydrogen ion concentra tions found in the endosomal compartments of antigen-presenting cells. We and others have proposed that class II molecules undergo a reversi ble conformational change at low pH that is associated with enhanced p eptide loading. However, no one has previously provided direct evidenc e for a structural change in class II proteins in the mildly acidic pH conditions in which enhanced peptide binding is observed. In this stu dy, susceptibility to denaturation induced by sodium dodecyl sulfate ( SDS) detergent or heat was used to probe the conformation of class II at different hydrogen ion concentrations. Class II molecules became se nsitive to denaturation at pH 5.5-6.5 depending on the allele and expe rimental conditions. The observed structural transition was fully reve rsible if acidic pH was neutralized before exposure to SDS or heat. Ex periments with the environment-sensitive fluorescent probe ANS (8-anil ino-1-naphthalene-sulfonic acid) provided further evidence for a rever sible structural transition at mildly acidic pH associated with an inc rease in exposed hydrophobicity in class II molecules. IA(d) conformat ion was found to change at a higher pH than IE(d), IE(k), or IA(k), wh ich correlates with the different pH optimal for peptide binding by th ese molecules. We conclude that pH regulates peptide binding by influe ncing the structure of class II molecules.