Self-assembly of Class II Hydrophobins on Polar Surfaces

Abstract

Hydrophobins are structural proteins produced by filamentous fungi that are amphiphilic and function through self-assembling into structures such as membranes. They have diverse roles in the growth and development of fungi, for example in adhesion to substrates, for reducing surface tension to allow aerial growth, in forming protective coatings on spores and other structures. Hydrophobin membranes at the air-water interface and on hydrophobic solids are well studied, but understanding how hydrophobins can bind to a polar surface to make it more hydrophobic has remained unresolved. Here we have studied different class II hydrophobins for their ability to bind to polar surfaces that were immersed in buffer solution. We show here that the binding under some conditions results in a significant increase of water contact angle (WCA) on some surfaces. The highest contact angles were obtained on cationic surfaces where the hydrophobin HFBI has an average WCA of 62.6° at pH 9.0, HFBII an average of 69.0° at pH 8.0, and HFBIII had an average WCA of 61.9° at pH 8.0. The binding of the hydrophobins to the positively charged surface was shown to depend on both pH and ionic strength. The results are significant for understanding the mechanism for formation of structures such as the surface of mycelia or fungal spore coatings as well as for possible technical applications.

Two Novel Class II Hydrophobins from Trichoderma spp. Stimulate Enzymatic Hydrolysis of Poly(Ethylene Terephthalate) when Expressed as Fusion Proteins

“…Also, several class II hydrophobins are characterized by long N-terminal hydrophilic (GNrich) extensions (19), suggesting that the core hydrophobin structure is not affected by the length of a hydrophilic N terminus. In line with these findings, a number of proteins have been expressed as fusions to the N terminus of hydrophobins without causing an effect on the binding properties of the latter (40,52). Although the conjugation to GST and intracellular production in E. coli is unlikely to be scaled-up for the industrial level, these findings nevertheless suggest that the fusion of other hydrophilic N termini containing secretion signals to HFB4 or HFB7 may render them to be secreted in a soluble form by respective producer organisms.…”

“…Although our binding studies were conducted under the conditions subsequently used for the cutinase assays and thus do not fully resemble the optimal conditions used to study surface binding of other hydrophobins, it is of interest to compare the behavior of HFB4 and HFB7 to that of the better-known T. reesei hydrophobins HFB1 and HFB2: whereas HFB1 stably immobilizes fusion proteins to hydrophobic surfaces, retaining the activity of the fusion partner, HFB2 has a fast off-rate that does not allow an efficient immobilization to the same surfaces (53), whereas both HFBs bind well to polar surfaces (52). In this regard, HFB4 resembles HFB1 by binding well to both types of surfaces, whereas HFB7-unlike both HFB1 and HFB2-seems to lack affinity to polar surfaces.…”

Hydrophobin as a Nanolayer Primer That Enables the Fluorinated Coating of Poorly Reactive Polymer Surfaces

“…HFBII features fi ve positively charged amino acids on the exposed hydrophilic side, [ 21,36,37 ] and its isoelectric point (pI) was measured to be 5.8 (see Figure S1 and Table S1 of the Supporting Information). Therefore, the hypothesized interaction between HFBII and F10 was expected to be electrostatic and pH dependent.…”

Enhanced Cutinase-Catalyzed Hydrolysis of Polyethylene Terephthalate by Covalent Fusion to Hydrophobins

“…However, the mechanisms for the assembly of hydrophobins on hydrophilic surfaces are not yet fully understood (29). It was therefore rational to assume that the stimulatory effect of hydrophobins on PET hydrolysis by cutinase is due to the creation of a hydrophobic surface that targets the enzyme to its substrate.…”