Src is the kinase of the Helicobacter pylori CagA protein in vitro and in vivo

The gastric pathogen Helicobacter pylori uses a type IV secretion system to inject the bacterial CagA protein into gastric epithelial cells. Within the host cell, CagA becomes phosphorylated on tyrosine residues and initiates cytoskeletal rearrangements. We demonstrate here that Src-like protein-tyrosine kinases mediate CagA phosphorylation in vitro and in vivo. First, the Src-specific tyrosine kinase inhibitor PP2 specifically blocks CagA phosphorylation and cytoskeletal rearrangements thereby inhibiting the CagA-induced hummingbird phenotype of gastric epithelial cells. Second, CagA is in vivo phosphorylated by transiently expressed c-Src. Third, recombinant c-Src and lysates derived from c-Src-expressing fibroblasts but not lysates derived from Src-, Yes-, and Fyn-deficient cells phosphorylated CagA in vitro. Fourth, a transfected CagA-GFP fusion protein is phosphorylated in vivo in Src-positive fibroblasts but not in Src-, Yes-, and Fyn-deficient cells. Because a CagA-GFP fusion protein mutated in an EPIYA motif is not efficiently phosphorylated in any of these fibroblast cells, the CagA EPIYA motif appears to constitute the major c-Src phosphorylation site conserved among CagA-positive Helicobacter strains.