Schmitt L, Boniface J J, Davis M M, McConnell H M
Department of Chemistry, Stanford University, California 94305, USA.
Biochemistry. 1998 Dec 15;37(50):17371-80. doi: 10.1021/bi9815593.
Class II major histocompatibility (MHC) molecules bind fragments of antigens and present them to T cells. The triggering of the T-cell receptor (TCR) of CD4(+) T-helper cells by these protein-peptide complexes is a key event in the generation of a cellular immune response. In the context of this interaction, it is generally assumed that class II MHC-peptide complexes adopt a single recognition structure at the cell surface. On the other hand, kinetic analysis has revealed that a number of class II MHC-peptide complexes show biphasic dissociation kinetics, indicating the presence of multiple kinetic isomers. Here, we demonstrate that a water-soluble version of the murine class II MHC molecule I-Ek complexed with an antigenic peptide derived from pigeon cytochrome c (PCC) displays monophasic as well as biphasic dissociation kinetics. While a simple monophasic dissociation curve was obtained at neutral pH, the complex showed biphasic dissociation behavior at acidic pH. This shift was independent of the ionic strength of the solution. Moreover, the short-lived isomer could be regenerated from a pool of kinetically homogeneous long-lived complexes. This demonstrates that the isomers interconvert and exist in a pH-sensitive equilibrium. Altering the peptide residue of PCC that occupies the P6 pocket of I-Ek results in a class II MHC-peptide complex that shows only monophasic dissociation, indicating that the glutamine at this position plays a key role in the kinetic heterogeneity of the complex.
II类主要组织相容性（MHC）分子结合抗原片段并将其呈递给T细胞。这些蛋白质 - 肽复合物触发CD4(+)辅助性T细胞的T细胞受体（TCR）是细胞免疫反应产生中的关键事件。在这种相互作用的背景下，通常认为II类MHC - 肽复合物在细胞表面采用单一的识别结构。另一方面，动力学分析表明，许多II类MHC - 肽复合物表现出双相解离动力学，表明存在多种动力学异构体。在这里，我们证明与源自鸽细胞色素c（PCC）的抗原肽复合的小鼠II类MHC分子I - Ek的水溶性版本显示单相以及双相解离动力学。在中性pH下获得简单的单相解离曲线，而该复合物在酸性pH下表现出双相解离行为。这种转变与溶液的离子强度无关。此外，短寿命异构体可以从动力学上均一的长寿命复合物池中再生。这表明异构体相互转化并存在于pH敏感的平衡中。改变占据I - Ek的P6口袋的PCC的肽残基会导致II类MHC - 肽复合物仅显示单相解离，表明该位置的谷氨酰胺在复合物的动力学异质性中起关键作用。
