Page 272 - Advances in Textile Biotechnology

Developments in recombinant silk and other elastic protein fi bers 253

Biotechnological strategies concerning silk-protein production have to
take into account several aspects: silk genes are large (up to 15 Mbp) and
the use of bacterial host, such as E. coli, as industrial production host is
hampered by the fact that the size of expressible genes is limited and bac-
terial host have a distinct codon usage different to that of spiders. Success-
ful expression of spider silks have been achieved in bovine mammary
epithelial and hamster kidney cells, in the milk of transgenic goats and in

baculovirus expression systems with their genome modified with silk genes
and selectively infecting insect cells. In addition to bacterial hosts, the yeast
Pichia pastoris and plants are currently under investigation as a cheap
alternative to bacterial host for silk protein production like tobacco, potato
and Arabidopsis thaliana (Vendrely and Scheibel, 2007).
Silk proteins have been shown to solubilize in water, organic solvents
and ionic liquids, indicating the versatile options available, and they can
then be processed into new materials including fi bers, films, gels, porous

sponges and other related systems, thus making spider silks potentially
promising for future materials. Silk-like proteins (SLP) based on the repeat-
ing motif (GAGAGS) produce crystalline structures similar to the β-sheet
structures of native silk proteins (Cappello et al., 1990). Several methods to
increase the water solubility have been developed such as the addition of
sterical crystallization triggers such as methionine residues or charged phos-
phate groups (Winkler et al., 1999), which interrupted the β-sheet formation

preventing insolubility. Subsequent modifications of spider silk sequences
have been performed in an attempt to control the mechanical properties
showing that the nature of the fifth and tenth amino acid (X) in the

(GPGGX) n elastic motif and temperature have a large impact on the self-
organization process and in the level of elasticity of the proteins in the fi lm
or fi bers (Teule et al., 2007). Recently, mechanical properties of a recombi-
nant miniature spidroin have been increased by genetic engineering with

disulfides increasing the stiffness and tensile strength without changes in
the ability to form fibers, or in fiber morphology (Grip et al., 2009).

Silkworm fibroin silk has been functionalized by coupling enzymes such

as horseradish peroxidase, cell-binding domains such as Arg–Gly–Asp
(RGD) peptides and cell signaling factors such as parathyroid hormone
(PTH) and bone morphogenetic protein-2 (BMP-2) to improve cell
interactions and function or to form gradients on silk material surfaces
(Karageorgiou et al., 2004, Sofi a et al., 2001, Vepari and Kaplan, 2006).

10.7.2 Collagens
Collagens represent a family of related structures widely distributed in
nature and commonly used in drug delivery and pharmaceutical applica-
tions as well as in many medical devices. In humans, collagens are present

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