Enteropathogenic Escherichia coli manipulates the host exocyst complex to enhance pedestal formation

Enteropathogenic Escherichia coli (EPEC), a major cause of diarrheal disease, produces plasma membrane pedestals that promote colonization of host cells. Critical for pedestal formation is EPEC’s type III secretion system, which injects ~ 20 effector proteins into human cells. One of these effectors is Tir, which inserts into the host plasma membrane and stimulates the assembly of actin filaments essential for pedestal generation. To date, actin polymerization is the only host process known to contribute to pedestal formation. Here we report that EPEC co-opts the membrane trafficking pathway of polarized exocytosis, which acts together with actin polymerization to allow the efficient production of pedestals. Polarized exocytosis is mediated by the exocyst — a human octameric complex that uses intracellular vesicles to expand the plasma membrane. We found that EPEC stimulated exocytosis at sites of pedestal formation in a manner dependent on the exocyst. The bacterial effector EspH recruited the exocyst and promoted exocytosis. Genetic inactivation of espH or RNA interference (RNAi)-induced depletion of exocyst components reduced both the frequency and size of pedestals and impaired EPEC colonization of host cells. Additional RNAi experiments indicated that the exocyst is dispensable for actin filament assembly in pedestals. Co-depletion of components of the exocyst and the Arp2/3 complex showed that exocytosis and actin polymerization make additive contributions to pedestal formation. Collectively, these results indicate that exocyst-mediated expansion of the plasma membrane acts together with actin polymerization to optimize the generation of pedestals.