The F pilus mediates a novel pathway of CDI toxin import

Contact-dependent growth inhibition (CDI) is a wide-spread form of inter-bacterial competition that requires direct cell-to-cell contact. CDI+ inhibitor cells express CdiA effector proteins on their surface. CdiA binds to specific receptors on susceptible target bacteria and delivers a toxin derived from its C-terminal region (CdiA-CT). Here, we show that purified CdiA-CT536 toxin from uropathogenic Escherichia coli 536 translocates into bacteria, thereby by-passing the requirement for cell-to-cell contact during toxin delivery. Genetic analyses demonstrate that the N-terminal domain of CdiA-CT536 is necessary and sufficient for toxin import. The CdiA receptor plays no role in this import pathway; nor do the Tol and Ton systems, which are exploited to internalize colicin toxins. Instead, CdiA-CT536 import requires conjugative F pili. We provide evidence that the N-terminal domain of CdiA-CT536 interacts with F pilin, and that pilus retraction is critical for toxin import. This pathway is reminiscent of the strategy used by small RNA leviviruses to infect F+ cells. We propose that CdiA-CT536 mimics the pilin-binding maturation proteins of leviviruses, allowing the toxin to bind F pili and become internalized during pilus retraction.