Shigella flexneri is an enteroinvasive bacterium which causes bacillary dysentery in humans. A major feature of its pathogenic potential is the capacity to invade epithelial cells. Shigella entry into epithelial cells is considered a parasite-induced internalization process requiring polymerization of actin. Here we describe the cytoskeletal rearrangements during S. flexneri invasion of HeLa cells. After an initial contact of the bacterium with the cell surface, distinct nucleation zones of heavy chain actin polymerization appear in close proximity to the contact site underneath the parasite with long filaments being polymerized. These structures then push cellular protrusions that rise beside the entering bacterium, being sustained by tightly bundled long actin filaments organized in parallel orientation with their positive ends pointing to the cytoplasmic membrane. Finally, the cellular projections coalesce above the bacterial body, leading to its internalization. In addition, we found the actin-bundling protein plastin to be concentrated in these protrusions. Since plastin is known to bundle actin filaments in parallel orientation, colocalization of parallel actin filaments and plastin in the cellular protrusions strongly suggested a functional role of this protein in the architecture of parasite-induced cellular projections. Using transfection experiments, we show the differential recruitment of the two plastin isoforms (T- and L-) into Shigella entry zones. By transient expression of a truncated T-plastin which is deprived of one of its actin-binding sites, we also demonstrate the functional role of T-plastin in Shigella entry into HeLa cells.

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