Hagfish Slime Threads - The Functional Fold: Amyloid Structures in Nature - page 22

reinforces the emerging idea that amyloid-like protein structures
may be useful for constructing biomaterials and adhesives with
unique mechanical properties, several of which are highlighted in
this topic (see chapters 8 and 10, this volume). It also suggests that
the advantages of employing structural proteins with amyloid motifs
within an animal's body may be outweighed by the physiological
risks of amyloidogenic disease.

2.3 Slime Threads Are Bundles of Intermediate

Filaments

To understand the amyloid-like nature of stretched slime threads, it
is worthwhile to take a step back and examine the kinds of proteins
that make up the threads. Early work on the ultrastructure of
slime threads, especially in immature GTCs within the slime gland,
demonstrated that the bulk of the thread is composed of filaments
approximately 10 nm in diameter. Often interspersed with these
filaments are microtubules or microtubule doublets, although these
become rarer as the thread cell matures and the 10 nm filaments
become more plentiful. In the mid-1990s, Koch

established
that the proteins that make up these 10 nm filaments belong to the
“intermediate filament” family of proteins.

et al.

14
Along with F-actin and microtubules, intermediate filaments
are important components of the cytoskeleton of most metazoan
cells. In living cells, cytoplasmic intermediate filaments often form
a cage of filaments around the nucleus that radiate out toward
the cell periphery, where they are anchored in desmosomes and
hemidesmosomes.

Intermediate filaments are also noteworthy
because they make up the fibrous phase of the fibre-reinforced
composite materials known as

15

α

16

α

-Keratins consist of
intermediate filaments embedded in an elastic protein matrix, that
make up not only the soft and pliable outer layers of epidermis in
amniotes (the stratum corneum), and also a diverse array of harder
epidermal appendages in mammals, such as hairs, nails, horns,
claws, and the baleen plates of mysticete whales.

-keratins.

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Intermediate filaments comprise one of the most diverse gene
families in humans, with at least 70 expressed genes now identified
belonging to six different types. Types I and II are the acidic

and basic keratins proteins which are coexpressed as obligate

18