Sticky, Tough, Fast-setting Glue: How Metal Ions Make Arion subfuscus Mucus into a Potent Defensive Secretion

The defensive secretion produced by the slug Arion subfuscus is an unusual hydrogel. It is remarkably sticky and tough, and it cross-links and sets within seconds. It gains most of these mechanical properties directly or indirectly through the action of metal ions. There are abundant metal ions in the glue and most of the proteins in the glue are known or predicted to be metal-binding proteins. A model of how metal ions interact with the components of slug glue to achieve its properties is presented. In this model, abundant calcium and magnesium ions bring together major components of the glue and maintain the polyanions in a compact form. These ions in addition to iron might also contribute to linking the bulk of the cross-linked glue to the priming layer of adhesive proteins. The glue itself appears to be cross-linked by a metal-catalyzed oxidation. This oxidation creates carbonyl groups that would react with ligands on other proteins to form the main cross-links that stiffen the glue. The high abundance of the heme enzyme catalase is intriguing in the context of a metal-catalyzed oxidation.