Interactions of bovine neurophysins with neurohypophyseal hormones. On the role of tyrosine-49

Salt-dependent structural changes of neurohormones: lithium ions induce conformational rearrangements of ocytocin to a vasopressin-like structure

The preferred average conformation and structural subdomain interactions of the nonapeptide hormones vasopressin and ocytocin have been analyzed through the determination of their hydrodynamic volume and the thermal coefficient of the frictional resistance to rotation of their tyrosine residue. A spherical gross shape and an ellipsoidal gross shape were assessed respectively for ocytocin and vasopressin by fluorescence polarization analysis. Investigation of the thermal coefficient of viscosity and the critical temperature of both hormones and analogues indicated that strong interactions hold together the two structural subdomains of ocytocin (the flexible six-membered ring and the COOH-terminal tripeptide tail). An opposite situation was found in the case of vasopressin where such interactions could not be detected between the rigid ring and the flexible COOH-terminal tail. Lithium ions were shown to promote ocytocin binding to specific neurophysin sites restricted, under standard conditions, to vasopressin. In the presence of lithium, the gross conformational shape of ocytocin becomes similar to that of vasopressin but in the absence of salt. In addition, the ocytocin ring becomes more rigid in the presence of lithium while decreasing interactions between the ring and the COOH-terminal tail were detected. It is proposed that lithium ions induce specific conformational rearrangements of ocytocin toward a vasopressin-like structure, allowing recognition of this hormonal ligand by a specific vasopressin binding domain of neurophysins.

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.

Evidence for an adenylate-cyclase activity in neurosecretory granule membranes from bovine neurohypophysis

Purified bovine neurosecretory granules and their corresponding membranes were prepared after fractionation and purification processes from bovine pituitaries. An adenylate cyclase activity was detected both in the granules (apparent Km = 0.5 mM) and the corresponding preparations of the membranes (apparent Km = 0.5 mM). This enzyme was activated by fluoride in a way markedly dependent on the concentration of this ion, and with a maximum for a concentration of F- = 3.5 mM. The cyclase activity was also significantly enhanced by GTP. The reaction rate showed a strong dependence on the molar ration [Mg2+]/[ATP] with maximal velocity for 7. It is suggested that this activity might play an important role in the control and regulation of neurosecretion in the neurohypophysis.