Type 3 secretion effectors - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 452

( Vingadassalom et al., 2010 ). The NPY motif is conserved in EPEC Tir (as
NPY454) but in typical EPEC lineage 1 strains this pathway accounts only for
low levels of actin polymerization as these strains do not have a TccP/EspF U
homolog ( Campellone and Leong, 2005 ). Both pathways appear to be utilized
simultaneously in vitro for most non-O157 EHEC strains, EPEC O119:H6
( Whale et al., 2006 ) and EPEC lineage 2 strains which carry the homolog
TccP2/EspF M ( Ogura et al., 2007 ). The current conundrum is that neither path-
way appears to be necessary for A/E lesion formation in vivo in EHEC animal
models (infant rabbit and gnotobiotic piglet models ( Ritchie et al., 2008 ) or
the C. rodentium murine infection model), or in EPEC and EHEC infection of
human intestinal in vitro organ cultures (IVOC) ( Schuller et al., 2007 ; Crepin
et al., 2010 ), indicating that the molecular mechanisms and function of A/E
lesion formation during infection are far from understood.
EPEC Tir is also responsible for membrane phosphoinositide signaling as
it binds host phosphoinositide 3-kinase (PI3K) at phosphorylated tyrosine 454
(Y454p) ( Sason et al., 2009 ; Selbach et al., 2009 ) and the inositol-5-phosphatase
SHIP2 ( Smith et al., 2010 ). These two enzymes convert PI(4,5)P 2 to the pre-
dominant membrane phosphoinositide found in wild-type pedestals; PI(3,4)P 2 .
An intermediate, PI(3,4,5)P 3 , which accumulates when SHIP2 recruitment is
prevented, results in multiple elongated pedestals ( Smith et al., 2010 ), suggest-
ing a regulated process of initial PI(3,4,5)P 3 -enhanced actin polymerization fol-
lowed by signaling down-regulation by PI(3,4)P 2 , may occur.
Other T3SS effectors implicated in this initial process of A/E lesions or ped-
estal formation are EspH, EspB, and EspL. EspH inhibits Rho GTPases activ-
ity and as such alters the actin cytoskeleton. In addition to modulating Rho
GTPases activity, EspH can also promote N-WASP and WASP interacting pro-
tein (WIP) recruitment leading to Arp2/3-mediated actin accumulation, which
is independent of Tir residues Y454 and Y474 (i.e. Tir:Nck and Tir:IRTKS/
IRSp53 pathways) ( Wong et al., 2012 ) revealing yet another Tir-mediated actin
polymerization pathway that may contribute to A/E lesions in vivo.
EspB forms part of the translocation pore at the tip of the T3SS apparatus
but in addition is translocated into the host cell where it has an effector function.
Ectopic expression of EspB can redistribute host actin in the absence of any
other effectors ( Taylor et al., 1999 ) and can bind to host cell proteins including
α-catenin ( Kodama et al., 2002 ) and myosins ( Iizumi et al., 2007 ), affecting
pedestal formation and initiation of phagocytosis respectively. Both α-catenin
and EspB can be seen in the pedestal of EHEC-infected tissue culture cells and
are thought to participate in the rearrangement of actin molecules in the pedestal
( Hamaguchi et al., 2008 ). Detailed understanding of EspB's role in pedestal
formation has been hampered by its importance in T3SS pore formation, while
the interaction of EspB and myosins has been separated from its pore-forming
function, this has not occurred for the interaction of EspB and α-catenin.
EspL2 of EHEC can interact with annexin 2 and increase the ability of
annexin 2 to aggregate the Tir-induced actin that accumulates underneath the