Neurophysin Dimer Research Articles

Contributions of the interdomain loop, amino terminus, and subunit interface to the ligand‐facilitated dimerization of neurophysin: Crystal structures and mutation studies of bovine neurophysin‐I

Current evidence indicates that the ligand-facilitated dimerization of neurophysin is mediated in part by dimerization-induced changes at the hormone binding site of the unliganded state that increase ligand affinity. To elucidate other contributory factors, we investigated the potential role of neurophysin's short interdomain loop (residues 55-59), particularly the effects of loop residue mutation and of deleting amino-terminal residues 1-6, which interact with the loop and adjacent residues 53-54. The neurophysin studied was bovine neurophysin-I, necessitating determination of the crystal structures of des 1-6 bovine neurophysin-I in unliganded and liganded dimeric states, as well as the structure of its liganded Q58V mutant, in which peptide was bound with unexpectedly increased affinity. Increases in dimerization constant associated with selected loop residue mutations and with deletion of residues 1-6, together with structural data, provided evidence that dimerization of unliganded neurophysin-I is constrained by hydrogen bonding of the side chains of Gln58, Ser56, and Gln55 and by amino terminus interactions, loss or alteration of these hydrogen bonds, and probable loss of amino terminus interactions, contributing to the increased dimerization of the liganded state. An additional intersubunit hydrogen bond from residue 81, present only in the liganded state, was demonstrated as the largest single effect of ligand binding directly on the subunit interface. Comparison of bovine neurophysins I and II indicates broadly similar mechanisms for both, with the exception in neurophysin II of the absence of Gln55 side chain hydrogen bonds in the unliganded state and a more firmly established loss of amino terminus interactions in the liganded state. Evidence is presented that loop status modulates dimerization via long-range effects on neurophysin conformation involving neighboring Phe22 as a key intermediary.

Crystals of ligand-free bovine neurophysin II.

A modified neurophysin, des 1-6 bovine neurophysin II, has been crystallized in the absence of bound hormone or hormone analogue. These crystals represent the first crystals of ligand-free neurophysin, and are essential for understanding neurophysin-hormone recognition as well as hormone-induced neurophysin dimerization. The crystals diffract to beyond 1.8 A resolution, belong to space group P3(1)21 (or P3(2)21) with a = 48.86, c = 78.61 A, and contain one molecule per asymmetric unit.

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.

Slowly interchanging conformers of bovine neurophysin-I in the unliganded dimeric state

The effect of neurophysin dimerization on Tyr-49, a residue adjacent to the hormone-binding site, was investigated by proton NMR in order to analyze the basis of the dimerization-induced increase in neurophysin hormone affinity. Dimerization-induced changes in Tyr-49 resonances, in two unliganded bovine neurophysins, suggested that Tyr-49 perturbation is an intrinsic consequence of dimerization, although Tyr-49 is distant from the monomer-monomer interface in the crystalline liganded state. To determine whether this perturbation reflects a conformational difference between liganded and unliganded states that places Tyr-49 at the interface in the unliganded state, or a dimerization-induced change in secondary (2 degrees) or tertiary (3 degrees) structure, the more general structural consequences of dimerization were further analyzed. No change in 2 degrees structure upon dimerization was demonstrable by CD. On the other hand, a general similarity of regions involved in dimerization in unliganded and liganded states was indicated by NMR evidence of participation of His-80 and Phe-35 in dimerization in the unliganded state; both residues are at the interface in the crystal structure and distant from Tyr-49. Consistent with a lack of direct participation of Tyr-49 at the monomer-monomer interface, dimerization induced at least two distinct slowly exchanging environmental states for the 3.5 ring protons of Tyr-49 without significantly increased dipolar broadening relative to the monomer. Two environments were also found in the dimer of des-1-8 neurophysin-I for the methyl protons of Thr-9, another residue distant from the monomer-monomer interface and close to the binding site in the liganded state.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallographic analysis of the neurophysin-oxytocin complex *1A preliminary report

Single crystals of a bovine neurophysin II-oxytocin complex have been obtained using (NH4)2SO4 as the precipitating agent. The crystals diffract to at least 2·7Åresolution, belong to Laue group 4/mmm and exhibit systematic absences consistent with either space group P41212 or P43212. The cell dimensions are a = b = 69·07Å and c = 113·26Å. The crystals contain one neurophysin-oxytocin dimer per asmymmetric unit. Based on a Vm of 2·9Å3/Da, the solvent content is calculated to be 58%. Chromatographic analysis of the dissolved crystals suggests the presence of three oxytocin molecules per neurophysin dimer.

Crystallographic analysis of the neurophysin-oxytocin complex: A preliminary report

Single crystals of a bovine neurophysin II-oxytocin complex have been obtained using (NH 4) 2SO 4 as the precipitating agent. The crystals diffract to at least 2·7Åresolution, belong to Laue group 4/ mmm and exhibit systematic absences consistent with either space group P4 12 12 or P4 32 12. The cell dimensions are a = b = 69·07Å and c = 113·26Å. The crystals contain one neurophysin-oxytocin dimer per asmymmetric unit. Based on a V m of 2·9Å 3/Da, the solvent content is calculated to be 58%. Chromatographic analysis of the dissolved crystals suggests the presence of three oxytocin molecules per neurophysin dimer.

Enthalpies of ligand binding to bovine neurophysins.

Flow microcalorimetry and batch microcalorimetry have been used to survey the energetics of ligand binding by bovine neurophysins I and II. Calorimetry studies were supplemented by van't Hoff analyses of binding constants determined by circular dichroism. Free energies of binding of a series of di- and tripeptides that bind to the strong hormone binding site of neurophysin were partitioned into their enthalpic and entropic components. The results indicate that, at 25 degrees C, the binding of most peptides is an enthalpy-driven reaction associated with negative entropy and heat capacity changes. Studies elsewhere, supported by evidence here, indicate that the principal component of the negative enthalpy change does not arise from the increase in neurophysin dimerization associated with peptide binding. Accordingly, the negative enthalpy change is attributed to direct bonding interactions with peptide and possibly also to peptide-induced changes in tertiary or quaternary organization. Comparison of the binding enthalpies of different peptides indicated two types of bonding interactions that contribute to the negative enthalpy change of peptide ligation. Substitution of an aromatic- or sulfur-containing side chain for an aliphatic side chain in position 1 of bound peptides led to increases in negative enthalpy of from 1 to 6 kcal/mol, demonstrating that interactions typically classified as hydrophobic can have a significant exothermic component at 25 degrees C. Similarly, loss of hydrogen bonding potential in the peptide decreased the enthalpy change upon binding, in keeping with the expected enthalpic contribution of hydrogen bonds. In particular, the data suggested that the peptide backbone between residues 2 and 3 and the phenolic hydroxyl group in position 2 participate in hydrogen bonding.(ABSTRACT TRUNCATED AT 250 WORDS)

Conformational flexibility of neurophysin as investigated by local motions of fluorophores. Relationships with neurohypophyseal hormone binding

Flexibility of various structural domains of neurophysin and neurophysin-neurohypophyseal hormone complexes has been investigated through the fast rotational motion of fluorophores in highly viscous medium. Despite seven intrachain disulfide links, it is shown that some domains of neurophysin remain highly flexible. Dimerization of neurophysin does not affect the structural integrity of the individual subunits, each subdomain being conformationally equivalent within each protomer of the unliganded dimer. The absence of heterogeneous fluorescence anisotropy precludes the existence of a dimer tautomerization equilibrium. Binding of the hormonal ligands to neurophysin dimer promotes a large conformational change over the whole protein structure as assessed by differential alterations of the flexibility-rigidity and intrasegmental interaction properties of domains that do not participate directly to the dimerization/binding areas. The order of free-energy coupling between ligand binding and protein subunit association has been evaluated. Data are consistent with a model in which the first mole of bound ligand stabilizes the dimer by increasing the intersubunit contacts while the second mole of ligand induces most of the described conformational change. Accordingly, the positive cooperativity between the two dimeric binding sites is linked mainly to the binding of the second ligand. The induced structural change is perceived differently by each subunit as assessed by opposite local motions of Tyr49 in each liganded protomer and leads to the formation of a dimeric complex with a global pseudospherical symmetry although containing domains of local asymmetry.

Effects of peptide-binding on the proton n.m.r. spectrum of bovine neurophysin-I.

The effects of binding L-phenylalanyl-L-phenylalanine amide and related peptides on the 220 MHz and 300 MHz proton n.m.r. spectra of bovine neurophysin-I were studied. Throughout both the aliphatic and aromatic proton regions, marked binding-induced changes in the protein spectrum occur which are best explained by invoking conformational change within the neurophysin dimer, in addition to direct perturbation of individual protein protons by bound peptide. In the region downfield from 6 p.p.m., a new resonance, centered at 6.45 p.p.m. was resolved in 300 MHz spectra. This resonance is tentatively assigned to a non-exchangeable -NH and undergoes a reversible binding-induced broadening. Also in this region, the binding-induced chemical shift change in the ortho ring protons of Tyr-49 was used to explore additional aspects of the kinetics of peptide-binding. The results indicate that peptides with affinities greater than or equal to 10(4) M-1 exhibit slow to intermediate exchange rates on the time scale of the Tyr-49 chemical shift change, but that fast exchange can be achieved with peptides having affinities approximately equal to 10(2) M-1.

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.

Side-by-side dimerization of neurophysin: sedimentation velocity, viscometry, and fluorescence polarization studies.

Side-by-side dimerization of neurophysin: sedimentation velocity, viscometry, and fluorescence polarization studies.

Interaction of bromophenol blue and related dyes with bovine neurophysin-I: use as a probe of neurophysin chemistry.

The interaction of bromophenol blue and related dyes with bovine neurophysin-I was studied by equilibrium dialysis and gel filtration, absorption and circular dichroism spectroscopy, and analytical ultracentrifugation. Binding isotherms for bromophenol blue showed positive cooperativity, with one strong site and one or more weaker sites present per polypeptide chain at pH 4 and an apparent increase in relative importance of the weaker sites of lower pH. Circular dichroism (CD) studies suggested displacement of bound dye by peptides that bind to the neurophysin hormone binding site. Titration of bound bromophenol blue indicated that the deprotonated dye was bound to the strong site with approximately 20-fold greater affinity than the protonated dye. The pH dependence of binding of bromophenol blue and of bromocresol purple, which has a higher pKa than bromophenol blue, indicated that binding was dependent on protonation of a protein residue with a pKa of 2.9. This residue was identified as a protein carboxyl, probably on an abnormal side chain, by studies of glycine ethyl ester modified neurophysin and carboxypeptidase-treated neurophysin. The presence of exciton interactions between bound dye molecules when only one dye was bound per polypeptide chain and analytical ultracentrifugation results indicated that dye was bound predominantly to the dimeric form of the protein. The implication of the data are discussed with respect to a kinetic model of dye-neurophysin interaction, used elsewhere in a study of neurophysin dimerization, that assumed interaction of protein monomers with protonated dye. Additionally, results are presented which suggest, in disagreement with conclusions based on the kinetic model, that there is a pH-dependent component of neurophysin dimerization which parallels low pH fluorescence and CD changes observed earlier.

Thermodynamics and kinetics of bovine neurophysins binding to small peptide analogues of oxytocin and vasopressin.

Thermodynamic binding constants for the interactions of mononitrated neurophysins with oxytocin, vasopressin, and peptide analogues of the hormones were determined by using a spectrophotometric titration technique. The data were fit to a binding model which included all known interactions in these systems. From an examination of the free energies for the binding reaction, we concluded that residues 1-3 contribute 84% of the binding energy for formation of the neurophysin dimer mono complex and 79% for the formation of the bis complex. Rate constants for complex formation and dissociation with native bovine neurophysin were determined by using temperature-jump relaxation. The association rate constants for neurophysin dimer binding to oxytocin, vasopressin, and the peptide analogues were all in the range of 1.3 X 10(6) M-1 s-1 for mono complexation and 1.5 X 10(6) M-1 s-1 for bis complexation. Thus, formation rate constants are identical for both mono and bis complexation, and no significant differences exist between formation constants for hormones and peptides. On the other hand, a clear distinction in dissociation rate constants is apparent when one compares the hormones (kr = 4 to 16 s-1) with the peptide analogues (kr = 54 to 182 s-1). There is rougly a tenfold increase in overall dissociation rate constant when one compares the peptides to the hormones. From these data, we conclude that the rate-determining step in the association reaction involves the first two or three residues on the hormone. After the initial binding takes place, only with intact hormone, i.e., oxytocin or vasopressin, can additional bonding interactions in the complex take place. These additional interactions are reflected in the slower off-rate of the hormone complexes relative to the peptide complexes.

Bovine neurophysin dimerization and neurohypophyseal hormone binding.

Bovine neurophysin dimerization and neurohypophyseal hormone binding.

Stopped-flow investigation of nitrated bovine neurophysin monomer binding to oxytocin.

By use of stopped-flow kinetic data, we have measured the kinetics of mononitrated neurophysin I monomer binding to oxytocin. The association constant was 1.3(+/-0.3) x 10(5) M-1s-1 and the dissociation rate constant was 2.0(+/-0.5)s-1 for protonated oxytocin binding. Both rates are significantly slower than those observed for neurophysin dimer. These data suggest that the binding process by which the monomer binds oxytocin is not identical to that of dimer.

Importance of neurophysin dimer and of tyrosine-49 in the binding of neurohypophyseal peptides.

Importance of neurophysin dimer and of tyrosine-49 in the binding of neurohypophyseal peptides.

Interaction of bovine neurophysin with oxytocin and vasopressin measured by temperature-jump relaxation.

The interaction between bovine neurophysins I and II and oxytocin and vasopressin was measured using temperature-jump relaxation. A single relaxation time in the 10 to 90 ms range was noted for each solution. This time depended upon the concentration of both neurophysin and hormone and increased with increasing pH. The formation rate constants (+/- SE) for the interaction of neurophysin I dimer with the protonated form of oxytocin and vasopressin at pH 7.4 in 0.1 M KNO3 and 25 degrees C were 2.8 (+/- 0.4) x 10(6) and 2.3 (+/- 0.5) x 10(6) M-1 s-1, respectively; the dissociation rate constants were 11 and 15 s-1, respectively.For neurophysin II dimer, formation rate constants were 6.0 (+/- 0.2) x 10(6) and 2.4 (+/- 0.3) x 10(6) M-1 s-1; dissociation rate constants were 24 and 16 s-1 for oxytocin and vasopressin, respectively. Formation rate constants for the interaction of neurophysin monomer with protinated hormone were approximatley an order of magniment with circular dichorism and pH titration studies which indicate that this interaction occurs between a negatively charged carboxyl group on the neurophysin and a protonated alpha-amino group on the hormone. Our results indicate that oxytocin and vasopressin bind with greater affinity to the neurophysin dimer than the monomer and that the binding of oxytocin and vasopressin in neurophysin dimer than the monomer and that the binding of oxytocin and vasopressin in neurophysin dimer at pH 7.4 and concentrations between 10(-4) and 10(-5) M is nearly identical for each hormone.

Interactions of oxytocin and vasopressin with bovine neurophysins I and II. Effects of hormone binding on the protein quaternary structure: a simple model.

The effects of hormone binding on the reversible monomer in equilibrium dimer equilibrium of bovine neurophysins I or II in solution have been studied by sedimentation equilibrium measurements performed in conjunction with equilibrium dialysis experiments. Under normal solution conditions saturating amounts of oxytocin displace the neurophysin dimerization equilibrium toward the associated form of the protein to give a dimeric complex with two oxytocin molecules bound per dimer. Vasopressin exerts different influences on this oligomerization process. At low fractional saturation this ligand exhibits a behavior similar to oxytocin with a higher affinity for the neurophysin dimer than the monomer. But in contrast, at higher fractional saturation, vasopressin strongly displaces the aggregation equilibrium toward a monomeric complex bearing two vasopressin molecules. However, in the presence of a high concentration of LiCl two oxytocin molecules are bound per neurophysin protomer (10,000 daltons). These observations, together with earlier data for vasopressin binding, suggest that each neurophysin molecule possesses two structurally distinct hormone binding sites. These observations can be rationalized in a simple schematic model of hormone binding to neurophysin in which oxytocin favors a dimeric form with one hormone binding site available per 10,000 daltons while vasopressin favors the monomeric form with two hormone binding sites available per 10,000 daltons.