Influence of the macromolecular crowder alginate in the fibrillar organization of the functional amyloid FapC from Pseudomonas aeruginosa

Bacterial biofilms are an alternative lifestyle in which communities of bacteria are embedded in an extracellular matrix manly composed by polysaccharides, nucleic acids and proteins, being the hallmark of bacterial survival in a variety of ecological niches. Amyloid fibrils are one of the proteinaceous components of such extracellular crowded environments. FapC is the main component of the functional amyloid recently discovered in Pseudomonas species, including the opportunistic pathogen P. aeruginosa, which is a major cause of nosocomial infections and contamination of medical devices. Considering that several functional roles have been attributed to this bacterial amyloid, FapC emerged as a novel target to control Pseudomonas biofilm formation and to design new treatments against chronic infections. In this study, we used complementary biophysical techniques to evaluate conformational signatures of FapC amyloids formed in the presence of alginate, the major exopolysaccharide associated with the mucoid phenotype of P. aeruginosa strains isolated from cystic fibrosis patients. We found that the this naturally occurring macromolecular crowder leads to morphological similar yet polymorphic FapC fibrils, highlighting the importance of considering the complexity of the extracellular matrix in order to improve our understanding of microbial functional amyloids.