One of the reasons why mussels often dominate wave-swept rocky intertidal zones is that they can securely attach themselves to rocks where food is abundant, and predators and competitors are simply swept away. Mussels manage to hang on where others can't via their byssal apparatus, which is a collection of stretchy and tough connective tissue threads that mussels use to glue themselves to the substrate. One particularly important aspect of byssal thread mechanics is their ability to nearly double in length without breaking. This allows for the recruitment of other byssal threads into tension, which increases the ultimate strength of the entire byssal apparatus. While much is known about the mechanical properties of byssal threads and the fibrous proteins that make them up, little is known about how the structure of the byssal thread proteins gives rise to the macroscopic threads' mechanical properties. In a recent paper in Biomacromolecules, Tue Hassenkam and colleagues report the results of their attempts to understand the stretchiness of byssal threads at the molecular level using a technique called atomic force microscopy, or AFM. AFM is a unique microscopy technique in that it constructs images with near atomic resolution by literally tracing the surface of a sample via direct contact with an ultra-fine stylus.
Byssal threads consist primarily of fibrous proteins made up of a long,central collagen-like domain that is flanked by globular terminal domains. These proteins form collagen-like triple helix trimers that are mostly straight, but that also possess a prominent kink due to a disruption of the collagen repeat. Previous work using transmission electron microscopy has shown that these trimers associate in the byssal gland into higher-order rod-like structures, termed `mesogens,' which consist of seven trimers. Mesogens then self-assemble into liquid crystalline arrays that are eventually locked into place via a poorly understood cross-linking process. Molecular models predict that the mesogens can adopt either a `flower' configuration, in which the kinked ends flay out in different directions, or a more compact`banana' configuration in which the kinked ends all point in the same general direction. Hassenkam hoped to distinguish which configuration mesogens adopt in the byssal apparatus using AFM, and by imaging threads stretched and held at a strain of 100%, he wished to find out which structures are responsible for the impressive extensibility of byssal threads.
AFM proved to be useful for answering all of these questions. The team only found mesogens in the banana form, joined end to end to form liquid crystalline arrays resembling crimped sheets of paper. They also found that stretching the threads correlated with three major structural changes that could account for the extensibility of the threads at the molecular scale. First, the banana-shaped mesogens straightened out, resulting in a flattening of the crimps in the liquid crystal arrays. Second, the mesogens adopted an orientation more in line with the axis of the thread, whereas before they had been off to an angle. And thirdly, the globular terminal domains that link the individual proteins end-to-end lengthened. Together, these results provide a satisfying picture of the structure of byssal thread proteins at several levels of organization, as well as how those structures contribute to the ability of the threads to stretch as far as they do without breaking. By probing the mechanics of byssal threads at the molecular scale, Hassenkam and colleagues are beginning to reveal some of the essential details of how mussels pull off not getting pulled off.