Diversified Material Designs in Biological Underwater Adhesives - Structural Interfaces and Attachments in Biology - page 179

9.6 Overview of Tubeworm Cement

The tube that the tubeworm inhabits is made of natural particulates that are bonded
together with tubeworm cement [ 60 ]. The cement attaches two hard materials
together in water using a thin adhesive layer (Fig. 9.1 ). The process is as follows:
The animal grasps an environmental particulate that is several hundred microns in
diameter, puckers to put the cement onto the particulate, and presses it against a tip
of building tube for attachment [ 61 ]. The cement, which is several tens of microns
in thickness, attaches to two hard particulates in water. Thus, it attaches particulates
with an area of several hundreds of

m 2 by a cement layer of several tens of min

m m

thickness.
The cement is uniform in structure at the macroscopic level, while the bulk has a
similar microscopic structure with that of the mussel byssal disk, i.e., a solid foam-
like structure. It is unclear whether a specific localization of different components at
the molecular level occurs.
The cement is a proteinaceous material that contains three proteins (Fig. 9.6 ).
Among the model organisms reviewed in this chapter, the tubeworm's cement has
the fewest components. Two proteins with basic pIs [ 62 ] have apparent molecular
weights of 21 and 18 kDa, and the third protein has an acidic pI [ 63 ] of nearly 1 and
an apparent molecular weight of about 10-30 kDa. Both basic proteins are rich in
Gly with its content at 42 and 30 mol%, respectively, and also contain a high
content of DOPA at 10 and 7 mol%, respectively. The acidic protein has at least
seven variants and is comprised of Ser at 60-90 mol%, with a very limited number
of Tyr. The Ser residues in the protein seem to be almost phosphorylated. Although
the occurrence of DOPA in the acidic protein has not yet been shown, sporadic Tyr
in the deduced sequence from cDNA may be converted to DOPA. The primary
structure of the basic 21 kDa-protein contains 15 tandem repetitive sequences of a
decapeptide with a consensus sequence, which are mostly composed of three amino

Outstanding amino acids

PC-1 DOPA

PC-2 DOPA

PC-3 PhoSer/DOPA

Charged Hydrophilic DOPA

Fig. 9.6 Schematic illustration of the proteins in tubeworm cement. Each relative length of
illustration roughly corresponds to the molecular weight of respective proteins. Tubeworm cement
is composed of three proteins, which is the minimum number among underwater adhesives of the
three model sessile organisms. Primary structures of PC-1 and PC-2 are composed of tandem
repetitions, and that of PC-3 is a very simplified one due to a high content of Ser ranging from 60 to
90 mol%