Synthesis of iodoamino acids during in vitro thyroglobulin iodination in different states of its molecule]

Iodotyrosine and iodothyronine residues are formed in the protein molecule during bovine thyroglobulin iodination in vitro. Dissociation and reassociation of the thyroglobulin molecule have no significant influence on its iodoaminoacid composition. Thyroglobulin iodination in the presence of 8 M urea does not result in thyroxine synthesis despite the increased formation of iodotyrosine residues. Similarly, during iodination of reassociated thyroglobulin the new molecules of thyroxine are not formed either. It is presumed that during reassociation of thyroglobulin subunits the native conformation of the protein is not completely reconstituted. The results obtained suggest that the structure of thyroglobulin controls the distribution of the iodine atoms incorporated by the iodoaminoacid residues.     

A novel complex from bovine visual cells of a 33,000-dalton phosphoprotein with beta- and gamma-transducin: purification and subunit structure

Photoreceptors of mammalian retinas contain a 33-kDa (33K) protein that is phosphorylated, in vitro, by cyclic nucleotide dependent protein kinases. The 33K protein is phosphorylated in the dark, in situ, and dephosphorylated upon illumination. The soluble 33K protein from bovine retinas has been purified to near homogeneity by extraction at pH 5.7 and chromatography on ion-exchange, gel filtration, and hydroxylapatite columns. In the native conformation, the 33K protein is associated with a 37-kDa (37K) and a 10-kDa (10K) protein, forming a trimeric complex with a sedimentation coefficient of 4.9 S and an apparent molecular mass of 77 kDa. The 33K protein can be dissociated from the 37K/10K complex by centrifugation in the presence of high pH and high salt; the subunits reassociate to form the trimeric complex upon recentrifugation in an isotonic buffer with neutral pH. The 33K protein is phosphorylated rapidly by exogenous kinase, in vitro, whereas the 37K and 10K subunits remain unphosphorylated. The 37K and 10K subunits cross-react with antibodies prepared against the beta- and gamma-subunits, respectively, of bovine transducin, indicating that the 37K and 10K subunits are immunologically identical with beta- and gamma-transducin, respectively. No immuno-cross-reactivity was observed between the 33K protein and an antibody against the alpha-subunit of bovine transducin. The 33K-beta-/gamma-transducin complex exhibits striking similarity to transducin in its subunit structure and mode of subunit interaction, suggesting it may play an important role in the metabolism and function of rod photoreceptor cells.     

Aggregation of C-reactive protein in solutions at acid pH

The hydrodynamic properties of the C-reactive protein (CRP) at different pH were studied using quasi-elastic light scattering, size-exclusion liquid chromatography, and nonreducing gel electrophoresis. It was shown that a CRP solution at pH 5.0-7.2 presents a polydisperse system the major component of which is the native pentameric CRP. At pH 4.0-4.5, CRP exists in two states having different hydrodynamic properties: the native pentameric form with a molecular mass of 120 kDa and with the hydrodynamic radius of 4.03 nm and high-molecular-weight aggregates with a wide range of their molecular weight distribution. The interaction of the C-reactive protein with monoclonal antibodies to it indicates that conformation-dependent surface epitopes of the protein lose the native structure at pH 5.0-5.5. The aggregation of CRP is an irreversible process, which begins in a narrow pH range of pH 5.0-4.5 and is not accompanied by the dissociation into subunits but is determined by intermolecular interactions of its quasi-native pentamers.     

Cross-linking with dimethylsuberimidate to study thyroglobulin conformation

The present investigation demonstrates that the cross-linking agent, dimethylsuberimidate, is an usefull tool to study thyroglobulin structure. In fact, while reproducible and discrete polymerization products are obtained in strictly controlled conditions, valuable information on the native assemblage of thyroglobulin subunits and the effects of its major post-translational modification (iodination) on its structure, are reported.     

Dissociation of thyroglobulin by tetraphenylborate ion

Purified bovine and ovine thyroglobulins (19 S) are partially dissociated into 12-S subunits after treatment with sodium tetraphenyl borate. The extent of dissociation obtained by sodium tetraphenyl borate or sodium dodecyl sulfate treatment is the same. The electrophoretic mobilities on acrylamide gels of sodium tetraphenyl borate-resistant molecules and of native thyroglobulin are identical. Sodium dodecyl sulfate-resistant molecules move more slowly than the native protein.     

Purification and characterization of a lectin from wild sunflower (Helianthus tuberosus L.) tubers

A lectin (HTTL) was isolated from Helianthus tuberosus L. (wild sunflower) tubers using ion-exchange chromatography, gel filtration, and affinity chromatography. The lectin agglutinated both untreated and trypsin-treated rabbit erythrocytes and did not agglutinate human blood cells of groups A, B, and O. The gel filtration showed the native molecular mass of 72 kDa and subunit molecular masses of 17 and 18.5 kDa on 12% SDS-PAGE. The lectin activity was inhibited by D-mannose. The tetrameric protein revealed a unique characteristic by forming a broad zone of protein in native PAGE at pH 8.3, which dissociated into seven subunits of varying e/m ratios on acid gel at pH 4.3. These seven bands revealed two polypeptide species of molecular masses 17 and 18.5 kDa on 12% SDS-PAGE, as in the case of the native protein. The result indicated that of the seven subunits, three were homotetramers of 17 kDa, one was a homotetramer of 18.5 kDa, and three were heterotetramers of 17 and 18.5 kDa. The lectin was thermostable with broad pH optima (pH 4-8) and had no requirement for divalent metal cations for its activity. The amino acid composition showed that the lectin contained higher amounts of glycine, alanine, and lysine, but no methionine. The sugar content was estimated to be 5.3% mannose equivalent. The HTTL was mitogenic to mouse spleen (total) cells at 25 microg/ml concentration. The lectin showed characteristics different from those of the earlier reported H. tuberosus tuber lectins and hence opens up a new avenue to investigate the structure-function relationship of lectin in Helianthus species.     

The negatively charged protein extracted from Tetrahymena pyriformis as an attractant in Amoeba proteus chemotaxis

We extracted a pure protein from Tetrahymena pyriformis as the chemotactic substance in Amoeba proteus. The protein was heat-stable, negatively charged at neutral pH (since its isoelectric point was near pH 4.5) and composed of three subunits. Its molecular weight was 3 × 8,000.     

Stability and subunit structure of human alpha2-macroglobulin.

The molecular weights of alpha2-macroglobulin and its non-covalent subunits have been determined by equilibrium centrifugation. The secondary structure of the native and the thermally denatured molecules has been analyzed by circular dichroic measurements. In contrast to most proteins the thermally denatured form contains a slightly more highly organized polypeptide chain than the native form. The relaxation time of the native protein, as determined by fluorescence polarization measurements, indicates that alpha2-macroglobulin is composed of domains smaller than that of the two subunits. The transitions in acid, alkali, and at high temperatures have been explored in order to establish the pH and thermal range of stability of alpha-macroglobin.     

The structure of toxic protein, the mistletoe lectin, at different pH: the study using intrinsic fluorescence method

The effects of pH on the conformation of mistletoe lectin I and its isolated A- and B-subunits has been investigated by using the methods of intrinsic fluorescence. By the denaturating action of guanidine hydrochloride and the influence of the quenchers (I-, Cs+, acrylamide) the structural stability of the native protein and its isolated subunits was estimated. Treatment of the protein with the denaturant and quenchers revealed its different structure at pH 7.0 and 4.0. At pH 4.0 tryptophan residues become more accessible to quenchers, positive charge of the surrounding area increases and the protein becomes more stable to the action of denaturant. The structure of the isolated A- and B-chains of mistletoe lectin I differs considerably from that of the whole protein: a) its stability to the action of guanidine hydrochloride is lower; b) it depends on the ionic strength of the solvent; c) it is characterized by increased accessibility of tryptophan residues to quenchers (for B-chain). Differences between the conformations of the isolated chains at pH 7.0 and 4.0 are marked more strongly; moreover, at pH 4.5 the B-chain undergoes structural transition. The possible relationship between structural peculiarities of mistletoe lectin I and the mechanism of its transmembrane transfer is discussed.     

The N-acetylglucosamine-specific receptor of the thyroid. Binding characteristics, partial characterization, and potential role

The binding characteristics of the GlcNAc binding protein present in thyroid membranes (Consiglio, E., Shifrin, S., Yavin, Z., Ambesi-Impiombato, F.S., Rall, J.E., Salvatore, G., and Kohn, L.D. (1981) J. Biol. Chem. 256, 10592-10599) were reinvestigated using neoglycoproteins as probes. Plasma membrane preparations from porcine thyroid specifically bound 125I-GlcNAc35-bovine serum albumin. Binding was dependent on the presence of calcium. Binding of ligand to receptor was minimal at neutral pH and maximal at pH 5.0. Equilibrium binding studies indicated positive cooperativity of binding and a site capacity of about 60 pmol/mg of protein. Competition studies were compatible with a specificity hierarchy of GlcNAc much greater than Gal; no recognition of mannose, fucose, or glucose moieties was noted. The receptor was detergent-solubilized from plasma membrane preparations and on the basis of the defined binding properties, purified by chromatography on a GlcNAc-Sepharose affinity column. The purified GlcNAc thyroid receptor has a subunit molecular size of about 45 kDa and appears to be an oligomer composed of three subunits. The receptor was identified as a component of thyrocytes by in situ cytochemical localization with fluorescent neoglycoproteins. In certain cases it was mainly present on, or near, the apical cell surface. It is suggested that this GlcNAc receptor functions in thyroglobulin metabolism, possibly involved in recycling of internalized thyroglobulin molecules back into the follicular lumen.     

Reconstitution of rapeseed high molecular weight protein from isolated subunits

The high molecular weight protein was isolated from rapeseed and characterised. Six subunits were isolated in SDS (0.01%) solution on polyacrylamide-gel electrophoresis and by gel filtration on Sephadex G-100. Reassociation by removing SDS by co-dialysis, against 10 mM sodium phosphate buffer (pH 7.9) was done and the yield was about 90%. The reconstituted protein was indistinguishable from the intact protein in all respects. The subunits isolated from the native protein and the reconstituted protein were found to have identical molecular weights and N-terminal amino acids. No disulphide bonds were observed in the subunit association. Amino acid analysis of the proteins and the six subunits was performed and the number of each amino acid residue calculated.     

Structure-activity relationship of a hemagglutinin from Moringa oleifera seeds

The hemagglutinin from the seeds of Moringa oleifera (MoL) agglutinates human as well as rabbit erythrocytes; the affinity for the latter is almost 250 times more than that for the former. MoL was inhibited by glycoproteins namely thyroglobulin, fetuin and holotransferin indicating the complex sugar specificity of the lectin. The protein is a homodimer with molecular mass of 14kDa, subunits (7.1kDa) linked by the disulfide bond(s). The secondary structure elements of MoL are alpha-helix, 28%; beta-sheet, 23%; turn 20% and unordered 28%. While the activity and secondary structure were not affected at extreme pH and high temperature, they were drastically affected in presence of dithiothreitol at and above pH 7.0, indicating that disulfide linkages hold the active conformation of the protein.     

Assembly of apoferritin from horse spleen: comparison of the protein in its native and reassembled state

The in vitro selfassembly of apoferritin after previous dissociation and unfolding in 7.2M guanidinium chloride, pH 3.5, yields up to 80% of a protein complex exhibiting the molecular mass of the native icositetramer of greater than or equal to 450 kDa. After removal of high molecular mass byproducts, the final reassembly product proves to be indistinguishable from native apoferritin with respect to its functional and conformational properties. These refer to the intrinsic fluorescence and to the far and near UV circular dichroism. The unfolding transitions of the native and reassembled protein in aqueous guanidinium chloride or at acid pH coincide within the range of error. The reassembled protein is also able to catalyze the oxidation of Fe(II). Higher polymers of the apoferritin complex represent most of the residual 20% of the reconstituted protein. They are stabilized by non-covalent (preferentially hydrophobic) interactions, and may be disassembled to the icositetramer by preferential solvation of the protein in the presence of less than or equal to 50% (v/v) ethylene glycol. The change in fluorescence emission accompanying polymerization reflects altered surface properties of the apoferritin subunits compatible with those reported for the ferritin----hemosiderin transition.     

The alternatively folded state of the antibody C(H)3 domain

The C(H)3 domain of antibodies is characterized by two antiparallel beta-sheets forming a disulfide-linked sandwich-like structure. At acidic pH values and low ionic strength, C(H)3 becomes completely unfolded. The addition of salt transforms the acid-unfolded protein into an alternatively folded state exhibiting a characteristic secondary structure. The transition from native to alternatively folded C(H)3 is a fast reaction. Interestingly, this reaction involves the formation of a defined oligomer consisting of 12-14 subunits. Association is completely reversible and the native dimer is quantitatively reformed at neutral pH. This alternatively folded protein is remarkably stable against thermal and chemical denaturation and the unfolding transitions are highly cooperative. With a t(m) of 80 degrees C, the stability of the alternatively folded state is comparable to that of the native state of C(H)3. The defined oligomeric structure of C(H)3 at pH 2 seems to be a prerequisite for the cooperative unfolding transitions.     

Thermal denaturation and gelation of rubisco: effects of pH and ions

In order to understand the mechanism of thermal gelation of rubisco, its native and heat denatured states were characterized by absorbance, fluorescence and circular dichroïsm spectroscopies as well as by differential scanning calorimetry in the presence of various salts. It appears that during the denaturation process, divalent anions are released while divalent cations are fixed by the protein, while it is disorganized and while the environment of its aromatic chromophores becomes more hydrophilic. The pH transition of gelation is shifted 1–2 pH units higher than the transition of denaturation temperature which occurs near the isoelectric point of the native molecule. This shift probably corresponds to the breaking of saline bridges within the protein molecule. Finally, a large effect of divalent cations on the phase diagram indicates that a particular denatured state is attained when these cations are in the denaturation medium.     

Stability and self-assembly of the S-layer protein of the cell wall of Bacillus stearothermophilus

The surface layer of the cell envelope of Bacillus stearothermophilus consists of a regular array of protein subunits. As shown by dodecyl sulfate polyacrylamide gel-electrophoresis and ultracentrifugation, the fully solubilized S-layer protein represents a homogeneous entity with a subunit molecular mass of 115 +/- 5 kDa. Solubilization of the protein may be accomplished at acid pH, or using high concentrations of urea or guanidine X HCl. It is accompanied by (partial) denaturation, thus interfering with the characterization of the protein in its unperturbed native state. Removal of the solubilizing agent by dialysis or dilution allows the S-layer to be reassembled into two-dimensional crystalline lattices identical to those observed in intact cells. To determine the kinetics of association, optimum conditions are found to be rapid mixing with 0.1 M sodium phosphate pH 7.0, 20 degrees C, final protein concentration greater than 10 micrograms/ml. If the time course of the self-assembly is monitored by light scattering, as well as by chemical cross-linking with glutardialdehyde, multiphasic kinetics with a rapid initial phase and slow consecutive processes of higher than second-order are observed. The rapid phase may be attributed to the formation of oligomeric precursors (Mr greater than 10(6) ). Concentration-dependent light scattering measurements give evidence for a "critical concentration" of association, suggesting that patches of 12-16 protein subunits fuse and recrystallize into the final (native) S-layer structure. Recrystallization tends to be complete.     

Phosphorylase kinase from chicken skeletal muscle. Quaternary structure, regulatory properties and partial proteolysis

Phosphorylase kinase has been purified from white and red chicken skeletal muscle to near homogeneity, as judged by sodium dodecyl sulphate (SDS) gel electrophoresis. The molecular mass of the native enzyme, estimated by chromatography on Sepharose 4B, is similar to that of rabbit skeletal muscle phosphorylase kinase, i.e. 1320 kDa. The purified enzyme both from white and red muscles showed four subunits upon polyacrylamide gel electrophoresis in the presence of SDS, corresponding to alpha', beta, gamma' and delta with molecular masses of 140 kDa, 129 kDa, 44 kDa and 17 kDa respectively. Based on the molecular mass of 1320 kDa for the native enzyme and on the molar ratio of subunits as estimated from densitometric tracings of the polyacrylamide gels, a subunit formula (alpha' beta gamma' delta)4 has been proposed. The antiserum against the mixture of the alpha' and beta subunits of chicken phosphorylase kinase gave a single precipitin line with the chicken enzyme but did not cross-react with the rabbit skeletal muscle phosphorylase kinase. The pH 6.8/8.2 activity ratio of phosphorylase kinase from chicken skeletal muscle varied from 0.3 to 0.5 for different preparations of the enzyme. Chicken phosphorylase kinase could utilize rabbit phosphorylase b as a substrate with an apparent Km value of 0.02 mM at pH 8.2. The apparent V (18 mumol min-1 mg-1) and Km values for ATP at pH 8.2 (0.20 mM) were of the same order of magnitude as that of the purified rabbit phosphorylase kinase b. The activity of chicken phosphorylase kinase was largely dependent on Ca2+. The chicken enzyme was activated 2-4-fold by calmodulin and troponin C, with concentrations for half-maximal activation of 2 nM and 0.1 microM respectively. Phosphorylation with the catalytic subunit of cAMP-dependent protein kinase (up to 2 mol 32P/mol alpha beta gamma delta monomer) and autophosphorylation (up to 8 mol 32P/mol alpha beta gamma delta monomer) increased the activity 1.5-fold and 2-fold respectively. Limited tryptic and chymotryptic hydrolysis of chicken phosphorylase kinase stimulated its activity 2-fold. Electrophoretic analysis of the products of proteolytic attack suggests some differences in the structure of the rabbit and chicken gamma subunits and some similarities in the structure of the rabbit red muscle and chicken alpha'.     

Polymeric structure of pig small-intestinal mucus glycoprotein. Dissociation by proteolysis or by reduction of disulphide bridges.

Pig small-intestinal mucus glycoprotein, of molecular weight 1.72 X 10(6), is cleaved by Pronase digestion into glycoprotein subunits of molecular weight 4.5 X 10(5). Of the protein component of the native glycoprotein 29% by weight was lost on Pronase digestion, with no loss of carbohydrate. The non-glycosylated region of the protein that was lost with proteolytic digestion had a broad spectrum of amino acid residues, in contrast with the glycosylated region of the protein, which was resistant to proteolysis and was rich in serine, threonine and proline residues. Reduction with 0.2M-mercaptoethanol dissociated the Pronase-digested glycoprotein subunits into smaller glycoprotein subunits of molecular weight 2.7 X 10(5). On reduction, the native glycoprotein was dissociated into subunits of molecular weight 2.4 X 10(5), a similar size to those obtained from reduction of the Pronase-digested glycoprotein. On reductive dissociation of the native glycoprotein, in addition to glycoprotein subunits, protein was also released principally as a component of 90000 molecular weight. This protein was separated quantitatively from the reduced glycoprotein in amounts compatible with one 90000-mol.wt. protein molecule per 1.72 X 10(6)-mol.wt. native glycoprotein molecule. No 90000-mol.wt. protein was released on reduction of the isolated Pronase-digested glycoprotein. Pig small-intestinal mucus glycoprotein is therefore a covalent polymer of glycoprotein subunits joined by disulphide bridges. This polymeric structure differs in important respects from that previously shown for gastric mucus, in particular with respect to the size and number of component subunits per native molecule.     

Profile of the disulfide bonds in acetylcholinesterase

The inter- and intrasubunit disulfide bridges for the 11 S form of acetylcholinesterase isolated from Torpedo californica have been identified. Localized within the basal lamina of the synapse, the dimensionally asymmetric forms of acetylcholinesterase contain either two (13 S) or three (17 S) sets of catalytic subunits linked to collagenous and noncollagenous structural subunits. Limited proteolysis of these molecules yields a tetramer of catalytic subunits that sediments at 11 S. Each catalytic subunit contains 8 cysteine residues which were identified following tryptic digestion of the reduced, 14C-carboxymethylated protein. The tryptic peptides were purified by gel filtration followed by reverse-phase high performance liquid chromatography (HPLC) and then sequenced. The disulfide bonding profile was determined by treating the native, nonreduced 11 S form of acetylcholinesterase with a fluorescent, sulfhydryl-specific reagent, monobromobimane, prior to tryptic digestion. Peptides again were resolved by gel filtration and reverse-phase HPLC. One fluorescent cysteine-containing peptide was identified, indicating that a single sulfhydryl residue, Cys231, was present in its reduced form. Three pairs of disulfide-bonded peptides were identified. These were localized in the polypeptide chain based on the cDNA-deduced sequence of the protein and were identified as Cys67-Cys94, Cys254-Cys265, and Cys402-Cys521. Hence, three loops are found in the secondary structure. Cys572, located in the carboxyl-terminal tryptic peptide, was disulfide-bonded to an identical peptide and most likely forms an intersubunit cross-link. Since the 6 cysteine residues in acetylcholinesterase that are involved in intrachain disulfide bonds are conserved in the sequence of the homologous protein thyroglobulin, it is likely that both proteins share a common folding pattern in their respective tertiary structures. Cys231 and the carboxyl-terminal cysteine residue Cys572 are not conserved in thyroglobulin.     

Erythrocruorin from the water-flea Daphnia magna. Quaternary structure and arrangement of subunits.

The subunit structure of erythrocruorin from the cladoceran Daphnia magna was studied. The native protein was found to have a sedimentation coefficient (S2(20), w) of 17.9 +/- 0.2 S and a molecular weight, as determined by sedimentation equilibrium, of 494 000 +/- 33 000. Iron and haem determinations gave 0.312 +/- 0.011% and 3.84 +/- 0.04%, corresponding to minimal molecular weights of 17900 +/- 600 and 16 100 +/- 200 respectively. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis gave one band with mobility corresponding to a molecular weight of 31 000 +/- 1 500. The molecular weight of the polypeptide chain determined by sedimentation equilibrium in 6 M-guanidinium chloride and 0.1 M-2-mercaptoethanol is 31 100 +/- 1300. On a molecular-weight basis, Daphnia erythrocruorin is composed of 16 identical polypeptide chains carrying two haem groups each. The native structure is stable between pH5 and 8.5. At alkaline and acidic pH, a gradual decrease in the sedimentation coefficient down to 9.8S occurs. Above pH 10 and below pH4, a slow component with S20, w between 2.7S and 4.0S is observed. The 2.7S, 4.0S and 9.8S species are identified as single-chain subunits, subunit dimers and half-molecules respectively. We propose a model for the molecule composed of 16 2.7S subunits grouped in two layers stacked in an eclipsed orientation, the eight subunits of each layer occupying the vertices of a regular eight-sided polygon. Support for this arrangement is provided from electron microscopy and from analysis of the pH-dissociation pattern.