Structural changes and molecular interactions of hydrophobin SC3 in solution and on a hydrophobic surface

The hydrophobin SC3 belongs to a class of small proteins functioning in the growth and development of fungi. Its unique amphipathic property and remarkable surface activity make it interesting not only for biological studies but also for medical and industrial applications. Biophysical studies have revealed that SC3 possesses at least three distinct conformations, named "soluble-state SC3" for the protein in solution, and "alpha-helical-state SC3" and "beta-sheet-state SC3" for the different states of the protein associated at a hydrophobic-water interface. The present fluorescence study shows that the microenvironment of the dansyl-labeled N terminus of soluble-state SC3 is relatively hydrophobic, whereas it is hydrophilic for alpha-helical-state and beta-sheet-state SC3. Fluorescence collisional quenching indicates that the N terminus of soluble-state SC3 is more solvent-accessible than those of alpha-helical-state and beta-sheet-state SC3, with Stern-Volmer constants for acrylamide of 4.63, 0.02, and 0.2 M(-1) for the different states, respectively. Fluorescence resonance energy transfer measurements show that soluble-state SC3 tends to associate in solution but dissociates in TFA. Fluorescence energy transfer was eliminated by conversion of soluble-state SC3 to alpha-helical-state SC3 on a hydrophobic surface, indicating a spatial separation of the molecules in this state. By inducing the beta-sheet state, structural changes were observed, both by CD and by fluorescence, that could be fit to two exponentials with lifetimes of about 10 min and 4 h. Molecules in the beta-sheet state also underwent a slow change in spatial proximity on the hydrophobic surface, as revealed by the reappearance of fluorescence resonance energy transfer in time.