Unzipping a Functional Microbial Amyloid

Abstract

Bacterial and fungal species produce some of the best-characterized functional amyloids, i.e. extracellular fibres that play key roles in mediating adhesion and biofilm formation. Yet, the molecular details underlying their mechanical strength remain poorly understood. Here, we use single-molecule atomic force microscopy to measure the mechanical properties of amyloids formed by Als cell adhesion proteins from the pathogen Candida albicans. We show that stretching Als proteins through their amyloid sequence yields characteristic force signatures corresponding to the mechanical unzipping of β-sheet interactions formed between surfacearrayed Als proteins. The unzipping probability increases with contact time, reflecting the time necessary for optimal inter β-strand associations. These results demonstrate that amyloid interactions provide cohesive strength to a major adhesion protein from a microbial pathogen, thereby strengthening cell adhesion. We suggest that such functional amyloids may represent a generic mechanism for providing mechanical strength to cell adhesion proteins. In nanotechnology, these single-molecule manipulation experiments provide new opportunities to understand the molecular mechanisms driving the cohesion of functional amyloid-based nanostructures.

Adhesins in Human Fungal Pathogens: Glue with Plenty of Stick

“…The formation of amyloid clusters could thus explain why Als proteins exhibit weak binding to many ligands but mediate strong adherence. These data provided evidence that Als-mediated adhesion largely depends on conformational modifications of existing adhesins rather than or in addition to signal transduction and expression of new adhesin molecules (92).…”

Amyloid Structures as Biofilm Matrix Scaffolds

“…To start cell adhesion, Als proteins undergo conformational changes, accompanied by surface protein clustering, that spread around the entire surface of the cells (73,75). These adhesin clusters, called nanodomains, are force induced (76,77) and are composed of adhesion molecules aggregated through side chain amyloid-like interactions on the cell surface (74,78). This clustering is possible even though Als adhesins are anchored to the cell wall polysaccharide, since it is facilitated by the length and conformational flexibility of their extended molecules (73,74).…”

Deciphering the scaling of single-molecule interactions using Jarzynski’s equality

“…Single-molecule atomic force microscope experiments (SM-AFM) are now well established to measure unbinding/binding dynamics of protein unfolding 7,8 and specific and non-specific surface binding of single macromolecules 9,10 . Complementary surface forces apparatus (SFA) experiments have been extensively used to study equilibrated thermodynamic interaction forces at macroscopic contacts between surfaces providing specific interaction sites [11][12][13][14] .…”