CdtC. In terms of function, it is an AB toxin. In this context, the CdtB subunit is actually the catalytically active "A" subunit, and the CdtA and CdtC together form the binding "B" subunit, which helps the toxin bind and enter target cells. Some literature refers to the toxin structure as AB2 to reflect the presence of both CdtA and CdtC. Different from all other CDTs, Salmonella enterica serovar Typhi CDT (SeCDT) has no CdtA and CdtC homologues. However, encoded closely to the active subunit cdtb, the Pertussis-like toxin A and B (pltA/pltB) have been shown to be essential for cellular intoxication. PltA and PltB have a different structure from CdtA and CdtC, thus promoting CdtB activity in a different way. Both PltA and PltB have been found to bind directly to CdtB in vitro. In addition, different from all other CDTs, Salmonella genotoxin is produced only upon bacterial internalization in infected cells, thus the SeCDT traffic may differ remarkably from the canonical ones. CdtB CdtB is considered the active subunit of the CDT holotoxin. Microinjection of CdtB into susceptible cells without CdtA or CdtC results in the G2/M cell cycle arrest and cytoplasmic distension characteristic of CDT toxins. The structure of CdtB is well-conserved between different bacteria. The CdtB subunit is the most sequentially conserved between species. The molecular weight of CdtB ranges from 28 kDa to 29 kDa, depending on the species. As the active subunit, CdtB is termed the "A" subunit according to the AB toxin model. This confusing nomenclature is due to the naming of the toxin's subunits before their individual functions were understood. Activity CdtB exhibits at least two enzymatic activities- DNase activity capable of introducing double-strand breaks in DNA, and a phosphatase activity that resembles phosphatidylinositol 3,4,5-triphosphatase. Both activities can be demonstrated in vitro in the absence of the other two subunits. The relative importance of each activity in vivo is unclear. Mutations that reduce either activity also reduce the toxin's ability to induce G2/M phase arrest in at least some of the susceptible cell lines. Similarities to mammalian DNase I CdtB is functionally homologous to mammalian DNase I and contains a conserved pentapeptide sequence found in all DNase I enzymes to date. In addition, several residues critical to DNase I's ability to break the phosphodiester bonds in the DNA backbone are found in the CdtB structure. A 2002 paper studying the effect of