N-terminal intrinsically disordered region mediates self-catalytic interfacial nucleation of Aspergillus oryzae hydrophobin RolA
Hydrophobins are biosurfactant proteins that coat the cell surfaces of filamentous fungi. On the conidial surface, hydrophobins self-assemble into rodlets, forming a dense hydrophobic film that promotes air-dispersibility. Although rodlet formation is closely associated with the physiology of filamentous fungi, its underlying molecular mechanisms remain largely unknown. Previously, we revealed that RolA, a hydrophobin derived from Aspergillus oryzae, forms rodlets at the air-water interface. In this study, we focused on the flexible N-terminal region of RolA, which lacks a well-defined tertiary structure, and hypothesized that this intrinsically disordered region regulates rodlet formation. To investigate its role, we used RolA mutants with reduced charges in the N-terminal region and analyzed the rodlet formation process on the surface of a water-in-air sessile droplet using atomic force microscopy. In addition, we quantitatively characterized rodlet formation at the air-water interface by applying a kinetic perspective to the interfacial tension change profiles obtained from dynamic surface tension measurements. The results suggested that RolA first forms a monolayer at the air-water interface, then rodlet formation proceeds through the continuous supply of free RolA monomers from the bulk phase to the interfacial RolA film. Our molecular dynamics simulations of RolA at the interface supported a model in which RolA molecules within the interfacial film interact with free monomers in the bulk phase through their N-terminal regions. These results reveal a previously unidentified role of the N-terminal region in rodlet formation and provide a more comprehensive framework for understanding the molecular mechanism underlying RolA rodlet formation.