Hagfish Slime Threads - The Functional Fold: Amyloid Structures in Nature - page 23

α

co-polymers in epithelia and -keratins. Type III intermediate

filaments are expressed in cells of mesenchymal origin and include
desmin (in muscle), vimentin (in embryonic cells and a variety
of other cells such as fibroblasts and endothelial cells), and glial
fibrillary acidic protein (in glial cells). Type IV intermediate filaments
are the neurofilaments, and these are important components of the
neuronal cytoskeleton, especially within axons and dendrites. Type V
intermediate filaments are the nuclear lamins and these are found
exclusively in the nucleus as a basket-like structure just under the
nuclear envelope. Type VI intermediate filaments include nestin and
paranemin as well as two proteins found in the vertebrate oculuar
lens, filensin, and phakosin.
The proteins that make up the 10 nm filaments within hagfish
slime threads clearly belong to the intermediate filament family,
and have been cautiously classified as “keratin-like”.

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Slime thread
intermediate filaments assemble from equal molar ratios of two
proteins (named ) and in this way they are heteropolymers

like the keratins. There are some important differences to consider
when comparing “true keratins” and the intermediate filaments
in hagfish slime threads. Hagfish

α

and

γ

have a higher threonine
content than other keratins (approximately 12% vs. 4%), and they
have a linker segment (L12) that is 21 residues long, rather than
the 16-17 residues found in keratins.

α

and

γ

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Slime threads are unique in
that they represent the only known case of a secreted intermediate
filament, and this may have released these proteins from certain
kinds of selective pressures that have shaped the evolution of the
other intermediate filament proteins. In spite of these differences,
there is mounting evidence that the mechanical properties of slime
thread intermediate filaments are similar to more conventional
intermediate filaments.

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2.4 Mechanical Properties

One of the themes of this topic is that amyloid is a mechanically
useful motif for building protein materials. What then are the
mechanical properties of slime threads, both before and after they
are converted to amyloid? Overall, slime threads are remarkably
extensible in water, being able to stretch on average to 220% strain
before failing

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(Fig. 2.5).