The effects of ligands on enzymic carboxyl-methylation of neurophysins

The methyl-acceptor activities of bovine neurophysins I and II for the enzyme protein carboxymethylase (EC 2.1.1.24) were found to be similar and as high as for other previously identified, biologically active protein substrates. Effects on the rate of methylation of these neurophysins were investigated with the posterior pituitary hormone ligands, oxytocin and vasopressin, and the hormone-related tripeptide ligand, methionyl-tyrosyl-phenylalaninamide. An increase in the rate of neurophysin II methylation was observed with both oxytocin and tripeptide. This ligand-induced response did not occur with either native neurophysin I or disulfide-scrambled neurophysin II.

Phage randomization in a charybdotoxin scaffold leads to CD4-mimetic recognition motifs that bind HIV-1 envelope through non-aromatic sequences

Binding of HIV-1 gp120 to T-cell receptor CD4 initiates conformational changes in the viral envelope that trigger viral entry into host cells. Phage epitope randomization of a beta-turn loop of a charybdotoxin-based miniprotein scaffold was used to identify peptides that can bind gp120 and block the gp120-CD4 interaction. We describe here the display of the charybdotoxin scaffold on the filamentous phage fUSE5, its use to construct a beta-turn library, and miniprotein sequences identified through library panning with immobilized Env gp120. Competition enzyme-linked immunosorbent assay (ELISA) identified high-frequency phage selectants for which specific gp120 binding was competed by sCD4. Several of these selectants contain hydrophobic residues in place of the Phe that occurs in the gp120-binding beta-turns of both CD4 and previously identified scorpion toxin CD4 mimetics. One of these selectants, denoted TXM[24GQTL27], contains GQTL in place of the CD4 beta-turn sequence 40QGSF43. TXM[24GQTL27] peptide was prepared using solid-phase chemical synthesis, its binding to gp120 demonstrated by optical biosensor kinetics analysis and its affinity for the CD4 binding site of gp120 confirmed by competition ELISA. The results demonstrate that aromatic-less loop-containing CD4 recognition mimetics can be formed with detectable envelope protein binding within a beta-turn of the charybdotoxin miniprotein scaffold. The results of this work establish a methodology for phage display of a charybdotoxin miniprotein scaffold and point to the potential value of phage-based epitope randomization of this miniprotein for identifying novel CD4 mimetics. The latter are potentially useful in deconvoluting structural determinants of CD4-HIV envelope recognition and possibly in designing antagonists of viral entry.

From receptor recognition mechanisms to bioinspired mimetic antagonists in HIV-1/cell docking

Understanding the ways in which two or more proteins interact may give insight into underlying binding and activation mechanisms in biology, methods for protein separation and structure-based antagonism. This review describes ways in which protein recognition has been explored in our laboratory for the HIV-1/cell entry process. Initial contact between an HIV-1 virion particle and a human cell occurs between gp120 (an HIV-1 envelope protein) and CD4 (a human extracellular signaling protein). This interaction leads to a sequence of events which includes a conformational change in gp120, fusion of the HIV-1 and cellular membranes and eventual infection of the cell. Using an optical biosensor and a reporter antibody, we have been able to measure the conformational change in gp120 that occurs upon CD4 binding. We also have used this biosensor system to characterize CD4 mimetics, obtained by peptide synthesis in miniprotein scaffolds. Phage display techniques have been employed to identify novel miniprotein sequences. The combination of biosensor interaction kinetics analysis and phage display provides a useful approach for understanding the recognition mechanisms involved in the HIV/cell docking process. This approach may also be useful in investigating other protein complexes of importance in health and disease.

Cleaved high molecular weight kininogen binds directly to the integrin CD11b/CD18 (Mac-1) and blocks adhesion to fibrinogen and ICAM-1

High molecular weight kininogen (HK) and its cleaved form (HKa) have been shown to bind to neutrophils. Based on studies using monoclonal antibodies (mAbs), we postulated that CD11b/CD18 (Mac-1) might be the receptor on the neutrophils for binding to HK/HKa. However, the direct interaction of HK/HKa and Mac-1 had not been demonstrated. We therefore transfected HEK 293 cells with human Mac-1. Cell binding assays using fluorescein isothiocyanate-labeled HKa showed increased binding to the Mac-1 transfected cells compared with the control transfected cells. The binding was specific because unlabeled HKa, Mac-1-specific antibody, and fibrinogen can inhibit the binding of biotin-HKa to Mac-1 transfected cells. HKa bound to Mac-1 transfected cells (20 000 molecules/cell) with a K(d) = 62 nmol/L. To demonstrate directly the formation of a complex between HKa and Mac-1, we examined the interaction of HKa and purified Mac-1 in a cell-free system using an IAsys resonant mirror optical biosensor. The association and dissociation rate constants (k(on) and k(off), respectively) were determined, and they yielded a dissociation constant (K(d)) of 3.2x10(-9) mol/L. The functional significance of direct interaction of HKa to Mac-1 was investigated by examining the effect of HKa on cellular adhesion to fibrinogen and intercellular adhesion molecule-1 (ICAM-1), molecules abundant in the injured vessel wall. HKa blocked the adhesion of Mac-1 transfected cells to fibrinogen and ICAM-1 in a dose-dependent manner. Thus, HKa may interrupt Mac-1-mediated cell-extracellular matrix and cell-cell adhesive interactions and may therefore influence the recruitment of circulating neutrophils/monocytes to sites of vessel injury. (Blood. 2000;95:3788-3795)

Construction and binding kinetics of a soluble granulocyte-macrophage colony-stimulating factor receptor alpha-chain-Fc fusion protein

Granulocyte-macrophage colony-stimulating factor (GM-CSF) activity is mediated by a cellular receptor (GM-CSFR) that is comprised of an alpha-chain (GM-CSFRalpha), which specifically binds GM-CSF, and a beta-chain (betac), shared with the interleukin-3 and interleukin-5 receptors. GM-CSFRalpha exists in both a transmembrane (tmGM-CSFRalpha) and a soluble form (sGM-CSFRalpha). We designed an sGM-CSFRalpha-Fc fusion protein to study GM-CSF interactions with the GM-CSFRalpha. The construct was prepared by fusing the coding region of the sGM-CSFRalpha with the CH2-CH3 regions of murine IgG2a. Purified sGM-CSFRalpha-Fc ran as a monomer of 60 kDa on reducing SDS-polyacrylamide gel electrophoresis but formed a trimer of 160-200 kDa under nonreducing conditions. The sGM-CSFRalpha-Fc bound specifically to GM-CSF as demonstrated by standard and competitive immunoassays, as well as by radioligand assay with 125I-GM-CSF. The sGM-CSFRalpha-Fc also inhibited GM-CSF-dependent cell growth and therein is a functional antagonist. Kinetics of sGM-CSFRalpha-Fc binding to GM-CSF were evaluated using an IAsys biosensor (Affinity Sensors, Paramus, NJ) with two assay systems. In the first, the sGM-CSFRalpha-Fc was bound to immobilized staphylococcal protein A on the biosensor surface, and binding kinetics of GM-CSF in solution were determined. This revealed a rapid koff of 2.43 x 10(-2)/s. A second set of experiments was performed with GM-CSF immobilized to the sensor surface and the sGM-CSFRalpha-Fc in solution. The dissociation rate constant (koff) for the sGM-CSFRalpha-Fc trimer from GM-CSF was 1.57 x 10(-3)/s, attributable to the higher avidity of binding in this assay. These data indicate rapid dissociation of GM-CSF from the sGM-CSFRalpha-Fc and suggest that in vivo, sGM-CSFRalpha may need to be present in the local environment of a responsive cell to exert its antagonist activity.

Kinetic analysis of a protein antigen-antibody interaction limited by mass transport on an optical biosensor

Using BIAcore technology, we determined the rate constants for a protein antigen-antibody interaction that was mass transport limited on the optical biosensor. The antigen consisted of a soluble form of the human T-cell receptor CD4 (two amino terminal domains, D1D2) and the antibody was an anti-CD4 monoclonal from monkey engineered with the constant domains from human IgG1. High quality response data were obtained for this interaction by orienting the attachment of the antibody on the sensor surface and correcting for instrument artifacts with control experiments. Using numerical integration and global fitting, we demonstrate that a mass transport limited reaction was the only model of those tested that described well D1D2 binding to three different surface densities of the antibody. Statistical profiling techniques showed that the error space and correlation for the parameters in the non-linear model were essentially linear, but only when the model was simultaneously fitted to data from multiple surface densities. The "on" and "off" rate constants (1.2 x 10(-6) M-1 s-1 and 2.9 x 10(-4) s-1) determined from the kinetic analysis predict an equilibrium dissociation constant (KD = 0.24 +/- 0.01 nM) that agrees with the value measured in solution by titration calorimetry (KD = 0.2 +/- 0.1 nM). The results indicate that, although the D1D2-antibody reaction is partially controlled by mass transport on the optical biosensor, by optimizing the experimental design and analyzing data from multiple surface densities it is possible to determine accurate estimates of the intrinsic equilibrium and kinetic rate constants.

Kinetic and structural analysis of mutant CD4 receptors that are defective in HIV gp120 binding

The T-cell antigen coreceptor CD4 also serves as the receptor for the envelope glycoprotein gp120 of HIV. Extensive mutational analysis of CD4 has implicated residues from a portion of the extracellular amino-terminal domain (D1) in gp120 binding. However, none of these proteins has been fully characterized biophysically, and thus the precise effects on molecular structure and binding interactions are unknown. In the present study, we produced soluble versions of three mutant CD4 molecules (F43V, G47S, and A55F) and characterized their structural properties, thermostability, and ability to bind gp120. Crystallographic and thermodynamic analysis showed minimal structural alterations in the F43V and G47S mutant proteins, which have solvent-exposed mutant side chains. In contrast, some degree of disorder appears to exist in the folded state of A55F, as a result of mutating a buried side chain. Real time kinetic measurements of the interaction of the mutant proteins with gp120 showed affinity decreases of 5-fold for G47S, 50-fold for A55F, and 200-fold for F43V. Although both rate constants for the binding reaction were affected by these mutations, the loss in affinity was mainly due to a decrease in on rates, with less drastic changes occurring in the off rates. These observations suggest the involvement of conformational adaptation in the CD4-gp120 interaction. Together, the structural and kinetic data confirm that F43V is a critical residue in gp120 recognition site, which may also include main chain interactions at residue Gly-47.

Identifying structure-function relationships in four-helix bundle cytokines: towards de novo mimetics design

Many cytokines (growth-factor proteins) are constructed from a common four-helix bundle structural framework. Rapid advances have been made in relating the structure and function of a growing number of four-helix bundle cytokines. This understanding opens the way to design de novo mimetics through such strategies as cytokine hybrids, structure-excerpted scaffolds and contact residue topology mimics. These may provide leads for agonists and antagonists of cell growth and differentiation.

Molecular interaction analysis in ligand design using mass transport, kinetic and thermodynamic methods

Ligand design in biotechnology is underpinned by the control of molecular affinity. Hence, measuring binding interactions is a key component in designing ligands for such uses as therapeutics, diagnostics, biomaterials and separation science. Mass transport, kinetic and thermodynamic methods have been used for macromolecular interaction analysis but also have potential applicability as direct methods for measuring small molecular interactions. They can enhance the ligand design process by providing the ability to choose ligands based on both their kinetic and thermodynamic binding properties.

Interdomain communication of T-cell CD4 studied by absorbance and fluorescence difference spectroscopy measurements of urea-induced unfolding

CD4 is a transmembrane glycoprotein expressed on T-lymphocytes. It is a receptor for class II major histocompatibility complex (MHC) molecules and for the HIV envelope glycoprotein gp120. The extracellular portion of CD4 (sCD4) is a rod-shaped molecule consisting of four domains designated D1 through D4. Denaturant-induced unfolding of sCD4 and of isolated CD4 domains, D1D2 and D3D4, was measured using both ultraviolet absorbance and fluorescence difference spectroscopy. Though both absorbance and fluorescence changes arise from changes in the solvent exposure of the intrinsic tryptophan chromophores, the unfolding curves obtained with the two techniques looked very different for sCD4 and D3D4. This dissimilarity is indicative of a greater than two-state unfolding mechanism. The global three-state fit for sCD4, which simultaneously fit both absorbance and emission data to a model with one thermodynamically stable unfolding intermediate, was significantly better than the best two-state fit, suggesting that there are two unfolding regions. Unfolding of isolated D3D4 also fit a three-state model while unfolding of isolated D1D2 fit satisfactorily to a two-state model. The unfolding of these two isolated fragments could not be summed to yield the unfolding profile of sCD4, implying that an interaction between D2 and D3 is lost by splitting sCD4 between these domains. The unfolding data of isolated D1D2 and D3D4 were used with the solvent-accessible surface area of tryptophans calculated from atomic crystal structure coordinates of human D1D2 and rat D3D4 to assign the unfolding steps. The data are consistent with a model for sCD4 unfolding wherein the one stable intermediate appears to contain only the D4 domain unfolded. The remaining three domains apparently unfold as a unit. Furthermore, interactions between domains D1, D2, and D3 appear to stabilize D4, suggesting that stabilizing interactions exist between D3 and D4 even though the unfolding of the D3D4 fragment is best fit by a three-state model. This report is the first to describe a thermodynamic basis for a wide range of biological properties implicated for CD4.

Interpreting complex binding kinetics from optical biosensors: a comparison of analysis by linearization, the integrated rate equation, and numerical integration

The binding kinetics recorded for many interactions using BIAcore and IAsys optical biosensors do not fit a simple bimolecular interaction model (A + B<-->AB). Three methods of analysis have been used to derive estimates for kinetic constants from such data:linearization, curve fitting using the integrated rate equation, and curve fitting using numerical integration. To test how well these methods could interpret complex binding kinetics, we generated and analyzed simulated data for two systems, one involving a two-state conformational change (A + B<-->AB<-->(AB)*) and a second involving surface heterogeneity (A + B<-->AB and A + B*<-->AB*). The linearization method assumed a simple bimolecular interaction and was inadequate at interpreting these systems as both produced complex kinetics in the association and dissociation phases. The sum of two integrated rate equations correctly modeled surface heterogeneity; but, when applied nonglobally, it fit the data from the conformational change system equally well and thus provided misleading results. Numerical integration allowed a choice of model for analysis and was therefore the only method capable of returning accurate estimates of rate constants for both complex systems. Global analysis, in combination with numerical integration, provided a stringent test of the assumed model. However, this stringency suggests that its application to experimental systems will require high-quality biosensor data.

Design and characterization of an intramolecular antiparallel coiled coil peptide

A 56-residue polypeptide was designed to fold into a stable intramolecular antiparallel coiled coil, referred to as a coiled coil stem loop. The antiparallel orientation of the alpha-helices was dictated by the alignment of hydrophobic and ionic residues in the heptad repeat sequence (a, b, c, d, e, f, g)n. The hydrophobic core at the coiled coil interface was occupied by leucine and valine residues in heptad positions d and a' and positions a and d', respectively. The interface border positions e and g were occupied by glutamic acid in the amino-terminal helix and lysine residues in the carboxy-terminal helix. A loop segment connecting the alpha-helices began and ended with the helix-breaking residues glycine and proline. Alanine and serine residues were placed on the exposed b, c, and f positions of both helices to increase the helical propensity and solubility of the peptide, respectively. Several lines of evidence argued that the synthetic peptide made with this design folded into a stable monomeric coiled coil stem loop conformation: (1) the peptide was highly soluble in 150 mM sodium chloride and 50 mM sodium phosphate, pH 7.4; (2) the circular dichroism spectrum was alpha-helical but with relative ellipticity minima at 222 and 208 nm characteristic of a coiled coil structure: (3) the peptide exhibited an alpha-helical content near 80%, which was independent of peptide concentration and unchanged in the presence of trifluoroethanol; (4) size exclusion chromatography and sedimentation equilibrium ultracentrifuge measurements confirmed that the peptide was monomeric in aqueous solution; (5) the peptide exhibited high helical content over a wide pH range; (6) the apparent Tm for unfolding the alpha-helical structure was greater than 65 degrees C, and 3.0 M urea was required to reduce the helical structure by 50%; (7) a disulfide bond was readily formed in the monomer between the amino- and carboxy-terminal cysteine residues, confirming the antiparallel orientation of the helices; and (8) the peptide competed with fibrinogen for the GPIIbIIIa receptor indicating that the RGD residues present in the loop sequence were available for binding. This work establishes that an antiparallel alignment of alpha-helices can be achieved by designing specific hydrophobic and ionic interactions within the coiled coil. The prototype coiled coil peptide represents a sequence-simplified scaffold into which residues from alpha-helices and loops of native proteins can be inserted to form conformationally constrained mimetic recognition molecules.

Analysis of the interaction between human interleukin-5 and the soluble domain of its receptor using a surface plasmon resonance biosensor

A surface plasmon resonance (SPR) biosensor was used to study the interaction of human interleukin-5 (hIL5) with its receptor. IL5 is a major growth factor in the production and activation of eosinophils. The receptor for IL5 is composed of two subunits, alpha and beta. The alpha subunit provides the specificity for IL5 and consists of an extracellular soluble domain, a single transmembrane region and a cytoplasmic tail. We expressed the soluble domain of the human IL5 receptor alpha subunit (shIL5R alpha) and human IL5 (hIL5) in Drosophila. Both hIL5 and shIL5R alpha were immobilized separately through amine groups onto the carboxylated dextran layer of sensor chips of the BIAcore (Pharmacia) SPR biosensor after N-hydroxysuccinimide/carbodiimide activation of the chip surface. Interactions were measured for the complementary macromolecule, either shIL5R alpha or hIL5, in solution. Kinetics of binding of soluble analyte to immobilized ligand were measured and from this the association rate constant, dissociation rate constant and equilibrium dissociation constant (Kd) were derived. With immobilized shIL5R alpha and soluble hIL5, the measured Kd was 2 nM. A similar value was obtained by titration calorimetry. The Kd for Drosophila expressed receptor and IL5 is higher than the values reported for proteins expressed in different systems, likely due to differences in the methods of interaction analysis used or differences in protein glycosylation. Receptor-IL5 binding was relatively pH independent between pH 6.5 and 9.5. Outside this range, the dissociation rate increased with comparatively little increase in association rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Controlled formation of model homo- and heterodimer coiled coil polypeptides

Sequence-simplified coiled coil polypeptides were synthesized and their folding properties characterized in order to define the role of charged border residues at the coiled coil interface for the controlled formation of homodimer and heterodimer structures. Three peptides were designed to form parallel coiled coils with valine and leucine occupying the hydrophobic interface positions a and d, respectively, of the heptad repeat abcdefg. The polypeptide designated E/K42, with the heptad repeat sequence VSSLESK, contained glutamate and lysine in the interface border positions e and g, respectively, and was designed to form a coiled coil homodimer at neutral pH. Two other polypeptides, designated E/E35 and K/K35, have the heptad repeats VSSLESE and VSSLKSK, respectively. E/E35 contains only glutamic acid at both e and g positions; K/K35, only lysine, E/E35 and K/K35 were designed to form a stable coiled coil heterodimer when combined at neutral pH. All three polypeptides were prepared by solid-phase synthesis and purified by reverse-phase high-performance liquid chromatography followed by size-exclusion chromatography. E/K42 formed a stable dimeric coiled coil structure as determined by circular dichroism and size-exclusion chromatography. The alpha-helical content of E/K42 was highest at neutral pH and decreased at extremes of pH. The alpha-helical structure of E/K42 at micromolar concentrations had a Tm of 62-65 degrees C and exhibited a concentration dependence of thermal denaturation consistent with dimer formation. In contrast to results with E/K42, a mixture of E/E35 and K/K35, but neither alone, forms alpha-helix at neutral pH.(ABSTRACT TRUNCATED AT 250 WORDS)

Making sense from antisense: a review of experimental data and developing ideas on sense--antisense peptide recognition

Peptides encoded in the antisense strand of DNA have been predicted and found experimentally to bind to sense peptides and proteins with significant selectivity and affinity. Such sense--antisense peptide recognition has been observed in many systems, most often by detecting binding between immobilized and soluble interaction partners. Data obtained so far on sequence and solvent dependence of interaction support a hydrophobic-hydrophilic (amphipathic) model of peptide recognition. Nonetheless, the mechanistic understanding of this type of molecular recognition remains incomplete. Improving this understanding likely will require expanding the types of characteristics measured for sense--antisense peptide complexes and hence the types of analytical methods applied to such interactions. Understanding the mechanism of sense--antisense peptide recognition also may provide insights into mechanisms of native (sense) peptide and protein interactions and protein folding. Such insight may be helpful to learn how to design macromolecular recognition agents in technology for separation, diagnostics and therapeutics.

Effects of monomethoxypoly(ethylene glycol) modification of ribonuclease on antibody recognition, substrate accessibility and conformational stability

The effects of modification of bovine pancreatic ribonuclease A by monomethoxypoly(ethylene glycol) (MPEG) were examined for changes in recognition by antiRNase antibodies, enzymatic activity against low and high molecular weight substrates and conformational stability to temperature elevation. Modified forms of RNase were prepared containing an average of 4, 9, and 11 mol of MPEG/mol protein, by amino group modification. These were analysed by binding to RNase antibodies crosslinked to solid phase-immobilized protein A. The affinity column was incorporated into a high performance liquid chromatograph and the RNase species were studied by both zonal and frontal analytical affinity chromatography. An antibody dissociation constant of 7.6 x 10(-8) M was found for unmodified RNase, as compared to values of 1.3 x 10(-7) and 1.2 x 10(-6) M for RNase with 4 and 9 covalently bound MPEG chains, respectively. Modification also led to progressive loss of enzymatic activity against RNA, down to 3% for the most highly modified enzyme. In contrast, enzymatic activity against cytidine-2',3'-cyclic monophosphate was suppressed to a maximum of only 33% at the highest modification level, and the stability to temperature, as followed by circular dichroism, was reduced only partially, from 67 degrees C for native protein to 57 degrees C for RNase with 11 mol equivalents MPEG incorporated. The above differential effects on enzymatic activity, antibody binding and temperature effects are consistent with the view that MPEG modification has relatively small effects on conformational stability and small molecule accessibility, but more dramatic effects on large molecule (substrate as well as antibody) accessibility.(ABSTRACT TRUNCATED AT 250 WORDS)

Antisense peptide recognition of sense peptides: sequence simplification and evaluation of forces underlying the interaction

Structural principles were studied which underlie the recognition of sense peptides (sense DNA encoded) by synthetic peptides encoded in the corresponding antisense strand of DNA. The direct-readout antisense peptides corresponding to ribonuclease S-peptide bind to an affinity matrix containing immobilized S-peptide with significant selectivity and with dissociation constants in the range of 10(-6) M as judged by analytical affinity chromatography. Synthetic, sequence-modified forms of antisense peptides also exhibit substantial binding affinity, including a "scrambled" peptide in which the order of residue positions is changed while the overall residue composition is retained. The antisense mutants, as the original antisense peptides, bind at saturation with greater than 1:1 stoichiometry to immobilized S-peptide. The data suggest significant sequence degeneracy in the interaction of antisense with sense peptide. In contrast, selectivity was confirmed by the inability of several control peptides to bind to immobilized S-peptide. The idea was tested that the hydropathic pattern of the amino acid sequence serves to induce antisense peptide recognition. A hydropathically sequence-simplified mutant of antisense peptide was made in which all strongly hydrophilic (charged) residues were replaced by Lys, all strongly hydrophobic residues by Leu, and all weakly hydrophilic and hydrophobic residues by Ala, except Gly which was unchanged. This "KLAG" mutant also binds to immobilized S-peptide, with an affinity only an order of magnitude less than that with the original antisense peptide and with multiple stoichiometry. Mutants of the KLAG model, in which the hydropathic pattern was changed substantially, exhibited a lower binding affinity for S-peptide.(ABSTRACT TRUNCATED AT 250 WORDS)

Recognition properties of antisense peptides to Arg8-vasopressin/bovine neurophysin II biosynthetic precursor sequences

We studied the interaction properties of synthetic antisense (AS) peptides encoded in the antisense strand of DNA corresponding to the N-terminal 20-residue sequence of the biosynthetic precursor of Arg8-vasopressin (AVP) and its binding protein bovine neurophysin II (BNPII). Binding affinities of sense polypeptides AVP and BNPII with AS peptides were measured by analytical affinity chromatography, in each case by the extent of chromatographic retardation of a soluble polypeptide interactor on an affinity matrix containing the other interactor as the immobilized species. Chromatographically calculated dissociation constants ranged from 10(-3) to 10(-6) M. Experiments were carried out to define the selectivity and underlying forces involved in the AS peptide interactions. For AS peptide elutions on sense peptide affinity supports, reduced binding affinity with increasing 1-propanol concentration and ionic strength suggested the presence of both ionic and hydrophobic contributions to AS peptide/immobilized sense peptide recognition. This same conclusion was reached with the antisense peptides as the immobilized species and measurement of elution of sequence-simplified, truncated, and charge-depleted forms of sense peptides. Immobilized AS 20-mer affinity matrix differentially retarded AVP versus oxytocin (OT) and BNPII versus BNPI (the neurophysin related biosynthetically to OT) and was used to separate these polypeptides from acid extracts of bovine posterior pituitaries. In addition, immobilized AS 12-mer corresponding to AVP-Gly-Lys-Arg could be used to separate AVP from OT. The results confirm that antisense peptides recognize sense peptides with significant selectivity in the AVP/BNPII precursor case.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequence simplification and the intra- and intermolecular self-recognition properties of vasopressin/neurophysin biosynthetic precursor

The self-assembly properties of the arginine 8-vasopressin/bovine neurophysin II (AVP/BNPII) biosynthetic precursor were studied using glycopeptide-deleted and sequence-redesigned semisynthetic derivatives. Semisynthetic precursors were prepared by chemically coupling synthetic vasopressinyl sequence domains and native protein-derived neurophysin II domain. Measurement of precursor-protein association by the extent of affinity chromatographic retardation on agarose-immobilized BNPII verified that the semisynthetic precursor with native AVP sequence has an enhanced self-association propensity similar to that predicted for native precursor. Here, the stabilizing contacts between hormone and neurophysin domains, mainly the positively charged protonated alpha-amino group and tyrosyl 2 side chain of the hormone, are retained. Semisynthetic precursor variants in which the hormone domain is sequence-simplified by introducing alanyl residues in positions not considered important for neurophysin recognition show non-reduced association to BNPII. In contrast, removal of one of the main contact elements between hormone and neurophysin by acetylation of the hormone alpha-amino group abolishes potentiation of precursor self-association. The results show that the presence of the C-terminal glycopeptide sequence domain of native vasopressin precursor is not required to promote self-assembly of the precursor. The data verify the view proposed for the oxytocinyl precursor that intramolecular domain interaction is the triggering event which promotes the increase in affinity of precursor self-association (intermolecular self-recognition). The data also define some of the intramolecular self-recognition elements in the folded precursor required for the high affinity intermolecular self-recognition.

Bioaffinity chromatography: synergy between interactive chromatography and molecular recognition for the separation and analysis of macromolecules

Affinity chromatography, commonly regarded as an integral tool in macromolecular separation sciences, also provides an analytical method to study structure-function relationships of macromolecular interaction processes and to design recognition molecules. The latter, as found recently for the case of antisense peptides, may be useful as affinity agents in immobilized forms to effect new types of biomolecular separation.

Structural requirements of peptide hormone binding for peptide-potentiated self-association of bovine neurophysin II

Site-specific, truncated, and sequence-simplified analogs of the hormone [Arg8]vasopressin were investigated for the relationship between their abilities to recognize immobilized bovine neurophysin and to promote neurophysin self-association. Peptide binding to neurophysin was measured quantitatively by analytical high performance affinity chromatography on immobilized bovine neurophysin II. Neurophysin self-association, measured as binding of soluble to immobilized neurophysin, was promoted (made higher affinity) by soluble peptide hormone and its analogs, with the effect of particular peptides being proportional to their binding affinities for neurophysin. Sequence-redesigned peptides able to recognize neurophysin, including dipeptide amides, were able to potentiate the self-association to the same extent as the natural hormone when tested at concentrations adjusted to effect equal degrees of saturation of neurophysin. The relationship between peptide affinity to neurophysin and the potentiation of self-association suggests that the latter is directly dependent on the former and can occur even with limited segments of hormone sequence. The data fit best to a model in which hormone binding and self-association surfaces of neurophysin are separate and linked through the neurophysin molecule to produce cooperativity (hormone-promoted self-association). Given that only limited structural elements of hormone are required for promoting self-association, the results fit less well with models in which cooperativity requires that hormone make dimer-stabilizing contacts with both self-associating subunits of neurophysin simultaneously.

Effect of neurophysin on enzymatic maturation of oxytocin from its precursor

We examined the extent to which rates of enzymatic conversion of the oxytocin biosynthetic precursor to mature peptide are modulated by intramolecular and intermolecular assembly of precursor and polypeptide intermediates. The biosynthesized precursor contains hormone and neurophysin sequences linked by a Gly-Lys-Arg sequence and undergoes enzymatic processing reactions which include endoproteolytic cleavage at the Lys-Arg dibasic sequence, carboxypeptidase B-like exoproteolytic cleavage, and enzymatic amidation. We evaluated the effect of neurophysin on such processing reactions using semisynthetic precursors of oxytocin/bovine neurophysin I and synthetic oxytocinyl precursor intermediates as substrates. Neurophysin I at high concentration (0.7 mM) reduced the rates of carboxy-peptidase B-like conversion of oxytocinyl-Gly-Lys-Arg to oxytocinyl-Gly and the enzymatic amidation of oxytocinyl-Gly to mature (C-terminal amidated) oxytocin. The dependence of rate suppression on the concentrations of peptide substrate and neurophysin I suggested that suppression is due to intermolecular formation of hormone-neurophysin complexes which are aggregated at least to dimers. An analogous intramolecular neurophysin effect was found for endoproteolytic processing of semisynthetic precursors. Endoproteinase Lys-C cleaved the Lys11-Arg12 peptide bond in a native-like semisynthetic precursor at a significantly slower rate than it did an assembly-deficient precursor analogue. The difference in semisynthetic precursor endoproteolysis rates is most substantial at the high concentrations at which the native-like precursor would form dimers but the assembly-deficient analogue would not. The native-like semisynthetic precursor was more stable than the assembly-deficient precursor analogue to tryptic digestion. The concentration-dependent effects of neurophysin, both intramolecularly as a precursor domain and intermolecularly as an interacting protein, are likely to occur in the secretory granules in which the biosynthetic precursors are packaged. The molecular organization of both hormone/neurophysin precursors and the noncovalently complexed hormone-neurophysin intermediates can be expected to play a role in modulating enzymatic processing reactions that lead to mature neurohypophysial hormones.

Sequence redesign and the assembly mechanism of the oxytocin/bovine neurophysin I biosynthetic precursor

The structural organization of neurohypophysial hormone biosynthetic precursors and the interdependence between intramolecular folding and precursor self-association were examined using sequence-engineered mutants of the semisynthetic oxytocin/bovine neurophysin precursor (pros-OT/BNPI). In [N alpha 1-Ac,N epsilon 30,71-diacetimidyl, Ala2,des-His106] Pro-Ot/BNPI or [N alpha 1-Ac,Ala2]pros-OT/BNPI), two structural elements (Tyr2 and free alpha-amino group) were eliminated which were predicted to be critical for intramolecular conformation by stabilizing contact between hormone and neurophysin domains. This mutant was used to test the dependence of precursor self-association on intramolecular conformation. In the second mutant precursor, [N alpha 30,71-diacetimidyl,D-Pro7,D-Leu8,des-His106]p ro-OT/BNPI (or [D-Pro7,D-Leu8]pros-OT/BNPI), the stereochemistry at L-Pro7-L-Leu8 was changed to test the extent to which precursor conformation depends on ordered structure in the processing/spacer sequence which connects the interacting hormone and neurophysin I domains. Intramolecular conformation was characterized for the precursor and mutants by analytical affinity chromatography on immobilized hormone analog Met-Tyr-Phe and by circular dichroism. Data obtained by both methods showed that, while pros-OT/BNPI is folded, with hormone domain occupying the hormone-binding site of the neurophysin domain, the alpha-acetyl-Ala2 mutant is not so organized intramolecularly. When pros-OT/BNPI and the alpha-acetyl-Ala2 mutant were eluted on immobilized BNPII to measure self-association propensity, the native-like precursor was found to bind with 12-15-fold higher affinity than the assembly mutant. Thus, while pros-OT/BNPI assumes a molecular structure containing a high-affinity self-association surface induced by intramolecular hormone domain-neurophysin domain interaction, [N alpha 1-Ac,Ala2]pros-OT/BNPI does not. The results with the alpha-acetyl-Ala2 mutant show that intramolecular domain-domain interaction is the obligatory "trigger" which induces the high-affinity precursor self-association that likely drives precursor to aggregated forms in the concentrated intragranular environment that exists in peptide hormone-synthesizing cells. In contrast, affinity chromatographic and circular dichroism properties of the D-Pro7,D-Leu8 mutant show that this intramolecular trigger is dependent, but only weakly, on the conformation of the peptide sequence between domains, as judged by native-like interaction properties below 40 degrees C but lowered stability to elevated temperature.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular diagnostics using analytical immuno high performance liquid affinity chromatography

Analytical immuno high performance liquid affinity chromatography (analytical immuno HPLAC) was evaluated as a molecular diagnostic tool. Antibodies raised in rabbits against bovine neurophysin II were immobilized through Protein A crosslinking onto coated silica. Interaction of immobilized antibody with mobile antigen was characterized by zonal and frontal elutions of 14C-labeled bovine neurophysin II under isocratic, nondenaturing conditions. The chromatographic behavior shows that analytical immuno HPLAC with immobilized antibodies can be used to detect the number and functional nature of matrix-interacting antigens in mixtures, thus providing a quantitative chromatographic technology for "antigen mapping."

Anti-sense peptide recognition of sense peptides: direct quantitative characterization with the ribonuclease S-peptide system using analytical high-performance affinity chromatography

The ability of peptides coded by the anti-sense strand of DNA to interact specifically with peptides coded by the sense strand has been evaluated. The sense peptide examined, ribonuclease S-peptide, was immobilized on a coated silica affinity chromatographic matrix. Anti-sense peptides were synthesized on the basis of the anti-sense DNA sequence for the S-peptide region in native pancreatic ribonuclease A. The interaction of synthetic anti-sense peptides with sense peptide was quantitated from the degree of retardation during chromatographic elution on the sense peptide affinity matrix in buffers with and without soluble competing sense peptide. Sense/anti-sense peptide interactions were found to occur with significant affinities with each of two anti-sense 20-residue peptides of opposite amino-to-carboxyl orientations and to weaken progressively with decreasing length of anti-sense peptide. The substantial chromatographic retardation of anti-sense peptides was specific, since it decreased as expected with increasing concentration of the soluble competing S-peptide, could not be mimicked by the elution of several control peptides (including S-peptide itself) on the S-peptide matrix, and did not occur with a blank chromatographic matrix (no S-peptide attached). The stoichiometry of anti-sense peptide binding to immobilized sense peptide was found to be far greater than 1:1, and at least 4-5:1, for the two 20-mer anti-sense peptides. In sum, the analytical affinity chromatographic experiments have established quantitatively that anti-sense peptide binding to sense peptides occurs in the ribonuclease S-peptide case and have identified some structural elements that govern these interactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Analytical high-performance affinity chromatography: evaluation by studies of neurophysin self-association and neurophysin-peptide hormone interaction using glass matrices

Bovine neurophysin II (BNP II) was covalently immobilized on both nonporous and porous (200-nm pore diameter) glass beads and incorporated in a high-performance liquid chromatograph to evaluate analytical high-performance affinity chromatography as a microscale method for characterizing biomolecular interactions. By extension of the theoretical treatment of analytical affinity chromatography, both the self-association of neurophysin and its binding of the peptide hormone vasopressin were characterized by using a single chromatographic column containing immobilized neurophysin predominantly in the monomer form. Both [3H] [Arg8]vasopressin (AVP) and 125I-BNP II were rapidly eluted (less than 25 min). The relatively symmetrical elution peaks obtained allowed calculation of both equilibrium dissociation constants and kinetic dissociation rate constants. The dissociation constant measured chromatographically for the AVP-immobilized neurophysin complex, KM/L = 11 microM with porous glass beads and 75 microM with nonporous glass (NPG) beads, was in reasonable agreement with those previously obtained by curve fitting of Scatchard plots (16-20 microM) and from binding to [BNP II]Sepharose (50 microM). The values obtained are larger than that for dissociation of AVP from BNP II dimer, by a factor consistent with the intended nature of immobilized BNP II as monomers. Chromatography of BNP II on the [BNP II]NPG gave a dimer dissociation constant of 166 microM, a value in excellent agreement with that derived from equilibrium sedimentation studies (172 microM). In contrast to the agreement of chromatographic equilibrium binding constants with those measured in solution, the dissociation rate, k-3, determined from the variance of the affinity chromatographic elution profile with nonporous beads, was several orders of magnitude smaller than the solution counterpart.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative high-performance affinity chromatography: evaluation of use for analyzing peptide and protein interactions

To examine analytical high-performance affinity chromatography as a microscale method for characterizing macromolecular interactions, the chromatographic behavior was evaluated of Arg8-vasopressin on bovine neurophysin II covalently immobilized in its monomer form on several new high-flow and pressure-resisting affinity supports. Zonal elution of both tritiated and unlabeled peptide hormone and an extension of theoretical treatment of analytical affinity chromatography allowed determination of equilibrium dissociation constants of hormone binding to immobilized bovine neurophysin II. Microamounts of hormone, ranging from 0.05 to 15 micrograms, were eluted within 20-30 min, with a quantitative recovery of the amount injected. For zones containing more than 5 micrograms, continuous elution monitoring was possible by ultraviolet absorbance, providing greater speed and accuracy in data analysis. The values obtained for the equilibrium dissociation constants were in good agreement with those previously measured in solution. The above hormone-protein evaluation system has led to identification of several pressure-resistant affinity supports, including silica-, agarose- and glass-based matrices, which are appropriate for use with high-performance liquid chromatographic instrumentation for affinity chromatographic analysis of macromolecular interactions.

Analytical affinity chromatography in studies of molecular recognition in biology: a review

Measuring macromolecular and cellular interactions remains central to the study of recognition in biology and its application in biotechnology. Analytical affinity chromatography provides a versatile methodology to detect and quantitate such interactions. Both zonal and frontal elution approaches have been developed, essentially in parallel, for analytical affinity chromatography. A close quantitative relatedness of chromatographically obtained equilibrium constants and analogous constants determined fully in solution has been found for a growing number of proteins. This consistently observed correlation has formed the basis for extending theoretical treatments in order to evaluate not only monovalent molecular systems of varying types but also multivalently interacting macromolecules, including those which exhibit cooperativity. The potential to measure chemical rate constants by affinity chromatography also has been recognized, and experimental tests of the available theory are being made. As a micromethod, the quantitative use of affinity chromatography has important applicability for biochemical analysis of an increasing array of biologically active molecules being discovered and isolated but available in only relatively small amounts. Analytical affinity chromatography thus provides a means to use matrix--mobile interactant systems to study mechanisms of biomolecular interactions and therein to attain an understanding of such interactions which often is not easily achieved by solution methods alone.

Isolation and sequence analysis of oxytocin from the sheep corpus luteum

Studies have revealed that the corpus luteum (CL) of the sheep releases oxytocin (OT) -like immunoreactivity under normal and physiological conditions. We have now purified and completely characterized the OT-like species from ovine CL and established by Edman degradation and comparative reverse-phase HPLC its identity with hypothalamic oxytocin. On the basis of radioimmunoassay, the characterized oxytocin was the only peptide possessing OT-like immunoreactivity. This study represents the first identification by sequence analysis of oxytocin outside the central nervous system.

Crystallographic structure of an active, sequence-engineered ribonuclease

X-ray diffraction methods were used to test a synthetic-modeling approach to the sequence engineering of bovine pancreatic ribonuclease. A model of RNase S-peptide (residues 1-20), having a simplified amino acid sequence but retaining elements deduced to be essential for conformation and function, was previously synthesized and found to form a catalytically active and stable complex with native S-protein (residues 21-24). We have now obtained a 3-A-resolution electron density map of this semisynthetic complex which reveals that the conformation of model peptide closely mimics that of native S-peptide, as intended by sequence design. Some small differences from the native structure are observed: Glu-2 and Arg-10 of the model complex are not close enough to form a salt bridge, the position of the His-12 imidazole ring is slightly shifted in the active site, and the peptide's amino terminus is reoriented. Nonetheless, the major structural features predicted to be essential by computer-aided peptide-design analysis are preserved in the model peptide portion of the complex. These include (i) the alpha-helical framework involving residues 3-13, (ii) the catalytically competent orientation of His-12, and (iii) complex-stabilizing non-bonding interactions involving Phe-8 and Met-13 of S-peptide and hydrophobic residues in the cleft region of S-protein. Further, sequence simplification has not introduced any non-native, potentially stabilizing contacts between the model peptide and S-protein. The results emphasize the usefulness, in redesigning native proteins, of categorizing sequence into residues providing conformational framework and those determining intra-and intermolecular surface recognition.

Pituitary enzyme conversion of putative synthetic oxytocin precursor intermediates

Neurosecretory granule lysate from bovine posterior pituitary was shown to contain both carboxypeptidase B and amidating activities. The former sequentially releases COOH-terminal basic residues from the oxytocin biosynthetic precursor fragment oxytocinyl-GKR (CYIQNCPLGKR) to form oxytocinyl-GK and then oxytocinyl-G. The amidating enzyme converts the resulting oxytocinyl-G into oxytocin (CYIQNCPLG-NH2). The carboxypeptidase B was separated from a less specific carboxypeptidase present in granule lysate by gel filtration on Sephacryl S-300. Percoll density gradient centrifugation (after preliminary differential centrifugation) also yielded granule fractions enriched in the specific carboxypeptidase B activity. The carboxypeptidase B which converts the oxytocinyl peptides showed a fairly sharp pH dependence with an optimum of 5.5-6, was activated by cobalt ion, and was inhibited by cupric ion, EDTA, and a thiol protease inhibitor, p-chloromercuribenzoate. The amidating activity which converts oxytocinyl-G to oxytocin was competed by degradation due to proteases and/or peptidases present in lysate of Percoll gradient-derived granules. Oxytocinyl-GKR was shown by analytical affinity chromatography to bind noncovalently to neurophysin with an affinity close to that of mature oxytocin. This binding activity and the observation of carboxypeptidase B activity in the presence of large concentrations of neurophysin are consistent with the view that the exoproteolytic processing and amidation steps which occur after initial endoproteolysis of pro-oxytocin/neurophysin likely take place on oxytocin intermediate peptides which are bound in noncovalent complexes with the neurophysin domain from the precursor.

Folding of thermolysin fragments. Correlation between conformational stability and antigenicity of carboxyl-terminal fragments

Circular dichroism (CD) and immunochemical measurements have been used to examine conformational properties of COOH-terminal fragments 121-316, 206-316 and 225(226)-316 of thermolysin, and to compare these properties to those of native thermolysin and thermolysin S, the stable partially active two-fragment complex composed of fragments 5-224(225) and 225(226)-316. In aqueous solution at neutral pH, all the COOH-terminal fragments attain a native-like conformation, as judged both by the content of secondary structure deduced from far-ultraviolet CD spectra and by the recognition of rabbit polyclonal antibodies specific for the COOH-terminal region in native thermolysin. The three fragments showed reversible cooperative unfolding transitions mediated by both heat and guanidine hydrochloride (Gdn X HCl). The phase transition curves were analyzed for Tm (temperature of half-denaturation) and Gibbs free energies (delta GD) of unfolding from native to denatured state. The observed order of thermal stability is 225(226)-316 less than or equal to 206-316 less than 121-316 less than thermolysin S less than thermolysin. The ranking of delta GD values for the three fragments correlates with the size of each fragment. Competitive binding studies by radioimmunoassay using 14C-labeled thermolysin and affinity purified antibodies specific for native antigenic determinants in segment 206-316 of native thermolysin indicate that the COOH-terminal fragments adopt native-like conformations which are in equilibrium with non-native conformations. These equilibria are shifted towards the native state as the fragment size increases from 225(226)-316, to 206-316, to 121-316. Fragment 225(226)-316, when combined with fragment 5-224(225) in the thermolysin S complex, adopts a more stable native-like conformation and becomes much more antigenic. It has been shown that the degree of antigenicity of COOH-terminal fragments towards thermolysin antibodies correlates directly with their conformational stability. The results of this study are discussed in relation to the recently proposed correlation between antigenicity and segmental mobility of globular proteins.

Molecular properties of the oxytocin/bovine neurophysin biosynthetic precursor. Studies using a semisynthetic precursor

An oxytocin/bovine neurophysin I biosynthetic precursor, [N epsilon-diacetimidyl-30,71, des-His106]pro-OT/BNPI, was synthesized from a synthetic oxytocinyl peptide, 1/2Cys-Tyr-Ile-Gln-Asn-1/2Cys-Pro-Leu-Gly-Gly-Lys-Arg, and native neurophysin by chemical semisynthesis. The semisynthetic precursor contains the entire sequence of the biosynthetic precursor deduced from the complementary DNA structure except for omission of the carboxyl-terminal histidine residue. The covalent structure of the semisynthetic product was verified by amino acid analysis and amino-terminal analysis. Analytical affinity chromatography was employed to evaluate noncovalent binding properties of the precursor. The precursor does not bind significantly to immobilized Met-Tyr-Phe, a hormone binding site ligand. In contrast, the acetimidated precursor binds to immobilized bovine neurophysin II, with a 13-fold higher affinity than does acetimidated neurophysin itself. When a hormonal ligand, [Lys8]vasopressin, was added to the elution buffer at the concentration of 0.1 mM so that a major portion of the immobilized BNPII was liganded, the affinity between the immobilized liganded BNPII and the precursor was enhanced 8-fold and approached the affinity for the liganded (bovine neurophysin I-immobilized BNPII) interaction. The data imply that the precursor can self-associate and that this self-association is closely related to that of liganded neurophysin. The tripeptide affinity matrix data argue that, in the precursor, the ligand binding site of the neurophysin domain is occupied intramolecularly by the hormone domain. The data verify the view that both the self-association surface and hormone binding site are established upon precursor folding. A disulfide stability analysis showed the resistance, to disulfide interchange by dithiothreitol, of semisynthetic precursor but not of neurophysin, as judged by protein association and peptide ligand binding activities, respectively. The results argue that the molecular structure of the precursor is established upon precursor folding and before enzymatic processing that produces mature hormone and neurophysin.

Quantitative affinity high-performance liquid chromatography of neuroendocrine polypeptides using porous and non-porous glass derivatives

Analytical affinity HPLC was developed to isolate and characterize neuroendocrine peptide/protein components. Bovine neurophysin II (NP-II) was covalently immobilized on succinamidopropyl derivatives of both controlled-pore glass (CPG) and non-porous glass (NPG). These derivatives were packed into 25 X 0.46 cm I.D. stainless-steel columns and incorporated into a high-performance liquid chromatograph. Interaction of [3H]Arg8-vasopressin ([3H]AVP) with NP-II was examined by chromatography of AVP on both CPG and NPG affinity matrices. Zonal elution profiles of [3H]AVP on NPG matrix showed, as predicted theoretically, a linear dependence of retardation on the concentration of hormone injected. The data permit calculation of the equilibrium dissociation constant for the NP-II/AVP interaction. Elution characteristics also were measured by frontal analysis of large-zone chromatography experiments, the results of which were in good agreement with the zonal elution analysis. Affinity resulting from dimerization also was studied by chromatography of [125I]NP-II on the NPG matrix. In this case, concentration dependence of retardation was non-linear, again as predicted theoretically. Off-rate kinetic constants for dissociation of the mobile interactant from the stationary phase also were obtained. The studies illustrate the utility of analytical affinity HPLC on non-porous beads for measuring relative affinities for various soluble ligands with small amounts of material. Chromatography on the CPG column proved useful for purification of microscale amounts of [3H]AVP.

Nature of the charged-group effect on the stability of the C-peptide helix

The residues responsible for the pH-dependent stability of the helix formed by the isolated C-peptide (residues 1-13 of ribonuclease A) have been identified by chemical synthesis of analogues and measurement of their helix-forming properties. Each of the residues ionizing between pH 2 and pH 8 has been replaced separately by an uncharged residue. Protonation of Glu-2- is responsible for the sharp decrease in helix stability between pH 5 and pH 2, and deprotonation of His-12+ causes a similar decrease between pH 5 and pH 8. Glu-9- is not needed for helix stability. The results cannot be explained by the Zimm-Bragg model and host-guest data for alpha-helix formation, which predict that the stability of the C-peptide helix should increase when Glu-2- is protonated or when His-12+ is deprotonated. Moreover, histidine+ is a strong helix-breaker in host-guest studies. In proteins, acidic and basic residues tend to occur at opposite ends of alpha-helices: acidic residues occur preferentially near the NH2-terminal end and basic residues near the COOH-terminal end. A possible explanation, based on a helix dipole model, has been given [Blagdon, D. E. & Goodman, M. (1975) Biopolymers 14, 241-245]. Our results are consistent with the helix dipole model and they support the suggestion that the distribution of charged residues in protein helices reflects the helix-stabilizing propensity of those residues. Because Glu-9 is not needed for helix stability, a possible Glu-9-...His-12+ salt bridge does not contribute significantly to helix stability. The role of a possible Glu-2-...Arg-10+ salt bridge has not yet been evaluated. A charged-group effect on alpha-helix stability in water has also been observed in a different peptide system [Ihara, S., Ooi, T. & Takahashi, S. (1982) Biopolymers 21, 131-145]: block copolymers containing (Ala)20 and (Glu)20 show partial helix formation at low temperatures, pH 7.5, where the glutamic acid residues are ionized. (Glu)20(Ala)20Phe forms a helix that is markedly more stable than (Ala)20(Glu)20Phe. The results are consistent with a helix dipole model.

Synthesis and properties of an all-D model ribonuclease S-peptide

In order to examine the effect of a defined enantiomeric sequence on protein structure, the all-D model ribonuclease S-peptide, H-Ala-Glu-Ala4-Lys-Phe-Ala-Arg-Ala-His-Met-Ala2-OH, has been synthesized by the solid phase method. The all-L peptide has been synthesized previously and shown to possess 36% of ribonuclease S activity when added to ribonuclease S-protein (Komoriya, A. & Chaiken, I.M. (1982) J. Biol. Chem 257, 2599-2604). The synthetic D-peptide was purified by gel filtration and semipreparative reverse phase HPLC. Amino acid composition of the synthetic peptide was in agreement with theory and gas chromatographic analysis showed that no significant racemization had occurred during synthesis. Circular dichroism (CD) studies of the D-peptide showed a peak of positive ellipticity in the 220-230 nm region, whereas a negative ellipticity peak for the L-peptide was observed. The effects of temperature and trifluoroethanol on the far-ultraviolet CD spectra of D- and L-peptides were similar but of opposite sign, confirming the expectation that the D-peptide has the propensity to form an alpha-helical structure which is enantiomeric with respect to that formed by the L-peptide. In the presence of S-protein, the L-peptide showed hydrolytic activity against the substrate cytidine-2':3'-monophosphate, whereas the D-peptide was inactive. Addition of the D-peptide to mixtures of L-peptide and S-protein did not lead to inhibition of enzymatic activity. These results indicate lack of binding of D-peptide to S-protein to produce either an active or inactive species.

High-performance liquid chromatography and studies of neurophysin-neurohypophysial hormone pathways

High-performance liquid chromatography (HPLC) is being used extensively to characterize active polypeptides, precursor processing mechanisms, and cooperative peptide-protein noncovalent complexes in neuroendocrine pathways for neurohypophysial peptide hormones, oxytocin and vasopressin, and the hormone-associated proteins, neurophysins. Reversed-phase and ion-exchange HPLC polypeptide mapping have been used to detect the hormones, associated proteins, and other molecular forms containing these. This mapping has provided a means not only to isolate these molecules when present in micro amounts but also ultimately to identify anatomical sites which contain the neurophysin/hormone molecular pathways and to define the relatedness of polypeptide forms contained in different pathways. Reversed-phase HPLC also has provided a means to study proteolytic precursor processing, both to isolate synthetic and semisynthetic polypeptides prepared for use as substrates in processing reactions and eventually to study the polypeptides and intermediates produced by these reactions. Finally, bioaffinity HPLC is being evaluated as a separatory and analytical tool. The latter includes its use to characterize the noncovalent peptide-protein and protein-protein interactions which occur among the molecular forms of the neurophysin/hormone pathways. These experiments typify the impact of HPLC for both analytical and preparative separations in studies of biologically active peptides and proteins.

High-performance liquid chromatographic mapping and structural characterization of neurophysin isoforms

Both ion-exchange and reverse-phase HPLC protocols for micromapping of neurophysins have been examined and the structural relationships among the major isoforms identified in the maps have been characterized. Reverse-phase HPLC was found to be especially useful for obtaining fingerprints of the isoforms within each of the two major families of neurophysins, I (oxytocin-related) and II (vasopressin-related), for both bovine and human neurophysins from posterior pituitary sources. From fractionation of the bovine proteins on octylsilyl columns, at least four neurophysins I were identified, one of which corresponds to the intact sequence of 93 residues and three of which vary from the parent by various degrees of carboxyl-terminal truncation. For bovine neurophysin II, two isoforms were identified in the reverse-phase HPLC maps, both of 95 residues, which vary from one another by the residue, either Ile or Val, at position 89. Isoforms were also detected for human neurophysins, including a carboxyl-terminal truncated form of human neurophysin II. All of the major neurophysin isoforms and several of the minor forms were shown to be functionally active as expressed by their binding to peptide ligand affinity matrices. Reverse-phase HPLC mapping on the octylsilyl matrix allowed neurophysin fingerprinting of crude tissue extracts by providing a narrow "window" within which the neurophysins elute but many other polypeptides expected to be present are excluded. The reverse phase HPLC method provides a useful way to obtain isolated neurophysin isoforms for physicochemical characterizations now usually carried out with mixtures of isoforms obtained by ion-exchange chromatography. The method also has characteristics amenable both for high-sensitivity fingerprinting of neurophysin isoforms, from different species and anatomical sources, and as a prelude to microstructural and -functional characterization of the isoforms so isolated.

Cooperative interactions in neurophysin-neuropeptide hormone complexes. Analytical affinity chromatography of native and covalently-modified neurophysins

The structural interdependence between neurophysin (NP) self-association and ligand binding surfaces has been studied by analytical affinity chromatography of several NP sequence variants and derivatives on Met-Tyr-Phe-aminobutyl-agarose and bovine NP-II Sepharose. Elutions of radiolabeled NP's from both matrices show that hybrid dimers can form between major bovine NP's (I and II, or VLDV- and MSEL-NP's, respectively), as well as between human and bovine NP's, with affinities close to that for homologous dimer formation. Such evidence supports the view that the region of NP involved in NP-NP contact is composed primarily of conserved structural elements of the protein. NP antibodies which recognize surfaces close to or in the NP-NP contact region have been detected by their effects on bovine NP-II elution on NP-II Sepharose. Elutions of [3-nitro-Tyr 49] BNP-II from Met-Tyr-Phe-aminobutyl-agarose showed that nitration has little effect on the chromatographic properties of NP-II. This evidence substantiates previous arguments (Angal, S. & Chaiken, I.M. (1982) Biochemistry 21, 1574-1580) that the chromatographic behavior of native NP's on the affinity matrices is an expression of the interdependence of NP self-association and ligand binding surfaces and not due to bivalent peptide binding by NP monomer. The affinity chromatographic properties of NP derivatives, including bovine NP-II photolabeled in the ligand binding site and tryptic fragments of bovine NP-I (NP-I-(9-93) and [des 19-20] NP-I-(9-93)), support the view that the surfaces for ligand binding and NP-NP contact are conformationally linked. The data argue that conformational changes that ensue upon noncovalent ligand binding and lead to enhanced NP self-association cannot occur favorably with the protein modified by either covalent ligand attachment or limited amino-terminal proteolysis.

Characterization of the size and 5' cap of messenger RNA encoding neurophysin precursors

The coding activity of bovine hypothalamic poly A+ mRNA for neurophysin I and II immunoreactive proteins was characterized with respect to size and 5' cap. The mRNA was fractionated by methylmercuric hydroxide agarose gel electrophoresis and subsequently translated in vitro in rabbit reticulocyte lysates. Alternatively, mRNA was fractionated by gel exclusion HPLC and translated in wheat germ extracts. Immunoprecipitated translation products were analyzed by gel exclusion HPLC. Neurophysin-immunospecific protein of approximately 17,000 daltons, the size expected for the neuropeptide hormone-neurophysin precursors, was encoded by mRNA species of two size classes. The smaller class of mRNA's was of the size expected from the size of the precursor proteins. The larger class was 5-10 times larger. The low K+ concentration optimum for translation of unfractionated mRNA encoding neurophysin I immunoreactive proteins and the inability of a cap analogue to inhibit this translation suggest that mRNA species encoding neurophysin I-immunoreactive translation products are incompletely capped. By contrast, the mRNA encoding neurophysin II-immunoreactive products appear to contain a normal cap structure.

Purification and characterization of ornithine transcarbamoylase from Saccharomyces cerevisiae

Ornithine transcarbamoylase (OTCase) has been purified in 100-mg quantities from a plasmid-containing, enzyme-overproducing strain of Saccharomyces cerevisiae. The specific activity of the homogeneous enzyme is 2.5-fold above that previously reported. The molecular weight and partial specific volume of OTCase were determined by sedimentation equilibrium in solutions containing H2O and D2O. Data from two rotor speeds were simultaneously fit using nonlinear least squares analysis with multiple independent variables giving a molecular weight of 110,000 +/- 2,200 and a partial specific volume of 0.732 +/- 0.006 ml g-1. The ultraviolet absorption spectrum of OTCase gives a specific absorbance at 280 nm of 0.36. This low value is consistent with a small number of aromatic residues. Amino acid analysis, fluorescence, and multicomponent analysis yield 1 tryptophan, 4 tyrosine, and 24 phenylalanine/polypeptide chain. From an analysis of the circular dichroic spectrum, it was determined that OTCase contained 22% alpha-helix, 43% beta-sheet, 8% beta-turn, and 27% random structure. The fluorescence of the single tryptophan/polypeptide chain has an emission maximum at 320 nm, indicating a hydrophobic environment.

Onset of neurophysin self-association upon neurophysin/neuropeptide hormone precursor biosynthesis

The potential of the common biosynthetic precursor of neurophysin and neuropeptide hormones to self-associate has been assessed by quantitative affinity chromatographic analysis. The precursor form, with the hormone sequence in the amino terminal region and assumed able to interact intramolecularly with the hormone binding site of the neurophysin domain of the folded precursor, exhibits an affinity for neurophysin-agarose which is intermediate between those of unliganded neurophysin and non-covalently hormone-liganded neurophysin. The results lead to a prediction that neurophysin self-association is established upon precursor synthesis and prior to limited proteolysis of the precursor to release mature neurophysin and hormone components. Such self-association could play a role in packaging of the precursor into secretory granules and in regulating subsequent precursor processing events within the granules.

An activity from Escherichia coli membranes responsible for the modification of nitrate reductase to its precursor form

An enzymatic activity which modifies nitrate reductase has been identified in the cytoplasmic membrane of Escherichia coli. This activity changes subunit B to a form with a slightly greater electrophoretic mobility on sodium dodecyl sulfate-polyacrylamide gels (B'). The B' polypeptide produced by this modifying enzyme was compared to an apparently identical polypeptide identified in the precursor form of nitrate reductase which can be found in the cytoplasm of all strains and in the membrane of mutants defective in the insertion of nitrate reductase. These B' polypeptides were all identical with respect to mobility on gradient sodium dodecyl sulfate gels and peptides produced by limited digests using trypsin, papain, and Staphylococcus aureus V8 protease. When compared to subunit B, the proteolytic gel maps of B' polypeptides showed minor differences. From the identity of the modified B' with precursor B', the ability to convert B into B' in vitro and the in vivo nature of B' as a precursor of B, it was concluded that the modification of B to B' is a reversible process and is due to the removal of one or more small nonprotein molecules.

Minimum information content and formation of interacting ribonuclease fragment complexes

The degree to which amino acid sequence can be simplified with retention of conformational and functional properties has been investigated by semisynthesis using non-covalent fragment complexes of bovine pancreatic ribonuclease as test cases. Based on the ribonuclease S system, a set of synthetic model sequences was defined for the S-peptide (1-20) region which interacted productively with native S-protein (21-124). The most simple sequence, an eicosapeptide containing helix-favoring Ala residues at all positions except Glu 1 and 14, Phe 8, His 12, and Met 13, effected at least 15% of ribonuclease catalytic activity (versus native ribonuclease S) when added to S-protein in saturating amounts. The data for model S-peptides define an alpha-helical framework and specific side chains at positions 8, 12, and 13 as the core of sequence information necessary for S-peptide to effect a productive non-covalent complex with S-protein. Previous ribonuclease fragment studies also were used as a basis for making the productive, non-overlapping complex, (1-15):(21-111):(116-124). Addition of synthetic (1-15) and (116-124) to (21-111) led to a 3 degrees increase in Tm and 4% (versus ribonuclease A) catalytic activity. The three-fragment complex, with the beta-bend residues 112-115 deleted, exhibited significantly lower stability to thermal denaturation than did related two-fragment complexes. The potential use of three-fragment complexes related to the above is discussed for semi-synthetic sequence modeling concomitantly in the N- and C-terminal regions of ribonuclease.