Salmonella effector kinase SteC is activated by phosphorylation at Serine 379

The pathogen Salmonella, which causes significant human morbidity and mortality, encodes an effector kinase, SteC, which mediates actin polymerisation and cell migration. Given the minimal nature of its kinase domain, it remains unclear how SteC is catalytically active and how this activity is regulated. Here, we show that SteC is activated following the phosphorylation of the highly conserved S379 residue, which can be carried out by a host kinase. Phosphorylation of S379 dramatically increases nucleotide binding affinity of SteC, enabling substrate phosphorylation and promoting actin polymerisation. Further mutational analysis identified the functional role of HD and DGD motifs that likely mimic the HxD and DFG motifs of eukaryotic kinases. Meanwhile, the C-tail of SteC, encompassing amino acids 429-457, is essential for function following translocation from Salmonella, but dispensable for catalysis in vitro. Overall, our findings uncover two previously unappreciated mechanisms that mediate the activity of the only Salmonella effector kinase within the host.