Hagfish Slime Threads - The Functional Fold: Amyloid Structures in Nature - page 28

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β

perpendicular to the long axis of the fibril. -sheets in

slime threads are arranged parallel to the long axis of the threads.
This is not surprising given that

In contrast,

β

-sheet structures in slime threads
arise from coiled coils that are parallel to the longitudinal axis of the
thread. These coiled coils are transformed into -sheets via tensile

stresses that further increase the alignment of the proteins along the
longitudinal axis.
Within coiled coil

β

α

-helices, hydrogen bonding occurs within

the helices. When tensile stresses disrupt these intra-helix bonds in
slime thread proteins, opportunities arise for the formation of inter-
chain hydrogen bonding, in the form of

β

-sheets, which in turn can
go on to stack into higher order structures like -sheet crystals. It is

this rise in inter-chain hydrogen bonding as well as the straightening
of polypeptides that likely account for the dramatic strain hardening
(i.e., increase in stiffness) that is seen in slime threads. In contrast,
amyloid fibrils exhibit high initial stiffness due to extensive

β

β

-sheet
bonding. Tensional stresses on amyloid fibres directly strain inter-
chain

β

-sheet hydrogen bonds, and it is the gradual unzipping of
these bonds that provides amyloid fibrils with high toughness
(or energy to break). These bonds have also been described as
“sacrificial” bonds that are required for longitudinal deformation,
but do not lead to failure of the fibril because of the “hidden length”
that is released when a row of hydrogen bonds has been broken.

13

In
slime threads, sacrificial bonds and hidden length exist within coiled
coil -sheets are formed, because they are oriented

parallel to the fibre axis, further straining of the material can only
occur via the straining or breakage of peptide bonds, or the shearing
of protein chains past one another, resulting in high stiffness of the
material.

α

-helices. When

β

2.6 Implications for Other Materials Containing

Intermediate Filaments

Although studies of the structure and mechanics of slime threads
were originally undertaken to better understand the biophysics of
hagfish slime, the results of this work have inspired novel research
into mechanical properties of intermediate filaments in other
systems. Since it was first recognized that 10 nm filaments within
living cells are from the same gene family as the filaments that