Curli Extracellular Matrix - The Functional Fold: Amyloid Structures in Nature - page 120

7.4 Curli Biogenesis: Mechanism, Kinetics, and

Fibril Structure

7.4.1 Evidence to Being Naturally Occurring Amyloids

Curli fibres satisfy several mechanistic, structural, and tinctorial
amyloid related features (Fig. 7.2). Curli fibres are insoluble in sodium
dodecyl sulfate (SDS)

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and resistant to protease digestion.

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In vivo

assembly depends on interaction with a nucleator protein (CsgB) to
form amyloid-like fibres.

7

Similar to other amyloidogenic proteins,
curli (and curli expressing bacteria) bind the dye Congo red (CR)
and exhibit green birefringence as viewed by cross-polarization
microscopy.

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Curli were confirmed as naturally occurring amyloid
fibres when they were shown to exhibit a red shift upon CR binding,
to produce a significant fluorescence signal typical to amyloids upon
binding to thioflavin T (ThT) dye, and to display spontaneous self-
assembly by the purified monomers in vitro into amyloid-like fibrils.

These fibres were substantially rich in -sheet content according to

electron microscopy and circular dichroism analysis.

β

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(a) (b)

A A B B

(c)

C C (d) D D

Figure 7.2 Amyloidal characteristics of curli. Transmission electron

microscopy (TEM) micrograph (a) and of high-resolution
deep-etch EM micrographs (b) of MC4100 expressing

curli (bars: 500 nm and 60 nm, respectively). (c) Congo red
dye binding by curliated (left) and non-curliated (right)

E . coli

E .

coli MC4100. (d) CD spectrum of curli purified from MC4100

compared with that of soluble unpolymerized CsgA-his,
indicating transition from random-coil to β-sheet rich

structure. (b and d) Reprinted from Chapman et al ., Science , 295

(2005) with permission from AAAS. See also Colour Insert.