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 M HC 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 r esidues 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 inde pendent of peptide structure (3-5 x 10(6) M(-1) s(-1)). The ternary co mplex assembly involves a rate-limiting step of beta(2)m association w ith 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-or der 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 tha n that of beta(2)m dissociation. The former process decreases the H af finity 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 behavio r is rationalized by the functional requirements of class I molecules: Peptide structure determines the ternary complex's lifetime, and pept ide 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.