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Contributed by Nancy Pierce on 01/24/2010
Andrew Smith (Biology) presented "Cross-linking in slug glue: gelled plaster of Paris?”, co-authored with Meghan Menges (Biology ’10). Society for Integrative and Comparative Biology. Seattle, WA. January 2010.
The defensive glue of the slug Arion subfuscus sets rapidly into a sticky, elastic mass. There appear to be several cross-linking mechanisms, but the initial gelation may occur through complex coacervation involving sulfate- and calcium-binding polymers. In this mechanism, electrostatic forces bring together charged polymers creating locally high concentrations. These may cross-link to create a reticular network. Calcium and sulfate are particularly interesting, as their interaction causes setting in plaster of Paris. Slug glue was shown to contain a strikingly high concentration of calcium (40 mM) as measured by atomic absorption spectrometry and energy dispersive SEM. It also contains a comparable amount of sulfate (40-50 mM) as measured by a colorimetric assay and SEM. The sulfate is likely bound to polysaccharides, while several of the proteins bind strongly to metals. The sulfate would create a high negative charge density, which would be neutralized by the calcium leading to coacervation, drawing metal-binding proteins and sulfated polysaccharides together. Several assays were developed to determine if any proteins in the glue bound to sulfate in this way. These assays identified a 15 kDa protein that was known to be unique to the glue and has been shown to stiffen gels. This protein precipitated with sulfated sugars, but only in the presence of metals. Furthermore, it bound to sulfate groups in column chromatography when metals were present. Chelating the metals often blocked this binding. Thus, there is a specific metal-based interaction between the primary cross-linking protein and sulfate groups. The glue is not soluble in acid, however, and the calcium is not tightly bound to the glue, suggesting that a strong solubility-based interaction, as seen in plaster, does not occur.