Investigation of the interaction among the components of the cytolethal distending toxin of Haemophilus ducreyi

The cytolethal distending toxin (CDT) of Haemophilus ducreyi is encoded by the cdtABC genes, but the composition of active CDT is not known. Both immu noaffinity and metal affinity chromatographic methods were used to purify H . ducreyi CDT from recombinant Escherichia coli strains bearing wild-type o r mutated H. ducreyi cdtABC genes. Both affinity-purified preparations cont ained CdtA, CdtB, and CdtC proteins. These purification efforts also reveal ed that the formation of a noncovalent CdtB-CdtC complex and production of a fully active CDT complex required the presence of a functional CdtA prote in. When purified recombinant CdtB and CdtC proteins were mixed, only very slight CDT activity was detected. In contrast, when a bacterial cell extrac t containing CdtA was mixed with purified preparations of both CdtB and Cdt C, full CDT activity was reconstituted in vitro. These results indicate tha t CdtA is essential for normal H. ducreyi CDT activity and that CdtA likely modifies or alters either CdtB or CdtC or both to form the active CDT comp lex. (C) 2001 Academic Press.