The mussels that cling obstinately to seaside rocks and piers use hundreds of strong but rubbery hairs, called byssal threads, to strengthen their grip. In the latest issue of Science, researchers reveal how these remarkable threads help the mussels stand the battering of waves: They are a hybrid of the inflexible molecule collagen, which provides strength, and more flexible molecules that provide resilience. This composition could someday help scientists build replacement tendons for people that would be stronger and more flexible than the original.
Interest in the byssus goes back a long way. The ancient Greeks wove byssal threads into fabrics because they combined a satiny texture with strength and durability. Modern researchers have been intrigued by the threads' elasticity. They can be stretched to double their original length without breaking, even though they consist mostly of collagen, which can normally stretch by only 10% of its length before it snaps. J. Herbert Waite and his colleagues at the University of Delaware in Newark began trying to pick apart the structure by figuring out how the mussel makes its wonder threads.
To learn which genes in the mussel make the byssal threads, the researchers first treated the threads with the enzyme pepsin to liquefy their interlocking networks of proteins. Next, they created antibodies that would recognize the proteins in the threads and searched through a mussel gene and protein library to pick out the responsible genes. The genes for the threads turned out to code for a mixture of two proteins. One, called Col-P, is identical to the triple-stranded twist of a collagen protein, except that the ends are replaced with more stretchable protein similar to elastin. The other protein, Col-D, was like Col-P, but the elastic ends were replaced with less rubbery sections.
The structure of Col-P is exciting, says structural biologist Jürgen Engel from the University of Basel in Switzerland, because it's the first example of collagen being combined with more flexible elastinlike structures in the same molecule. However, Engel says the group needs to confirm the structure by actually synthesizing the elastinlike section from the genes. Waite hopes the byssus structures could reveal a successful formula for a stronger, more flexible tendon to serve as a prosthesis in humans. "They might be good models for what we need to imitate," he says.