Tough Slug Glue: How Transition Metal Binding Proteins Cross‐link an Unusual Bioadhesive

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In this study, we characterize the role of transition metals in the structure and function of the adhesive secretion of the slug Arion subfuscus. A deeper understanding of this glue's cross-linking mechanisms is useful because of its potential to guide the design of novel medical adhesives. We hypothesized that transition metal ions, in particular iron, play an important role in cross-linking and thus strengthening the glue. We used immobilized metal ion affinity chromatography to identify iron and zinc binding proteins in the glue, and to determine the pH at which these proteins bind and dissociate. Atomic absorption spectroscopy was also used to measure how tightly zinc was bound to the glue at different pH values. The glue was then subjected to tensile tests at pH values where the proteins were bound to metals, and at pH values where they dissociated, to determine if loss of metal-binding made the glue weaker. An iron-binding protein and several zinc-binding proteins were identified. The zinc was readily removed from the glue as the pH dropped from 6.5 to 4.5, but this had no effect on the glue's strength. In contrast, the iron-binding protein stayed bound across a pH range from 3 to 8, but dissociated from iron at a pH of 9. This dissociation appears to impact the glue's mechanical properties. The precise nature of this effect is still under investigation. These data suggest that iron may be more important to the glue's strength than zinc. This information should be valuable in guiding the design of synthetic glues that can adhere to wet surfaces, and deform with tissues but maintain toughness.