Hagfish Slime Threads - The Functional Fold: Amyloid Structures in Nature - page 31

>2.5% strain) in wool are completely reversible (at least under
physiological conditions), and do not lead to the formation of stable
β

-sheets. Interestingly, these deformations lead to the disruption
of

α

-helical secondary structure, but somehow the formation of

23

β

Although the mechanism is not fully known,
the matrix proteins surrounding the intermediate filaments in wool
are likely responsible. The matrix is believed to form an elastomeric
network that acts in parallel to the intermediate filaments, and thus
may provide a restoring force that inhibits formation of

-sheets is inhibited.

β

-sheets

and returns disrupted regions to their former

α

-helical state. If

α

intermediate filaments in hard -keratins are in fact maintained

in a dehydrated state, even in water, this too could contribute to
the reluctance of hard

α

-keratins to undergo a strain-induced

α

transition. Textile scientists have known for decades that
wool fibres can be persuaded to undergo an

-to-

β

transition by
holding them in a stretched state in the presence of steam.

α

-to-

β

30

This
treatment likely disrupts the matrix and allows water to penetrate the
intermediate filaments, which may lower the activation energy of the
α

transition.
While mammalian hard

-to-

β

-keratin epidermal appendages are
reinforced with intermediate filaments that are dominated by

α

α

-helix, birds and reptiles evolved a unique hard keratin material
that is reinforced with a completely different fibrous component
dominated by -keratins make up scales,

feathers, and beaks in these groups. Because the fibrils within
β

β

-sheet.

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These so-called

β

β

-sheets with the chains aligned
primarily with the fibril axis, these materials exhibit high stiffness
and do not exhibit a dramatic yield point like hard

-keratins consist of proteins in

α

-keratins do

when the hidden length within

α

-helices is released.

2.7 Conclusions and Outlook

Hagfish slime threads are a case of a structural biomaterial that
adopts amyloid-like properties. The fact that slime threads only
take on an amyloid-like structure outside of the animal's body, and
only after they have been draw-transformed by an external force,
underscores the paucity of amyloid structures found within cells
and within organisms. Slime threads are essentially bundles of
intermediate filaments, and as such, they have proven to be a valuable