Kinetic isomers of a class II MHC-peptide complex

Class II major histocompatibility (MHC) molecules bind fragments of antigen s 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 int eraction, it is generally assumed that class II MHC-peptide complexes adopt a single recognition structure at the cell surface. On the other hand, kin etic analysis has revealed that a number of class IT MHC-peptide complexes show biphasic dissociation kinetics, indicating the presence of multiple ki netic isomers. Here, we demonstrate that a water-soluble version of the mur ine class II MHC molecule I-E-k complexed with an antigenic peptide derived from pigeon cytochrome c (PCC) displays monophasic as well as biphasic dis sociation kinetics. While a simple monophasic dissociation curve was obtain ed at neutral pH, the complex showed biphasic dissociation behavior at acid ic pH, This shift was independent of the ionic strength of the solution. Mo reover, the short-lived isomer could be regenerated from a pool of kinetica lly homogeneous long-lived complexes. This demonstrates that the isomers in terconvert and exist in a pH-sensitive equilibrium. Altering the peptide re sidue of PCC that occupies the P6 pocket of I-E-k results in a class II MHC -peptide complex that shows only monophasic dissociation, indicating that t he glutamine at this position plays a key role in the kinetic heterogeneity of the complex.