Biology Reference
In-Depth Information
sugars into a lipid carrier located at the plasma membrane. The newly
synthesized repeating units are then translocated to the periplasmatic
face of the plasma membrane by the integral protein Wzx, where they
are polymerized by another integral membrane protein - Wzy. This
reaction is influenced by the activity of the polysaccharide copolymer-
ase (PCP) protein Wzc since it requires its interaction with the outer
membrane polysaccharide export (OPX) protein Wza. These proteins
form a complex that spans the cell envelope, thereby providing a molec-
ular scaffold for the export of the polymer (
Cuthbertson et al., 2009
;
Whitfield, 2006
).
2.2. ABC-Dependent Pathway
The assembly and export of
E. coli
group 2 capsules follow the ABC-depen-
dent pathway (
Fig. 7.1
B). In this process, the polysaccharide is fully polym-
erized in the cytoplasmatic face of the plasma membrane by the sequential
addition of sugar residues to the nonreducing terminus of the nascent poly-
mer. The complete molecule is then exported through the plasma mem-
brane by an ABC transporter, comprising two transmembrane domains and
two nucleotide-binding domains, which are encoded by
kpsM
and
kpsT
genes, respectively. Other proteins, such as KpsC, KpsS, KpsF, and KpsU, also
participate in this process, but their precise role is still unclear. The export of
the polymer through the periplasm and outer membrane requires the activ-
ity of the PCP protein, KpsE, and the OPX protein, KpsD, which may form
a transenvelope assembly machine analogous to that of Wza and Wzc in the
Wzy-dependent pathway (
Cuthbertson et al., 2010
,
2009
;
Whitfield, 2006
).
2.3. Role of PCP and OPX Proteins in EPS Assembly and
Export
Despite the differences observed for the Wzy- and ABC-dependent assem-
bly and export pathways, in the Gram-negative bacteria, both processes
require the involvement of members of the PCP and OPX protein families
for the translocation of the polymer to the cell surface. The structure, func-
tion, and phylogeny of the PCP and OPX proteins involved in this process
were recently reviewed, providing new insights into its putative functions
(
Cuthbertson et al., 2009
).
2.3.1. Characteristics of the PCP proteins
The PCP proteins are essential for the assembly and export of several bac-
terial surface polysaccharides (
Morona,Van Den Bosch, & Daniels, 2000
).
