Repeated cycles of rapid actin assembly and disassembly on epithelial cell phagosomes

We have found that early in infection of the intracellular pathogen Listeria monocytogenes in Madin-Darby canine kidney epithelial cells expressing actin conjugated to green fluorescent protein, F-actin rapidly assembles (approximately 25 s) and disassembles (approximately 30 s) around the bacteria, a phenomenon we call flashing. L. monocytogenes strains unable to perform actin-based motility or unable to escape the phagosome were capable of flashing, suggesting that the actin assembly occurs on the phagosome membrane. Cycles of actin assembly and disassembly could occur repeatedly on the same phagosome. Indirect immunofluorescence showed that most bacteria were fully internalized when flashing occurred, suggesting that actin flashing does not represent phagocytosis. Escherichia coli expressing invA, a gene product from Yersinia pseudotuberculosis that mediates cellular invasion, also induced flashing. Furthermore, polystyrene beads coated with E-cadherin or transferrin also induced flashing after internalization. This suggests that flashing occurs downstream of several distinct molecular entry mechanisms and may be a general consequence of internalization of large objects by epithelial cells.