Curli Extracellular Matrix - The Functional Fold: Amyloid Structures in Nature - page 123

both CsgA and CsgB subunits are secreted to the outer membrane
as soluble proteins. While CsgA is released to the cellular milieu,
CsgB is anchored to the membrane to provide CsgA subunits with
the optimal nucleation point from the cell surface and to accelerate
their precipitation into insoluble surface fibres. Though CsgA
polymerization is feasible in the absence of CsgB nucleator, curli

biogenesis is impractical with its absence. This model is supported
by the following observations and demonstrations. (i) Interbacterial
complementation of curli structural genes. A CsgA-deficient, CsgB-
expressing cell (i.e., “receptor” cell) can direct the assembly of curli
on its surface using CsgA monomers provided by a CsgB-deficient,
CsgA-expressing cell (i.e., “donor” cell).

in vivo

7

nucleation of
CsgA by CsgB. Nevertheless, when secreted together with CsgA to
the cellular milieu

(iii) In vitro

(defected in membrane localization), it
displays diminished ability to nucleate CsgA into normal curli fibres.

in vivo

8

(iii) Partial inhibition of curli formation by short peptides, rationally
designed to block CsgA self-recognition motifs. These peptides were
assumed to prevent CsgA monomers extracellular assembly into
fibres.

34

(iv) Partial inhibition of curli formation in the presence of
CsgA- or CsgB-specific antiserum.

9

Findings (iii) and (iv) provide
firm evidence that curli structural subunits are secreted as soluble
monomers.

7.5 Functional Role of Curli

Functional amyloids are unique since proteins are activated in
their aggregated form. Before specifying curli functions, it may be
appropriate to discuss functional amyloids and cell toxicity. Curli
fibres, similar to other functional amyloids formed

in vivo , are not

38

known to display cell toxicity. This may be explained by several

features of curli. First, their unstructured conformation within the
cells facilitates high turnover to avoid intracellular uncontrolled
polymerization. Second, the structural monomers are hypothesized
to preferentially adopt

β

-helix structures, allowing fast transition
into its amyloidal fold. Third, curli assembly is highly regulated
by a nucleator protein dedicated to localize and accelerate CsgA
polymerization. Taken together, the presence of the assumed
cytotoxic intermediates is significantly decreased.