ADP-riboxanation upregulates the E3 ISG15 ligase activity of
HERC5
To determine the biological function of HERC5 ADP-riboxanation catalyzed
by S. flexneri effector OspC1, we overexpressed the
ISG15-congjugating system together with WT or MUT OspC1 in 293T cells.
As reviewed in the previous publication and observed in our experiment,
only free ISG15, but not ISGylation, was induced upon type I IFN
stimulation (Zhang et al., 2011; Figure 4a). That being said, the
ISGylation observed in the overexpression experiments were all caused by
exogenous expression of ISG15-congjugating system (Figure 4b). Comparing
to the MUT group, the ISGylation generated in the WT group were much
stronger (Figure 4c), which indicated that ADP-riboxanation of HERC5
catalyzed by S. flexneri effector OspC1 promoted the activity of
its E3 ISG15 activity.
To further verify this conclusion, we tested the ISGylation of HERC5
itself and RIG-I in the overexpression system in the presence of WT or
MUT OspC1, and found that the ISGylation of HERC5 itself was enhanced
slightly, whereas the ISGylation of RIG-I was promoted by S.
flexneri effector OspC1 significantly. Given that the ISGylation of
RIG-I negatively regulates its activation as a feedback mechanism to
fine-tune host innate immune response (Kim et al., 2008),Shigella may subvert host innate anti-bacterial defence by
hijacking this negative mechanism via OspC1-catalyzed HERC5
ADP-riboxanation, which results in the accumulation of non-functional
ISGylated RIG-I in the host cells.