EVIDENCE FOR INVARIANT CHAIN-85-101 (CLIP) BINDING IN THE ANTIGEN-BINDING SITE OF MHC CLASS-II MOLECULES

The region of invariant chain encompassing residues 81-104 is critical for association with MHC class II molecules. This segment of invarian t chain, termed CLIP for CLass II-associated Invariant chain Peptides, has been shown to inhibit antigenic peptide binding and T cell stimul ation. Polymorphism affects the ability of CLIP to inhibit antigenic p eptide binding, suggesting that CLIP may occupy the MHC II antigen bin ding site directly. However, CLIP may also mediate inhibition by bindi ng to an alternate site causing an allosteric change to prevent antige nic peptide binding. The relationship between the apparent dissociatio n constant in the presence of a competitor (K-app) and the competitor concentration can be examined to determine the nature of competition b etween two ligands. In competitive binding experiments between CLIP an d antigenic peptide we find a linear dependence of K-app on competitor concentration. These data are consistent with CLIP and antigenic pept ide competing for the same site on the MHC class II molecule, thus arg uing against an allosteric mechanism of CLIP inhibition. Mildly acidic conditions are thought to promote peptide loading in the endosome com partment by facilitating CLIP dissociation and enhancing antigenic pep tide association. We have compared the effect of acidic pH on the equi librium binding of murine CLIP and antigenic peptide to MHC class II m olecules. Like antigenic peptide, CLIP binding can be greatly enhanced at mildly acidic pH, suggesting that a passive competitive mechanism for CLIP removal may not be sufficient to achieve loading of antigenic peptide in the endosome.