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Contact-dependent growth inhibition (CDI) systems enable the direct transfer of protein toxins between competing Gram-negative bacteria. CDI +  strains produce cell surface CdiA effector proteins that bind specific receptors on neighboring bacteria to initiate toxin delivery. Three classes of CdiA effectors that recognize different outer membrane protein receptors have been characterized in  Escherichia coli  to date. Here, we describe a fourth effector class that uses the lipopolysaccharide (LPS) core as a receptor to identify target bacteria. Selection for CDI-resistant target cells yielded  waaF  and  waaP  “deep-rough” mutants, which are unable to synthesize the full LPS core. The CDI resistance phenotypes of other  waa  mutants suggest that phosphorylated inner-core heptose residues form a critical CdiA recognition epitope. Class IV  cdi  loci also encode putative lysyl acyltransferases (CdiC) that are homologous to enzymes that lipidate repeats-in-toxin (RTX) cytolysins. We found that catalytically active CdiC is required for full target cell killing activity, and we provide evidence that the acyltransferase appends 3-hydroxydecanoate to a specific Lys residue within the CdiA receptor-binding domain. We propose that the lipid moiety inserts into the hydrophobic leaflet of lipid A to anchor CdiA interactions with the core oligosaccharide. Thus, LPS-binding CDI systems appear to have co-opted an RTX toxin-activating acyltransferase to increase the affinity of CdiA effectors for the target cell outer membrane.

Lipidation of Class IV CdiA Effector Proteins Promotes Target Cell Recognition during Contact-Dependent Growth Inhibition