Molecular aspects of complement-mediated bacterial killing. Periplasmic conversion of C9 from a protoxin to a toxin.

As part of the membrane attack complex complement protein C9 is responsible for direct killing of bacteria. Here we show that in the periplasmic space of an Escherichia coli cell C9 is converted from a protoxin to a toxin by periplasmic conditions missing in spheroplasts. This conversion is independent of the pathway by which C9 enters the periplasm. Both, C9 shocked into the periplasm and plasmid-expressed C9 targeted to the periplasm via a signal sequence are toxic. Toxicity requires disulfide-linked C9 because export into the periplasm of cells defective in disulfide bond synthesis (dsbA and dsbB mutants) is not toxic unless N-acetylcysteine is added externally to promote cystines. A N-terminal fragment, C9[1-144], is not toxic nor is cytoplasmically expressed C9, even in trxB mutants that are able to form disulfide bonds in the cytoplasm. Importantly, expression of full-length C9 in complement-resistant cells has no effect on their viability. Expression and translocation into the periplasm may provide a novel model to identify molecular mechanisms of other bactericidal disulfide-linked proteins and to investigate the nature of bacterial complement resistance.