Many organisms fabricate protein fibers exhibiting interesting combination of lightweight and high mechanical properties defined by the hierarchical organization of their diverse protein building blocks. For example the highly ordered collagen protein of Mytilus mussel byssus, a set of holdfast fibers that helps mussels survive strong marine currents, achieve self-healing and energy dissipation. However, other bivalves use different protein building blocks to make byssus: fan shell mussels Pinnidae have been observed to produce a non-collagenous byssus, and their distant relatives, the pearl oysters Pteriidae possess a byssus that is made out of globular proteins. Their nanostructure remains so far unknown, but preliminary evidence suggests that these two mussel families living in calm water share a common byssus protein structure that is very different from those of other investigated species (e.g. Mytilus spp.), which survive in wave-beaten habitats.
In order to gain insights into the biomolecular organisation of the byssus of Pinnidae and Pteriidae mussels, we employed a range of analytical techniques (EM, XRD, FTIR, Raman spectroscopy, AAA, ICP) to investigate the structure and composition of the byssus of two fan mussels, Pinna nobilis and Atrina pectinata, and two pearl oysters Pinctada margaritifera and Pinctada fucata. Our findings suggest that threads are comprised of a highly unusual helical organization of globular proteins, which differs not only from Mytilus byssus, but from any extra-organismic protein fiber studied thus far and seems more reminiscent of intracellular filamentous proteins. Moreover, the different species assemble these helical building blocks in a slightly different way on the micro- and macro scale, which might be related to their different anchoring strategies; fan shell mussels being buried in sandy area while pearl oysters stick to hard surfaces. Future investigations will attempt to understand the assembly of the globular protein into helical structures, and of these protein helixes into larger structures; however, it is clear in any case that the fan mussel and pearl oyster byssus represents a new paradigm in protein-based materials.