Interfacial self-assembly of a bacterial hydrophobin

Significance In the natural environment the majority of bacteria live within the confines of a structured social community called a biofilm. The stability of biofilms arises from the extracellular matrix, which consists of proteins, polysaccharides, and extracellular DNA. One of these proteins, BslA, forms a hydrophobic “raincoat” at the surface of the biofilm. We have uncovered the mechanism that enables this protein to function, revealing a structural metamorphosis from a form that is stable in water to a structure that prefers the interface where it self-assembles with nanometer precision to form a robust film. Our findings have wide-ranging implications, from the disruption of harmful bacterial biofilms to the generation of nanoscale materials. The majority of bacteria in the natural environment live within the confines of a biofilm. The Gram-positive bacterium Bacillus subtilis forms biofilms that exhibit a characteristic wrinkled morphology and a highly hydrophobic surface. A critical component in generating these properties is the protein BslA, which forms a coat across the surface of the sessile community. We recently reported the structure of BslA, and noted the presence of a large surface-exposed hydrophobic patch. Such surface patches are also observed in the class of surface-active proteins known as hydrophobins, and are thought to mediate their interfacial activity. However, although functionally related to the hydrophobins, BslA shares no sequence nor structural similarity, and here we show that the mechanism of action is also distinct. Specifically, our results suggest that the amino acids making up the large, surface-exposed hydrophobic cap in the crystal structure are shielded in aqueous solution by adopting a random coil conformation, enabling the protein to be soluble and monomeric. At an interface, these cap residues refold, inserting the hydrophobic side chains into the air or oil phase and forming a three-stranded β-sheet. This form then self-assembles into a well-ordered 2D rectangular lattice that stabilizes the interface. By replacing a hydrophobic leucine in the center of the cap with a positively charged lysine, we changed the energetics of adsorption and disrupted the formation of the 2D lattice. This limited structural metamorphosis represents a previously unidentified environmentally responsive mechanism for interfacial stabilization by proteins.