Type 3 secretion effectors - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 459

nucleus. Here, NF-κB binds specific DNA consensus sequences and activates
expression of pro-inflammatory cytokines.
EPEC initially activates the NF-κB signaling pathway through a T3SS-
independent mechanism, and subsequently utilizes a T3SS-dependent mechanism
to inhibit NF-κB activation and production of pro-inflammatory cytokines ( Hauf
and Chakraborty, 2003 ; Maresca et al., 2005 ) ( Figure 15.2 A). In recent years an
increasing number of studies have demonstrated that EPEC and EHEC encode
and translocate into cells a set of protein effectors that target specific regulators
of the NF-κB pathway to disarm the host inflammatory response ( Nadler et al.,
2010 ; Newton et al., 2010 ; Vossenkamper et al., 2010 ; Yen et al., 2010 ; Baruch
et al., 2011 ; Muhlen et al., 2011 ; Pearson et al., 2011 ; Zhang et al., 2012 ).
NleE from EPEC/EHEC and OspZ from Shigella ( Zurawski et al., 2008 )
belong to the same family and play a key role in modulating the innate immune
response during infection by blocking the translocation of activated NF-κB (p65
and c-Rel but not p50) to the nucleus, thereby decreasing the expression and
production of IL-8 ( Nadler et al., 2010 ; Newton et al., 2010 ; Zhang et al., 2012 ).
This effect is seen in both epithelial cells and dendritic cells ( Vossenkamper
et al., 2010 ). NleE prevents IκB degradation in response to TNFα or IL-1β sug-
gesting that the effector targets components shared by these inflammatory sig-
naling pathways.
NleE has a unique S -adenosyl-L-methionine-dependent methyltransferase
(SAM) activity that specifically modifies a zinc-coordinating cysteine in the
Npl4 zinc finger (NZF) domains in TAB2 and TAB3. This cysteine methylation
disrupts ubiquitin chain binding of TAB2/3 and blocks TRAF6-induced activa-
tion of TAK1 ( Zhang et al., 2012 ). A 6-aa motif IDSYMK 209-214 has been shown
to be critical for the immunosuppressive function of NleE ( Newton et al., 2010 ).
Indeed, the crystal structure of NleE revealed a SAM-binding pocket, which
involves Y212 and R107. Mutation of either residue abolished NleE methyla-
tion of TAB3-NZF ( Zhang et al., 2012 ). Given the abundance of biological pro-
cesses relying on the activation of zinc-finger motifs by cysteine-zinc binding,
specific methylation of zinc-coordinating cysteine residues might regulate other
eukaryotic pathways in addition to NF-κB signaling.
Another effector involved in the stabilization of IκB is the highly conserved
effector NleB. NleB1 from EPEC inhibits NF-κB activation as effectively as
NleE in response to TNFα but not IL-1β suggesting that NleB1 may act at a dif-
ferent point of the NF-κB signaling pathway and independently from MyD88/
IRAK activation of TRAF6 ( Kelly et al., 2006 ; Petty et al., 2010 ). The mecha-
nism, however, is still unknown.
The NF-κB pathway is also targeted by the effector NleC. This effector con-
tains a consensus zinc metalloprotease motif 183 HEIIH 187 that is responsible for
its proteolytic activity. Indeed NleC was recently described as a potent protease
of EPEC that cleaves and inactivates the NF-κB p65 subunit ( Petty et al., 2010 ;
Yen et al., 2010 ; Baruch et al., 2011 ; Muhlen et al., 2011 ; Pearson et al., 2011 ).
Site-directed mutagenesis of the histidines within the consensus sequence is