Structure intermediates in the reactions of antigenic peptides with MHC molecules Cold Spring Harbor

Molecular complexes of class I and class II major histocompatibility complex (MHC) molecules and anti-genic peptides are recognized by specific ytotoxic and helper lymphocytes (Schwartz 1985; Townsend and Bodmer 1989). Class II MHC/peptide complexes re-constituted in planar lipid bilayers are recognized by specific T-helper cells (Watts et al. 1984,1985). That is, specific antigenic peptides bind to affinity-purified class II MHC molecules in lipid membranes, the resulting MHC/peptide complexes have long lifetimes (Watts and McConnell 1986), and even after extensive wash-ing, such MHC/peptide complexes trigger specific T-helper cells to release interleukin-2. However, there are major questions as to the mechanism by which exogenous peptides bind to MHC molecules in bilayer membranes. Such questions are raised by the difficulty of preparing MHC/peptide complexes of defined stoichiometry, with many preparations only showing 5-10 % binding of added peptide (Watts and McCon-nell 1986; Sadegh-Nasseri and McConnell 1989). This raises the issue of the possible role of unidentified additional peptides, multiple conformational states of the MHC protein, the role of lipids, and so forth. To the extent hat they are comparable, results obtained with solubilized MHC/peptide complexes in detergent micelles are similar to those obtained in the bilayer membranes (Babbitt et al. 1985; Buus et al. 1986; Guillet et al. 1987). Thus, peptide/MHC binding does not appear to be strongly affected by peptide/lipid or protein/lipid interaction. Kinetic studies of FpCytc and I-E k in planar bilayer membranes provide evidence for sequential steps in both the association and dissociation reactions, with peptide/MHC specificity being already shown in the more rapid binding step (Sadegh-Nasseri and McConnell 1989). These kinetic data thus provide evidence for significant structural intermediates. The pre-sent work was undertaken to explore further the poss-ibility that peptide binding and dissociation involves a protein-folding intermediate or even disassembly of the class II molecules