Unique features in the intracellular transport of typhoid toxin revealed by a genome-wide screen.

Typhoid toxin is a virulence factor for Salmonella Typhi and Paratyphi, the cause of typhoid fever in humans. This toxin has a unique architecture in that its pentameric B subunit, made of PltB, is linked to two enzymatic A subunits, the ADP ribosyl transferase PltA and the deoxyribonuclease CdtB. Typhoid toxin is uniquely adapted to humans, recognizing surface glycoprotein sialoglycans terminated in acetyl neuraminic acid, which are preferentially expressed by human cells. The transport pathway to its cellular targets followed by typhoid toxin after receptor binding is currently unknown. Through a genome-wide CRISPR/Cas9-mediated screen we have characterized the mechanisms by which typhoid toxin is transported within human cells. We found that typhoid toxin hijacks specific elements of the retrograde transport and endoplasmic reticulum-associated degradation machineries to reach its subcellular destination within target cells. Our study reveals unique and common features in the transport mechanisms of bacterial toxins that could serve as the bases for the development of novel anti-toxin therapeutic strategies. Author summary Typhoid toxin is an important virulence factor for the human pathogen Salmonella Typhi, the cause of typhoid fever. This toxin is composed of a pentameric B subunit linked to two enzymatic A subunits, resulting in an unusual A2B5 configuration. The B subunit targets the toxin's enzymatic activities by interacting with specific surface receptors. Once internalized, the toxin must be transported to its final subcellular destination by specific transport mechanisms. Here we have used a multidisciplinary approach to define the details of the intracellular transport mechanisms utilized by typhoid toxin. Through a genome-wide screen, we found that typhoid toxin utilizes components of the retrograde transport cellular machinery to arrive to the endoplasmic reticulum, from where it is transported to the cell cytosol by the endoplasmic reticulum-associated degradation pathway. By comparing typhoid toxin's transport pathway with the transport mechanisms utilized by other toxins we have defined unique a common components that transport these toxins to their cellular destinations. These studies may provide the based for the development of novel anti-toxin therapeutic strategies.