Anyone who has gathered mussels from the seashore will have been impressed by their ability to stick to rocks — even when lashed by waves.
Mussels can fix themselves to almost any organic or inorganic surface using a series of threads. Phillip Messersmith and colleagues (Proc. Natl Acad. Sci. USA doi: 10.1073/pnas0605552103; 2006) now explain the role played in this gripping drama by an unusual amino acid named L-DOPA, which is found in at least five adhesive proteins in the mussel's threads.
Essentially, L-DOPA acts as the glue that sticks the adhesive proteins to surfaces. Using the technique of atomic force microscopy, the authors showed that L-DOPA molecules bind surprisingly strongly to a titanium surface, which is a good model for inorganic surfaces such as rock.
The combined effect of large numbers of these interactions is so great that if a whole mussel protein were pulled away from a surface, the protein itself would break apart before the binding to the surface gave way. These interactions are reversible in ambient conditions, which implies that they cannot be covalent bonds.
In the marine environment, some of the L-DOPA units in the adhesive proteins would become oxidized. Messersmith et al. show that oxidized L-DOPA binds weakly to titanium but apparently forms strong covalent bonds to organic surfaces. The solidification of mussel glues was long suspected to be related to this oxidation process. The double stickiness accounts for the strength of the bonding in natural conditions and also means that L-DOPA might be a suitable adhesive for attaching biological molecules to surfaces — and thus invaluable for medical applications.
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Mitchinson, A. Mussel muscle. Nature 442, 877 (2006). https://doi.org/10.1038/442877b
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