Resolution of the glycosyltransferase activities from two strains of Streptococcus mutans by polyacrylamide gel electrophoresis in the presence of Tween 80

Role of the C-terminal region of dextransucrase from Leuconostoc mesenteroides IBT-PQ in cell anchoring

dsrP, a gene that encodes a cell-associated dextransucrase produced by Leuconostoc mesenteroides IBT-PQ, was isolated, sequenced and expressed in Escherichia coli. From sequence analysis, seven repeat units in the N-terminal region were found, as well as five cell wall binding repeats in the C-terminal region. A model of the C-terminal domain of dextransucrase was built based on the solenoid structure of the cell wall binding domain already described in LytA. By experiments involving direct interactions of the enzyme with L. mesenteroides cells, as well as among the cells and the single C-terminal domain expressed in E. coli, evidence was obtained concerning the anchoring function of this region in cell-associated dextransucrase, a function which may be independent of its capacity to bind dextran.

Structural and functional features of fructansucrases present in Leuconostoc mesenteroides ATCC 8293

Glycosyltransferases produced by Leuconostoc mesenteroides subsp. mesenteroides ATCC 8293 (equivalent to NRRL B-1118) were identified. Two glucansucrases and one fructansucrases were observed in batch culture while levC and levL genes, corresponding to two fructansucrases, were isolated from information obtained from the released draft sequence of this Leuconostoc strain genome and cloned in Escherichia coli. The recombinant enzymes were shown to be fructansucrases producing a polymer identified by NMR as levan, confirming our recent report stating that these are also mosaic levansucrases bearing structural features of glucansucrases in the amino and carboxy terminal regions, as is also the case of inulosucrase (IslA) from Leuconostoc citreum CW28 and levansucrase (LevS) from L. mesenteroides NRRL B-512F. The recombinant levansucrase LevC was purified and characterized in terms of pH, temperature, and kinetic properties. The enzyme exhibits Michaelis-Menten kinetic properties with a K(m) = 27.3 mM and a k(cat) = 282.9 s(-1). This levansucrase behaves mainly as a transferase as only 30% of the substrate is hydrolyzed in a wide range of sucrose concentrations, with higher hydrolytic activities at low substrate concentrations. With this report we experimentally confirm the unusual structural pattern displayed by fructansucrases present in Leuconostoc species that group as a novel sub family of fructansucrases.

Identification and functional characterization of levS, a gene encoding for a levansucrase from Leuconostoc mesenteroides NRRL B-512 F

A Leuconostoc mesenteroides NRRL B-512 F levansucrase gene, (levS), was isolated, sequenced and cloned in Escherichia coli. The recombinant enzyme was shown to be a fructosyltransferase producing a polymer identified by (13)C-NMR as levan. Based on sequence analysis, we found that this levansucrase is a mosaic protein, bearing structural features of glucosyltransferases in the amino and carboxy terminal regions similarly to inulosucrase from Leuconostoc citreum. The phylogenetic analysis of the C-terminal region domain of levansucrases from L. mesenteroides demonstrates that they group together into a novel putative sub-family of genes and evolved long before all other glucosyltransferases, while their catalytic domain structure is species related.

Stabilization of the glucan-binding lectin of Streptococcus sobrinus by specific ligand

Cell suspensions of Streptococcus sobrinus can be aggregated by high molecular-weight alpha-1,6 glucans. The aggregation depends on the fidelity of a cell wall-bound, glucan-binding lectin (GBL). It is thought that the lectin may play a part in the sucrose-dependent accretion of streptococci in dental plaques. Results showed that the anionic detergent, sodium dodecyl sulphate (SDS) was a potent inhibitor of the lectin. When cells were incubated in SDS and washed to remove the detergent, lectin activity was diminished. Following incubation of the cells with SDS in the presence of glucan T-10, a low molecular-weight alpha-1,6 glucan, the loss of activity was less pronounced, suggesting that the glucan afforded partial protection against denaturation. Urea and guanidine hydrochloride were good inhibitors of the lectin, but, unlike SDS, were not able to inhibit it irreversibly, except at very high concentrations. Cationic detergents, such as cetylpyridinium bromide (and chloride), also irreversibly denatured the streptococcal lectin, but were not as effective as SDS in abolishing its activity. The results suggest that alpha-1,6 glucan stabilizes the GBL of S. sobrinus, rendering it more resistant to the effect of chaotropes. This may be one reason why dental plaques tend to resist detergents in dentrifices.

Functional molecular size and structure of dextransucrase by radiation inactivation and gel electrophoresis

Robyt et al. have proposed a mechanism for dextransucrase in which dextran is synthesized by the cooperative action of two equivalent nucleophiles (Robyt, J.F., Kimble, B.K. and Walseth, T.F. (1974) Arch. Biochem. Biophys. 165, 634-640). To distinguish between the possibilities that the enzyme is a monomer bearing both nucleophiles, or a dimer with each subunit bearing one nucleophile, the molecular weight of the enzyme was determined by SDS-polyacrylamide gel electrophoresis and by radiation inactivation. Two major forms of dextransucrase from Leuconostoc mesenteroides NRRL B-512F were found on SDS-polyacrylamide gel electrophoresis, with Mr 177 000 and 158 000, and sometimes a minor form with Mr 168 000. No form of dextransucrase smaller than Mr 158 000 was found, either in the presence or absence of dextran T10, although levansucrase was detected at Mr 92 000 and 116 000. On irradiation with 60Co, dextransucrase behaved as a single species with a maximum size of Mr 201 000. Because Mr 201 000 is much smaller than the minimum dimer size of Mr 316 000 (= 2 X 158 000), it is concluded that both nucleophiles are probably located on the same peptide, rather than one on each subunit of a dimer, and that peptide association is probably not required for dextran synthesis.

Purification, and comparison, of two forms of dextransucrase from Streptococcus sanguis

A procedure has been developed whereby native and proteolyzed forms of dextransucrase have been purified; it involves gel filtration, and hydroxylapatite chromatography in the presence of 0.10% sodium dodecyl sulfate. This procedure is highly reproducible, and permits approximately 30% recovery of high purity (94% homogeneous) enzyme as an inactive, SDS complex that can be reactivated by the addition of Triton X-100. The purified enzymes have been compared with regard to amino acid compositions, and isoelectric and catalytic properties. An analysis of the structure of their product D-glucans was also made. Although the structural characteristics of the enzyme forms differ, proteolysis does not cause alterations in their catalytic properties.

The origin and composition of multiple forms of dextransucrase from Streptococcus sanguis

Multiple forms of purified dextransucrase have been observed in the presence of low detergent concentrations ( Luzio , G.A., Grahame , D. A. and Mayer, R.M. (1982) Arch. Biochem. Biophys. 216, 751-757). We now show these forms to arise partly as a result of proteolysis, and partly due to incomplete dissociation of the enzyme. Upon 25 degrees C incubation of the crude enzyme, several new bands appeared with little or no change in total activity. The electrophoretic pattern of aged, crude enzyme was similar to that of partially purified enzyme. Specific detection of dextransucrase on SDS gels revealed a single polypeptide of 174 kDa, which is converted to a 156 kDa protein during the aging process. The observation indicates the occurrence of proteolysis. The polypeptide composition of several of the enzyme forms was determined by two-dimensional electrophoresis. Forms Ia and IIa are composed exclusively of 174 kDa polypeptides. Forms III and IVa consist of 156 kDa units, as does the newly observed form Ic. It is likely that form Ib contains both 174 and 156 kDa polypeptides. The results indicate that incomplete dissociation of aggregates of the 174 kDa unit accounts for all of the bands observed on native gels run on fresh culture extracts. Additional enzyme forms result from aggregation of the 156 kDa proteolysis product alone, and from aggregation with unproteolyzed units to form hybrid aggregates.

Stabilization of dextransucrase from Leuconostoc mesenteroides NRRL B-512F by nonionic detergents, poly(ethylene glycol) and high-molecular-weight dextran

Dextransucrase (sucrose: 1,6-alpha-D-glucan 6-alpha-D-glucosyltransferase, EC 2.4.1.5) (3 IU/ml culture supernatant) was obtained by a modification of the method of Robyt and Walseth (Robyt, J.F. and Walseth, T.F. (1979) Carbohydr. Res. 68, 95-111) from a nitrosoguanidine mutant of Leuconostoc mesenteroides NRRL B-512F selected for high dextransucrase production. Dialyzed, concentrated culture supernatant (crude enzyme) was treated with immobilized dextranase (EC 3.2.1.11) and chromatographed on a column of Bio-Gel A-5m. The resulting, purified enzyme lost activity rapidly at 25 degrees C or on manipulation, as did the crude enzyme when diluted below 1 U/ml. Both enzyme preparations could be stabilized by low levels of high-molecular-weight dextran (2 micrograms/ml), poly(ethylene glycol) (e.g., 10 micrograms/ml PEG 20 000), or nonionic detergents (e.g., 10 micrograms/ml Tween 80). The stabilizing capacity of poly(ethylene glycol) and of dextran increased with molecular weight. Calcium had no stabilizing action in the absence of other additions, but reduced the inactivation that occurred in the presence of 0.5% bovine serum albumin or high concentrations (greater than 0.1%) of Triton X-100. In summary, dextransucrase could be stabilized against activity losses caused by heating or by dilution through the addition of low concentrations of nonionic polymers (dextran, PEG 20000, methyl cellulose) or of nonionic detergents at or slightly below their critical micelle concentrations.

Purification and characterization of basic glucosyltransferase from Streptococcus mutans serotype c

Streptococcus mutans Ingbritt (serotype c) was found to secrete basic glucosyltransferase (sucrose: 1,6-alpha-D-glucan 3-alpha and 6-alpha-glucosyltransferase). The enzyme preparation obtained by ethanol fractionation, DEAE Bio-Gel A chromatography, chromatofocusing and preparative isoelectric focusing was composed of three isozymes with slightly different isoelectric points (pI 8.1-8.4). The molecular weight was estimated to be 151000 by SDS-polyacrylamide gel electrophoresis. The specific activity of the enzyme was 9.8 IU per mg of protein and the optimum pH was 6.5. The enzyme was activated 2.4-fold by commercial dextran T10, and had Km values of 7.1 micro M for the dextran and 4.3 mM for sucrose. Glucan was de novo synthesized from sucrose by the enzyme and found to be 1,6-alpha-D-glucan with 17.7% of 1,3,6-branching structure by a gas-liquid chromatography-mass spectroscopy.

D-Glucosyltransferase of Streptococcus mutans: isolation of two forms of the enzyme that bind to insoluble dextran

The D-glucosyltransferase from Streptococcus mutans 6715 has been separated into three enzymic fractions that differ in their binding to dextran and in their synthesis of dextran from sucrose. One enzymic fraction (AFF-I) does not bind to insoluble dextran, and it produces an insoluble D-glucan. Fraction AFF-IIU was eluted from a dextran affinity-column by either dextran or urea, whereas fraction AFF-IID was eluted only by dextran. Both of these fractions produce insoluble D-glucans from sucrose.

Improved purification procedure for the extracellular D-glucosyltransferase from Streptococcus mutans 6715

The exocellular D-glucosyltransferase from Streptococcus mutans 6715 has been highly purified with minimal loss of enzymic activity. The organisms were cultured in trypticase soy-broth that had been treated with invertase and filtered through an ultrafilter fitted with a membrane having a cut-off molecular weight at 10,000. To the growth medium was added Tween 80, which prevented the enzyme from aggregating. The final step in the purification employed insoluble, streptococcal dextran as an affinity support. Two D-glucosyltransferase activities were detected, viz., one that did not adsorb to the insoluble dextran and one that did. The enzymic fraction that had adsorbed to the insoluble dextran in the affinity column was strongly inhibited by added insoluble dextran.