Hagfish Slime Threads - The Functional Fold: Amyloid Structures in Nature - page 30

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tools. These researchers found that not only can keratin (Type

I/II), desmin (Type III), and neuro filament (Type IV) intermediate
filaments deform to 250% strain before breaking, they also found
that the filaments did not return to their resting length after these
kinds of deformations. This result is in perfect alignment with the
post-yield plastic behaviour of hagfish slime threads.
This new model of intermediate filament mechanics in living
cells raises the obvious question of why the properties of hard
α

-keratins are so different if they consist mostly of intermediate
filaments. The main differences that need to be explained are the
high elastic modulus of hard

α

-keratins like wool (approximately
2 GPa) compared to slime threads (approximately 6 MPa), the
low yield strain of 2.5% (compared to 35% in slime threads), the
high yield stress of approximately 40 MPa (compared to 3 MPa in
slime threads), and the low extensibility of approximately 45%
(compared to 220% in slime threads). One factor that could explain
these differences is that the proteins within intermediate filaments
in hard

α

-keratins are covalently cross-linked so that lines of force
bypass the softer components that lend greater compliance to slime
threads. We have hypothesized that these softer components are the
terminal domains that flank the central

α

11

-helical rod domain, but

this has yet to be tested.
Another possibility is that the intermediate filaments in hard
α

Support for
this idea comes from the fact that slime threads tested in air have a
stress-strain curve that is reminiscent of the curve for hard

-keratins are maintained in a semi-dehydrated state.

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α

-keratins
tested in water. Specifically, dry slime threads exhibit high elastic
modulus (in the several GPa range) and a low yield strain (2.4%)
like hard -keratins. The dehydration hypothesis is also supported

by the fact that the yield stress of hard

α

-keratins tested in water
is considerably higher than one would predict from cross-linking
alone. One possible mechanism by which hard

α

-keratins could be
maintained in a dehydrated state is via initial air drying, after which
point they could be squeezed down upon by the polymerization of
the keratin “matrix”, which consists of a highly cross-linked network
of proteins that surround the intermediate filaments.

α

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Another major difference between slime thread mechanics and
hard

α

-keratins like wool is that post-yield deformations in the
former lead to the conversion of

α

-helical secondary structure into

amyloid-like

β

-sheets. In contrast, post-yield deformations (i.e.,