Involvement of ionic interactions in self-assembly and resultant rodlet formation of class I hydrophobin RolA from Aspergillus oryzae

Hydrophobins are small amphiphilic proteins that are conserved in filamentous fungi. They localized on the conidial surface to make it hydrophobic, which contributes to conidial dispersal in the air, and helps fungi to infect plants and mammals and degrade polymers. Hydrophobins self-assemble and undergo structural transition from the amorphous state to the rodlet (rod-like multimeric structure) state. However, it remains unclear whether the amorphous or rodlet state is biologically functional and what external factors regulate state transition. In this study, we analyzed the self-assembly of hydrophobin RolA of Aspergillus oryzae in detail and identified factors regulating this process. Using atomic force microscopy, we observed RolA rodlet formation over time, and determined "rodlet elongation rate" and "rodlet formation frequency." Changes in these kinetic parameters in response to pH and salt concentration suggest that RolA rodlet formation is regulated by the strength of ionic interactions between RolA molecules.