Cdc42 facilitates invasion but not the actin-based motility of Shigella

The enteric pathogen Shigella utilizes host-encoded proteins to invade the gastrointestinal tract. Efficient invasion of host cells requires the stimulation of Rho-family GTPases and cytoskeletal alterations by Shigella-encoded IpaC [1]. Following invasion and lysis of the phagosome, Shigella exploits the host's actin-based polymerization machinery to assemble an actin tail that serves as the propulsive force required for spreading within and between cells (2,3]. The Shigella surface protein IcsA stimulates actin-tail formation by recruiting host-encoded N-WASP to drive Arp2/3-mediated actin assembly [4-7]. N-WASP is absolutely required for Shigella motility, but not for Shigella invasion [4, 6]. Although Rho-family GTPases have been implicated in both the invasion and motility of Shigella, the role of Cdc42, an N-WASP activator, in this process has been controversial [8, 9]. In these studies, we have examined the role of Cdc42 in Shigella invasion and actin-based motility using Cdc42-deficient cells. We demonstrate that Cdc42 is required for efficient Shigella invasion but reveal a minor Cdc42-independent pathway that can permit Shigella invasion. However, the actin-based motility of Shigella, as well as vaccinia, proceeds unperturbed in the absence of Cdc42. These data further support the involvement of distinct host-encoded proteins in the steps regulating invasion and intercellular spread of Shigella.