Biology Reference
In-Depth Information
Soulat et al., 2007
;
Walker, Saraste, Runswick, & Gay, 1982
). Interestingly,
in some of these organisms, the additional C-terminal domain is encoded
by a separate gene, similar to what is observed for the PCP-2 proteins
of Gram-positive bacteria, referred to as PCP-2b (
Cuthbertson et al.,
2009
;
Morona et al., 2000
). Most PCP-2a and PCP-2b proteins also con-
tain a tyrosine kinase domain located in the additional carboxy-terminal
cytoplasmic region. In the case of Wzc, the tyrosine kinase domain is
autophosphorylated at several C-terminal tyrosine residues, being sub-
sequently dephosphorylated by the Wzb phosphatase (
Wugeditsch et al.,
2001
). Current working models propose that Wzc must cycle between
its phosphorylated and nonphosphorylated forms to sustain the synthesis
of the
E. coli
group 1 capsule polysaccharides. However, other PCP-
2a proteins, such as the BcenP_01003624 from
Burkholderia cenocepacia
lack the C-terminal tyrosine residues, and others, including the
Xan-
thomonas campestris
GumC, have no kinase domain at all, even though
they participate in the EPS assembly and export. These observations
raised the hypothesis that the PCP-2a kinase activity may, in fact, be
related to earlier steps of the biosynthetic pathway. Despite these infer-
ences, more information is needed to unveil the exact role of the kinase
domain. The last group of PCP proteins, PCP-3 family, refers to the
PCP representatives that participate in the ABC-dependent assembly
and export systems, including the KpsE. These proteins are smaller than
the PCP-2a, lacking the additional carboxy-terminal cytoplasmic region
(
Morona et al., 2000
).
2.3.2. Characteristics of the OPX proteins
The last steps of EPS assembly and export also require a member of the
OPX protein family. These proteins are characterized by the presence
of a polysaccharide export sequence (PES) domain (IPR003715; pfam
02563) (
Cuthbertson et al., 2009
). The Wza protein of the Wzy-depen-
dent pathway is an integral outer membrane lipoprotein, and structural
studies revealed that it forms a ring-like octameric complex that pro-
vides the translocation channel across the outer membrane (
Dong et al.,
2006
;
Drummelsmith & Whitfield, 2000
). The periplasmatic domain of
Wza specifically interacts with the periplasmatic domain of Wzc, leading
to a conformational change in both proteins during the complex forma-
tion (
Collins et al., 2006
,
2007
;
Ford et al., 2009
). The existing data sug-
gest that in Wza, it is the PES domain that undergoes a conformational
change (
Collins et al., 2007
), allowing the translocation of the polymer
