Biology Reference
In-Depth Information
EscU is predicted to contain two TM helices and a C-terminal ∼20 kDa
cytoplasmic domain. EscU orthologs have also been implicated in the temporal
regulation of injectisome assembly, acting as part of a molecular 'switch' that
regulates the chronological secretion of apparatus components and virulence
effectors. It has been demonstrated that flagellar and non-flagellar homologs
undergo a spontaneous autocleavage event in their C-terminal cytoplasmic
domain leaving a cleaved fragment still associated with the C-terminal domain
(
Minamino and Macnab, 2000a
;
Lavander et al., 2002
). The recent structures
of the cytoplasmic domain of EscU along with orthologs from
Salmonella
typhimurium
(SpaS),
Yersinia enterocolitica
(YscU) and
Shigella
(Spa40) show
a small mixed alpha/beta domain (
Deane et al., 2008a
;
Zarivach et al., 2008
;
Lountos et al., 2009
;
Wiesand et al., 2009
). The structures revealed the molecu-
lar basis for an intein-like autocleavage reaction via asparagine cyclization on
a highly conserved surface exposed loop, resulting in the remodeling of sur-
face features and concomitant changes in electrostatics in the EscU cytosolic
domain. Non-cleavable mutants show aberrant secretion of the tip or transloca-
tor proteins suggesting cleavage is required for their recognition and a role in
switching from translocon to effector secretion (
Zarivach et al., 2008
).
In addition to the membrane-embedded components of the export apparatus,
several membrane-associated proteins are also involved in the regulation of secre-
tion, notably the T3SS ATPase EscN. The identification of an ATPase associated
with the flagellar T3SS (
Vogler et al., 1991
) and subsequently a homolog in the
T3SS injectisome of
Yersinia
(
Woestyn et al., 1994
) led to the proposal that it was
the energy source for protein translocation. More recently however, several studies
have suggested that the proton-motive force (PMF), the electrochemical potential
difference of protons across a membrane, provides the energy for protein unfold-
ing and translocation (
Minamino and Namba, 2008
;
Paul et al., 2008
) supported
by the observation that secretion can still occur, albeit less efficiently, in absence
of the ATPase in both the flagellar T3SS (
Minamino and Namba, 2008
) and the
T3SS injectisome (
Wilharm et al., 2004
). Based on the ability of the
Salmonella
SPI-1 T3SS ATPase to dissociate chaperone-effector complexes in an ATP-depen-
dent manner (
Akeda and Galan, 2005
), it is currently believed that the ATPase is
required for the effective targeting of T3S chaperone-effector complexes to the
base of injectisome and subsequent effector release and initial unfolding prior to
translocation. One intriguing feature of the ATPase is the similarity to the F/V/A-
ATPases, rotary motors that couple transmembrane ion flow to ATP catalysis, first
documented some 20 years ago (
Vogler et al., 1991
;
Woestyn et al., 1994
). Con-
firming the close structural similarity, structures of the EPEC T3SS ATPase EscN
(lacking the N-terminal domain) (
Zarivach et al., 2007
) and the flagellar ATPase
FliI (
Imada et al., 2007
) were published several years ago. FliI crystallized as a
homo-dimer conserved in nature to the hetero-dimer found in the F/V/A-ATPases
and experimental evidence has confirmed that, like the F/V/A-ATPases, the type 3
ATPases likely function as hexamers in the physiological context (
Kazetani et al.,
2009
). Initially reported for the flagellar T3SS, this evolutionary relationship has
been extended to further soluble export apparatus proteins: firstly, the flagellar
Search WWH ::
Custom Search