The second cholera toxin, Zot, and its plasmid-encoded and phage-encoded homologues constitute a group of putative ATPases with an altered purine NTP-binding motif.

It is shown that the second cholera toxin, Zot, ORF3 product of Pseudomonas plasmid pKB740, and ORF424 product of bacteriophage Pf1 are a group of closely related proteins containing a modified version of the purine NTP-binding motif, with a drastic substitution of tyrosine for a conserved glycine. They are distantly but reliably related to the product of gene I of filamentous bacteriophages which is a putative ATPase containing the classical NTP-binding motif and is involved in bacteriophage assembly and exit from the bacterial cell. Hydropathy analysis suggests that the Zot and gene I product may have a similar transmembrane topology. It is hypothesized that Zot may possess ATPase activity and modify the membrane structure of its target cells in an ATP-dependent fashion. Genes for Zot and the related protein of pKB740 are likely to have evolved from gene I of a Pf1-like bacteriophage.