Dimerization of native and proteolytically modified neurophysins as monitored by proton magnetic resonance spectroscopy: proximity of tyrosine-49 to the subunit interface

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.

Two novel mutations of the vasopressin gene associated with familial diabetes insipidus and identification of an asymptomatic carrier infant

Familial diabetes insipidus (FDI) is a syndrome of central vasopressin deficiency that is inherited in an autosomal dominant manner and that typically becomes clinically apparent in the first decade of life. Two novel mutations of the vasopressin gene have been identified in two previously unstudied kindreds with FDI. In each kindred, the inheritance of the FDI phenotype was consistent with an autosomal dominant mode of inheritance. In each proband, the diagnosis of central diabetes insipidus had been confirmed previously with a water deprivation protocol. After extraction of genomic DNA from each individual, the three exons of the vasopressin gene were separately amplified by PCR and directly sequenced using an automated dye termination method. In the proband and two other carriers of one kindred, a heterozygous C to T mutation was identified at nucleotide 1857. This is predicted to produce a serine to phenylalanine substitution at residue 56 of the vasopressin-related neurophysin peptide encoded by the mutated allele. The mutation also abolished an MspI site in the vasopressin sequence, and analysis of genomic DNA from eight members of the kindred (five with FDI) confirmed segregation of the mutation with the FDI phenotype. Another member of the kindred, a 13-month-old infant, also has the heterozygous C to T mutation, but a formal water balance study showed no evidence of diabetes insipidus. In the proband of the other kindred, a heterozygous G to A mutation was identified at nucleotide 1873. This mutation would be predicted to cause a cysteine to tyrosine substitution at residue 61 of the neurophysin encoded by the mutated allele. This heterozygous mutation was confirmed by the presence of an RsaI restriction site in one vasopressin allele in two members of the kindred. Therefore, two novel heterozygous mutations of the vasopressin gene have been identified in FDI kindreds. In one kindred, an asymptomatic carrier infant was identified and will require continued observation to determine whether she will develop clinical diabetes insipidus. The presence of these two novel mutations in a region of the vasopressin gene where other FDI mutations have been reported suggests that the part of the neurophysin peptide encoded by these sequences may be critically important in the appropriate expression of vasopressin.

Amino acid sequence and properties of vasopressin-associated elephant neurophysin

The primary structure of an elephant neurophysin, homologous to vasopressin-associated neurophysins, is reported. The protein contains a Tyr for Asn substitution at position 75, a position in direct contact with residues 77 and 78 of the monomer-monomer interface. This Tyr residue therefore serves as a potential reporter of the path involved in the long-range linkage between peptide binding and dimerization in this system. NMR studies of the protein in unliganded and liganded states demonstrated normal dimerization properties and the expected increase in dimerization associated with binding peptide. In keeping with an elevated pKa of 11.1 assigned to Tyr-75 by UV spectrophotometric titration, the NMR signals from the 3,5 and 2,6 ring protons of Tyr-75 were shifted 0.3 and 0.2 ppm upfield, respectively, relative to their positions in small peptides, indicating significant shielding and/or hydrogen bonding. The Tyr-75 ring proton signals narrowed slightly, with no discernible change in chemical shift, on conversion from dimer to monomer in the unliganded state. Ring protons of Tyr-49, distant from the monomer-monomer interface, but adjacent to the peptide-binding site, were markedly perturbed by dimerization, in accord with their behavior in bovine neurophysins. The results suggest that the secondary and tertiary structure of the region 75-78 is largely unchanged by dimerization, and argue against an important role for this region in dimerization-mediated conformational changes that alter the binding site in the unliganded state.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurophysin-neurohypophyseal hormone interactions: studies using a dansylated vasotocin analogue

We have synthesized a neurohypophyseal hormone analogue containing an extrinsic fluorescence probe by linking a dansyl (DNS) group to the epsilon-amino group of the lysine at residue 8 of vasotocin. The fluorescence properties of this analogue have been characterized by steady-state and time-resolved spectroscopic methods and compared with those of epsilon-DNS-lysine and the dansylated carboxyl terminal tripeptide Pro-Lys(DNS)-GlyNH2. The binding of this hormone analogue to purified isoforms of bovine neurophysins, the natural carrier proteins of the neurohypophyseal hormones, results in changes in several fluorescence parameters of the dansyl probe. These changes include an increase in intensity and average lifetime, a shift of the emission band to higher energies, and an increase in the emission anisotropy. Anisotropy changes have been used to determine dissociation constants for binding to these neurophysin isoforms. Based on the changes in the fluorescence properties of the dansyl probe, the dansyl group itself interacts with the protein. The degree of the dansyl-neurophysin interaction, however, appears to be different for the full sequence isoform of neurophysin I and the Val89 isoform of neurophysin II.

Crystal structure of a bovine neurophysin II dipeptide complex at 2.8 A determined from the single-wavelength anomalous scattering signal of an incorporated iodine atom

The crystal structure of a dipeptide complex of bovine neurophysin II has been solved at 2.8 A resolution solely by using single-wavelength anomalous scattering data from a single iodinated derivative. The asymmetric unit is an elongated tetramer of dimensions 110 x 40 x 30 A, composed of two dimers related by pseudo twofold symmetry. Each monomer consists of two homologous layers, each with four antiparallel beta-strands. The two regions are connected by a helix followed by a long loop. Monomer-monomer contacts involve antiparallel beta-sheet interactions, which form a dimer with two layers of eight beta-strands. One peptide per monomer occupies the principal hormone-binding pocket formed by part of the amino-terminal region and parts of the connecting helix and loop, with binding to protein consistent with conclusions drawn from solution studies. Dimer-dimer contacts involve the Tyr49 region adjacent to this site. A fifth dipeptide, of unknown biological significance, helps to stabilize one of the monomer-monomer interfaces and the tetramer-tetramer network in the crystal.

A single base substitution in the coding region for neurophysin II associated with familial central diabetes insipidus

To elucidate the molecular mechanism of familial central diabetes insipidus (FDI), we sequenced the arginine vasopressin-neurophysin II (AVP-NPII) gene in 2 patients belonging to a pedigree that is consistent with an autosomal dominant mode of inheritance. 10 patients with idiopathic central diabetes insipidus (IDI) and 5 normals were also studied. The AVP-NPII gene, locating on chromosome 20, consists of three exons that encode putative signal peptide, AVP, NPII, and glycoprotein. Using polymerase chain reaction, fragments including the promoter region and all coding regions were amplified from genomic DNA and subjected to direct sequencing. Sequences of 10 patients with IDI were identical with those of normals, while in 2 patients with FDI, a single base substitution was detected in one of two alleles of the AVP-NPII gene, indicating they were heterozygotes for this mutation. It was a G----A transition at nucleotide position 1859 in the second exon, resulting in a substitution of Gly for Ser at amino acid position 57 in the NPII moiety. It was speculated that the mutated AVP-NPII precursor or the mutated NPII molecule, through their conformational changes, might be responsible for AVP deficiency.

Influence of neurophysin residues 1-8 on the optical activity of neurophysin-peptide complexes. Direct evidence that the 1-8 sequence alters the environment of bound peptide

Circular dichroism was used to compare the environment of peptides bound to native and des 1-8 neurophysin in order to further elucidate the role of the neurophysin 1-8 sequence in peptide-binding. A very large positive ellipticity (approximately 6000 deg cm2 dmol-1), shown earlier to be induced in tyrosine at position 2 of peptides bound to the native protein, was determined by the present study to be paralleled by similar induced changes in tyrosine at peptide position 1. Deletion of the neurophysin 1-8 sequence led to loss of half of the induced optical activity at peptide positions 1 and 2 and changes in binding-induced optical activity in the protein, the latter partially assignable to protein disulfides. In the mononitrated native and des 1-8 proteins, the optical activity of neurophysin Tyr-49, a residue at the peptide-binding site, was reduced by 80% in complexes of the des 1-8 protein relative to those of the native protein. The results suggest a role for neurophysin Arg-8 in modulating the optical activity at the binding site by directly placing a charge proximal to the binding site and/or by altering binding site conformation. The data provide the first unambiguous evidence of a difference in the environment of bound peptide between the native and des 1-8 proteins.

Crystals of modified bovine neurophysin II

An enzymatically modified form of bovine neurophysin II has been crystallized in three unique crystal forms. The orthorhombic form crystallizes in space group P2(1)2(1)2 with a = 15.33 nm, b = 6.92 nm, c = 3.63 nm, with four molecules in the asymmetric unit. The monoclinic form crystallizes in space group P2(1) with a = 6.22 nm, b = 9.55 nm, c = 5.45 nm and beta = 110.2 degrees, with eight molecules in the asymmetric unit. The tetragonal form crystallizes in space group P4(1)2(1)2 or P4(3)2(1)2 with a = 14.1 nm and c = 14.2 nm, with twelve molecules in the asymmetric unit. We report here the crystallization conditions, as well as the crystal data.

Application of peptide-mediated ring current shifts to the study of neurophysin-peptide interactions: a partial model of the neurophysin-peptide complex

Perdeuteriated peptides were synthesized that are capable of binding to the hormone binding site of neurophysin but that differ in the position of aromatic residues. The binding of these peptides to bovine neurophysin I and its des-1-8 derivative was studied by proton nuclear magnetic resonance spectroscopy in order to identify protein residues near the binding site through the observation of differential ring current effects on assignable protein resonances. Phenylalanine in position 3 of bound peptides was shown to induce significant ring current shifts in several resonances assignable to the 1-8 sequence, including those of Leu-3 and/or Leu-5, but was without effect on Tyr-49 ring protons. The magnitude of these shifts was dependent on the identity of peptide residue 1. By contrast, the sole demonstrable direct effect of an aromatic residue in position 1 was a downfield shift in Tyr-49 ring protons. Study of peptide binding to des-1-8-neurophysin demonstrated similar conformations of native and des-1-8 complexes except for the environment of Tyr-49, confirmed the peptide-induced ring current shift assignments in native neurophysin, and indicated an effect of binding on Thr-9. These observations are integrated with other results to provide a partial model of neurophysin-peptide complexes that places the ring of Tyr-49 at a distance 5-10 A from residue 1 of bound peptide and that places both the 1-8 sequence and the protein backbone region containing Tyr-49 proximal to each other and to peptide residue 3.(ABSTRACT TRUNCATED AT 250 WORDS)