Insertion of a disulfide-containing neurotoxin into E. coli alkaline phosphatase: the hybrid retains both biological activities.

We have inserted a disulfide-containing snake neurotoxin into the N-terminal end of Escherichia coli alkaline phosphatase, between residues +6 and +7 of the mature enzyme. For this purpose, we have designed a cloning and expression vector which allows insertion of foreign DNA between the corresponding codons, and visual selection of the desired recombinant clones upon recovery of phosphatase activity. The hybrid protein is exported to the bacterial periplasm, the alkaline phosphatase signal peptide is correctly processed, and both domains are functionally conformed. The phosphatase domain displays catalytic activity, and the inserted toxin is able to bind to its biological target, the nicotinic acetylcholine receptor. The hybrid molecule is remarkably stable and resistant to proteolysis. Crude periplasmic extract containing the hybrid can be used as a tracer-containing reagent in competitive enzymo-immuno and enzymo-receptor assays. We propose to use the system described in this paper for fast preparation of properly folded disulfide-containing enzymatic probes.     

Production of extracellular alkaline phosphatase by Escherichia coli K-12 periplasmic-leaky mutants carrying phoA + Plasmids

Summary Col E1 hybrid plasmids carrying the phoA ⁺ structural gene of alkaline phosphatase, a periplasmic enzyme of Escherichia coli K-12, were identified from the Clarke and Carbon genomic bank. Wild-type (lky ⁺) phoA ⁺ plasmid-bearing strains synthesized 14 times more intracellular enzyme than the haploid lky ⁺ strain. Phosphate-induced repression was maintained in transformed strains. PhoA ⁺ plasmids carrying the phoB and phoR regulatory genes were introduced into a periplasmic-leaky (lky) recipient strain able to release alkaline phosphatase into the extracellular medium. Transformed lky mutants excreted up to 90% of total enzyme activity which corresponded to 3.5 times the amount of intracellular alkaline phosphatase made by the haploid lky ⁺ strain. The protein composition analysis of periplasmic and extracellular fractions showed that: (i) wild-type phoA ⁺ hybrid plasmidbearing clones did not excrete alkaline phosphatase but had a modified periplasmic content; (ii) alkaline phosphatase was the major excreted protein by transformed lky mutants. The use of periplasmic-leaky phoA ⁺ hybrid plasmid-bearing mutants for an easier production and purification of alkaline phosphatase is discussed.     

Phage-enzymes: expression and affinity chromatography of functional alkaline phosphatase on the surface of bacteriophage.

We have demonstrated that an active enzyme can be expressed on the surface of a bacteriophage. The gene encoding alkaline phosphatase from Escherichia coli was cloned upstream of gene 3, which encodes a minor coat protein of the filamentous bacteriophage, fd. A fusion protein of the correct size was detected from viral particles by Western blotting. Ultrafiltration confirmed that the enzyme fusion behaves as part of a larger structure as would be expected of an enzyme fused to a viral particle. Both wild-type alkaline phosphatase (Arg166) and an active site mutant (Ala166) expressed in this way retain catalytic activity and have qualitatively similar kinetic properties to free enzyme. Values were obtained for Km of 72.7 and 1070 microM respectively whilst relative kcat for the mutant was 36% of that for wild-type. Phage particles expressing alkaline phosphatase were bound to an immobilized inhibitor (arsenate-Sepharose) and eluted with product (20 mM inorganic phosphate). In this way, the functional enzyme is co-purified with the DNA encoding it. This may permit a novel approach to enzyme engineering based on affinity chromatography of mutant enzymes expressed on the phage surface.     

Targeted entry via somatostatin receptors using a novel modified retrovirus glycoprotein that delivers genes at levels comparable to those of wild-type viral glycoproteins

Here we report a novel viral glycoprotein created by replacing a natural receptor-binding sequence of the ecotropic Moloney murine leukemia virus envelope glycoprotein with the peptide ligand somatostatin. This new chimeric glycoprotein, which has been named the Sst receptor binding site (Sst-RBS), gives targeted transduction based on three criteria: (i) a gain of the use of a new entry receptor not used by any known virus; (ii) targeted entry at levels comparable to gene delivery by wild-type ecotropic Moloney murine leukemia virus and vesicular stomatitis virus (VSV) G glycoproteins; and (iii) a loss of the use of the natural ecotropic virus receptor. Retroviral vectors coated with Sst-RBS gained the ability to bind and transduce human 293 cells expressing somatostatin receptors. Their infection was specific to target somatostatin receptors, since a synthetic somatostatin peptide inhibited infection in a dose-dependent manner and the ability to transduce mouse cells bearing the natural ecotropic receptor was effectively lost. Importantly, vectors coated with the Sst-RBS glycoprotein gave targeted entry of up to 1 × 10(6) transducing U/ml, a level comparable to that seen with infection of vectors coated with the parental wild-type ecotropic Moloney murine leukemia virus glycoprotein through the ecotropic receptor and approaching that of infection of VSV G-coated vectors through the VSV receptor. To our knowledge, this is the first example of a glycoprotein that gives targeted entry of retroviral vectors at levels comparable to the natural capacity of viral envelope glycoproteins.     

Analysis of binding centers in nicotinic receptors with the aid of synthetic peptides

We studies the receptor-binding specificity of the synthetic peptide HAP (High Affinity Peptide) and its analogues, which are regarded as a model of the orthosteric site nicotinic acetylcholine receptors (nAChR). Using radioligand analysis, electrophysiology tests, and calcium imaging, we assessed the ability of HAP to interact with nAChR antagonists: long α-neurotoxins and α-conotoxins. A high affinity of HAP for α-bungarotoxin and the absence of its interaction with α-cobratoxin and α-conotoxins was found. The synthesized analogues of HAP in general retained the properties of the original peptide. Thus, HAP cannot be a model of a ligand-binding site.     

Molecular cloning and sequence analysis of human placental alkaline phosphatase

The complete amino acid sequence of the precursor and mature forms of human placental alkaline phosphatase have been inferred from analysis of a cDNA. A near full-length PLAP cDNA (2.8 kilobases) was identified upon screening a bacteriophage lambda gt11 placental cDNA library with antibodies against CNBr fragments of the enzyme. The precursor protein (535 amino acids) displays, after the start codon for translation, a hydrophobic signal peptide of 21 amino acids before the amino-terminal sequence of mature placental alkaline phosphatase. The mature protein is 513 amino acids long. The active site serine has been identified at position 92, as well as two putative glycosylation sites at Asn122 and Asn249 and a highly hydrophobic membrane anchoring domain at the carboxyl terminus of the protein. Significant homology exists between placental alkaline phosphatase and Escherichia coli alkaline phosphatase. Placental alkaline phosphatase is the first eukaryotic alkaline phosphatase to be cloned and sequenced.     

Inhibitory effects of somatostatin on growth and differentiation in cultured intestinal mucosa

The effect of somatostatin on mucosal DNA, protein and brush border enzymes was studied in organ cultured rabbit jejunum and ileum. Compared to control cultures, somatostatin reduced the biopsy DNA and protein content in parallel in the jejunum, but was ineffective in the ileum. This was probably due to a direct growth inhibition, since DNA and brush border enzyme activity from desquamated cells in the postculture medium were unaffected. In addition, a direct inhibition of jejunal villous cell differentiation by somatostatin was reflected in a significant decrease of sucrase, maltase and alkaline phosphatase activity. In the ileum, only the specific activity of alkaline phosphatase was reduced. The key enzyme of cholesterol synthesis, HMG-CoA-reductase, was measured as an intracellular enzyme control and was not influenced by the hormone. The high somatostatin concentrations necessary to achieve the effects are not an artefact of hormone degradation during culture, as shown by radioimmunoassay, and suggest a local or "paracrine" rather than systemic, inhibitory action of somatostatin on intestinal growth and differentiation.     

A deletion that includes the signal peptidase cleavage site impairs processing, glycosylation, and secretion of cell surface yeast acid phosphatase.

We transformed Saccharomyces cerevisiae with a high-copy-number plasmid carrying either the wild-type gene coding for a repressible cell surface acid phosphatase or two modified genes whose products lack a 13- or 14-amino-acid segment spanning or immediately adjacent to the signal peptidase cleavage site. The wild-type gene product underwent proteolytic cleavage of the signal peptide, core glycosylation, and outer chain glycosylation. The deletion spanning the signal peptidase cleavage site led to an unprocessed protein. This modified protein exhibited core glycosylation, whereas its outer chain glycosylation was severely inhibited. Secretion of the deleted protein was impaired, and active enzyme accumulated within the cell. The deletion immediately adjacent to the signal peptidase cleavage site exhibited only a small decrease in the efficiency of processing and had no effect on the efficiency of secretion.     

Fluorine-19 nuclear magnetic resonance study of fluorotyrosine alkaline phosphatase: the influence of zinc on protein structure and a conformational change induced by phosphate binding.

19F nuclear magnetic resonance (NMR) spectroscopy has been used to study a fully active E. coli fluorotyrosine alkaline phosphatase. The fluorotyrosine resonances provide sensitive probes of the conformational states of the protein. They were used to follow the addition of zinc or cobalt to the apoprotein, and the titration of the protein with inorganic phosphate or the inhibitor 2-hydroxy-5-nitrobenzylphosphonate. The results indicate that 2 molecules of inorganic phosphate per dimer of alkaline phosphatase are required to complete a general conformational change in the protein involving perturbations to the environment of several tyrosines. Spectra of the cobalt enzyme indicate that on specific tyrosine per subunit may be near the metal site. The 19F NMR results, combined with the 31P NMR results in the accompanying paper, lead directly to the conclusion that dissociation of noncovalently bound inorganic phosphate from the enzyme is the rate-limiting process in enzyme catalysis at high pH. The local environment of the individual fluorotyrosines is also discussed.     

Mutations that alter the signal sequence of alkaline phosphatase in Escherichia coli.

A phoA-lacZ gene fusion was used to isolate mutants altered in the alkaline phosphatase signal sequence. This was done by selecting Lac+ mutants from a phoA-lacZ fusion strain that produces a membrane-bound hybrid protein and is unable to grow on lactose. Two such mutant derivatives were characterized. The mutations lie within the phoA portion of the fused gene and cause internalization of the hybrid protein. When the mutations were genetically recombined into an otherwise wild-type phoA gene, they interfered with export of alkaline phosphatase to the periplasm. The mutant alkaline phosphatase protein was found instead in the cytoplasm in precursor form. DNA sequence analysis demonstrated that both mutations lead to amino acid alterations in the signal sequence of alkaline phosphatase.     

On the binding determinants of the glutamate agonist with the glutamate receptor ligand binding domain

Ionotropic glutamate receptors (GluRs) are ligand-gated membrane channel proteins found in the central neural system that mediate a fast excitatory response of neurons. In this paper, we report theoretical analysis of the ligand-protein interactions in the binding pocket of the S1S2 (ligand binding) domain of the GluR2 receptor in the closed conformation. By utilizing several theoretical methods ranging from continuum electrostatics to all-atom molecular dynamics simulations and quantum chemical calculations, we were able to characterize in detail glutamate agonist binding to the wild-type and E705D mutant proteins. A theoretical model of the protein-ligand interactions is validated via direct comparison of theoretical and Fourier transform infrared spectroscopy (FTIR) measured frequency shifts of the ligand's carboxylate group vibrations [Jayaraman et al. (2000) Biochemistry 39, 8693-8697; Cheng et al. (2002) Biochemistry 41, 1602-1608]. A detailed picture of the interactions in the binding site is inferred by analyzing contributions to vibrational frequencies produced by protein residues forming the ligand-binding pocket. The role of mobility and hydrogen-bonding network of water in the ligand-binding pocket and the contribution of protein residues exposed in the binding pocket to the binding and selectivity of the ligand are discussed. It is demonstrated that the molecular surface of the protein in the ligand-free state has mainly positive electrostatic potential attractive to the negatively charged ligand, and the potential produced by the protein in the ligand-binding pocket in the closed state is complementary to the distribution of the electrostatic potential produced by the ligand itself. Such charge complementarity ensures specificity to the unique charge distribution of the ligand.     

Studies on the receptors mediating responses of osteoblasts to thrombin

The serine protease thrombin stimulates proliferation in osteoblasts, but decreases alkaline phosphatase (ALP) activity, a marker of osteoblast differentiation. Three thrombin receptors have been identified, protease activated receptor (PAR)-1, PAR-3 and PAR-4; we have previously demonstrated that mouse osteoblasts express PAR-1 and PAR-4. The effect of thrombin on osteoblast proliferation and differentiation was studied to determine which of the thrombin receptors is responsible for the primary effects of thrombin. Primary mouse calvarial osteoblasts from PAR-1-null and wild-type mice, and synthetic peptides that specifically activate PAR-1 (TFFLR-NH2) and PAR-4 (AYPGKF-NH2) were used. Both the PAR-1-activating peptide and thrombin stimulated incorporation of 5-bromo-2'-deoxyuridine (two to four-fold, P < 0.001) and reduced alkaline phosphatase activity (approximately three-fold, P < 0.05) in cells from wild-type mice. The PAR-4-activating peptide, however, had no effect on either alkaline phosphatase activity or proliferation in these cells. Neither thrombin nor PAR-4-activating peptide was able to affect osteoblast proliferation or alkaline phosphatase activity in cells isolated from PAR-1-null mice. The results demonstrate that thrombin stimulates proliferation and inhibits differentiation of osteoblasts through activation of PAR-1. No other thrombin receptor appears to be involved in these effects.     

A lipopeptide-encoding sequence upstream from the IysA gene of Pseudomonas aeruginosa

An open reading frame (ORF) of 141 bp was observed upstream from the Pseudomonas aeruginosa lysA gene. The translation product of this ORF contains a signal peptide with a lipoprotein box, Ile-Ala-Ala-Cys, at the predicted signal peptidase cleavage site. The Escherichia coli phoA gene without its signal sequence was fused in frame to this ORF in a broad host-range plasmid. The resulting construct expressed a hybrid protein exhibiting alkaline phosphatase activity in phoA mutants of both E. coli and P. aeruginosa. This indicates that the ORF encodes a peptide, part of which acts as an export signal. The hybrid peptide was identified by immunoblotting with alkaline phosphatase antiserum. The accumulation of a precursor form was observed when P. aeruginosa cells carrying this gene fusion on a plasmid were treated with globomycin. Moreover, the mature form could be labelled with 2-[3H]-glycerol, indicating that lipidic residues may be linked to the hybrid protein. Taken together, these results strongly suggest that the ORF encodes a lipopeptide. We propose that the gene is called IppL.     

Protease peptide mapping of affinity-labeled rat pancreatic cholecystokinin-binding proteins

Affinity-labeling probes with sites of cross-linking distributed along the ligand have been used to biochemically characterize the pancreatic cholecystokinin (CCK) receptor. Probes with photolabile sites spanning the receptor-binding domain have labeled a Mr = 85,000-95,000 plasma membrane protein, while a probe cross-linked via the amino terminus of CCK-33, far removed from the carboxyl-terminal receptor-binding domain, has labeled a distinct Mr = 80,000 protein. In this work, protease peptide mapping of the pancreatic proteins labeled by each of these probes has been performed to gain insight into the identities of the bands and to define domains of the labeled proteins. Photolabile decapeptide probes with sites of cross-linking at the amino terminus, mid region, and carboxyl terminus of the receptor-binding domain each labeled a Mr = 85,000-95,000 glycoprotein with a Mr = 42,000 core protein and similar Staphylococcus aureus V8 protease peptide maps. This confirms that each probe labels the same binding protein and the same domain of that protein. Serial slices through the broad labeled band were separately deglycosylated and protease-treated, demonstrating a single protein core with differential glycosylation. The CCK-33-based probe, however, labeled predominantly two proteins, one having similar sizes in its native and deglycosylated forms to that labeled by the decapeptide probes and a distinct Mr = 80,000 protein. Of note, the peptide map of the protein believed to be the same as that labeled by the shorter probes was different, suggesting that this probe labeled the binding subunit at a site distinct from that which was labeled by the short probes.     

Design of a specific peptide tag that affords covalent and site-specific enzyme immobilization catalyzed by microbial transglutaminase

Transglutaminase-mediated site-specific and covalent immobilization of an enzyme to chemically modified agarose was explored. Using Escherichia coli alkaline phosphatase (AP) as a model, two designed specific peptide tags containing a reactive lysine (Lys) residue with different length Gly-Ser linkers for microbial transglutaminase (MTG) were genetically attached to N- or C-termini. For solid support, agarose gel beads were chemically modified with beta-casein to display reactive glutamine (Gln) residues on the support surface. Recombinant APs were enzymatically and covalently immobilized to casein-grafted agarose beads. Immobilization by MTG markedly depended on either the position or the length of the peptide tags incorporated to AP, suggesting steric constraint upon enzymatic immobilization. Enzymatically immobilized AP showed comparable catalytic turnover (k(cat)) to the soluble counterpart and comparable operational stability with chemically immobilized AP. These results indicate that attachment of a suitable specific peptide tag to the right position of a target protein is crucial for MTG-mediated formulation of highly active immobilized proteins.     

The active-site and amino-terminal amino acid sequence of bovine intestinal alkaline phosphatase

The active site of bovine intestinal alkaline phosphatase (orthophosphoric-monoester phosphohydrolase (alkaline optimum), EC 3.1.3.1) was labeled with [32P]Pi, a radioactive CNBr peptide was isolated and the amino acid sequence was determined. The sequence of the active-site peptide has limited homology (26%) with the active-site sequence of Escherichia coli alkaline phosphatase except for the ten residues immediately flanking the active-site serine (70%). A possible amino acid sequence deduced from the amino acid composition of an active-site tryptic peptide from human placental alkaline phosphatase is very similar to the bovine intestinal active-site sequence. The amino-terminal sequence of bovine intestinal alkaline phosphatase is homologous (69%) with the human placental enzyme but not with the E. coli phosphatase.     

Thermodynamic study of ligand binding to protein-tyrosine phosphatase 1B and its substrate-trapping mutants

The binding of several phosphonodifluoromethyl phenylalanine (F(2)Pmp)-containing peptides to protein-tyrosine phosphatase 1B (PTP1B) and its substrate-trapping mutants (C215S and D181A) has been studied using isothermal titration calorimetry. The binding of a high affinity ligand, Ac-Asp-Ala-Asp-Glu-F(2)Pmp-Leu-NH(2), to PTP1B (K(d) = 0.24 microm) is favored by both enthalpic and entropic contributions. Disruption of ionic interactions between the side chain of Arg-47 and the N-terminal acidic residues reduces the binding affinity primarily through the reduction of the TDeltaS term. The role of Arg-47 may be to maximize surface contact between PTP1B and the peptide, which contributes to high affinity binding. The active site Cys-215 --> Ser mutant PTP1B binds ligands with the same affinity as the wild-type enzyme. However, unlike wild-type PTP1B, peptide binding to C215S is predominantly driven by enthalpy change, which likely results from the elimination of the electrostatic repulsion between the thiolate anion and the phosphonate group. The increased enthalpic contribution is offset by reduction in the binding entropy, which may be the result of increased entropy of the unbound protein caused by this mutation. The general acid-deficient mutant D181A binds the peptide 5-fold tighter than the C215S mutant, consistent with the observation that the Asp to Ala mutant is a better "substrate-trapping" reagent than C215S. The increased binding affinity for D181A as compared with the wild-type PTP1B results primarily from an increase in the DeltaH of binding in the mutant, which may be related to decreased electrostatic repulsion between the phosphate moiety and PTP1B. These results have important implications for the design of high affinity PTP1B inhibitors.     

Phosphate binding in the active site of alkaline phosphatase and the interactions of 2-nitrosoacetophenone with alkaline phosphatase-induced small structural changes

To monitor structural changes during the binding of Pi to the active site of mammalian alkaline phosphatase in water medium, reaction-induced infrared spectroscopy was used. The interaction of Pi with alkaline phosphatase was triggered by a photorelease of ATP from the inactive P(3)-[1-(2-nitrophenyl)]ethyl ester of ATP. After photorelease, ATP was sequentially hydrolyzed by alkaline phosphatase giving rise to adenosine and three Pi. Although a phosphodiesterase activity was detected prior the photorelease of ATP, it was possible to monitor the structural effects induced by Pi binding to alkaline phosphatase. Interactions of Pi with alkaline phosphatase were evidenced by weak infrared changes around 1631 and at 1639 cm(-1), suggesting a small distortion of peptide carbonyl backbone. This result indicates that the motion required for the formation of the enzyme-phosphate complex is minimal on the part of alkaline phosphatase, consistent with alkaline phosphatase being an almost perfect enzyme. Photoproduct 2-nitrosoacetophenone may bind to alkaline phosphatase in a site other than the active site of bovine intestinal alkaline phosphatase and than the uncompetitive binding site of L-Phe in bovine intestinal alkaline phosphatase, affecting one-two amino acid residues.     

The normally periplasmic enzyme β-lactamase is specifically and efficiently translocated through the Escherichia coli outer membrane when it is fused to the cell-surface enzyme pullulanase

Hybrid proteins were constructed in which C-terminal regions of the bacterial cell surface and extracellular protein pullulanase were replaced by the mature forms of the normally periplasmic Escherichia coli proteins beta-lactamase or alkaline phosphatase. In E. coli strains expressing all pullulanase secretion genes, pullulanase-beta-lactamase hybrid protein molecules containing an N-terminal 834-amino-acid pullulanase segment were efficiently and completely transported to the cell surface. This hybrid protein remained temporarily anchored to the cell surface, presumably via fatty acids attached to the N-terminal cysteine of the pullulanase segment, and was subsequently specifically released into the medium in a manner indistinguishable from that of pullulanase itself. These results suggest that the C-terminal extremity of pullulanase lacks signal(s) required for export to the cell surface. When beta-lactamase was replaced by alkaline phosphatase, the resulting hybrid also became exposed at the cell surface, but exposition was less efficient and specific release into the medium was not observed. We conclude that proteins that do not normally cross the outer membrane can be induced to do so when fused to a permissive site near the C-terminus of pullulanase.     

Effect of membrane moiety and magnesium ions on the inhibition of matrix-induced alkaline phosphatase by zinc ions

1. The inhibition of matrix-induced alkaline phosphatase by zinc ions is due to the displacement of magnesium ions from its binding site. 2. Binding of magnesium ions to alkaline phosphatase induces conformational changes which activate the enzyme. 3. Binding of zinc ions to alkaline phosphatase induces conformational changes which impair the catalytic action of the enzyme. 4. The inhibition of the enzyme by zinc ions is affected by membrane environment and magnesium ions.