Peptide Interaction with a Class I Major Histocompatibility Complex-Encoded Molecule: Allosteric Control of the Ternary Complex Stability

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Abstract
Thermodynamics and kinetics of interaction between a soluble class I MHC heterodimer composed of the H-2Kd heavy chain (H) and human β2microglobulin (β2m) with a dansylated peptide series based on residues 147−155 of influenza virus nucleoprotein sequence were studied by means of real-time fluorescence measurements. Peptide−heterodimer binding is a second-order process with specific rates practically independent of peptide structure (3−5 × 106 M-1 s-1). The ternary complex assembly involves a rate-limiting step of β2m association with H to yield an unstable heterodimer (τ ≤ 5 s, 37 °C). Peptide binding provides a positive feedback enhancing H's affinity for β2m, thus stabilizing the ternary complex. The latter decays by either peptide or β2m dissociation. The first-order rate constants of peptide dissociation ((0.5 × 10-2)−(0.4 × 10-3) s-1, 37 °C) depend on their structures and are faster than that of β2m dissociation. The former process decreases the H affinity for β2m and induces their dissociation. This dissociation, in turn, drastically lowers H affinity for peptide. Thus, these three components produce a system which is stable as a trimer. This behavior is rationalized by the functional requirements of class I molecules: Peptide structure determines the ternary complex's lifetime, and peptide rebinding on the cell surface is rendered unlikely by the limited stability of the empty heterodimers and the very low peptide affinity of the heavy chains.