Bangia N, Watts T H
Department of Immunology, University of Toronto, Ontario, Canada.
Int Immunol. 1995 Oct;7(10):1585-91. doi: 10.1093/intimm/7.10.1585.
The region of invariant chain encompassing residues 81-104 is critical for association with MHC class II molecules. This segment of invariant chain, termed CLIP for Class II-associated invariant chain Peptides, has been shown to inhibit antigenic peptide binding and T cell stimulation. Polymorphism affects the ability of CLIP to inhibit antigenic peptide binding, suggesting that CLIP may occupy the MHC II antigen binding site directly. However, CLIP may also mediate inhibition by binding to an alternate site causing an allosteric change to prevent antigenic peptide binding. The relationship between the apparent dissociation constant in the presence of a competitor (Kapp) and the competitor concentration can be examined to determine the nature of competition between two ligands. In competitive binding experiments between CLIP and antigenic peptide we find a linear dependence of Kapp on competitor concentration. These data are consistent with CLIP and antigenic peptide competing for the same site on the MHC class II molecule, thus arguing against an allosteric mechanism of CLIP inhibition. Mildly acidic conditions are thought to promote peptide loading in the endosome compartment by facilitating CLIP dissociation and enhancing antigenic peptide association. We have compared the effect of acidic pH on the equilibrium binding of murine CLIP and antigenic peptide to MHC class II molecules. 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.
恒定链中包含81 - 104位氨基酸残基的区域对于与MHC II类分子的结合至关重要。恒定链的这一片段，称为II类相关恒定链肽（CLIP），已被证明能抑制抗原肽结合和T细胞刺激。多态性影响CLIP抑制抗原肽结合的能力，这表明CLIP可能直接占据MHC II类抗原结合位点。然而，CLIP也可能通过结合到另一个位点引起变构变化以阻止抗原肽结合来介导抑制作用。可以研究在存在竞争者时的表观解离常数（Kapp）与竞争者浓度之间的关系，以确定两种配体之间竞争的性质。在CLIP与抗原肽的竞争性结合实验中，我们发现Kapp与竞争者浓度呈线性关系。这些数据与CLIP和抗原肽竞争MHC II类分子上的同一位点一致，因此反对CLIP抑制的变构机制。弱酸性条件被认为通过促进CLIP解离和增强抗原肽结合来促进内体区室中的肽装载。我们比较了酸性pH对小鼠CLIP和抗原肽与MHC II类分子平衡结合的影响。与抗原肽一样，在弱酸性pH下CLIP结合可大大增强，这表明CLIP去除的被动竞争机制可能不足以在内体中实现抗原肽的装载。
