pH-dependent peptide binding properties of the type I diabetes-associated I-A(g7) molecule: Rapid release of CLIP at an endosomal pH

MHC class II molecules and invariant chain assemble at a neutral pH in the endoplasmic reticulum and are transported to a low pH compartment where the invariant chain is trimmed to the class II-associated invariant chain pept ide (CLIP). For many major histocompatibility complex class II molecules, D M is required for rapid removal of CLIP, which allows binding of antigenic peptides. Since I-A(g7) confers susceptibility to type I diabetes in NOD mi ce, the biochemical requirements for peptide loading were examined using so luble I-A(g7) expressed in insect cells. I-A(g7) formed long-lived complexe s with naturally processed peptides from transferrin and albumin, whereas s everal peptides that represent T cell epitopes of islet autoantigens were p oor binders. I-A(g7)-peptide complexes were not sodium dodecyl sulfate (SDS ) resistant, indicating that SDS sensitivity may be an intrinsic property o f I-A(g7). Complexes of I-A(g7) and CLIP formed at a neutral pH, but rapidl y dissociated at pH 5. This rapid dissociation was due to a poor fit of M98 of CLIP in the P9 pocket of I-A(g7), since substitution of M98 by a negati vely charged residue greatly enhanced the stability of the complex. These b iochemical properties of I-A(g7) result in the rapid generation of empty mo lecules at an endosomal pH and have a global effect on peptide binding by I -A(g7).