A novel glucan-binding protein with lipase activity from the oral pathogen Streptococcus mutans

Abstract

Streptococcus mutansproduces extracellular glucosyltransferases (GTFs) that synthesize glucans from sucrose. These glucans are important in determining the permeability properties and adhesiveness of dental plaque. GTFs and the GbpA glucan-binding protein are characterized by a binding domain containing a series of 33-amino-acid repeats, called ‘A’ repeats. TheS. mutansgenome sequence was searched for ORFs containing ‘A’ repeats, and one novel gene,gbpD, which appears to be unique to the mutans group of streptococci, was identified. The GbpD sequence revealed the presence of three ‘A’ repeats, in the middle of the protein, and a novel glucan-binding assay showed that GbpD binds to dextran with aKDof 2–3 nM. Construction of truncated derivatives of GbpD confirmed that the ‘A’ repeat region was essential for binding. Furthermore, agbpDknockout mutant was modified in the extent of aggregation induced by polymers derived from sucrose. The N-terminus of GbpD has a signal sequence, followed by a region with no homologues in the public databases, while the C-terminus has homology to theα/βhydrolase family (including lipases and carboxylesterases). GbpD contains the two regions typical of these enzymes: a GxSxG active site ‘lipase box’ and an ‘oxyanion hole’. GbpD released free fatty acids (FFAs) from a range of triglycerides in the presence of calcium, indicating a lipase activity. The glucan binding/lipase bifunctionality suggested the natural substrate for the enzyme may be a surface macromolecule consisting of carbohydrate linked to lipid. ThegbpDmutant was less hydrophobic than wild-type and pure recombinant GbpD reduced the hydrophobicity ofS. mutansand another plaque bacterium,Streptococcus sanguinis. GbpD bound to and released FFA from lipoteichoic acid (LTA) ofS. sanguinis, but had no effect on LTA fromS. mutans. These results raise the intriguing possibility that GbpD may be involved in direct interspecies competition within the plaque biofilm.