Biology Reference
In-Depth Information
within these hollow filaments along their considerable lengths to the transloca-
tion pore at the host cell membrane. Recombinant, purified EspA spontaneously
oligomerizes into filament-like structures, and the crystal structure of EspA in
complex with its T3SS encoded anti-polymerization chaperone CesA (
Yip et al.,
2005a
) revealed the canonical two-helix coiled-coil domain, suggesting an oligo-
merization mechanism similar to that of the needle, as well as an unstructured
region between these two domains that is likely surface-exposed.
The T3SS translocon is a pore-forming complex proposed to insert directly
into the eukaryotic host cell membrane, allowing T3S effector proteins entry
into the cytoplasm (for review see
Mattei et al., 2011
). The pore is a proposed
hetero-dimeric complex composed of two integral membrane proteins (EspB,
EspD in EPEC), which interact with the tip structure at the apical end of the
needle (
Dorman, 2004
). Like the needle and tip components, the translocon
proteins are exported by the injectisome itself, a process likely initiated upon
host-cell contact (
Enninga et al., 2005
). EspD is predicted to contain a two
transmembrane region and a coiled-coil domain (
Dasanayake et al., 2011
) while
EspB has one transmembrane spanning segment, a coiled-coil domain, and an
additional amphipathic helix (
Luo and Donnenberg, 2011
). Presumably the
coiled-coil domains in both pore components facilitate interactions with the
preceding needle tip/filament proteins.
At present there is limited structural information for the translocon, hindered
by poor expression of this pore-forming complex using standard recombinant
methods. Low-resolution EM and AFM analysis of the EPEC translocon in trans-
fected sheep red blood cell membranes appears to suggest a non-symmetrical six
to eight subunit hetero-oligomeric stoichiometry (
Ide et al., 2001
) although it
remains to be confirmed if the structures observed in this study are indeed the
translocon. The translocon proteins have so far been recalcitrant to higher-reso-
lution structural study, however Barta et al. recently published the X-ray crystal-
lographic structures of the protease-resistant domain of the small subunit from
Shigella
(IpaB) and
Salmonella
SPI-1 (SipB) components (
Barta et al., 2012
).
The structures revealed a coiled-coil motif with observed conservation between
the various species (despite the relatively low sequence identity of ∼ 22%). Fur-
ther, structural comparisons revealed similarity to other bacterial pore-forming
proteins, notably colicin Ia, suggesting an evolutionary relationship.
Prior to secretion, the two hydrophobic translocon proteins are stabilized
within the bacterial cytoplasm by a common T3SS chaperone, CesD (classified
as a type II chaperone; for a recent review of T3SS chaperone architecture/func-
tion see
Wilharm et al., 2007
). The localized interaction interface of the trans-
locon proteins with their cognate chaperone has recently been characterized,
revealing independent binding to a common surface on the chaperone. Although
these studies are beginning to illuminate our understanding of translocon func-
tion, many critical questions such as oligomerization state and stoichiometric
make-up of the pore and the consequent effect on translocon insertion into the
host membrane and effector secretion remain unanswered.
Search WWH ::
Custom Search