Disruption of an internal membrane-spanning region in Shiga toxin 1 reduces cytotoxicity

Shiga toxin type 1 (Stx1) belongs to the Shiga family of bipartite AB toxins that inactivate eukaryotic 60S ribosomes. The A subunit of Stxs are N-glycosidases that share structural and functional features in their catalytic center and in an internal hydrophobic region that shows strong transmembrane propensity. Both features are conserved in ricin and other ribosomal inactivating proteins. During eukaryotic cell intoxication, holotoxin likely moves retrograde from the Golgi apparatus to the endoplasmic reticulum. The hydrophobic region, spanning residues I224 through N241 in the Stx1 A subunit (Stx1A), was hypothesized to participate in toxin translocation across internal target cell membranes. The TMpred computer program was used to design a series of site-specific mutations in this hydrophobic region that disrupt transmembrane propensity to various degrees. Mutations were synthesized by PCR overlap extension and confirmed by DNA sequencing. Mutants StxAF226Y, A231D, G234E, and A231D-G234E and wild-type Stx1A were expressed in Escherichia coli SY327 and purified by dye-ligand affinity chromatography. All of the mutant toxins were similar to wild-type Stx1A in enzymatic activity, as determined by inhibition of cell-free protein synthesis, and in susceptibility to trypsin digestion. Purified mutant or wild-type Stx1A combined with Stx1B subunits in vitro to form a holotoxin, as determined by native polyacrylamide gel electrophoresis immunoblotting. StxA mutant A231D-G234E, predicted to abolish transmembrane propensity, was 225-fold less cytotoxic to cultured Vero cells than were the wild-type toxin and the other mutant toxins which retained some transmembrane potential. Furthermore, compared to wild-type Stx1A, A231D-G234E Stx1A was less able to interact with synthetic lipid vesicles, as determined by analysis of tryptophan fluorescence for each toxin in the presence of increasing concentrations of lipid membrane vesicles. These results provide evidence that this conserved internal hydrophobic motif contributes to Stx1 translocation in eukaryotic cells.     

Imaging the intracellular trafficking and state of the AB5 quaternary structure of cholera toxin

The subcellular localization and corresponding quaternary state of fluorescent labelled cholera toxin were determined at different time points after exposure to living cells by a novel form of fluorescence confocal microscopy. The compartmentalization and locus of separation of the pentameric B subunits (CTB) from the A subunit (CTA) of the toxin were evaluated on a pixel-by-pixel (voxel-by-voxel) basis by measuring the fluorescence resonance energy transfer (FRET) between CTB labelled with the sulfoindocyanine dye Cy3 and an antibody against CTA labelled with Cy5. The FRET efficiency was determined by a new technique based on the release of quenching of the Cy3 donor after photodestruction of the Cy5 acceptor in a region of interest within the cell. The results demonstrate vesicular transport of the holotoxin from the plasma membrane to the Golgi compartment with subsequent separation of the CTA and CTB subunits. The CTA subunit is redirected to the plasma membrane by retrograde transport via the endoplasmic reticulum whereas the CTB subunit persists in the Golgi compartment.     

Evidence for two different types of P2 receptors stimulating insulin secretion from pancreatic B cell

Escherichia coli heat-labile enterotoxin (LT) consists of an A subunit and five B subunits. These subunits oligomerize into an assembled holotoxin within the periplasm. Structural analysis of LT has revealed that the A subunit interacts with the B subunit through its carboxy terminus. This indicates that the carboxy-terminal portion of the protein is required for assembly of holotoxin in the periplasm. However, it is not known whether other regions of the A subunit contribute to the assembly. The A subunit constituting the holotoxin contains a disulfide bond between Cys-187 and Cys-199. It has been observed in many proteins that the intramolecular disulfide bond is deeply involved in the function and tertiary structure of the protein. We speculated that the disulfide bond of the A subunit contributes to the assembly in the periplasm, although the bond is not a structural element of the carboxy-terminal portion of the A subunit. We replaced these cysteine residues of the A subunit by oligonucleotide-directed site-specific mutagenesis and analyzed the LTs produced by cells containing the mutant LT genes. The amount of the mutant holotoxin produced was small compared with that of the wild-type strain, indicating that the disulfide bond of the A subunit contributes to the structure which functions as the site of nucleation in the assembly. A reconstitution experiment in vitro supported the notion. Subsequently, we found that the mutant A subunit constituting holotoxin is easily degraded by trypsin and that in cells incubated with mutant LTs, the lag until the intracellular cyclic AMP begins to accumulate is longer than in cells incubated with native LTs. These results might be useful for the analysis of the interaction of LT with target cells at the molecular level.     

Conversion of native oligomeric to a modified monomeric form of human C-reactive protein

C-reactive protein (CRP) is a pentameric oligoprotein composed of identical 23 kD subunits which can be modified by urea-chelation treatment to a form resembling the free subunit termed modified CRP (mCRP). mCRP has distinct physicochemical, antigenic, and biologic activities compared to CRP. The conditions under which CRP is converted to mCRP, and the molecular forms in the transition, are important to better understand the distinct properties of mCRP and to determine if the subunit form can convert back to the pentameric native CRP form. This study characterized the antigenic and conformational changes associated with the interconversion of CRP and mCRP. The rate of dissociation of CRP protomers into individual subunits by treatment in 8 M urea-10 mM EDTA solution was rapid and complete in 2 min as assayed by an enzyme-linked immunofiltration assay using monoclonal antibodies specific to the mCRP. Attempts to reconstitute pentameric CRP from mCRP under renaturation conditions were unsuccessful, resulting in a protein retaining exclusively mCRP characteristics. Using two-dimensional urea gradient gel electrophoresis, partial rapid unfolding of the pentamer occurred above 3 M urea, a subunit dissociation at 6 M urea, and further subunit unfolding at 6-8 M urea concentrations. The urea gradient electrophoresis results suggest that there are only two predominant conformational states occurring at each urea transition concentration. Using the same urea gradient electrophoresis conditions mCRP migrated as a single molecular form at all urea concentrations showing no evidence for reassociation to pentameric CRP or other aggregate form. The results of this study show a molecular conversion for an oligomeric protein (CRP) to monomeric subunits (mCRP) having rapid forward transition kinetics in 8 M urea plus chelator with negligible reversibility.     

Purification and characterization of a lectin from Wistaria floribunda seeds

A lectin from Wistaria floribunda seeds which specifically binds to N-acetyl-D-galactosamine was purified to homogeneity as judged by polyacrylamide gel electrophoresis. Its molecular weight was estimated to be 68,000 by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. It dissociated into subunits on reduction with 2-mercaptoethanol with concomitant loss of hemagglutinating activity. On oxidation in air, the subunits reassociated into the lectin molecules with hemagglutinating activity. Carboxymethylation of the subunits with iodoacetic acid prevented their reassociation on oxidation in air. The molecular weight of the subunits was 32,000, which is about one-half that of the native lectin, suggesting that the lectin consists of two subunits. The results total, NH2-terminal, and COOH-terminal amino acid analyses, and mapping of the tryptic digest of the lectin indicated that these two subunits were indistinguishable and were probably identical, and that they were linked together covalently through a single disulfide bond. Equilibrium dialysis experiments show that the lectin and its subunit molecules are divalent and monovalent, respectively, with respect to sugar binding. The lectin is a glycoprotein, containing 3.2% carbohydrate. The carbohydrate moiety is composed of mannose, galactose, and glucosamine is a molar ratio of 1:2:1 and these sugars seem to be linked as a single oligosaccharide chain to each subunit of the protein.     

Phenylalanine 30 plays an important role in receptor binding of verotoxin-1

The homopentameric B subunit of verotoxin 1 (VT1) binds to the glycosphingolipid receptor globotriaosylceramide (Gb3). We produced mutants with alanine substitutions for residues found near the cleft between adjacent subunits. Substitution of alanine for phenylalanine 30 (Phe-30) resulted in a fourfold reduction in B subunit binding affinity for Gb3 and a 10-fold reduction in receptor density in a solid-phase binding assay. The interaction of wild-type and mutant B subunits with Pk trisaccharide in solution was examined by titration microcalorimetry. The carbohydrate binding of the mutant was markedly impaired compared with that of the wild type and was too weak to allow calculation of a binding constant. These results demonstrate that the mutation significantly impaired the carbohydrate-binding function of the B subunit. To ensure that the mutation had not caused a significant change in structure, the mutant B subunit was crystallized and its structure was determined by X-ray diffraction. Difference Fourier analysis showed that its structure was identical to that of the wild type, except for the substitution of alanine for Phe-30. The mutation was also produced in the VT1 operon, and mutant holotoxin was purified to homogeneity. The cytotoxicity of the mutant holotoxin was reduced by a factor of 10(5) compared to that of the wild type in the Vero cell cytotoxicity assay. The results suggest that the aromatic ring of Phe-30 plays a major role in binding of the B subunit to the Galalpha1-4Galbeta1-4Glc trisaccharide portion of Gb3. Examination of the VT1 B crystal structure suggests two potential carbohydrate-binding sites which lie on either side of Phe-30.     

Studies on the disulfide bonds of glycoprotein hormones. Complete reduction and reoxidation of the disulfide bonds of the alpha subunit of bovine luteinizing hormone

Reoxidation of the disulfide bonds of the alpha subunit of bovine luteinizing hormone (LH) after their complete reduction both in the presence and absence of denaturing agent yields a product which is indistinguishable from the native subunit in its electrophoretic pattern on polyacrylamide gels and in its ability to recombine with the beta subunits of both luteinizing hormone and thyrotropin. The circular dichroism spectrum of the reoxidized alpha subunit is essentially identical to that of native alpha subunit except that its maximum at 233 nm is smaller than observed with native LHalpha. The intact hormone preparations obtained by recombination of reoxidized alpha subunit with native LH-beta exhibit electrophoretic patterns in polyacrylamide gels, elution profiles on gel filtration, binding activities to a membrane fraction from rat testes, and circular dichroism spectra identical to those of native LH and recombinants of native LH-alpha with the beta subunit. Recombinants of native or reoxidized LH-alpha with the beta subunit of thyrotropin are also indistinguishable in their electrophoretic patterns on polyacrylamide gels and in their in vivo activities of stimulating 32P uptake in thyroids of day-old chicks. While this study does not preclude that the alpha subunit may be biosynthesized as part of a larger precursor protein, the data demonstrate that sufficient information is present in the linear sequence of the alpha subunit to allow folding and formation of disulfide bonds to yield a functional alpha subunit.     

The origin of congenital cholesteatoma

Transgenic potatoes were engineered to synthesize a cholera toxin B subunit (CTB) pentamer with affinity for GMI-ganglioside. Both serum and intestinal CTB-specific antibodies were induced in orally immunized mice. Mucosal antibody titers declined gradually after the last immunization but were restored following an oral booster of transgenic potato. The cytopathic effect of cholera holotoxin (CT) on Vero cells was neutralized by serum from mice immunized with transgenic potato tissues. Following intraileal injection with CT, the plant-immunized mice showed up to a 60% reduction in diarrheal fluid accumulation in the small intestine. Protection against CT was based on inhibition of enterotoxin binding to the cell-surface receptor GMI-ganglioside. These results demonstrate the ability of transgenic food plants to generate protective immunity in mice against a bacterial enterotoxin.     

Mucosal immunoadjuvant activity of recombinant Escherichia coli heat-labile enterotoxin and its B subunit: induction of systemic IgG and secretory IgA responses in mice by intranasal immunization with influenza virus surface antigen

The Escherichia coli heat-labile enterotoxin (LT) is a very potent mucosal immunogen. LT also has strong adjuvant activity towards coadministered unrelated antigens and is therefore of potential interest for development of mucosal vaccines. However, despite the great demand for such mucosal vaccines, the use of LT holotoxin as an adjuvant is essentially precluded by its toxicity. LT is composed of an A subunit, carrying the toxic ADP-ribosylation activity, and a pentamer of identical B subunits, which mediates binding to ganglioside GM1, the cellular receptor for the toxin. In this paper, we demonstrate that recombinant enzymatically inactive variants of LT, including the LTB pentamer by itself, retain the immunoadjuvant activity of LT holotoxin in a murine influenza model. Mice were immunized intranasally (i.n.) with influenza virus subunit antigen, consisting mostly of the isolated surface glycoprotein hemagglutinin (HA), supplemented with either recombinant LTB (rLTB), a nontoxic LT mutant (E112K, with a Glu112-->Lys substitution in the A subunit), or LT holotoxin, and the induction of systemic IgG and local S-IgA responses was evaluated by direct enzyme-linked immunosorbent assay (ELISA). Immunization with subunit antigen alone resulted in a poor systemic IgG response and no detectable S-IgA. However, supplementation of the antigen with E112K or rLTB resulted in a substantial stimulation of the serum IgG level and in induction of a strong S-IgA response in the nasal cavity. The adjuvant activity of E112K or rLTB under these conditions was essentially the same as that of the LT holotoxin. The present results demonstrate that nontoxic variants of LT, rLTB in particular, represent promising immunoadjuvants for potential application in an i.n. influenza virus subunit vaccine. Nontoxic LT variants may also be used in i.n. vaccine formulations directed against other mucosal pathogens. In this respect, it is of interest that LT(B)-stimulated antibody responses after i.n. immunization were also observed at distant mucosal sites, including the urogenital system. This, in principle, opens the possibility to develop i.n. vaccines against sexually transmitted infectious diseases.     

An alpha-amanitin-resistant DNA-dependent RNA polymerase II from the fungus Aspergillus nidulans

An alpha-amanitin-resistant DNA-dependent RNA polymerase II has been purified from the lower eukaryote Aspergillus nidulans to apparent homogeneity by extraction of the enzyme at low salt concentration, polymin P (polyethylene imine) fractionation, binding to ion-exchangers and density gradient centrifugation. By this procedure 0.4 mg of RNA polymerase II can be purified over 6,000-fold from 500 g (wet weight) of starting material with a yield of 25% and a specific activity of 550 units/mg. The subunit composition has been resolved by polyacrylamide gel electrophoresis in the presence of sodium dodecylsulphate and by two-dimensional gel electrophoresis using a non-denaturing gel in the first dimension and a dodecylsulphate slab gel in the second dimension. The putative subunits have molecular weights of 170,000, 150,000, 33,000, 27,000, 24,000, 19,000, 18,000 and 16,000. Only one form of RNA polymerase II could be resolved by electrophoresis. The chromatographic and catalytic properties and the subunit composition of the purified RNA polymerase II are clearly different from RNA polymerase I from A. nidulans but throughout comparable with other class II enzymes. It differs from all other class II enzymes by its insensitivity towards the toxin alpha-amanitin, even at concentrations up to 400 micrograms/ml, and appears to be unable to bind O-[14C]methyl-gamma-amanitin at a concentration of 10 micrograms/ml of the toxin. We conclude that the purified RNA polymerase from A. nidulans is a real, but exceptional, type of the class II RNA polymerases.     

Pantothenate synthetase of Escherichia coli B. I. Physicochemical properties

Using a new apparatus for preparative polyacrylamide gel electrophoresis, pantothenate synthetase (D-pantoate: beta-alanine ligase (AMP-forming), [EC 6.3.2.1] was purified about 500-fold from Escherichia coli B. It was found to be homogeneous in analytical disc gel electrophoresis and sedimentation ultracentrifugation (so20, w=4.9). From sedimentation equilibrium ultracentrifugation, a molecular weight of 70,100 was obtained, which is in good agreement with the value obtained by the Sephadex G-150 gel filtration method (69,000); the diffusion constant was calculated to be 5.88X10(-7) cm2/sec. The minimum molecular weight calculated from the amino acid composition of this enzyme protein was 19,700, a value in reasonable accord with molecular weight of the enzyme subunit, 18,000, obtained by gel electrophoresis in the presence of sodium dodecylsulfate. The partial specific volume, v, was calculated to be 0.71 cm3/g. The enzyme had an amino-terminal glycyl residue and a Leu-Ala-Ser-OH sequence at the carboxyl end. Electrophoresis of the enzyme with carrier ampholine gave an isoelectric point of pH 4.6.     

Subunit composition of delta-crystallin from embryonic chick lens. Analysis of methionine-containing tryptic peptides and cyanogen bromide peptides

The predominant protein in the embryonic chick lens, delta-crystallin, is composed of four subunits with molecular weights near 50,000. The degree to which these 4 polypeptides are the same or dissimilar was explored in delta crystallin purified from 15-day-old embryonic chick lenses by relating the numbers of methionine-containing tryptic peptides and cyanogen bromide (CNBr) peptides derived from the native protein to the average number of methionine residues per subunit. Amino acid analyses indicated that 1 mol of native delta-crystallin contains approximately 32 methionine residues, leading to an average of 8 methionine residues per subunit. Approximately equal amounts of 8 methionine-containing tryptic peptides were resolved by two-dimensional thin layer separation on cellulose sheets and by isoelectric focusing in polyacrylamide gels. Nine CNBr peptides were resolved by a combination of electrophoresis in sodium dodecyl sulfate (SDS)-polyacrylamide gels and chromatography on SDS-hydroxylapatite columns. The additive molecular weight of the 9 CNBr peptides was very close to the delta-crystallin subunit molecular weight of 50,000. Thus, the subunits of 15-day-old embryonic chick delta-crystallin have similar sequence of encoded amino acids.     

Subclinical vitamin A deficiency in undersix children in Nagpur, India

The CP-I subunit of calf kidney adenosine deaminase complexing protein (ADCP), isolated by affinity chromatography based on Sepharose-4B immobilized adenosine deaminase, is identical with dipeptidyl peptidase IV. This finding is based on the following results: (a) Its M(r) = 110 kD, as determined by sodium dodecyl sulphate polyacrylamide gel electrophoresis; (b) its catalytic activity toward Gly-Pro-p-nitroanilide; (c) its inhibition by serine protease inhibitor; and (d) by two peptide sequences resulting from its trypsin proteolysis. Accordingly, the CP-I subunit of ADCP isolated from bovine kidney is DPPIV (CD26). Thus, as anticipated, the high affinity between ADA subunits prevails even when they originate in different species.     

Toxin and subunit specificity of blocking affinity of three peptide toxins for heteromultimeric, voltage-gated potassium channels expressed in Xenopus oocytes

The ability of voltage-gated potassium channel alpha-subunits to form heteromultimers has complicated efforts to use toxins to characterize potassium channels in native cells. Here I investigate the effects of subunit composition on toxin blocking affinity, using three members of the Shaker subfamily of potassium channel alpha-subunits (Kv1.1, Kv1.2 and Kv1.4), which are known to form heteromultimers in vivo, in the Xenopus oocyte expression system. These subunits were coexpressed as pairs in which one member was toxin-sensitive and the other relatively insensitive. The blocking affinity of two dendrotoxins (DTX-I and delta-DTX) and a scorpion toxin (tityustoxin-Kalpha) on the resulting mixed population of channels was measured to evaluate three models of toxin block. The single subunit model, in which a single toxin-sensitive subunit renders the channel toxin sensitive, best described all of the data for the two dendrotoxins and the block of tityustoxin-Kalpha for a mixed population of channels composed of Kv1.1 and Kv1.2 subunits. However, with tityustoxin-Kalpha, the data for a mixed population of Kv1.2 and Kv1.4 subunits was fit best by a model in which the toxin interacts with all four subunits for high-affinity block. The data suggest that subunit composition of potassium channels can have a large effect on toxin block and that different toxins yield strikingly diverse results with the same pair of subunits, even when they are nearly identical in blocking affinity for the toxin-sensitive subunit.     

Formation of hybrid concanavalin A molecules by subunit exchange

Mixing of native concanavalin A (Con A) and its dimeric succinylated derivative (succinyl-Con A) in glycine-HCl buffer, pH 4.5, resulted in the formation of a new chemical species that could be separated as a unique fraction by DEAE-cellulose chromatography or by gel electrophoresis. The molecular weight of this new component, which contained the subunits (Mr= 26,000) of native Con A and its succinyl derivative in equimolar amounts, was 50,000 at both pH 5 and pH 7. These data suggest that subunit exchange between 2 chemically distinct Con A molecules yields a hybrid molecule consisting of one protomer of native Con A and one protomer of succinyl-Con A. Similar exchange reactions and hybrid molecules were also observed after mixing acetyl-Con A and succinyl-Con A. These procedures provide several new chemical variants of the Con A molecule that may be useful for the analysis of lectin-cell surface interactions.     

A novel 2-aminomuconate deaminase in the nitrobenzene degradation pathway of Pseudomonas pseudoalcaligenes JS45

2-Aminomuconate, an intermediate in the metabolism of tryptophan in mammals, is also an intermediate in the biodegradation of nitrobenzene by Pseudomonas pseudoalcaligenes JS45. Strain JS45 hydrolyzes 2-aminomuconate to 4-oxalocrotonic acid, with the release of ammonia, which serves as the nitrogen source for growth of the microorganism. As an initial step in studying the novel deamination mechanism, we report here the purification and some properties of 2-aminomuconate deaminase. The purified enzyme migrates as a single band with a molecular mass of 16.6 kDa in 15% polyacrylamide gel electrophoresis under denaturing conditions. The estimated molecular mass of the native enzyme was 100 kDa by gel filtration and 4 to 20% gradient nondenaturing polyacrylamide gel electrophoresis, suggesting that the enzyme consists of six identical subunits. The enzyme was stable at room temperature and exhibited optimal activity at pH 6.6. The Km for 2-aminomuconate was approximately 67 microM, and the Vmax was 125 micromol x min(-1) x mg(-1). The N-terminal amino acid sequence of the enzyme did not show any significant similarity to any sequence in the databases. The purified enzyme converted 2-aminomuconate directly to 4-oxalocrotonate, rather than 2-hydroxymuconate, which suggests that the deamination was carried out via an imine intermediate.     

Effect of mild acid treatment on the survival, enteropathogenicity, and protein production in vibrio parahaemolyticus

Vibrio parahaemolyticus is an important food-borne enteropathogen that encounters various adverse conditions in its native environment or during infection. Effects of mild acid treatment on survival under stress conditions, enteropathogenicity, and protein production in this pathogen were investigated. Logarithmically grown cells, at pH 7.5 shifted to pH 5.0 for 30 min, were more resistant to subsequent acid challenge at pH 4.4. A two-phase adaptive procedure (pH 5.8 for 30 min; pH 5.0 for 30 min) was better than a single-phase procedure for enhancing the acid tolerance of this pathogen. The acid-adapted cells were cross-protected against the challenges of low salinity and thermal inactivation. One-dimensional polyacrylamide gel electrophoresis revealed that proteins with molecular masses of 6.4, 9.0, 13.6, 16.3, 18.9, 22.9, 24.4, 28.3, 33. 9, 36.9, 41.2, 47.6, 58.1, 65.6, 80.5, 88.2, and 96.9 kDa were induced or significantly enhanced, while proteins of 25.3, 30.1, 30. 7, and 91.7 kDa were significantly inhibited. Two-dimensional polyacrylamide gel electrophoresis revealed that 20 species of proteins were induced or significantly enhanced, while 26 species were inhibited. In assays conducted using the suckling mouse model, enteropathogenicity of the acid-adapted cells was significantly enhanced in terms of intestine/body weight ratio and in vivo recovery of infected cells.     

Alpha-Aminoadipate aminotransferase and kynurenine aminotransferase. Purification, characterization, and further evidence for identity

Alpha-Aminoadipate aminotransferase and kynurenine aminotransferase activities were co-purified from the rat kidney supernatant fraction. The resulting preparation was determined to be nearly homogenous by analytical disc gel electrophoresis at pH 8.9 and 7.5 isoelectric focusing on polyacrylamide gels, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A molecular weight of approximately 85,000 was determined on Sephadex G-200 chromatography and sucrose density gradient analysis. The enzyme was determined to be comprised of two subunits of approximately the same molecular weight (45,500 +/- 850) by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. An isoelectric pH of 6.56 +/- 0.06 was determined by focusing on polyacrylamide gels. Further evidence is provided to support the idea that the alpha-aminoadipate aminotransferase and kynureine aminotransferase activities are properties of a single protein: (a) co-purification of the two activities from the rat kidney supernatant fraction with the ratio of their specific activities remaining constant, (b) similar chromatographic behavior, (c) a similarity in their dependence on added pyridoxal-P for activity, and (d) a similar pattern of heat inactivation.     

Biochemical characterization of the human arsenite-stimulated ATPase (hASNA-I)

Arsenic is a potent toxin and carcinogen. In prokaryotes, arsenic detoxification is accomplished by chromosomal and plasmid-borne operon-encoded efflux systems. We have previously reported the cloning of hASNA-I, a human homologue of arsA encoding the ATPase component of the Escherichia coli arsenite transporter. Purified glutathione S-transferase (GST)-hASNA-I fusion protein was biochemically characterized, and its properties were compared with those of ArsA. The GST-hASNA-I exhibited a basal level of ATPase activity of 18.5 +/- 8 nmol/min/mg in the absence of arsenite. Arsenite produced a 1.6 +/- 0.1-fold stimulation of activity (p = 0. 0044), which was related to an increase in Vmax; antimonite did not stimulate activity. Two lines of evidence suggest that an oligomer is the most likely native form of hASNA-I. First, lysates of human embryo kidney 293 cells overproducing recombinant hASNA-I produced a single monomeric 37-kDa band on SDS-polyacrylamide gel electrophoresis (PAGE) and two distinct species when analyzed using nondenaturing PAGE. Second, chemical cross-linking of the 63-kDa GST-hASNA-I resulted in the formation of dimeric and tetrameric protein forms. The results indicate that hASNA-I is a distinct human arsenite-stimulated ATPase belonging to the same superfamily of ATPases represented by the E. coli ArsA protein.     

Studies of the subunit structure of wheat germ ribonucleic acid polymerase II

We have previously presented a rapid, high yield method for the large scale purification of homogeneous RNA polymerase II from wheat germ (Jendrisak, J.J., and Burgess, R.R.(1975), Biochemistry 14, 4639), and we now report a detailed study of its subunit structure. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate indicates that polypeptides with molecular weights of 220 000, 140 000, 40 000, 27 000, 25 000, 21 000, 20 000, 17 800, 17 000, 16 500, 16 000, and approximately 14 000 are associated with the enzyme. Two-dimensional polyacrylamide gel electrophoresis, by which the subunits were separated in the first dimension in the presence of 8 M urea at pH 8.7 and in the second dimension in the presence of 0.1% sodium dodecyl sulfate, indicates that the 40 000 molecular weight component is composed of two nearly identical polypeptides and that the low molecular weight components (smaller than or equal to 40 000) are acidic proteins except for the 25 000 molecular weight polypeptide.