Intrinsic aggregation propensity of the CsgB nucleator protein is crucial for curli fiber formation

Several organisms exploit the extraordinary physical properties of amyloid fibrils forming natural protective amyloids, in an effort to support complex biological functions. Curli amyloid fibers are a major component of mature biofilms, which are produced by many Enterobacteriaceae species and are responsible, among other functions, for the initial adhesion of bacteria to surfaces or cells. The main axis of curli fibers is formed by a major structural subunit, known as CsgA. CsgA self-assembly is promoted by oligomeric nuclei formed by a minor curli subunit, known as the CsgB nucleator protein. Here, by implementing AMYLPRED2, a consensus prediction method for the identification of ‘aggregation-prone’ regions in protein sequences, developed in our laboratory, we have successfully identified potent amyloidogenic regions of the CsgB subunit. Peptide-analogues corresponding to the predicted ‘aggregation-prone’ segments of CsgB were chemically synthesized and studied, utilizing several biophysical techniques. Our experimental data indicate that these peptides self-assemble in solution, forming fibrils with characteristic amyloidogenic properties. Using comparative modeling techniques, we have developed three-dimensional models of both CsgA and CsgB subunits. Structural analysis revealed that the identified ‘aggregation-prone’ segments may promote gradual polymerization of CsgB. Briefly, our results indicate that the intrinsic self-aggregation propensity of the CsgB subunit, most probably has a pivotal role in initiating the formation of curli amyloid fibers by promoting the self-assembly process of the CsgB nucleator protein.