NleH1 inhibits RPS3 phosphorylation in vitro
EHEC pathogens are important causative agents of both foodborne disease and pediatric renal failure31. EHEC utilize T3SS to inject effector proteins directly into intestinal epithelial cells32, a subset of which inhibit NF-κB-dependent innate responses9, 33-38. The E. coli O157:H7 EDL933 effector protein NleH1 binds to and attenuates RPS3 nuclear translocation, thus impairing RPS3-dependent NF-κB signaling9. We therefore hypothesized that NleH1 may function by inhibiting RPS3 S209 phosphorylation. As expected, transfecting increasing amounts of NleH1-HA plasmid blocked TNFα-induced NF-κB activation in a dose-dependent manner (Fig. 6a-b)9. Remarkably, NleH1 reduced both TNF-induced, as well as basal RPS3 phosphorylation to roughly 20% of vehicle control (Fig. 6c). Expressing NleH1 does not interfere with either TNF-induced IKK activation or IκBα degradation, consistent with the lack of NleH1 impact on p65 nuclear translocation9 (Fig. 6c).
To determine if NleH1 inhibits RPS3 phosphorylation, we infected HeLa cells with E. coli O157:H7 strains possessing or lacking either nleH1 (ΔnleH1) or with a strain lacking a functional T3SS unable to inject NleH1 into mammalian cells (ΔescN). In uninfected cells, TNF-treatment stimulated a ∼7-fold increase in RPS3 S209 phosphorylation, peaking at 30 minutes (Fig. 6d). By contrast, RPS3 S209 phosphorylation was substantially impaired in cells infected with wild-type E. coli O157:H7 (Fig. 6d). However, TNF-induced RPS3 S209 phosphorylation was unimpaired in cells infected with either ΔnleH1 or ΔescN (Fig. 6d). We showed previously that wild-type, but not ΔnleH1 or ΔescN E. coli O157:H7 significantly attenuated TNF-induced RPS3 nuclear translocation9. The parallel between RPS3 phosphorylation and its nuclear translocation during E. coli infection provides evidence in the context of an NF-κB-dependent disease process that RPS3 S209 phosphorylation is important for nuclear translocation.
Our discovery that NleH1 inhibits RPS3 S209 phosphorylation suggested that it should also blocks RPS3-dependent NF-κB target gene transcription (e.g. IL8, NFKBIA, and TNFAIP3). Indeed, these genes were only modestly upregulated in cells infected with wild-type E. coli O157:H7, but significantly induced in cells infected with either ΔnleH1 or ΔescN strains (Fig. 6e). In contrast, deleting nleH1 had no impact on the expression of RPS3-independent genes, including CD25 and TNFSF13B (Supplementary Fig. 12). Together these results demonstrate that NleH1 specifically inhibits the protective immune response by directly blocking RPS3 S209 phosphorylation and thereby impairing critical RPS3-dependent NF-κB target genes.