Escherichia coli CdtB mediates cytolethal distending toxin cell cycle arrest

We previously reported that the CdtB polypeptide of Escherichia coli cytolethal distending toxin (CBT) shares significant pattern-specific homology with mammalian type I DNases. In addition, the DNase related residues of CdtB are required for cellular toxicity. Here we demonstrate that purified CdtB converts supercolied plasmid DNA to relaxed and linear forms and promotes cell cycle arrest when combined with an E. coli extract containing CdtA and CdtC. CdtB alone had no effect on HeLa cells, however; introduction of the polypeptide into HeLa cells by electroporation resulted in cellular distension, chromatin fragmentation, and cell cycle arrest, all of which are consequences of CDT action. In contrast to these findings, purified CdtB(H154A) lacked both D;DNA-nicking and cell cycle arrest activities. These results suggest a functional relationship between DNase-related residues in CdtB and CDT biological activity.