SteC is a Salmonella kinase required for SPI-2-dependent F-actin remodelling

Salmonella enterica serovar Typhimurium (S. Typhimurium) replicates inside mammalian cells within membrane‐bound compartments called Salmonella‐containing vacuoles. Intracellular replication is dependent on the activities of several effector proteins translocated across the vacuolar membrane by the Salmonella pathogenicity island 2 (SPI‐2)‐type III secretion system (T3SS). This is accompanied by the formation in the vicinity of bacterial vacuoles of an F‐actin meshwork, thought to be involved in maintaining the integrity of vacuolar membranes. In this study, we investigated the function of the SPI‐2 T3SS effector SteC. An steC mutant strain was not defective for intracellular replication or attenuated for virulence in mice. However, the steC mutant was defective for SPI‐2‐dependent F‐actin meshwork formation in host cells, although the vacuolar membranes surrounding mutant bacteria appeared to be normal. Expression of SteC in fibroblast cells following transfection caused extensive rearrangements of the F‐actin cytoskeleton. Sequence analysis identified amino acid similarity between SteC and the human kinase Raf‐1. A His‐tagged SteC fusion protein had kinase activity in vitro and a point mutant lacking kinase activity was unable to induce F‐actin rearrangements in vivo. We conclude that SPI‐2‐dependent F‐actin meshwork formation depends on the kinase activity of SteC, which resembles more closely eukaryotic than prokaryotic kinases.

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