The enterohaemorrhagic Escherichia coli (serotype O157 : H7) Tir molecule is not functionally interchangeable for its enteropathogenic E-coli (serotype O127 : H6) homologue

A major virulence determinant of enteropathogenic Escherichia coil (EPEC) i s the Tir molecule that is translocated into the plasma membrane where it o rchestrates cytoskeletal rearrangements. Tir undergoes several phosphorylat ion events within host cells, with modification on a tyrosine essential for its actin-nucleating function. The EHEC (serotype O157:H7) Tir homologue i s not tyrosine phosphorylated implying that it uses an alternative mechanis m to nucleate actin. This is supported in this study by the demonstration t hat EHEC Tir is unable to functionally substitute for its EPEC homologue. L ike EPEC, the EHEC Tir molecule is phosphorylated within host cells, with t he actin-nucleating dysfunction correlated to an altered modification profi le. In contrast to EHEC Tir, the EPEC Tir molecule mediated actin nucleatio n whether delivered into host cells by either strain. Thus, it would appear that EHEC encodes specific factor(s) that facilitate the correct modificat ion of its Tir molecule within host cells. Domain-swapping experiments reve aled that the N-terminal, alpha -actinin binding, Tir domains were function ally interchangeable, with both the actin-nucleating dysfunction and altere d modification profiles linked to the EHEC C-terminal Tir domain. This tyro sine-independent modification process presumably confers an advantage to EH EC O157:H7 and may contribute to the prevalence of this strain in EHEC dise ase. The presented data are also consistent with EPEC and EHEC sharing non- phosphotyrosine phosphorylation event(s), with an important role for such m odifications in Tir function. An EHEC-induced phosphotyrosine dephosphoryla tion activity is also identified.