Conformational isomers of a class II MHC-peptide complex in solution

A number of kinetic measurements of peptide dissociation from class II MHC- peptide complexes provide compelling evidence for the existence of conforma tional isomers in solution. There is evidence that T-lymphocytes can distin guish such isomers. However, virtually nothing is known about the structure of these isomers. Accordingly, we have investiugated water-soluble version of the murine class II MHC molecule I-E complexed with an antigenic peptid e derived from pigeon cytochrome c residues 89-104 (PCC) by F-19-NMR. Two f luorine labels were placed on the PCC peptide; one fluorine label was place d at a MHC contact site, the other at a position involved in T-cell recepto r (TCR) recognition. Introduction of these labels did not alter the observe d kinetics of the PCC/I-E-k complex. The NMR data show two conformational i somers of this immunogenic complex. The presence of conformational isomers at a TCR contact site suggests that these structures may be recognized diff erently by the TCR. The agreement between the dissociation kinetics and the F-19-NMR data demonstrate that kinetic heterogeneity is correlated with st ructural counterparts observed by NMR. Dissociations in the presence of dim ethyl sulfoxide were used to show that the rate of interconversion of these conformational isomers at pH 7.0 is low, with a lifetime on the order of h ours or more. Modification of a peptide residue of PCC occupying the minor MHC binding pocket P6 alters the F-19-NMR spectra of both labels. This demo nstrates that distant changes of amino acid residues can influence the conf ormation of the whole antigenic peptide inside the MHC binding cleft. (C) 1 999 Academic Press.