the majority of bacteria in the natural environment love within the confines of biofilm. the gram-positive bacterium bacillus subtilis form biofilms that exhibit a characteristic wrinkled morphology and a highly hydrophobic surface. a critical component in generating these properties is the protein bsia, which forms a coat across the surface of the sessile community . we recently reported the structure of bsia, 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, bsia shares no sequence nor structural similarity, and here we show thar 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 B-sheet. this form then self-assembles into a wellordered 2D rectangular lattic 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 adsoption and disrupted the formation of 2D lattice. this limited structural metamorphosis represent a previously unidentified environmentally responsive mechanism for interfacial stabilization by proteins.