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Likewise, silks reveal a quite ancient structure, and silk
structures probably have evolved several times independently.
5
Comparing silks and functional amyloids, similarities can be found
concerning their structure-function relationships. Both exhibit a low
sequence complexity of the respective proteins and a trend towards
repetitive sequence compositions allowing a high conformational
flexibility without a strict prevalence for a specific conformation. All
amyloids exhibit a cross-β structure when assembled into fibrous
structures,
71
5
while silks in contrast can feature different folds.
Strikingly, those silks with α-helical or cross-β structure are extruded
or deposited, while the spinning process (as described earlier) leads
exclusively to parallel-β silks.
5
Cross-β silks can be converted into
parallel-β structures upon stretching,
33
indicating that mechanical
stress and shear forces are crucial events leading to the parallel-β
arrangement.
It can be speculated that “ancient” silk proteins primarily
adopted an amyloid-like structure which can still be observed in
nanofibrils as found in the B-zone of the major ampullate gland
16
21
and
On the basis
of the amyloid structure, spiders probably improved the spinning
process to adopt the mechanical properties of silks according to
special needs. Instead of forming a network of short amyloid fibrils
with limited length, spiders produce long and continuous threads.
The embedding of β-sheet crystals in an amorphous matrix within
these threads leads to elaborate mechanical features combining
the high strength provided by densely packed β-sheets with elastic
properties. The highly complex process of stretching and aligning
of silk proteins inside the spinning duct seems to lead unavoidably
to a transition of any cross-β structure to the observed parallel-β
structure which is found in all spun silk.
in vitro
in fibrils of engineered spider silk proteins.
Acknowledgement
We thank Markus Heim for critical comments on our manuscript.
This work was supported by the
Deutsche Forschungsgemeinschaft
.
References
1. Dobson, C. M. (2001)
Philos. Trans. R. Soc. Lond. B Biol. Sci.
,
356
, 133.
2. Smith, A. M., and Scheibel, T. (2010)
Macromol.
Chem. Phys.
,
211
, 127.
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