Functional complementation between bacterial MDR-like export systems: Colicin V, alpha-hemolysin, and Erwinia protease

The antibacterial protein Colicin V (ColV) is secreted from gram-negative bacteria by a signal sequence-independent pathway. The proteins that mediate the export of ColV share sequence similarities with components from other signal sequence-independent export systems such as those for α-hemolysin (Hly) and Erwinia protease (Prt). We report here that the intact HlyBD export system can export active ColV from Escherichia coli strains lacking the ColV export proteins CvaA and CvaB. The individual Hly export genes complement mutations in their respective ColV homologs, but do so at a lower efficiency. When CvaA or CvaB is expressed along with the intact HlyBD exporter, the Cva export protein interferes with export of ColV through the HlyBD system. Gene fusions and point mutations in the ColV structural gene were used to define signals in ColV recognized by the Hly exporter. An export signal in ColV recognized by HlyBD is localized to the amino-terminal 57 amino acids of the protein. In addition, mutations in the ColV export signal differentially affect export through CvaAB and HlyBD, suggesting differences in signal specificity between the Cva and Hly systems. The three Erwinia protease export proteins can also export active ColV, and interference is seen when CvaA or CvaB is expressed along with the intact Prt exporter. Functional complementation is not reciprocal; α-hemolysin is not exported through either the ColV system or the Prt system.