Structural and functional characterization of the NF-κB-targeting toxin AIP56 from Photobacterium damselae subsp. piscicida reveals a novel mechanism for membrane interaction and translocation

Bacterial AB toxins are secreted virulence factors that are internalized by target cells through receptor-mediated endocytosis, translocating their enzymatic domain to the cytosol from endosomes (short-trip) or the endoplasmic reticulum (long-trip). To accomplish this, they evolved a multidomain structure organized into either a single polypeptide chain or non-covalently associated polypeptide chains. The prototypical short-trip single-chain toxin is characterized by a receptor-binding domain that confers cellular specificity and a translocation domain responsible for pore formation whereby the catalytic domain translocates to the cytosol in an endosomal acidification-dependent way. In this work, the resolution of the three-dimensional structure of AIP56 showed that instead of a two-domain organization, as previously predicted, AIP56 has a three-domain organization, with a NleC-like catalytic domain associated with a small middle domain that contains the linker-peptide, followed by the receptor-binding domain. In contrast to prototypical single-chain AB toxins, AIP56 does not comprise a typical structurally complex translocation domain; instead, the elements involved in translocation are scattered across its domains. Thus, the catalytic domain contains a helical hairpin that serves as a molecular switch for triggering the conformational changes necessary for membrane insertion only with endosomal acidification, whereas the middle and receptor-binding domains are required for pore formation. Considering the increasing number of putative AIP56 homologous toxins in databases, the data presented here have implications far beyond the AIP56 intoxication process and the virulence of Phdp.