EspFU is a translocated EHEC effector that interacts with Tir and N-WASP and promotes nck-independent actin assembly

Several microbial pathogens including enteropathogenic E. coli (EPEC) exploit mammalian tyrosine-kinase signaling cascades to recruit Nck adaptor proteins and activate N-WASP-Arp2/3-mediated actin assembly. To promote localized actin "pedestal formation," EPEC translocates the bacterial effector protein Tir into the plasma membrane, where it is tyrosine-phosphorylated and binds Nck. Enterohemorrhagic E. coli (EHEC) also generates Tir-dependent pedestals, but in the absence of phosphotyrosines and Nck recruitment. To identify additional EHEC effectors that stimulate phosphotyrosine-independent actin assembly, we systematically generated EHEC mutants containing specific deletions in putative pathogenicity-islands. Among 0.33 Mb of deleted sequences, only one ORF was critical for pedestal formation. It lies within prophage-U, and encodes, a protein similar to the known effector EspF. This proline-rich protein, EspF(U), is the only EHEC effector of actin assembly absent from EPEC. Whereas EHEC Tir cannot efficiently recruit N-WASP or trigger actin polymerization, EspF(U) associates with Tir, binds N-WASP, and potently stimulates Nck-independent actin assembly.