Inhibition of Protein Aggregation by the Curli Accessory Proteins CsgE and CsgF

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Illness due to pathogenic bacteria such Salmonella and E. Coli pose a significant public health problem. The Centers for Disease Control estimates that every year Salmonella alone causes one million foodborne illnesses leading to 19,000 hospitalizations, and 380 deaths. Compounds that are able to mitigate the infectivity of pathogenic bacteria, such as Salmonella and E. Coli, could play an important role in our ability to prevent, or at least reduce the incidence of, food borne illness. Curli are a class of cell surface filaments found in Escherichia and Salmonella spp. Curli, comprised mainly of the oligomerized protein subunit CsgA, are thought to facilitate bacterial surface colonization, long-term survival, and bacterial cell-cell association, and appear to play a vital role in host infection. Given its apparent role in bacterial survival, inhibition of CsgA oligomerization to form curli is thought to be a promising avenue for the development of novel agents that reduce bacterial viability and infection. However, our ability to exploit curli biogenesis in the development of novel therapeutic agents is limited due to scant mechanistic information available on curli assembly. The overall goal of our investigations in our laboratory is to determine the mechanism of curli biogenesis. Curli formation is thought to be mediated by at least six different proteins: CsgA, CagB, CsgC, CsgE, CsgF, and CsgG. CsgA and CsgB are the structural subunits of curli with CsgA present as the major, and CsgB the minor, protein components. Although CsgA and CsgB are synthesized in the cytosol, the assembly of these proteins into curli occurs on the cell surface. It has been postulated that two proteins, CsgE and CsgF act as chaperones to prevent the improper aggregation of CsgA and/or CsgB until these proteins are properly localized on the extracellular surface where they form curli. Based on sequence analysis, and circular dichroism, we found that CsgE and CsgF contain significant regions that lack defined secondary structure, and posited that the two proteins may be intrinsically disordered with the disordered regions serving as sites of protein interaction. In the studies proposed herein we aim determine the details of the interaction between the curli assembly chaperon proteins, CsgE and CsgF, and the curli structural proteins CsgA and CsgB. Determining the molecular details of CsgA assembly may help us elucidate the details of curli formation and, in the long-term, aid in the development of novel, fimbriae based, therapies to reduce long-term pathogen survival and persistence.