A role for cortactin in Listeria monocytogenes invasion of NIH 3T3 cells, but not in its intracellular motility

Cortactin is an F-actin binding protein that binds to the Arp2/3 complex, stimulates its actin nucleation activity, and inhibits actin filament debranching. Using RNA interference directed against cortactin, we explored the importance of cortactin for several processes involving dynamic actin assembly. Silencing cortactin expression was efficiently achieved in HeLa and NIH 3T3 cells, with less than 5% of cortactin expression in siRNA-treated cells. Surprisingly, endocytosis in HeLa and NIH 3T3 cells, and cell migration rates, were not altered by RNAi-rnediated cortactin silencing. Listeria utilizes actin-based motility to move within and spread among mammalian host cells; its actin-clouds and tails recruit cortactin. We explored the role of cortactin during the Listeria life cycle in cortactin "knockdown" NIH 3T3 cells. Interestingly, cortactin siRNA-treated cells showed a significant reduction in the efficiency of the bacteria invasion in NIH 3T3 cells. However, cortactin depletion did not interfere with assembly of Listeria actin clouds or actin tails, or Listeria intracellular motility or speed. Therefore, our findings suggest that cortactin plays a role in Listeria internalization, but not in the formation of actin clouds and tails, or in bacteria intracellular motility.