Results Here we address the question of how the large surface-exposed hydrophobic patch of BslA is stabilized in aqueous environments to mediate the surface activity of the protein. One possible trivial mechanism is the formation of higher order oligomers, such as the decamer observed in the BslA crystal structure, or the tetramer observed for the class II fungal hydrophobin HFB II (15). However, although size exclusion chromatography indicated that purified BslA forms a mixture of monomers, dimers, and higher order oligomers (SI Appendix, Figs. S1 and S2), addition of a reducing agent such as dithiothreitol or β-mercaptoethanol yielded a sample containing purely monomeric protein (SI Appendix, Figs. S2 and S3) stable on the timescale of days. BslA contains two closely spaced cysteine residues, and the role of these residues in biofilm formation and/or maturation is as yet unclear. That said, here we show that the oxidation state of the cysteine residue is not important to the in vitro biophysical properties of the protein; dimeric BslA gives equivalent results to that of monomer if we assume that the concentration of surface-active species comprises 50% of the protein in the sample (i.e., only one subunit of the disulfide-bonded dimer is available for surface activity; SI Appendix, Fig. S4). We have therefore established that the monomeric protein is stable and soluble over a timescale of days, suggesting that the large hydrophobic cap we observed in the crystal structure must be shielded in aqueous solution, likely through some form of structural rearrangement. All experiments presented here used monomeric protein unless otherwise stated.
Results Here we address the question of how the large surface-exposed hydrophobic patch of BslA is stabilized in aqueous environments to mediate the surface activity of the protein. One possible trivial mechanism is the formation of higher order oligomers, such as the decamer observed in the BslA crystal structure, or the tetramer observed for the class II fungal hydrophobin HFB II (15). However, although size exclusion chromatography indicated that purified BslA forms a mixture of monomers, dimers, and higher order oligomers (SI Appendix, Figs. S1 and S2), addition of a reducing agent such as dithiothreitol or β-mercaptoethanol yielded a sample containing purely monomeric protein (SI Appendix, Figs. S2 and S3) stable on the timescale of days. BslA contains two closely spaced cysteine residues, and the role of these residues in biofilm formation and/or maturation is as yet unclear. That said, here we show that the oxidation state of the cysteine residue is not important to the in vitro biophysical properties of the protein; dimeric BslA gives equivalent results to that of monomer if we assume that the concentration of surface-active species comprises 50% of the protein in the sample (i.e., only one subunit of the disulfide-bonded dimer is available for surface activity; SI Appendix, Fig. S4). We have therefore established that the monomeric protein is stable and soluble over a timescale of days, suggesting that the large hydrophobic cap we observed in the crystal structure must be shielded in aqueous solution, likely through some form of structural rearrangement. All experiments presented here used monomeric protein unless otherwise stated.
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