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Research Article

Interfacial self-assembly of a bacterial hydrophobin

Keith M. Bromley, Ryan J. Morris, Laura Hobley, Giovanni Brandani, Rachel M. C. Gillespie, Matthew McCluskey, Ulrich Zachariae, Davide Marenduzzo, Nicola R. Stanley-Wall, and Cait. E. MacPhee
PNAS April 28, 2015 112 (17) 5419-5424; first published April 13, 2015; https://doi.org/10.1073/pnas.1419016112
Keith M. Bromley
aSchool of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom;
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Ryan J. Morris
aSchool of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom;
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Laura Hobley
bDivision of Molecular Microbiology, College of Life Sciences, University of Dundee, Dundee DD1 5EH, United Kingdom; and
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Giovanni Brandani
aSchool of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom;
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Rachel M. C. Gillespie
bDivision of Molecular Microbiology, College of Life Sciences, University of Dundee, Dundee DD1 5EH, United Kingdom; and
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Matthew McCluskey
aSchool of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom;
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Ulrich Zachariae
cDivision of Computational Biology, College of Life Sciences, University of Dundee, Dundee DD1 5EH, United Kingdom
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Davide Marenduzzo
aSchool of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom;
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Nicola R. Stanley-Wall
bDivision of Molecular Microbiology, College of Life Sciences, University of Dundee, Dundee DD1 5EH, United Kingdom; and
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  • For correspondence: cait.macphee@ed.ac.uk n.r.stanleywall@dundee.ac.uk
Cait. E. MacPhee
aSchool of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom;
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  • For correspondence: cait.macphee@ed.ac.uk n.r.stanleywall@dundee.ac.uk
  1. Edited by Gregory A. Petsko, Weill Cornell Medical College, New York, NY, and approved March 24, 2015 (received for review October 2, 2014)

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Significance

In the natural environment the majority of bacteria live within the confines of a structured social community called a biofilm. The stability of biofilms arises from the extracellular matrix, which consists of proteins, polysaccharides, and extracellular DNA. One of these proteins, BslA, forms a hydrophobic “raincoat” at the surface of the biofilm. We have uncovered the mechanism that enables this protein to function, revealing a structural metamorphosis from a form that is stable in water to a structure that prefers the interface where it self-assembles with nanometer precision to form a robust film. Our findings have wide-ranging implications, from the disruption of harmful bacterial biofilms to the generation of nanoscale materials.

Abstract

The majority of bacteria in the natural environment live within the confines of a biofilm. The Gram-positive bacterium Bacillus subtilis forms biofilms that exhibit a characteristic wrinkled morphology and a highly hydrophobic surface. A critical component in generating these properties is the protein BslA, which forms a coat across the surface of the sessile community. We recently reported the structure of BslA, and noted the presence of a large surface-exposed hydrophobic patch. Such surface patches are also observed in the class of surface-active proteins known as hydrophobins, and are thought to mediate their interfacial activity. However, although functionally related to the hydrophobins, BslA shares no sequence nor structural similarity, and here we show that the mechanism of action is also distinct. Specifically, our results suggest that the amino acids making up the large, surface-exposed hydrophobic cap in the crystal structure are shielded in aqueous solution by adopting a random coil conformation, enabling the protein to be soluble and monomeric. At an interface, these cap residues refold, inserting the hydrophobic side chains into the air or oil phase and forming a three-stranded β-sheet. This form then self-assembles into a well-ordered 2D rectangular lattice that stabilizes the interface. By replacing a hydrophobic leucine in the center of the cap with a positively charged lysine, we changed the energetics of adsorption and disrupted the formation of the 2D lattice. This limited structural metamorphosis represents a previously unidentified environmentally responsive mechanism for interfacial stabilization by proteins.

  • BslA
  • interfacial self-assembly
  • bacterial hydrophobin
  • Bacillus subtilis
  • biofilm

Footnotes

  • ↵1To whom correspondence may be addressed. Email: cait.macphee{at}ed.ac.uk or n.r.stanleywall{at}dundee.ac.uk.
  • Author contributions: K.M.B., R.J.M., L.H., G.B., U.Z., D.M., N.R.S.-W., and C.E.M. designed research; K.M.B., R.J.M., L.H., G.B., R.M.C.G., M.M., D.M., and C.E.M. performed research; L.H., G.B., and N.R.S.-W. contributed new reagents/analytic tools; K.M.B., R.J.M., L.H., G.B., R.M.C.G., M.M., U.Z., D.M., and N.R.S.-W. analyzed data; and K.M.B., R.J.M., L.H., N.R.S.-W., and C.E.M. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1419016112/-/DCSupplemental.

Freely available online through the PNAS open access option.

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Interfacial self-assembly of BslA
Keith M. Bromley, Ryan J. Morris, Laura Hobley, Giovanni Brandani, Rachel M. C. Gillespie, Matthew McCluskey, Ulrich Zachariae, Davide Marenduzzo, Nicola R. Stanley-Wall, Cait. E. MacPhee
Proceedings of the National Academy of Sciences Apr 2015, 112 (17) 5419-5424; DOI: 10.1073/pnas.1419016112

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Interfacial self-assembly of BslA
Keith M. Bromley, Ryan J. Morris, Laura Hobley, Giovanni Brandani, Rachel M. C. Gillespie, Matthew McCluskey, Ulrich Zachariae, Davide Marenduzzo, Nicola R. Stanley-Wall, Cait. E. MacPhee
Proceedings of the National Academy of Sciences Apr 2015, 112 (17) 5419-5424; DOI: 10.1073/pnas.1419016112
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