Phosphorylation of IcsA by cAMP-dependent protein kinase and its effect on intercellular spread of Shigella flexneri

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Abstract
Shigella flexneri, a Gram‐negative bacillus belonging to the family Enterobacteriaceae, causes bacillary dysentery in humans by invading colonic epithelial cells. Processes by which epithelial cells, which are not professional phagocytes, may limit the spread of the invading microorganisms are poorly understood. This paper shows that IcsA (VirG), a 120kDa bacterial outer membrane protein responsible for intracellular and cell‐to‐cell spread through polymerization of actin, is a major substrate for phosphorylation by cyclic‐dependent protein kinases. Site‐directed mutagenesis of a sequence encoding phosphorylation consensus motif SSRRASS, located at residues 754–760, almost completely abolished the ability of this protein to be phosphorylated by protein kinase A. Such mutants expressed a ‘super Ics’ phenotype, characterized by an increased capacity to spread from cell‐to‐cell during the first three hours of infection in the HeLa cell infection assay. These data suggest that host‐cell phosphorytation of key virulence proteins located on the bacterial surface may represent a significant host defence mechanism during the invasion process.