Peptide interaction with a class I major histocompatibility complex-encoded molecule: Allosteric control of the ternary complex stability

Thermodynamics and kinetics of interaction between a soluble class I MHC heterodimer composed of the H-2K(d) heavy chain (H) and human beta(2)microglobulin (beta(2)m) 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 x 10(6) M(-1) s(-1)). The ternary complex assembly involves a rate-limiting step of beta(2)m association with H to yield an unstable heterodimer (tau less than or equal to 5 s, 37 degrees C). Peptide binding provides a positive feedback enhancing H's affinity for beta(2)m, thus stabilizing the ternary complex, The latter decays by either peptide or beta(2)m dissociation. The first-order rate constants of peptide dissociation ((0.5 x 10(-2))-(0.4 x 10(-3)) s(-1), 37 degrees C) depend on their structures and are faster than that of beta(2)m dissociation. The former process decreases the H affinity for beta(2)m 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.