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Infect. Immun. doi:10.1128/IAI.00186-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Amylase-binding protein B (AbpB) of Streptococcus gordonii is an Extracellular Dipeptidyl-Peptidase

Department of Oral Biology, School of Dental Medicine, State University of New York at Buffalo, New York, USA; Department of Periodontics and Endodontics, School of Dental Medicine, State University of New York at Buffalo, New York, USA; Department of Oral Health and Diagnostic Sciences, School of Dental Medicine, University of Connecticut Health Center, Connecticut, USA

^* To whom correspondence should be addressed. Email: fas1{at}buffalo.edu.

	Abstract

The oral commensal bacterium Streptococcus gordonii interacts with salivary amylase via two amylase-binding proteins, AbpA and AbpB. Based on sequence analysis, the 20-kD AbpA protein is unique to S. gordonii, whereas the 82-kDa AbpB protein appears to share sequence homology with other bacterial dipeptidases. The aim of this study was to verify the peptidase activity of AbpB and further explore its potential functions. The abpB gene was cloned, expressed as histidine-tagged (His)-AbpB in Escherichia coli and purified. Its amylase-binding activity was verified in an amylase ligand binding assay and its cross-reactivity with an anti-AbpB antibody. Both recombinant His-AbpB and partially purified native AbpB displayed dipeptidase activity and degraded human type VI collagen and fibrinogen, but not salivary amylase. Salivary amylase precipitates not only AbpA and AbpB, but also glucosyltransferase (Gtf-G) from S. gordonii supernatants. Since Streptococcus mutans also releases Gtf enzymes that could also be involved in multispecies plaque interactions, the effect of S. gordonii AbpB on S. mutans Gtf-B activity was also tested. Salivary amylase and/or His-AbpB caused a 1.4- to 2-fold increase of S. mutans Gtf-B sucrase activity and a 3- to 6- fold increase in transferase activity. Enzyme-linked immunosorbent assay (ELISA) verified the interaction of His-AbpB and amylase with Gtf-B. In summary, AbpB demonstrates proteolytic activity and interacts with and modulates Gtf activity. These activities may help explain the crucial role AbpB appears to play in S. gordonii oral colonization.