Functional amyloid formation in streptococcus mutans

ABSTRACT
Dental caries, the most common microbial disease, is caused by overgrowth of acidogenic and aciduric bacteria
including Streptococcus mutans. Childhood caries incidence in the U.S. is high and there is a clear imperative to
better understand caries pathogenesis. Cariogenic organisms thrive in biofilm environments. Amyloid was
first identified in the context of pathology but does not always represent a protein mis-folding pathway.
Functional amyloid is also recognized. Amyloid aggregates are evolutionarily conserved cross -sheet
quaternary structures with common biophysical properties enabling their detection and study. Multiple
microorganisms are now known to produce functional amyloids within biofilm environments. Our group was
the first to discover Streptococcus mutans amyloids. We have now identified four amyloid-forming proteins in
this bacterium. Three of these, P1 (AgI/II), WapA, and Cnm are sortase-localized adhesins whose extracellular
truncation derivatives are amyloidogenic. The previously unknown fourth protein, Smu_63, serves as a
negative regulator of biofilm cell density and genetic competence. We have provided extensive tertiary and
quaternary structural characterization of adhesin P1 and structural characterization of other proteins is in
progress. We have provided definitive X-ray fiber diffraction evidence of a classical stacked -sheet amyloid
structure for S. mutans amyloids. Furthermore, our work contributes to a new paradigm for multiple
streptococcal and staphylococcal amyloids. Naturally-occurring truncation products play two key roles within
these organisms' biofilm life cycles. First by promoting adherence to cognate ligands in their monomeric
forms via quaternary interactions with the parent adhesins linked to the cell surface, and second by facilitating
detachment of biofilm cells and extracellular matrix components from aging biofilms in their amyloid form.
The left-handed Z-configuration of extracellular DNA within biofilms was recently associated with biofilm
stability whereas right-handed B-DNA disrupted extant biofilms. The amyloid, but not monomeric form of
neuropathologic A, drives conversion of Z-DNA to B-DNA. Cardiolipin-rich mitochondrial membranes
modulate amyloidogeneis of -synuclein and Htt involved in Parkinson's and Huntingtin Diseases. We have
identified cardiolipin as a prevalent lipid in S. mutans cytoplasmic membranes and extracellular membrane
vesicles. In this renewal application we will explore relationships between S. mutans amyloid-forming
proteins and B- and Z-forms of DNA in vitro and in vivo within adherent and detaching biofilms (Aim 1),
determine the impact of membrane lipid composition on S. mutans amyloid formation within aging biofilms
and assess interactions of amyloidogenic proteins with specific lipids of interest (Aim 2), and continue to use
state of the art methods including solution and solid-state NMR to identify and characterize structural
transitions reflective of monomer to amyloid conversion and determine if amyloid signatures for each protein
are impacted by exposure to different DNA configurations, lipids, or other amyloidogenic proteins (Aim 3).

Grant Number: 2R56DE021789-10A1
NIH Institute/Center: NIH
Principal Investigator: L. Jeannine Brady