Watch a sandcastle worm building its marine home and you will see the busy little builders use a curious cement to construct their underwater castle. This incredible adhesive is strong enough to withstand the constant battering of waves in the inter-tidal zone where the worms live. The worm waves its tentacles in the water to capture sand and shell fragments, which it sweeps to the building organ near its mouth. The worm sizes up each particle, holding and turning them over with its tentacles. When the worm is satisfied, it carefully dabs the particle with adhesive from its cement glands and presses it into place in its tube home. Intrigued by this glue that works under water,Herbert Waite has spent the past decade working out the intricate details of its structure and composition. His team's latest findings have led to a new model suggesting how the adhesive mechanism might work(p. 4727).
To unlock the secrets of sandcastle worms' cement, Waite and colleagues at the University of California, Santa Barbara, needed worms to supply cement. Luckily, sandcastle worms live off the Californian coast, so the team could collect worms right on their doorstep. The worms quickly adjusted to life in the lab, happily building tube homes using tiny glass beads provided in their new habitat. The team harvested worm cement by collecting these newly constructed homes and prying cement disks off the glass beads. They were now ready to examine the structure of the cement using a scanning electron microscope, and were surprised to see that the cement had a foam-like structure. They decided to analyse the composition of the cement using spectrometry and amino acid analysis, and found that the cement's major component was an acidic phosphoserine-rich protein. `We already knew that the cement was rich in the amino acid serine' says Waite, `but we didn't expect to find that most of the serine in the cement is phosphorylated.'
Using their new knowledge of the structure and composition of the cement,the team are now in a position to explain the glue's adhesive mechanism. Waite explains that the worm mixes positively charged cement-precursor proteins and negatively charged phosphoserine-rich proteins in its cement glands, in a ratio that neutralizes the charges. When oppositely charged macromolecules are mixed in this way, a process known as complex coacervation happens: the solution separates into a dense liquid phase and a less dense liquid phase.`The worm stockpiles the dense liquid until it finds a suitable particle'Waite explains, `then applies a sticky kiss of the liquid to the particle before pressing it into place'. The cement droplets then harden to form a solid cement disk.
To explain the origin of the cement's foamy structure, Waite suggests that water released during the coacervation process might be trapped in little pockets or vacuoles in the dense liquid. He points out that the pharmaceutical industry uses an equivalent process to produce encapsulated drugs.`Discovering that coacervation also happens in nature provides us with a golden opportunity to study proteins that are naturally adapted for this process' Waite says, `and may result in insights that could be applied in industry'.