Prerna Gopa, Chandran Ragunath, Vishal Vyas, Mayilvahanan Shanmugam and Narayanan Ramasubbu
Amylase binding protein A (AbpA) of Streptococcus gordonii serves as a major receptor for human salivary α-amylase (HSAmy), the predominant enzyme in human salivary secretions, to bind to the bacterial cell surface. On enamel surfaces, the binding of AbpA to HSAmy renders S. gordonii act as a scaffold for other oral bacteria to attach to the acquired enamel pellicle leading to complex bacterial communities and invasion of host tissues. While the role of AbpA in adhesion, starch metabolism and biofilm formation in influencing the ecology of the oral biofilms has been established, the structure function relationships of AbpA are yet to be defined. Since distally located aromatic residues of HSAmy are involved in the interaction with AbpA, we hypothesized that AbpA might use separate structural regions to bind to HSAmy. To test this, several deletion mutants of AbpA were constructed and studied to correlate the effect of deletions in binding to HSAmy in the following: 1) their secondary structural features through circular dichroism studies; 2) their ability to alter the capacity of HSAmy to hydrolyze starch and several oligosaccharides after complex formation and 3) their binding to HSAmy using surface plasmon resonance spectroscopy. Our results showed that a) AbpA does not bind at the active site of salivary α-amylase; b) both the N-terminal region (24-56 residues) and a central region (residues 124-165) are required for binding to HSAmy; and c) the C-terminal end residues (166- 195) are not necessary for binding. These results clearly show that AbpA binding to HSAmy encompasses distinct and distal regions of the structure.
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