SMITH, A.M.: The use of mucous gels as glues

Many marine animals secrete visco-elastic, mucus-based adhesives. These adhesive gels form attachments that can approach the strength of barnacle and mussel glues, yet they are temporary. The ability to form such strong, temporary attachments with a gel containing more than 95% water makes these adhesives a promising biomaterial for further study. Recent research on adhesive forms of molluscan mucus has changed our understanding of these gels. Limpet mucus differs substantially from the structural paradigm established for vertebrate mucus. Rather than being built from megadalton-sized protein-polysaccharide complexes, the key constituents of limpet mucus are relatively small proteins ranging from 20 to 200 kD. Chromatography experiments suggest that these proteins form a gel by cross-linking into large aggregates via non-covalent bonds. This also differs from mammalian mucus, which depends solely on tangling interactions between the giant subunits. Like limpet mucus, periwinkle adhesive mucus also depends heavily on relatively small proteins. These are critical differences from mammalian mucus, and their impact on function needs further study. Another key finding is that, in both limpets and periwinkles, normal mucus does not seem inherently adhesive; instead, the process of forming an adhesive bond involves substantial modification of the gel. Limpet mucus becomes adhesive when a 118 kD protein is added to the gel network. Periwinkle mucus becomes adhesive when a 64 kD protein is added to a weak carbohydrate-based gel. The putative adhesive proteins share significant similarities; they both have isoelectric points around 5, and have a similar amino acid composition. It is likely that many other animals adhere using a similar pattern.