CdiA Effectors Use Modular Receptor-Binding Domains To Recognize Target Bacteria

Contact-dependent growth inhibition (CDI) systems encode CdiA effectors, which bind to specific receptors on neighboring bacteria and deliver C-terminal toxin domains to suppress target cell growth. Two classes of CdiA effectors that bind distinct cell surface receptors have been identified, but the molecular basis of receptor specificity is not understood. Alignment of BamA-specific CdiA(EC93) from Escherichia coli EC93 and OmpC-specific CdiA(EC536) from E. coli 536 suggests that the receptor-binding domain resides within a central region that varies between the two effectors. In support of this hypothesis, we find that CdiA(EC93) fragments containing residues Arg1358 to Phe1646 bind specifically to purified BamA. Moreover, chimeric CdiA(EC93) that carries the corresponding sequence from CdiA(EC536) is endowed with OmpC-binding activity, demonstrating that this region dictates receptor specificity. A survey of E. coli CdiA proteins reveals two additional effector classes, which presumably recognize distinct receptors. Using a genetic approach, we identify the outer membrane nucleoside transporter Tsx as the receptor for a third class of CdiA effectors. Thus, CDI systems exploit multiple outer membrane proteins to identify and engage target cells. These results underscore the modularity of CdiA proteins and suggest that novel effectors can be constructed through genetic recombination to interchange different receptor-binding domains and toxic payloads.