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The invasion protein InlB from Listeria monocytogenes activates PLC‐γ1 downstream from PI 3‐kinase

Entry of the bacterial pathogen Listeria monocytogenes into non‐phagocytic mammalian cells is mainly mediated by the InlB protein. Here we show that in the human epithelial cell line HEp‐2, the invasion protein InlB activates sequentially a p85β‐p110 class IA PI 3‐kinase and the phospholipase C‐γ1 (PLC‐γ1) without detectable tyrosine phosphorylation of PLC‐γ1. Purified InlB stimulates association of PLC‐γ1 with one or more tyrosine‐phosphorylated proteins, followed by a transient increase in intracellular inositol 1,4,5‐trisphosphate (IP3) levels and a release of intracellular Ca2+ in a PI 3‐kinase‐dependent manner. Infection of HEp‐2 cells with wild‐type L. monocytogenes bacteria also induces association of PLC‐γ1 with phosphotyrosyl proteins. This interaction is undetectable upon infection with a ΔinlB mutant revealing an InlB specific signal. Interestingly, pharmacological or genetic inactivation of PLC‐γ1 does not significantly affect InlB‐mediated bacterial uptake, suggesting that InlB‐mediated PLC‐γ1 activation and calcium mobilization are involved in post‐internalization steps.