Three-dimensional modelling of the catalytic domain of Streptococcus mutans glucosyltransferase GtfB

Norspermidine changes the basic structure of S. mutans biofilm

The factors regulating the assembly of the three-dimensional structure of Streptococcus mutans biofilms remain obscure. Polyamines are essential in biofilm formation of certain bacteria. Norspermidine, an unusual polyamine, has been a controversial polyamine that can lead to biofilm disassembly. However, the role of norspermidine in S. mutans biofilms remains unknown. Therefore, the present study investigated the impact of norspermidine on S. mutans biofilms. The different architectures of the biofilms in norspermidine and control groups indicated that the basic units, bacteria-exopolysaccharide units (BEUs), represent the exopolysaccharide (EPS) and bacterial assembly pattern in S. mutans biofilms. In addition, norspermidine inhibited S. mutans biofilm formation and changed the basic composition of the biofilm, which led to an unusual EPS architecture. Therefore, 5 mM norspermidine inhibited biofilm formation both by decreasing the rate of cell viability and changing the biofilm structure. Gene-expression microarray analysis indicated that the formation of an irregular architecture in the norspermidine group was potentially attributable to the downregulation of elements of the quorum-sensing system (by 2.7–15-fold). The present study suggested that the BEUs are a basic structure of S. mutans biofilm and its assembly is regulated majorly by the quorum-sensing system. Norspermidine can lead to structure change in BEUs by influencing S. mutans quorum-sensing system.

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 analysis of the functional influence of the surface peptide Gtf-P1 on Streptococcus mutans glucosyltransferase C activity

Glucosyltransferases (GtfB/C/D) in Streptococcus mutans are responsible for synthesizing water-insoluble and water-soluble glucans from sucrose and play very crucial roles in the formation of dental plaque. A monoclonal antibody against a 19-mer peptide fragment named Gtf-P1 was found in GtfC to reduce the enzyme activity to 50%. However, a similar experiment suggested almost unchanged activity in GtfD, despite of the very high sequence homology between the two enzymes. No further details are yet available to elucidate the biochemical mechanism responsible for such discrimination. For a better understanding of the catalytic behavior of these glucosyltransferases, structural and functional analyses were performed. First, the exact epitope was identified to specify the residue(s) required for monoclonal antibody recognition. The results suggest that the discrimination is determined solely by single residue substitution. Second, based on a combined sequence and secondary structure alignment against known crystal structure of segments from closely related proteins, a three-dimensional homology model for GtfC was built. Structural analysis for the region communicating between Gtf-P1 and the catalytic triad revealed the possibility for an "en bloc" movement of hydrophobic residues, which may transduce the functional influence on enzyme activity from the surface of molecule into the proximity of the active site. Figure Side chain interactions between Gtf-P1 and catalytic Asp-477 in GtfC. Calpha-tracing of GtfC with the two crucial peptides (Gtf-P1, orange; Gtf-P2, blue) and the catalytic triad residues ( red) highlighted to show their relative spatial organization. Side chains for the residues are also depicted according to their atom types. The structure is viewed with the barrel opening facing down

Facilitated intranasal induction of mucosal and systemic immunity to mutans streptococcal glucosyltransferase peptide vaccines

Synthetic peptide vaccines which are derived from functional domains of Streptococcus mutans glucosyltransferases (GTF) have been shown to induce protective immunity in Sprague-Dawley rats after subcutaneous injection in the salivary gland region. Since mucosal induction of salivary immunity would be preferable in humans, we explored methods to induce mucosal antibody in the rat to the GTF peptide vaccines HDS and HDS-GLU after intranasal administration. Several methods of facilitation of the immune response were studied: the incorporation of peptides in bioadhesive poly(D,L-lactide-coglycolide) (PLGA) microparticles, the use of monoepitopic (HDS) or diepitopic (HDS-GLU) peptide constructs, or the use of mucosal adjuvants. Salivary immunoglobulin A (IgA) responses were not detected after intranasal administration of diepitopic HDS-GLU peptide constructs in alum or after incorporation into PLGA microparticles. However, significant primary and secondary salivary IgA and serum IgG antibody responses to HDS were induced in all rats when cholera holotoxin (CT) or a detoxified mutant Escherichia coli heat-labile enterotoxin (R192G LT) were intranasally administered with HDS peptide constructs in PLGA. Coadministration of LT with HDS resulted in predominantly IgG2a responses in the serum, while coadministration with CT resulted in significant IgG1 and IgG2a responses to HDS. Serum IgG antibody, which was induced to the HDS peptide construct by coadministration with these adjuvants, also bound intact mutans streptococcal GTF in an enzyme-linked immunosorbent assay and inhibited its enzymatic activity. Thus, immune responses which are potentially protective for dental caries can be induced to peptide-based GTF vaccines after mucosal administration if combined with the CT or LT R192G mucosal adjuvant.