ABSTRACT
Functional amyloid has recently emerged as an underlying strategy for providing mechanical strength to many natural adhesives and cements from both prokaryotic and eukaryotic organisms. Specifically, functional amyloid has been identified in adhesives extruded from marine invertebrate, algae, and as a component of barnacle cement. In addition, cursory evidence exists to suggest that a number of other adhesive systems may also utilize functional amyloid. Atomic force microscopy has allowed nanoscale mechanical responses of functional amyloid fibrils to be made directly from the natural adhesive matrix, and has led to unexpected insights into the function of amyloid in natural materials. The molecular level origin of cohesive strength was found to be associated with the
generic amyloid intermolecular β-sheet structure, while adhesive strength was related to adhesive residues external to the amyloid core. The mechanistic link of amyloid-based cohesive and adhesive strength is now expected to be more widespread amongst natural adhesives than previously thought. These remarkably robust and highly ordered protein assemblies in biological adhesives provide inspiration for the biomimetic development of a new generation of adhesives for use in medicine, biotechnology, and a range of other applications. 8.1 Introduction to Natural AdhesivesNature has evolved extraordinary adhesive mechanisms to promote the attachment of a wide diversity of organisms to many materials. These strategies are essential for successful completion of an organism’s life cycle, thereby ensuring its survival in aquatic or terrestrial habitats. Interest in natural adhesives has primarily focused on organisms from aquatic environments, particularly marine, to develop anti-fouling strategies, or to develop aqueous-based biomimetic adhesives.1,2 These marine adhesives are remarkably effective in attaching to virtually any damp, wet, or submerged surface, with strong, flexible bonds. Sessile marine organisms depend upon tenacious permanent adhesion (e.g., barnacles, mussels, and algae) to deal with shear forces in the ocean, or at the tidal interface, while motile organisms rely instead on strong, but transitory, adhesion to different surfaces (e.g., echinoderms, gastropods, gliding diatoms) to readily enable locomotion.3 Whether temporary or permanent, nature provides many examples of successful attachment to various materials in a moist environment2 (Fig. 8.1). Due to the chemical and structural complexity of biological adhesives and cements, precise mechanisms have been clarified for only a few systems. For example, the strong, permanent attachment of the marine mussel Mytilus edulis has been found to be reliant upon an adhesive with a high content of the modified amino acid 3,4-dihydroxy-l-phenylalanine,4,5 whereas barnacles adhere initially in cyprid larval form via o-quinone cross-linking,6 followed by an adult cement comprising three groups of proteins that contain high levels of serine, threonine, glycine, and alanine.6,7
applications, there are a number of reasons why many biological adhesives are only recently becoming better understood. Although some organisms secrete a relatively large quantity of extracellular polymeric substances to adhere, there are many smaller attachment organisms (e.g., protists, fungi, and microalgae) which only secrete miniscule amounts of complex adhesive. Thus only very small amounts are available which can be isolated for biochemical analyses. This poses an enormous challenge for the identification and characterization of specific adhesive polymers, and further difficulties arise due to their high insolubility and rapid curing. Overcoming these challenges signals the turning point for extracting the exact working mechanisms and developing a full understanding of biological adhesives and cements. Significant progress can be made by approaching the field in an interdisciplinary way, combining a knowledge of ecology to identify potentially useful model organisms with biochemistry expertise to analyse the adhesives and cements, and mechanics to interpret their structure and material properties. By using modern methods such as the tools of nanotechnology for studying the molecular mechanisms of adhesives, we have an opportunity to build upon the basic biochemical and mechanical
principles involved in adhesion.8 As research into natural adhesives and cements progresses, similarities in underlying principles have begun to emerge, while concurrently more variations in the details becomes clear. This has provided some surprising insights into an unexpected commonality amongst many adhesives and cements, as well as emphasizing the multitude of ways that adhesion can be achieved. This chapter focuses on the identification and mechanical properties of one such structural commonality between otherwise unconnected organisms: Functional amyloid. 8.2 Amyloid in Natural Adhesives
The previous chapter (Chapter 7) describes the detection of amyloid amongst prokaryotes, where Escherichia coli was shown to produce biofilm-associated amyloid fibrils known as curli.9 A specific functionality was not identified for curli at that time. Curli was described as a surface protein polymer that mediated interactions important for biofilm formation, host cell colonization, adhesion, and cell aggregation.9,10 Several other bacteria (e.g., Salmonella) have since been found to produce other families of amyloid fibrils (see review of Otzen and Nielsen, 2007),11 and it is believed that multiple roles exist for amyloid as its widespread occurrence was discovered in further bacterial species from different habitats.12,13 Functional amyloid was not identified in natural adhesives until 2006 when a small, green algae, Prasiola linearis, was shown to produce amyloid in the adhesive matrix associated with the holdfast region that attaches permanently to surfaces in coastal lagoons and saltmarshes.14 This was the first report of an amyloid-based biological adhesive and a specific mechanical functionality was associated with the amyloid component.14 A mechanical response was proposed relative to the molecular structure of the generic amyloid fibril form found in natural adhesives using atomic force microscopy (AFM). The following year, amyloid was found in the adhesive of another algal species, the filamentous terrestrial green algae Klebsormidium flaccidum.15 Additional characteristics of benefit to the mechanical properties of the adhesive were revealed, leading to the suggestion that amyloid fibrils may provide a generic mechanism for mechanical strength in natural adhesives.
