In vitro reconstitution implicates multiple OMP assembly factors in CdiA

Contact-dependent inhibition is a mechanism of interbacterial competition in gram-negative bacteria. A large cell surface protein called CdiA is the key component of CDI systems; it inhibits the growth of neighboring, nonsibling bacteria by delivering a protein toxin through their cell envelopes. Here, we describe an in vitro reconstitution of the initial steps of the toxin delivery process, which normally occur in the outer membrane of a target cell. We show that a large, purified fragment of a CdiA protein from Escherichia coli EC93 spontaneously assembles into proteoliposomes containing the target cell receptor, BamA. BamA normally performs the essential function of assembling β-barrel proteins in the outer membrane using a lateral gate in its own β-barrel, but our data indicate that the CdiA assembly process does not depend on BamA's gating action. Rather, CdiA assembles into the lipid bilayer once it has formed a stable association with the receptor. Our data further indicate that CdiA's relatively rapid insertion into the membrane is followed by a slower autoproteolytic cleavage of the toxin domain at its C-terminal end. The periplasmic chaperones SurA and Skp increase the rate of that toxin cleavage process in vitro, and SurA overexpression in vivo sensitizes cells to CdiA intoxication. Collectively, our results suggest that CdiA may co-opt multiple factors in the β-barrel assembly pathway to facilitate efficient delivery of its toxin into target cells.