EcxAB Is a Founding Member of a New Family of Metalloprotease AB5 Toxins with a Hybrid Cholera-like B Subunit

AB5 toxins are composed of an enzymatic A subunit that disrupts cellular function associated with a pentameric B subunit required for host cell invasion. EcxAB is an AB5 toxin isolated from clinical strains of Escherichia coli classified as part of the cholera family due to B subunit homology. Cholera-group toxins have catalytic ADP-ribosyltransferases as their A subunits, so it was surprising that EcxA did not. We confirmed that EcxAB self-associates as a functional toxin and obtained its structure. EcxAB is a prototypical member of a hybrid AB5 toxin family containing metzincin-type metalloproteases as their active A subunit paired to a cholera-like B subunit. Furthermore, EcxA is distinct from previously characterized proteases and thus founds an AB5-associated metzincin family that we term the toxilysins. EcxAB provides the first observation of conserved B subunit usage across different AB5 toxin families and provides evidence that the intersubunit interface of these toxins is far more permissive than previously supposed. •EcxA and CfxA are bacterial homologues of animal matrix metalloproteases•EcxAB has a cholera toxin-like B subunit but unique A subunit•EcxB binds to GM1 with similar affinity to CtxB•EcxA is a founding member of a new family of metzincin proteases: the toxilysins AB5 toxins are major virulence factors in pathogenic bacteria that cause diseases such as Cholera, Typhoid, whooping cough, and dysentery. Ng et al. report a fundamentally new type of AB5 toxin that shares some features with those of Cholera but is otherwise surprisingly different.