Biology Reference
In-Depth Information
The inner-membrane export apparatus
The T3SS inner-membrane export apparatus consists of five integral membrane
proteins (EscRSTUV), predicted to localize within the dual inner-membrane
rings (EscJ/EscD) at the base of the basal body, along with several cytoplasmic
membrane-associated proteins including the T3SS ATPase (EscN; see
Figure
14.2
). It should be noted that these proteins, which regulate secretion across the
inner membrane, have been termed the export apparatus in the literature to dis-
tinguish them from the distinct and downstream translocating pore (the translo-
con) that passages substrate through the host membrane. The export apparatus
proteins are amongst the most highly conserved proteins of the T3SS and have
closely related homologs in the flagellar T3SS. EscRSTU are encoded on the
same operon whilst the larger EscV is encoded separately (
Figure 14.1
A). Given
their membrane-spanning nature, it is perhaps not surprising that little is cur-
rently understood at the molecular level regarding the coordinated function of
the export apparatus, although it is generally believed to be central to recruitment
and regulation of initial insertion of effectors into the injectisome (for review
of the related flagellar export apparatus see
Minamino et al., 2008b
). Presum-
ably, packing of EscRSTUV within the EscJ/D rings maintains sufficient room
for passage of downstream needle/translocon and effector proteins through the
inner membrane, as suggested by recent high-resolution EM studies in
Salmo-
nella typhimurium
(
Schraidt and Marlovits, 2011
). The significant cytoplasmic
domains of two of the export apparatus proteins, EscU and EscV, have been sug-
gested to form a functional export gate controlling access to the inner channel
(
Saijo-Hamano et al., 2010
). The soluble ATPase is thought to dock to this export
gate (
Minamino et al., 2012
), where it uses the energy derived from ATP hydro-
lysis to dissociate effectors from their cognate chaperones prior to insertion into
the inner-membrane channel.
Structural models for soluble domains of the export apparatus proteins have
begun to emerge in recent years. EscV is the largest of the export apparatus pro-
teins (∼70 kDa) with eight predicted TM helices and a large C-terminal cytoplas-
mic domain (∼40 kDa). X-ray crystallographic analysis of the analogous domain
has recently been published for the
Salmonella
SPI-1 T3SS ortholog InvA (
Lilic
et al., 2010
;
Worrall et al., 2010
) in addition to three structures of the flagellar
homolog FlhA (
Bange et al., 2010
;
Moore and Jia, 2010
;
Saijo-Hamano et al.,
2010
). These structures show a highly conserved overall fold consisting of four
subdomains. Of interest, the structures capture the protein in different conforma-
tional states with variation in the degree of opening of a central cleft, although
the functional relevance of this is at present unclear. The cytoplasmic domain of
FlhA has been shown to bind subunits destined for the formation of the flagellar
filament suggesting a role in substrate recognition and regulation of assembly
(
Bange et al., 2010
;
Diepold et al., 2011
). Using a fluorescent tag, Cornelis and
colleagues demonstrated that the
Yersinia
homolog YscV co-localizes with other
T3SS proteins in multiple copies, unlike the other export apparatus proteins, sug-
gesting it forms higher-order oligomers (
Diepold et al., 2011
).
Search WWH ::
Custom Search