Involvement of hydrophobic amino acid residues in C7-C8 loop of Aspergillus oryzae hydrophobin RolA in hydrophobic interaction between RolA and a polyester

Hydrophobins are amphipathic secretory proteins with eight conserved cysteine residues and are ubiquitous among filamentous fungi. The Cys3-Cys4 and Cys7-Cys8 loops of hydrophobins are thought to form hydrophobic segments involved in adsorption of hydrophobins on hydrophobic surfaces. When the fungus Aspergillus oryzae is grown in a liquid medium containing the polyester polybutylene succinate-co-adipate (PBSA), A. oryzae produces hydrophobin RolA, which attaches to PBSA. Here, we analyzed the kinetics of RolA adsorption on PBSA by using a PBSA pull-down assay and a quartz crystal microbalance (QCM) with PBSA-coated electrodes. We constructed RolA mutants in which hydrophobic amino acids in the two loops were replaced with serine, and we examined the kinetics of mutant adsorption on PBSA. QCM analysis revealed that mutants with replacements in the Cys7-Cys8 loop had lower affinity than wild-type RolA for PBSA, suggesting that this loop is involved in RolA adsorption on PBSA.