Diversified Material Designs in Biological Underwater Adhesives - Structural Interfaces and Attachments in Biology - page 174

protein with abundant Cys is considered to form Cys-DOPA cross-linkage
intermolecularly [ 12 ]. More recently, SFA-measurements suggested that one of
the surface coupling proteins directly interacts with the bulk protein of the disk,
with the strength of 1.3 mJ/m 2 by a yet unknown mechanism [ 25 ]. It is intriguing
to know how nature links distinct microscopic domains because this mechanism is
important for the design of artificial adhesives. Another important question to be
addressed is the molecular mechanism to localize the proteins at the specific
localization(s) of the joint.
9.4.3 Molecular Design Beyond DOPA
Almost all of the chemistry employed by the mussel adhesive has been interpreted
based on the versatile single amino acid DOPA or the functional group catechol.
There is no doubt about the significance of this amino acid and functional group in
the natural system. The mussel, however, employs several DOPA-proteins for
different portions of its byssal thread. The proteins have different characters and
thus other amino acids and primary structures should have several roles in cooper-
ation with the functionalities of DOPA. This was noticed by a study using SFA-
measurement on both a surface coupling protein and the coating protein [ 30 ], which
have similar DOPA-contents. The measurement indicated different properties
between them, thus it was suggested that amino acids other than DOPA or primary
structures may also play essential roles for the individual functions of each protein
in underwater attachment. Moreover, coupling to hydrophobic surfaces such as
synthetic polymers cannot be interpreted by the functioning of DOPA alone. The
principles employed for fine design of the underwater attachment need to be further
unraveled.
An EGF-like sequence motif [ 14 ] typically found in proteins with a cellular
function, as illustrated by its name, was also found in the bulk protein of the disk.
Because amino acids essential for cellular function are lacking in the byssal bulk
protein, the sequence motif is considered to serve as a structure unit for possible
protein-protein interactions in curing of the bulk. Similar alternative use of the
structure has been known in cases such as the bio-silicification protein in the
marine sponge [ 31 ]. The EGF-motif in proteins with a cellular function
usually has a defined conformation, which is essential for the function. In exten-
sion, bulk-formation of the byssal disk may also depend on the conformation of
the EGF-like motif, though no reports on the significance of the conformation
have appeared.
The thread portion of the mussel byssus is composed of three related proteins
[ 32 ]. The proteinaceous thread is among the most well-known biomolecular
materials. Silks of insects and fibrous proteins in extracellular matrix are typical
examples. The proteins in the thread portion of byssus are chimera of different
known fibrous proteins such as in collagen, spider silk, and elastin. Each protein
domain is assumed to have individual mechanical properties, thus the chimeras