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2.5
Structural Considerations
The first indication that slime thread proteins adopt an amyloid like
conformation came from Congo red staining of whole hagfish slime
that had been mechanically deformed in some way (Fig. 2.4). Further
experiments established that the onset of positive Congo red staining
(i.e., metachromasia and birefringence) corresponds with the yield
point of slime threads in water, or a tensile strain of approximately
35% strain (Fig. 2.7).
a
b
c
d
e
f
Figure 2.7
Congo red staining of slime threads stretched in sea water
visualized with dark field polarized light microscopy. 11 (a)
Unstrained and unstained threads are birefringent. Congo red
staining of control threads and threads stretched to less than
strains of 0.35 lose their birefringence and swell considerably
(data not shown). (b-f) Threads stained with Congo red after
extension to strains greater than 0.35 retained birefringence
and mechanical integrity, and displayed increasing metach-
romasia as strain increased. Threads appeared orange-yellow
when strained to 0.35 (b), green when strained to 0.50 (c),
blue at 0.75 (d), blue-violet at 1.0 (e), and pale magenta to
colourless at 1.50 (f). Scale bar is 10
ยต
m. With permission from
11
Biophysical Journal
.
See also Colour Insert.
Curiously, the nature of the Congo red staining changes as a
function of the strain history of the threads, with the resulting
metachromasia becoming increasingly blue shifted with increasing
strain. As far as we know, slime threads are the only system in which
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