Antibodies as probes for detection of conformational changes in proteins. A model study with the alkaline phosphatase of Escherichia coli

Preparation and characterization of monoclonal antibodies to the extracellular hemoglobin of Lumbricus terrestris

Murine monoclonal antibodies to the extracellular hemoglobin of Lumbricus terrestris were prepared by a modification of the method of Kohler and Milstein. 224 hybridomas were found to produce antibodies which bound to the hemoglobin; they were tested for binding to the four subunits of the hemoglobin: M (chain I, 16 kDa), D1 (chain V, 31 kDa), D2 (chain VI, 37 kDa) and T (50 kDa), a disulfide-bonded trimer of chains II, III and IV, each of about 17 kDa. 150 hybridomas bound to all four subunits and 40 hybridomas bound to various combinations of subunits. The remaining 34 hybridomas combined only with the hemoglobin. The twelve hybridomas obtained after subculturing and cloning were tested for their binding to the two fractions II and III, consisting of subunits D1 + D2 + T and M, respectively, obtained by dissociation at pH 9.5 and at pH 4.0 and to the reassociated whole molecules, obtained subsequent to return to neutral pH. Eight hybridomas which combined only with the hemoglobin also combined with all the reassociated molecules but not with any of the fractions: these monoclonal antibodies probably recognize conformation-dependent antigenic sites that are present only in the hexagonal bilayer structure characteristic of the native and reassociated hemoglobin molecules. Of the remaining four hybridomas, two bound to subunit T and two combined with subunits T and D2; they also combined with the reassociated molecules and with the fractions II. In addition, the hybridomas did not bind to the hemoglobins of Tubifex, Limnodrilus, Arenicola, Tylorrhynchus and Macrobdella or to the chlorocruorins of Myxicola and Eudistylia.

A molecular, genetic and immunological approach to the functioning of colicin A, a pore-forming protein

We have constructed, by recombinant DNA techniques, one hybrid protein, colicin A-beta-lactamase (P24), and two modified colicin As, one (P44) lacking a large central domain and the other (PX-345) with a different C-terminal region. The regulation of synthesis, the release into the medium and the properties of these proteins were studied. Only P44 was released into the medium. This suggests that both ends of the colicin A polypeptide chain might be required for colicin release. None of the three proteins was active on sensitive cells in an assay in vivo. However, P44 was able to form voltage-dependent channels in phospholipid planar bilayers. Its lack of activity in vivo is therefore probably caused by the inability to bind to the receptor in the outer membrane. PX-345 is a colicin in which the last 43 amino acids of colicin A have been replaced by 27 amino acids encoded by another reading frame in the same region of the colicin A structural gene; it was totally unable to form pores in planar bilayers at neutral pH but showed a very slight activity at acidic pH. These results confirm that the C-terminal domain of colicin A is involved in pore formation and indicate that at least the 43 C-terminal amino acid residues of this domain play a significant role in pore formation or pore function. Fifteen monoclonal antibodies directed against colicin A have been isolated by using conventional techniques. Five out of the 15 monoclonal antibodies could preferentially recognize wild-type colicin A. In addition, the altered forms of the colicin A polypeptide were used to map the epitopes of ten monoclonal antibodies reacting specifically with colicin A. Some of the antibodies did not bind to colicin A when it was pre-incubated at acidic pH suggesting that colicin A undergoes conformational change at about pH 4. The effects of monoclonal antibodies on activity in vivo of colicin A were investigated. The degree of inhibition observed was related to the location of the epitopes, with monoclonal antibodies reacting with the N terminus giving greater inhibition. The monoclonal antibodies directed against the C-terminal region promoted an apparent activation of colicin activity in vivo.

A colicin A fragment containing the receptor binding domain can be directed to the periplasmic space in E. coli through gene fusion

The central region of the colicin A polypeptide chain has been fused to the N-terminal part of beta-lactamase through genetic recombination. This region comprising amino acid residues 70-335 confers on the hybrid protein the ability to protect sensitive cells from the lethal action of colicin A. Although colicin A belongs to the cytoplasmic compartment of E. coli, export of the hybrid protein to the periplasmic space was promoted by the signal peptide of beta-lactamase.

A protease as a possible sensor of environmental conditions in E. coli outer membrane

A mutant strain devoid of the colicin A protease activity (cpr) located at the external face of the outer membrane has been isolated. The location of the mutation (about 74 min on the genetic map), its pleiotropic effects concerning mainly the protein composition of the outer membrane and sensitivity to phages and colicins, are very similar in cpr and in tpo, envZ, perA strains that are affected in the ompB locus. Conditions resulting in inhibition of the colicin A protease activity also result in transcriptional regulation of OmpF, OmpC, and LamB protein synthesis. The possibility for this protease as an osmosensor of the cell's external environment is discussed.

Immunological studies on bovine milk lipoprotein lipase. Effects of Fab fragments on enzyme activity

Rabbit antiserum was prepared against purified bovine mild lipoprotein lipase. Immunoelectrophoresis of lipoprotein lipase gave a single precipitin line against the antibody which was coincident with enzyme activity. The gamma-globulin fraction inhibited heparin-releasable lipoprotein lipase activity of bovine arterial intima, heart muscle and adipose tissue. The antibody also inhibited the lipoprotein lipase activity from adipose tissue of human and pig, but not that of rat and dog. Fab fragments were prepared by papain digestion of the gamma-globulin fraction. Fab fragments inhibited the lipoprotein lipase-catalyzed hydrolysis of dimyristoylphosphatidylcholine vesicles and trioleoylglycerol emulsions to the same extent. The Fab fragments also inhibited the lipolysis of human plasma very low density lipoproteins. The change of the kinetic parameters for the lipoprotein lipase-catalyzed hydrolysis of trioleoylglycerol by the Fab fragments was accompanied with a 3-fold increase in Km and a 10-fold decrease in Vmax. Preincubation of lipoprotein lipase with apolipoprotein C-II, the activator protein for lipoprotein lipase, did not prevent inhibition of enzyme activity by the Fab fragments. However, preincubation with dipalmitoylphosphatidylcholine-emulsified trioleoylglycerol or Triton X-100-emulsified trioleoylglycerol had a protective effect (remaining activity 7.0 or 25.8%, respectively, compared to 1.0 or 0.4% with no preincubation). The addition of both apolipoprotein C-II and substrate prior to the incubation with the Fab fragments was associated with an increased protective effect against inhibition of enzyme activity; remaining activity with dipalmitoylphosphatidylcholine-emulsified trioleoylglycerol was 40.6% and with Triton X-100-emulsified trioleoylglycerol, 45.4%. Human plasma very low density lipoproteins also protected against the inhibition of enzyme activity by the Fab fragments. These immunological studies suggest that the interaction of lipoprotein lipase with apolipoprotein C-II in the presence of lipids is associated with a conformational change in the structure of the enzyme such that the Fab fragments are less inhibitory. The consequence of a conformational change in lipoprotein lipase may be to facilitate the formation of an enzyme-triacylglycerol complex so as to enhance the rate of the lipoprotein lipase-catalyzed turnover of substrate to products.

Exclusive localization of colicin A in cell cytoplasm of producing bacteria

The production of colicin A in Citrobacter freundii and in Escherichia coli was studied. After induction with low concentrations of mitomycin C, these organisms differed with regards to cell growth, cell viability, and kinetics of colicin A biosynthesis. Despite these differences, immunoferritin labelling on ultra-thin sections of induced frozen cells demonstrated that colicin A was located exclusively within the cell cytoplasm in both types of bacteria. By using protein markers, it was shown that at no time after induction was colicin A accumulated in the periplasmic space or in inner or outer membranes. These results were confirmed by a biochemical approach. For at least 3 h after induction, colicin A remained associated with producing cells and no colicin A activity was found in the periplasmic space. These results are discussed with reference to the synthesis and export of other bacteriocins.

Biosynthesis and export of colicin A in Citrobacter freundii CA31

Synthesis of colicin A after induction with mitomycin C was studied. Specific inhibition of chromosomal protein synthesis occurred very shortly after mitomycin addition. There was no coordinate synthesis of colicin A (61000 Mr) and low-molecular-weight protein. Free and membrane-bound polysome fractions were isolated from cells induced with mitomycin C. Colicin A is synthesized in vitro in the free polysomes and not in the membrane-bound polysomes. Conditions are described which allow a practically specific labelling of colicin A in vivo. By using this system it was possible to demonstrate that colicin A is not transferred cotranslationally across the cytoplasmic membrane. In contrast, this protein leaves the cell where it was made long after synthesis. Preliminary evidence, suggesting that pauses occur during synthesis of colicin A, is presented.

Antigen-antibody interactions and the anomalous kinetics of arylsulfatase A

Antibodies against homogeneous rabbit liver arylsulfatase A (aryl-sulfatase sulfohydrolase, EC 3.1.6.1) were produced in a goat and the effects of these antibodies on the kinetic parameters of the enzyme have been studied. The results indicate that the binding of antibody to the enzyme does not alter the enzyme active site, since Km and -ki values are unaffected. However, a small reduction in the enzyme activity was observed as the result of a reduction of V in the enzyme-antibody complex. The binding of antibodies led to a change in the pH-rate profile, giving one broad pH optimum shifted toward higher pH value. The enzyme-antibody complex still showed the characteristic arylsulfatase A anomalous kinetics at pH 5.5, but the inactivation was significantly slower than for the native enzyme. As calculated from quantitative immuno-precipitation data, the native enzyme bound 5--7 molecules of IgG. The number of IgG molecules which bound to the turnover-modified enzyme was reduced to 3--4. The loss of antigenic determinants from the turnover-modified enzyme indicates that significant conformational changes occur during the turnover-induced modification, or that a covalent modification of residues present at the antigenic sites has occurred, or both.

Purification and molecular properties of a new colicin

The process of isolation and purification of a new colicin isolated from a Citrobacter strain is described. Escherichia coli sensitive cells are protected by vitamin B12 from the action of this bacteriocin; this suggests that it belongs to the E group of colicins. Therefore, we have called it colicin E4. It has a molecular weight of 56 000 and two molecular forms of isoelectric points 9.4 and 8.2 are separated in electrofocusing on polyacrylamide gels. It has a sedimentation coefficient of 3.4 S and the absorption coefficient A1(280%) nm is 6.23 cm(-1). Using an antibody raised against pure colicin E4, no cross-reaction was detected against colicins A, E1 or K. The physiological effect of colicin E4 on sensitive cells is very similar to that of colicins E1, K or I which disrupt the energized membrane state.

Procaine, a local anesthetic interacting with the cell membrane, inhibits the processing of precursor forms of periplasmic proteins in Escherichia coli

Treatment of Escherichia coli cells with procaine (0.55%, w/v) results in the accumulation of precursor in addition to mature forms of two periplasmic proteins, alkaline phosphatase and glutamine-binding protein. The precursor form of alkaline phosphatase has a higher molecular weight than the mature form by about 2600. An experimental technique is described to isolate and purify precursor forms of any presumably exported protein. After the membrane solubilization step in the presence of nonionic detergent, a peptidase is stimulated, resulting in partial cleavage of the precursors. The products of this cleavage have been identified as the mature protein and presumably the signal peptide in the case of alkaline phosphatase. The amino acid composition of this peptide, which is comprised of 25 residues, has been determined. Procaine (0.55%, w/v) causes an increase in molecular packing of lipid molecules in the membrane which might result in an alteration of membrane fluidity sufficient for selective inhibition of processing of precursors of exported proteins.

Evidence for synthesis of alkaline phosphatase on membrane-bound polysomes in Escherichia coli

Polysomes containing nascent chains of alkaline phosphatase have been isolated from a membrane-bound polysome preparation. Indirect immunoprecipitation using conformation-specific antibodies has been employed. This technique provides a good enrichment of these polyribosomes since routinely no more than than 10--15% of non-specific immunoprecipitation was observed. The yield of the procedure is generally 40% but can be increased if higher non-specific immunoprecipitation is tolerated. Antibodies, previously described, directed against uncoiled or folded monomers of alkaline phosphatase can be used as primary antibody to recognize the nascent chains contained in membrane-bound polysomes which suggests that these chains are partially folded.

Antigenic sites related to human serum proteins in HBsAg: isolation from normal human serum of a high-molecular-weight glycoprotein reacting with antialbumin

A novel minor constituent was isolated from normal human serum by affinity chromatography on columns of insolubilized concanavalin A and antibodies to albumin, followed by rate zonal centrifugation. This component has the following properties: a sedimentation coefficient of approximately 31; a diameter of 14--22 nm, and a buoyant density of 1.302 gm/cu cm. It contains about 1% neutral sugars and is electrophoretically heterogeneous, since two populations of particles with isoelectric points of pH 4.95 and 5.75 were separated by isoelectric focusing. It contains a single major glycopeptide with an apparent molecular weight of 72,000 daltons. Its amino acid composition is distinct from that of albumin. It elicited the formation of antibodies that also reacted with HBsAg.

Aminopeptidase N from Escherichia coli. Unusual interactions with the cell surface

The subcellular localization of aminopeptidase N (previously called aminoendopeptidase) has been investigated. This enzyme was found to be partially released (30-40%) by osmotic shock or by converting Escherichia coli K10 cells to spheroplasts. However, in all other E. coli strains (K12, B/r, MRE 600, ML 308) tested, this enzyme is not released at all by these procedures and thus behaves like a cytoplasmic enzyme. The crypticity of aminopeptidase N is surprisingly low, 75-85% of the enzyme activity is directly assayable in intact cells of any E. coli strain. Various inhibitors of transport systems do not interfer with this assay. Aminopeptidase activity could also be assayed in spheroplasts, even when an insolubilized substrate was used, which suggests a surface location of this enzyme. As well, N-ethylmaleimide (0.4 mM), under conditions which do not allow penetration in the cytoplasm, caused 70% inhibition of aminopeptidase N. Binding of 125I-labeled antiaminopeptidase N antibody to spheroplasts (from K12 strain) was used to assay the orientation of aminopeptidase N in the membrane. This enzyme is exposed on the outer surface of the cytoplasmic membrane. Confirmation of this orientation was obtained by comparing the accessibility of aminopeptidase, alkaline phosphatase and beta-galactosidase to fluorescamine in intact cells. Only 16% of the total beta-galactosidase was labeled with this fluorescent reagent whereas 44-45% of the aminopeptidase N and 59% of the alkaline phosphatase were labeled. Electron microscopic visualization of insolubilized reaction products of aminopeptidase N within the cells showed that these products are located at the poles of the cells. Neither mutant cells which were devoid of aminopeptidase N activity nor parental strains with the enzyme activity inhibited with phenylmercuric chloride contained the characteristic black caps. Thus, it appears that the periplasm is enlarged at the poles of the cells and that the reaction product is mainly located in these places. Investigation of the type of interactions of aminopeptidase N with the plasma membrane only revealed that aminopeptidase N has mainly an electrostatic interaction with the outer surface, probably mediated by magnesium ion bridges. Additional interactions are involved since disruption of the integrity of the cytoplasmic membrane is required to totally release this enzyme.

Metal ion-induced conformational changes in Escherichia coli alkaline phosphatase

Ultraviolet difference spectra are produced by the binding of divalent metal ions to metal-free alkaline phosphatase (EC 3.1.3.1). The interaction of the apoprotein with Zn2+, Mn2+, Co2+ and Cd2+, which induce the tight binding of one phosphate ion per dimer, give distinctly different ultraviolet spectra changes from Ni2+ and Hg2+ which do not induce phosphate binding. Spectrophotometric titrations at alkaline pH of various metallo-enzymes reveal a smaller number of ionizable tyrosines and a greater stability towards alkaline denaturation in the Zn2+- and Mn2+-enzymes than in the Ni2+-, Hg2+- and apoenzymes. The Zn2+- and Mn2+-enzymes have CD spectra in the region of the aromatic transitions that are different from the CD spectra of the Ni2+-, Hg2+- and apoenzymes. Modifications of arginines with 2,3-butanedione show that a smaller number of arginine residues are modified in the Zn2+-enzyme than in the Hg2+-enzyme. The presented data indicate that alkaline phosphatase from Escherichia coli must have a well-defined conformation in order to bind phosphate. Some metal ions (i.e. Zn2+, Co2+, Mn2+ and Cd2+), when interacting with the apoenzyme, alter the conformation of the protein molecule in such a way that it is able to interact with substrate molecules, while other metal ions (i.e. Ni2+ and Hg2+) are incapable of inducing the appropriate conformational change of the apoenzyme. These findings suggest an important structural function of the first two tightly bound metal ions in enzyme.

Structural and topological homology between porcine intestinal and renal brush border aminopeptidase

A method for the preparation of closed, right-side-out vesicles from the brush border membrane of the kidney proximal tubules is described. The aminopeptidase known to be bound to this membrane was investigated in order to compare its properties with those already reported for the intestinal enzyme. Both are composed of a hydrophilic, catalytically active part lying on the external side of the membrane and a short hydrophobic domain probably located in the N-terminal region of one of the subunits ensuring fixation to the lipid matrix. The enzyme were also found to be clinically similar. Moreover, a quantitative immunological technique showed that they contained 6 cross-reacting determinants, consistent with a very high degree of homology. Four of these determinants were accessible in the bound form of the enzymes in the region of the active site. The other two, probably related to the junction between the hydrophilic moiety and the hydrophobic anchor were completely masked in the bound form. The remainder (6 in the intestinal and 4 in the renal enzyme), were heterologous. The accessibility of two well determinants in this latter group was substantially reduced, perhaps by the proximity of the lipid and/or of other enzyme molecules.

Effects of antibodies to various molecular forms of a mutationally altered Escherichia coli alkaline phosphatase on its activation by zinc

Immunogenic and antigenic properties of a Zn2+ -deficient alkaline phosphatase produced in a mutant (U-47) of Escherichia coli have been studied. The native U-47 enzyme, that exists in a monomerdimer equilibrium, was used as immunogen. From the antisera obtained, four antibody populations directed to the various molecular forms of U-47 enzyme have been purified by affinity chromatography using specific antigens coupled to glutaraldehyde-activated beads of indubiose. 70% of the total antibody obtained was directed both to the monomeric and the dimeric forms, 9% was directed to the dimer but showed a low affinity for the monomer; 10% and 11% were specifically directed respectively to the monomer and the dimer. Each antibody population purified had a specific effect on the catalytic activity of the Zn2+ -activated U-47 enzyme. The anti-monomer-dimer and the anti-dimer-monomer inhibit to the same extent whereas the specific anti-monomer does not alter the activity significantly and the specific anti-dimer causes a 30% activation. The catalytic activity of the alkaline phosphatase produced in wild-type strains was also reduced by these anti-U-47 enzyme antibodies. However, whereas the anti-monomer had again very little effect, the anti-dimer-monomer and the anti-monomer-dimer inhibited this enzyme to different extents. The specific antidimer also inhibited this wild-type alkaline phosphate. Antibodies of high affinity to the dimeric form of U-47 enzyme, i.e. specific anti-dimer or anti-dimer-monomer, caused a 30% activation when they were added prior to the reactivation process by Zn2+. Specific anti-monomer strongly inhibited this reactivation process. The Fab fragment of the anti-wild-type phosphatase antibody, under the same conditions, caused a 300% activation. The extents of interactions of the various molecular forms of U-47 enzyme and of the wild-type enzyme with the anti-monomer-dimer and with the anti-dimer have been determined. U-47 enzyme monomeric form has three determinants exposed and the dimeric form has five determinants exposed for interacting with the anti-monomer-dimer antibody, the free wild-type enzyme has only two determinants exposed to this antibody. These determinants might be close to the active site or in another critical location since this antibody can reduce the catalytic activity of the wild-type enzyme to half the original value. The anti-dimer antibodies can interact with three determinants exposed at the surface of the free Zn2+ -reactivated U-47 enzyme and the non-covalent binding of one mole of inorganic phosphate results in the exposure of one more antigenic determinant.

Existence, localization and regulation of the biosynthesis of aminoendopeptidase in gram-negative bacteria

An enzyme capable of hydrolyzing the substrate L-alanine p-nitroanilide has been found in 18 gram-negative Enterobacteriaceae and Pseudomonas that we tested. This enzyme might be located near the cell surface. Contrastingly we have not detected this activity on colonies of the 8 gram-positive Bacillus that we tested, which suggest a taxonomic value for this test. The aminoendopeptidases found in the various gram-negative bacteria showed similar electrophoretic mobilities and immunological cross-reactivity when tested again Escherichia coli anti-aminoendopeptidase antiserum. The regulation of aminoendopeptidase biosynthesis by the endogenous level of inorganic phosphate that we previously demonstrated in E. coli has been found to exist in all gram-negative organisms tested.

Purification of canine plasma renin by affinity chromatography

An affinity column for the purfication of canine plasma renin was prepared using goat anti-renin (dog kidney) gammaG gloublins. The antiserum was prepared against a purified kidney renin preparation. The anti-renin globulins were coupled to cyanogen bromide activated Sepharose. Using the anti-renin globulin-coupled Sepharose as an immuno-adsorbant, a method was devised allowing purification of plasma renin to a 1,000-fold purity.

Purification and properties of a periplasmic aminoendopeptidase from Escherichia coli

A periplasmic aminoendopeptidase from Escherichia coli has been purified to hemogeneity. It is a monomer of molecular weight 45000 and containing one -- SH group that is necessary for catalytic activity. The study of its substrate specificity indicated that the enzyme has both aminopeptidase and endopeptidase activity. The pH optimum for L-alanine p-nitroanilide hydrolysis is between 7 and 7.5 and that for 125I-labeled casein proteolysis between 7.3 and 7.6. The activation energy for the hydrolysis of L-anine p-nitroanilide was calculated to be 5.3 kcal X mol-1 (22.2 kJ X mol-1).