Actin dynamics during bacterial invasion of eukaryotic cells

One of the exiting models to study actin filaments dynamics is invasion of eukaryotic cells with bacteria. Adhering the cell surface, enteropathogenic bacteria enter the cell either by interaction of a specific bacterial protein with eukaryotic surface receptor followed by the bacteria internalization via the mechanism of signal transduction or by the bacteria-induced localized membrane ruffling, in which the bacterium is trapped and internalized via the process resembling macropinocytosis. Both processes involve actin polymerization and cytoskeleton rearrangements. Once intracellular, bacteria Listeria monocytogenes and Shigella flexneri escape from vacuole and form at their apical ends a comet-like tails of actin filaments used for intra- and intercellular movement, whose rate is equal to the rate of actin polymerization. It is shown that actin-based propulsion is driven by the free energy released by ATP hydrolysis linked to actin polymerization, and does not require myosin. In addition to actin, activated Arp 2/3 complex, actin depolymerizing factor (ADF/cofilin) and capping protein are required for motility. Eukaryotic cells can also be invaded by non-pathogenic bacteria of Escherichia coli A2 strain producing a new metalloprotease (protease ECP 32) that specifically cleaves actin. Similar protease activity and ability to invade the cells were found in mutants of Shigella flexneri. Strains E. coli and Sh. flexnery of that do not produce ECP 32 were not taken up by the cells. We assume, therefore, that protease ECP 32 may be a factor involved in invasion of eukaryotic cells by these bacteria. ECP 32-cleaved actin does not polymerize if it contains Ca2+ as a tightly bound cation. The ECP 32-cleaved actin containing tightly bound Mg2+ polymerizes less efficiently than intact actin and forms more dynamical polymers. The cleavage diminishes the rate constant of the addition of monomers to and increases the rates of ATP- and ADP-subunit dissociation from the polymer ends. However, the efficiency of the cleaved actin polymerization can be affected by gelsolin and myosin subfragment 1. Thus, cleavage with ECP 32 changes actin properties in a manner promoting fast and regulated dynamics of the system, which is required for bacterial invasion.