Binding of neurohypophyseal peptides to neurophysin dimer promotes formation of compact and spherical complexes.
Previous hydrodynamic studies [Rholam, M., & Nicolas, P. (1981) Biochemistry 20, 5837-5843] have demonstrated that the dimerization of a neurophysin monomer (prolate ellipsoid with an axial ratio, due to asymmetry, of 5.2) results in a decreased asymmetry (axial ratio, due to asymmetry, of 3.6) as the consequence of a side-by-side association process. By a combination of hydrodynamic measurements, including the use of sedimentation velocity, viscometry, and fluorescence polarization spectroscopy, the influence of hormone binding on the shape and asymmetry properties of the neurophysin dimer was evaluated. The binding of ocytocin, vasopressin, and the tripeptide analogue of the N-terminal sequence of ocytocin, Cys(S-Me)-Tyr-Ile-NH2, results in an increase of S020,W and a decrease in both the reduced viscosity and rotational relaxation time of the bis-liganded dimeric species vs. the nonliganded form. The axial ratio (a/b) due to asymmetry of the ligand-bound dimers was found in each case to be equal to, or slightly greater than, 1.0, indicating a compact spherical shape (Stokes radius 21 A). The profound alteration on molecular dimensions observed upon ligand binding is shown to be the consequence of a ligand-induced conformational change and might explain the intradimeric binding sites positive cooperativity. It is tentatively proposed that the pseudospherical shape of the neurophysin-hormone complexes may enhance the stability of neurophysin and contribute to the prevention of leakage of neuropeptides through the membrane of neurosecretory granules. The data provide a remarkable example of a small protein with a high content in disulfide links and that undergoes conspicuous changes in conformation under the influence of nonapeptide, or tripeptide, ligands.
D001665:Binding Sites; D008024:Ligands; D046911:Macromolecular Substances; D008956:Models, Chemical; D009481:Neurophysins; D009842:Oligopeptides; D010121:Oxytocin; D010446:Peptide Fragments; D011487:Protein Conformation; D014667:Vasopressins; D014783:Viscosity
D008024:Ligands; D046911:Macromolecular Substances; D009481:Neurophysins; D009842:Oligopeptides; D010446:Peptide Fragments; D014667:Vasopressins; D010121:Oxytocin