E. coli secreted protein F promotes EPEC invasion of intestinal epithelial cells via an SNX9-dependent mechanism

Enteropathogenic Escherichia coli (EPEC) infection requires the injection of effector proteins into intestinal epithelial cells (IECs) via type 3 secretion. Type 3-secreted effectors can interfere with IEC signalling pathways via specific protein-protein interactions. For example, E. coli secreted protein F (EspF) binds sorting nexin 9 (SNX9), an endocytic regulator, resulting in tubulation of the plasma membrane. Our aim was to determine the mechanism of EspF/SNX9-induced membrane tubulation. Point mutation of the SNX9 lipid binding domains or truncation of the EspF SNX9 binding domains significantly inhibited tubulation, as did inhibition of clathrin coated pit (CCP) assembly. Although characterized as non-invasive, EPEC are known to invade IECs in vitro and in vivo. Indeed, we found significant invasion of Caco-2 cells by EPEC, which, like tubulation, was blocked by pharmacological inhibition of CCPs. Interestingly, however, inhibition of dynamin activity did not prevent tubulation or EPEC invasion, which is in contrast to Salmonella invasion, which requires dynamin activity. Our data also indicate that EPEC invasion is dependent on EspF and its interaction with SNX9. Together, these findings suggest that EspF promotes EPEC invasion of IECs by harnessing the membrane-deforming activity of SNX9.