NMR behavior of the aromatic protons of bovine neurophysin-I and its peptide complexes: Implications for solution structure and for function
The NMR behavior of the aromatic protons of bovine neurophysin-I and its complexes was interpreted with reference to the 2.8 A crystal structure of the dipeptide complex of bovine neurophysin-II and to mechanisms underlying the thermodynamic linkage between neurophysin dimerization and peptide binding. Large binding-induced shifts in the ring proton signals of Tyr-2 of ligand peptides (approximately 0.5 ppm upfield and approximately 0.35 ppm downfield at 25 degrees C for the 3,5- and 2,6-ring protons, respectively) were demonstrated. Consistent with the crystal structure, and in disagreement with conclusions by other investigators, evidence is presented indicating the absence of dipolar contact between Tyr-2 ring protons and protein Phe ring protons. The large binding-induced shifts are attributed to a strong influence of proximal neurophysin carbonyl and disulfide groups on the bound Tyr-2 ring, of potential importance in binding specificity. Resolution of the behavior of neurophysin Phe residues -22 and -35 and of their proton NOE contacts provided insights into the conformational changes associated with peptide binding and with dimerization. Within the amino domain of the protein, as evidenced by the behavior of interface residue Phe-35 and its NOE contacts, binding-induced changes in the subunit interface appeared to involve principally the junction between this interface region and the 3,10-helix that connects it to the binding site in the liganded state. By contrast, as judged by the NOE contacts of His-80, the corresponding interface participant of the carboxyl domain, peptide binding induced a marked decrease in side-chain mobility within the carboxyl domain segment of the interface. Interactions of Phe-22 with protons assigned to Ala-68, neither of which is an interface participant, were demonstrated to be markedly altered both by dimerization in the unliganded state and by peptide binding to the dimer. Since Phe-22 is adjacent to the peptide-binding site, the results collectively support a model in which conformational differences between unliganded monomer and dimer are important contributors to the preferential binding of peptide to the dimer and indicate that the amino and carboxyl domain segments of the interface, which are homologous, are affected differently by peptide binding.