Biology Reference
In-Depth Information
Heat-labile toxin (LT)
The heat-labile toxin is a heteroheximeric molecule composed of a B-subunit
pentamer and a single A-subunit. Toxin activity resides in the A-subunit, while
the B-subunit is largely responsible for host cell binding. The A-subunit is
comprised of two principal domains, the A1 domain, responsible for the tox-
icity, and A2 which non-covalently anchors the A-subunit in the center of the
B pentamer (
Sixma et al., 1991
). Thus the overall structure (
Figure 6.3
a) has
been compared to a 'ring (the B-pentamer) on a finger' (the A-subunit) (
van
Heyningen, 1991
).
The B-subunit engages GM-1 gangliosides centered in lipid rafts on the
surface of eukaryotic cells to trigger internalization of the toxin into the host
cell following proteolytic 'nicking' of A1 and A2 into separate polypeptides
(of approximately 22 and 5 kDa, respectively) by serine proteases located
at the apical surface of epithelial cells (
Lencer et al., 1997
). The subunits
remain joined by a single disulfide bond following cleavage (
Lencer et al.,
1997
). Following uptake of the A1 subunit into the host cell cytoplasm, it
acts in concert with cellular ADP ribosylating factors (ARFs) to transfer
ADP-ribose from NAD to the alpha subunit of the heterotrimeric GTPase,
(Gsα) (
Spangler, 1992
). Formation of a ternary complex of ADP-ribosylated
Gsα GTP and adenylate cyclase catalyses cAMP production from ATP. The
resulting increases in intracellular concentrations of cAMP activate protein
kinase A, which subsequently phosphorylates the cystic fibrosis transmem-
brane conductance regulator (CFTR) activating the chloride channel (
Cheng
et al., 1991
). This promotes efflux of chloride into the intestinal lumen
paralleled by net decreases in Na
+
and water absorption resulting in watery
diarrhea characteristic of ETEC.
LT secretion requires a functional type II secretion system
Secretion of LT from ETEC requires the presence of a functional type II secre-
tion system (see Chapter 13), similar to the secretion apparatus for CT in
Vibrio
cholerae
(
Tauschek et al., 2002
). In the protypical ETEC strain
H10407
, LT
secretion appears to be further modulated by one or more genes encoded by
a pathogenicity island including
leoA
(
Fleckenstein et al., 2000
), a GTPase
(
Brown and Hardwidge, 2007
). Interestingly, much of the LT secreted by ETEC
in vitro remains associated with lipopolysaccharide of outer membrane vesicles
produced by the bacteria (
Horstman and Kuehn, 2000
).
Heat-stable toxins
ETEC may secrete one or more plasmid-encoded heat-stable toxins (ST), small
cysteine-rich peptides that engage an extracellular domain of guanylyl cyclase
C (GC-C) on the apical surface of intestinal epithelial cells (
Potter, 2011
). This
binding triggers activation of the intracellular portion of GC-C which con-
verts GTP to the intracellular messenger cGMP. Elevations in cGMP lead to
Search WWH ::
Custom Search