Specific Interaction of a Naturally Occurring Amyloidogenic Fragment of Streptococcus Mutans Adhesin P1 with Intact P1 on the Cell Surface Measured by Solid State Nuclear Magnetic Resonance Spectroscopy

Abstract

The P1 adhesin of the cariogenic bacterium Streptococcus mutans is a cell surface-localized protein involved in sucrose-independent adhesion and colonization of the tooth surface. The immunoreactive and adhesive properties of S. mutans suggest an unusual functional quaternary ultrastructure comprised of intact P1 that is covalently attached to the cell wall interacting with non-covalently associated proteolytic fragments thereof, particularly the ∼50-kDa C-terminal fragment C123 previously identified as Antigen II. S. mutans is capable of amyloid formation when grown in a biofilm and P1 is among its amyloidogenic proteins. The C123 fragment of P1 readily forms amyloid fibers in vitro suggesting it may play a role in the formation of functional amyloid during biofilm development. Using wild-type and P1-deficient strains of S. mutans, we have demonstrated that solid state NMR spectroscopy can be used to 1) globally characterize cell walls isolated from a Gram-positive bacterium; 2) quantify P1 localized on the cell surface; and 3) characterize the specific binding of heterologously expressed, isotopically-enriched C123 to cell wall-anchored P1. Our results lay the groundwork for future high-resolution characterization of the C123/P1 ultrastructure and subsequent adhesion and biofilm formation via NMR spectroscopy, and they support an emerging model of S. mutans colonization whereby quaternary P1-C123 interactions confer adhesive properties important to binding to immobilized human salivary agglutinin.