Enteropathogenic E. coli relies on collaboration between the formin mDia1 and the Arp2/3 complex for actin pedestal biogenesis and maintenance.

Enteropathogenic and enterohemorrhagic E. coli (EPEC and EHEC) are closely related extracellular pathogens that reorganize host cell actin into "pedestals" beneath the tightly adherent bacteria. This pedestal-forming activity is both a critical step in pathogenesis, and it makes EPEC and EHEC useful models for studying the actin rearrangements that underlie membrane protrusions. To generate pedestals, EPEC relies on the tyrosine phosphorylated bacterial effector protein Tir to bind host adaptor proteins that recruit N-WASP, a nucleation-promoting factor that activates the Arp2/3 complex to drive actin polymerization. In contrast, EHEC depends on the effector EspFU to multimerize N-WASP and promote Arp2/3 activation. Although these core pathways of pedestal assembly are well-characterized, the contributions of additional actin nucleation factors are unknown. We investigated potential cooperation between the Arp2/3 complex and other classes of nucleators using chemical inhibitors, siRNAs, and knockout cell lines. We found that inhibition of formins impairs actin pedestal assembly, motility, and cellular colonization for bacteria using the EPEC, but not the EHEC, pathway of actin polymerization. We also identified mDia1 as the formin contributing to EPEC pedestal assembly, as its expression level positively correlates with the efficiency of pedestal formation, and it localizes to the base of pedestals both during their initiation and once they have reached steady state. Collectively, our data suggest that mDia1 enhances EPEC pedestal biogenesis and maintenance by generating seed filaments to be used by the N-WASP-Arp2/3-dependent actin nucleation machinery and by sustaining Src-mediated phosphorylation of Tir.