Watching a coastal winter storm, no one can fail to marvel at the persistence of mussels battered by the waves. Tethered to the rocks by robust fibres known as byssal threads, scientists have long been intrigued by the fibre's mechanical properties. The thread's major component is a form of collagen, similar to that found in muscle tendon, but at strains where muscle tendons fail, mussel byssal threads just keep on stretching, easily doubling their length. And more remarkably, a few hours after yielding the stretchy threads self-heal, regaining most of their original stiffness. Intrigued, J. Herbert Waite and student Matthew Harrington began investigating the byssal thread's collagen like proteins, focusing in particular on preCol D to find out how it contributes to the thread's unique performance(p. 4307).
Harrington explains that preCol D is the main component of the thread nearest the end anchored to the rocks. Each PreCol D protein comprises several subdomains, and suspecting that the individual domains contributed different properties to the thread's remarkable sturdyness, Waite and Harrington decided to compare preCol D proteins from Mytilus edulis, M. galloprovincialis and M. californianus to see if they could learn more about each domains' functions.
Having sequenced the three M. californianus preCols, Harrington compared them with the preCols from the other two species. The collagen domains were essentially identical across all three species, but the sequences from some of the flanking domains differed dramatically.
Harrington explains that one of the PreCol D domains is very similar to another superprotein: spider silk. Spider silk is incredibly stiff, and when Harrington aligned the amino acid sequences of the silk-like domain from the three species, he found that the californianus protein had gained an extra stretch of amino acids, similar to the insertions found in the stiffest spider silks, which could explain why californianus byssal threads are so much stiffer than the other two. `Mussels have taken a page from the spiders' book' says Harrington `using the same types of insertion to stiffen the thread'.
Shifting focus to another small PreCol D domain, peppered with histidine amino acids, the pair realised that the positions of the histidines in the domain were essentially the same across all three species: `we thought the histidine residues must be important' says Harrington. As histidine amino acids coordinate metal ions, the team wondered whether the histidines from neighbouring preCols could cross-link with each other via metal ions, adding to the stiffness of the thread. Harrington began testing, altering the histidine sidechains' ability to form cross-links and measuring the impact on the thread's stiffness. Sure enough the byssal thread's stiffness varied as the team altered the histidine residues, confirming that the histidine-rich domain probably contributes to the thread's stiffness by cross-linking.
The pair also suspect that the histidine domain contributes to the thread's self-healing. Harrington explains that when collagen in tendon ruptures,strong covalent links in protein chains have been ripped apart; they cannot reform, so the tendon is permanently damaged. But when byssal threads are stretched to breaking point, the team suspect that the weaker histidine crosslinks fail, reforming once the storm has passed `giving the thread the ability to yield and give' says Harrington.