Structural basis of <i>Pseudomonas</i> biofilm-forming functional amyloid FapC formation

Biofilm-protected <i>Pseudomonas aeruginosa</i> causes chronic infections that are difficult to treat. FapC, the major biofilm-forming functional amyloid in <i>Pseudomonas</i>, is essential for biofilm integrity, yet its structural details remain unresolved. Using an integrative structural biology approach, we combine a solution nuclear magnetic resonance-based structural ensemble of unfolded monomeric FapC, a ~3.3-angstrom-resolution cryo-electron microscopy (cryo-EM) density map of FapC fibril, and all-atom molecular dynamics (MD) simulations to capture the transition from the unfolded to folded monomer to the fibrillar fold, providing a complete structural view of FapC biogenesis. Cryo-EM reveals a unique irregular triple-layer β solenoid cross-β fibril composed of a single protofilament. MD simulations initiated from monomeric and fibrillar FapC mapped structural transitions, offering mechanistic insights into amyloid assembly and disassembly. Understanding FapC reveals how <i>Pseudomonas</i> exploits functional amyloids for biofilm formation, and establishes a structural and mechanistic foundation for developing therapeutics targeting biofilm-related infection and antimicrobial resistance.