Diversified Material Designs in Biological Underwater Adhesives - Structural Interfaces and Attachments in Biology - page 177

100kDa-protein
52kDa-protein
68kDa-protein
19kDa-protein

20kDa-protein Charged Hydrophilic Hydrophobic

Fig. 9.5 Schematic illustration of proteins in barnacle cement. Each relative length of illustration
roughly corresponds to the molecular weight of respective proteins. Hydrophobic proteins occur
only in the barnacle cement

functioning at the coupling surface are smaller ones with molecular weights of
20 kDa [ 52 ] and 19 kDa [ 53 ]. The last one is a 68 kDa-protein [ 46 ] whose function
in underwater attachment is not yet clear. The primary structures of the two bulk
proteins are highly complex and not as simple as those found in mussel byssal
proteins and tubeworm cement proteins. The amino acid compositions and primary
structures roughly look like those of typical cellular proteins such as enzymes,
whose hydrophobic amino acids form the core of the molecule and hydrophilic
amino acids largely occur in the external area of the molecule. The primary
structure of the 52 kDa-protein is composed of four long degenerated repeats,
whereas no clear repetitions are found in the 100 kDa-protein. The two proteins
with possible surface functions are simpler in the amino acid compositions than
those of the two bulk cement proteins. The 20 kDa-protein is rich in charged amino
acids such as His and Asp/Glu, and Cys, whereas the content of hydrophobic amino
acids is limited. The primary structure is composed of six degenerated repeats.
The 19 kDa-protein also has a simple amino acid composition in which six amino
acids, Ser, Thr, Gly, Ala, Lys, and Val, occupy more than 60% of the total amino
acids, though no clear repetitions in the primary structure is found. The 68 kDa-
protein is composed of two domains whose longer N-terminal domain has some-
what similar amino acid composition to that of the 19 kDa-protein. The two surface
functional proteins are found not to be posttranslationally modified, whereas the
52 kDa-bulk protein is glycosylated. It is unclear whether the other proteins
are posttranslationally modified or not; however, no DOPA is found in all of the
barnacle cement proteins.

9.5.3 Bulk of the Cement

Though the bulk of the cement is generally insoluble in most solvents and by any
treatments, two methods have been found to render almost all of it soluble [ 46 , 51 ].
This is remarkably different from what is seen in the two other model organisms,
where very limited amounts of protein are rendered soluble from the byssal thread
and the tubeworm cement. Investigation of the processes that rendered a bulk