In a process reminiscent of Spiderman's wall-clinging abilities, the mussel casts out a bundle of thread-like fibres that are attached to a sticky disc called a plaque. This protein-based structure is collectively called a byssus.
It's been known for a number of years that a key ingredient of mussel glue is dihydroxyphenylalanine (DOPA). To date, at least 10 mussel foot proteins (mfp) that contain different amounts of DOPA have been identified in the byssus.
However, sea water has a pH of eight and contains dissolved oxygen, which compromises DOPA-mediated adhesion.
In their study, which appears in Nature Chemical Biology , Dr Jing Yu and colleagues describe how mussels overcome the adverse effects of their watery surrounds.
Works in water
To do this, the scientists inserted a microelectrode under the mussel's foot and mimicked plaque protein secretion while measuring the pH changes. Within two minutes, the pH had dropped to 5.8 from a resting value of 7.3.
"We always thought that the plaques were forming at the pH of seawater," says Dr Yu. "But we found that the mussels control the local pH when they apply adhesive."
The next step was to look more closely at the interaction between the first two foot proteins that the mussel squirts out: mfp-3 and mfp-6.
By isolating these proteins from the tongues of shucked mussels, Yu and colleagues were able to demonstrate that the acidic pH of 5.5 led to an increase in stickiness of the mfp-3 protein.
Furthermore, the mfp-6 protein, which has comparatively weak adhesion, was found to act as an 'antioxidant' that could restore adhesion in oxidized mfp-3.
Controlling bond strength
"We found that mussels are able to control the redox (reduction oxidation) reaction," says Dr Yu. "The proteins work together and tune their strength by slowly releasing antioxidants."
Dr Yu says there is still a lot to learn about the bonding process in mussels, for example the 3D structure and interaction between all the mfps. He also plans to look at ways we can control the local redox environment, which could lead to the design of better glues.
Dr Helmut Thissen, a principal research scientist at CSIRO Materials Science and Engineering, says there are already a number of synthetic glues that are based on DOPA protein chemistry.
"There are a number of beneficial effects of having an adhesive that works in aqueous environments - they have a wide range of applications," he says.
Applications include glues to attach anti-fouling compounds to underwater pipes or boats, as well as to repair human tissue.
But, Dr Thissen says we don't yet know how long the glues will last.
"The fate of the synthetic DOPA coatings is not clear, but the mussels certainly achieve adhesion for years," he says.
Secret To Mussel's Strength Exposed
US - Mussels' ability to remain steadfast even when being battered by ocean waves is due the unique properties of glue secreted from their feet. Now, scientists at the University of California Santa Barbara have revealed how these remarkable molluscs control their adhesive strength.
