The diverse landscape of AB5-type toxins.

AB-type toxins are a group of secreted protein toxins that are central virulence factors for bacterial pathogens such as and certain lineages of pathogenic and . AB toxins are composed of an active (A) subunit that manipulates host cell biology in complex with a pentameric binding/delivery (B) subunit that mediates the toxin's entry into host cells and its subsequent intracellular trafficking. Broadly speaking, all known AB-type toxins adopt similar structural architectures and employ similar mechanisms of binding, entering and trafficking within host cells. Despite this, there is a remarkable amount of diversity amongst AB-type toxins; this includes different toxin families with unrelated activities, as well as variation within families that can have profound functional consequences. In this review, we discuss the diversity that exists amongst characterized AB-type toxins, with an emphasis on the genetic and functional variability within AB toxin families, how this may have evolved, and its impact on human disease.