STABILITY OF EMPTY AND PEPTIDE-LOADED CLASS-II MAJOR HISTOCOMPATIBILITY COMPLEX-MOLECULES AT NEUTRAL AND ENDOSOMAL PH - COMPARISON TO CLASS-I PROTEINS

The structure and thermal stability of empty and peptide-filled forms of the murine class II major histocompatibility complex (MHC) molecule I-E(k) were studied at neutral and mildly acidic pH, The two forms ha ve distinct circular dichroic spectra, suggesting that a conformationa l change may accompany peptide binding, Thermal stability profiles ind icate that binding of peptide significantly increases the thermal stab ility of the empty heterodimers at both neutral and mildly acidic pH, Free energies calculated from these data provide a direct measure of t his stabilization and show that the empty form of I-E(k) is significan tly more stable than that of class I MHC proteins, Furthermore, for th e two MHC class II proteins that were analyzed (I-E(k) and I A(d)), th ermal stability was not significantly altered by acidification, In con trast, of four class I MHC molecules studied, three have shown a signi ficant loss in complex stability at low pH, The marked stability exhib ited by their empty form, as well as their resistance to low pH, as ob served in this study, correlate well with the ability of class II MHC molecules to traverse and bind peptides in acidic endosomal vesicles.