Biology Reference
In-Depth Information
28
transcription
HN-S (histone-like nucleoid
structuring) — the function of this protein is still unclear as it was
demonstrated both to positively
factor
of
csgD
;
28
and negatively
29
regulate the
expression of curli genes in
Salmonella typhimurium
and
E
.
coli
,
respectively.
7.3.2
Stability and Secretion Machinery
Once expressed, the structural curli proteins (CsgA and CsgB) are
directed to the periplasm. In the unpolymerized state, at least
in
vitro
, these subunits were shown to adopt a predominant random
coil (CsgA) or
α
β
-helical (AgfA) conformation rather than a
-rich
4,30
one.
Furthermore, these molecules are secreted as soluble
monomers.
4,7,9
The exact mechanism of their maintenance in this
conformation is still unclear. It is expected that the aggregated form
is toxic to the cell. The prevention of their aggregation might be
achieved by (i) assigning a chaperon protein that would usher and/
or accelerate CsgA and CsgB secretion through the outer membrane;
(ii) assuring a short half-life of free subunits inside the cell.
csgCEFG
) were suggested to possess a chaperon-like
activity. AgfC was shown to be localized to the periplasm and be
important for AgfA extracellular assembly.
(
afgCEFG
9
csgE
−
mutant produces
significantly less abundant and somewhat structurally distinct
fibrils.
A
4
In the absence of CsgG, a sharp decrease in the steady state
levels of CsgA and CsgB in the periplasm was observed, indicating
rapid degradation of the proteins.
11,12
Absence of CsgF results in
4
aberrant or no secretion of CsgB.
Interestingly, overexpression of
CsgG lipoprotein can compensate for CsgE deficiency (but not
vice
versa
), suggesting that CsgE facilitates CsgG activity.
The
accessory genes are essential for the proper
secretion and biogenesis of curli. However, it is the CsgG lipoprotein
that plays the principal role of secretion. CsgG forms oligomeric
structures in the outer membrane, apparently forming pores in the
membrane through which the CsgA and CsgB are transported to the
cell surface.
cgsEFG
CsgE and CsgF form a complex with CsgG at the outer
membrane, and are considered to assist CsgG proper and efficient
secretion.
12
12
The direction of CsgA to CsgG secretion machinery
is solely dependent on a 22 amino acid sequence located at its N
terminus. When assigned to a heterologous protein, this sequence
can direct it to the cell surface as well.
12
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