To colonize their hosts successfully, enteropathogenic Escherichia coli (EPEC) must attach to and form intimate associations with host intestinal cells, a process that requires rearrangements of the host cytoskeleton at the site of attachment. A recent study by Gad Frankel and colleagues now reveals that the host protein cytokeratin 18 (CK18) is essential for this, the first time that an intermediate filament protein has been implicated in EPEC pathogenesis.

EPEC adhere to host cells through a mechanism that involves a translocated bacterial protein — Tir (translocated intimin receptor) — that is inserted into the host cell plasma membrane and acts as a receptor for the bacterial outer-membrane protein intimin. Translocated Tir binds the host adaptor protein, Nck, which recruits neural Wiskott–Aldrich syndrome protein (N-WASP), the ARP2/3 complex and other actin-binding proteins. The recruitment of these proteins causes rearrangements of the cytoskeleton to form an actin-rich, pedestal-like structure at the site of attachment.

Frankel and colleagues carried out a yeast two-hybrid screen to identify other host proteins that interact with Tir and pulled out CK18. Tir and CK18 were shown to co-immunoprecipitate from cells infected with EPEC, confirming the relevance of the interaction in vivo. CK18 was shown to be recruited to sites of EPEC attachment in infected intestinal epithelial cells, where it colocalizes with Tir and ARP3, and this recruitment was found to be dependent on Tir, as it was abolished in cells infected with tir− EPEC.

The role of CK18 in EPEC attachment was examined by downregulating its expression in host cells using small interfering RNAs. Cells were then infected with EPEC, and the levels of CK18 expression and the formation of pedestals were analysed. In untransfected cells, with normal levels of CK18, ∼63% of adherent bacteria formed pedestals, whereas in cells in which CK18 expression was reduced or abolished, only ∼29% did so. In addition, in cells in which CK18 expression was downregulated, but not completely abolished, pedestals were shorter than normal and less ARP3 was associated with these structures.

These results indicate that intermediate filament proteins such as CK18 are required for the formation of the actin-rich pedestal at sites of EPEC attachment, and therefore have an essential role in the pathogenesis of these bacteria. How intermediate filaments modulate actin dynamics is not clear at present, and further studies are needed to elucidate the precise function of CK18 in the modification of the host cytoskeleton by EPEC.