Enteropathogenic Escherichia coli - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 75

rabbit infection ( Donnenberg et al., 1992b ; Marchès et al., 2000 ). There are
29 subtypes of intimin described to date. These subtypes are differentiated
by a variable C-terminal region comprising the last 280 amino acids that is
required for intimin-Tir binding and controlling specific host and tissue tro-
pism ( Fitzhenry et al., 2002 ; Ito et al., 2007 ; Mundy et al., 2007 ). Intimin β
is the most prevalent subtype found amongst isolates of EPEC from humans
and animals ( Ramachandran et al., 2003 ; Nakazato et al., 2004 ; Blanco et al.,
2005 ). Mutation of the eae gene leads to loss of A/E and host cell invasion
( Francis et al., 1991 ), in addition to preventing the promotion of pseudopod
formation ( Rosenshine et al., 1996b ) and EPEC attachment to human mucosal
explants ( Hicks et al., 1998 ). Unlike T3SS mutants, eae mutants retain the
ability to redistribute host cell actin but have a blunted capacity to concentrate
host cell cytoskeletal elements beneath the attachment site ( Donnenberg et al.,
1990a ; Rosenshine et al., 1992 ). Mutant eae strains of EPEC can complement
strains carrying mutations in espA , espB or other T3SS genes, allowing them
to establish A/E ( Rosenshine et al., 1992 ; Donnenberg et al., 1993b ; Kenny
et al., 1996 ). In addition, pre-infection with an eae mutant allows a laboratory
E. coli strain carrying the cloned intimin gene ( Rosenshine et al., 1996a ) or
beads coated with intimin ( Liu et al., 1999 ) to bind to epithelial cells. Such
trans complementation is possible because the eae mutant remains capable of
translocating Tir into the host cell membrane. Apart from binding Tir, intimin
may also bind to h ost i intimin r eceptors (Hir) such as nucleolin via its variable
C-terminus ( Mundy et al., 2007 ).
Tir is a multifunctional protein which, after injection into the host cell via
the T3SS, becomes integrated into the host cell membrane in a protease-sensi-
tive conformation ( Rosenshine et al., 1996a ; Kenny et al., 1997 ). This protein
exhibits a dimeric hairpin topology, such that an extracellular loop from each
monomer binds an intimin molecule ( De Grado et al., 1999 ; Hartland et al.,
1999 ; Kenny, 1999 ) with the N-terminal and C-terminal domains projecting
into the host cell cytoplasm ( Figure 4.2 B). Tir serves as the focus for the actin
reorganization that occurs at the site of EPEC pedestals ( Figure 4.1 D). Some,
but not all EPEC strains express Tir variants that have a tyrosine at position
474 (Y474). Upon translocation of this form of Tir into the host cell, it is phos-
phorylated at Y474 ( Kenny et al., 1997 ) by host cell kinases ( Nakazato et al.,
2004 ; Swimm et al., 2004 ; Blanco et al., 2005 ). This event is essential for actin
remodeling in such strains ( Kenny, 1999 ), as mutation of Tir results in the loss
of the A/E phenotype and reduced virulence in a rabbit model ( Kenny et al.,
1997 ; Marchès et al., 2000 ). As discussed below, there are other pathways lead-
ing to indistinguishable actin remodeling.
Apart from Tir, other translocated LEE encoded effector molecules include
EspF, EspG, EspH, EspZ, and Map ( Pallen et al., 2005 ). More recently, a rep-
ertoire of n on- L EE e ncoded effectors (Nles) that are secreted by the T3SS have
been discovered. For more detailed information on the type 3 effectors, see
Chapter 15.