INTERACTION BETWEEN HLA-DM AND HLA-DR INVOLVES REGIONS THAT UNDERGO CONFORMATIONAL-CHANGES AT LYSOSOMAL PH

Antigenic peptide loading of major histocompatibility complex class II molecules is enhanced by lysosomal pH and catalyzed by the HLA-DM mol ecule. The physical mechanism behind the catalytic activity of DM was investigated by using time-resolved fluorescence anisotropy (TRFA) and fluorescence binding studies with the dye 8-anilino-1-naphthalenesulf onic acid (ANS), We demonstrate that the conformations of both HLA-DM and HLA-DR3, irrespective of the composition of bound peptide, are pH sensitive, Both complexes reversibly expose more nonpolar regions upon protonation. Interaction of DM with DR shields these hydrophobic doma ins from the aqueous environment, leading to stabilization of the DM a nd DR conformations, At lysosomal pH, the uncovering of additional hyd rophobic patches leads to a more extensive DM-DR association, We propo se that DM catalyzes class II peptide loading by stabilizing the low-p H conformation of DR, favoring peptide exchange. The DM-DR association involves a larger hydrophobic surface area with DR/class II-associate d invariant chain peptides (CLIP) than with stable DR/peptide complexe s, explaining the preferred association of DM with the former, The dat a support a release mechanism of DM from the DM-DR complex through red uction of the interactive surface, upon binding of class II molecules with antigenic peptide or upon neutralization of the DM-DR complex at the cell surface.