Amino acid puts the muscle in mussel glue.

The proteins that mussels use to anchor themselves underwater derive their strength and stickiness from a single amino acid, according to a new study. The finding could help researchers develop moisture-resistant glues for biomedical and industrial purposes.

The modified amino acid, called dihydroxyphenylalanine (DOPA), has two functions. It allows the proteins containing it to stick to a variety of surfaces and also to set into a tough, rubbery cement, report Miaoer Yu, Jungyeon Hwang, and Timothy J. Deming of the University of California, Santa Barbara (UCSB).

"The fact that they have identified one [compound] doing those two completely different things is quite intriguing to me," says Herbert Waite, a marine biochemist at UCSB not connected with the current study. "In industry, people generally use different reagents for those two functions."

Mussels, before they end up on a seafood lover's plate, use fibers known as byssal threads to tether themselves (SN: 1/5/91, p. 8). Flat adhesive plaques at the ends of the threads bind to surfaces, allowing the bivalves to hang on even while buffeted by strong waves.

To make the glue, mussels first connect standard amino acids into chains and then modify them chemically, says Waite. DOPA results when an enzyme adds a second hydroxyl group to the amino acid tyrosine. Though not the most abundant amino acid in the adhesive, DOPA makes up 5 to 20 percent of its content.

To investigate DOPA's role, Deming's group synthesized model proteins containing 5 percent DOPA and the rest glutamic acid. These molecules bound much more strongly to aluminum than did proteins containing only glutamic acid.

The researchers also discovered that DOPA, in an oxidized form, is responsible for cross-linking protein strands. "That cross-linking has to kick in in order for that adhesive to perform," says Waite. "Otherwise, it's only bound to the surface, not to itself." Deming and his colleagues report their findings in the June 23 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY.

Unlike industrial adhesives used today, such protein-based glues would be made using water instead of harsh organic solvents. Also, the ability of the mussel adhesive to stick and set in water makes it attractive for medical and dental applications. In animals and laboratory cultures of human cells, the mussel adhesive does not appear to be toxic, Waite notes. Only further studies will reveal how DOPA makes the protein waterproof.