Leidreiter, F. | bioRxiv Science

bioRxiv2019

Glycan-dependent two-step cell adhesion mechanism of Tc toxins

Toxin complex (Tc) toxins are virulence factors widespread in insect and human bacterial pathogens. Tcs are composed of three subunits: TcA, TcB and TcC. TcA facilitates receptor-toxin interaction and membrane permeation, TcB and TcC form a toxin-encapsulating cocoon. While the mechanisms of holotoxin assembly and prepore-to-pore transition have been well-described, little is known about receptor binding and cellular uptake of Tcs. Here, we identify two classes of glycans, heparins/heparan sulfates and Lewis antigens, that act as receptors for different TcAs from insect- and human pathogenic bacteria. Glycan array screening and electron cryo microscopy (cryo-EM) structures reveal that all tested TcAs bind unexpectedly with their -helical part of the shell domain to negatively charged heparins. In addition, TcdA1 from the insect-pathogen Photorhabdus luminescens binds to Lewis antigens with micromolar affinity. A cryo-EM structure of the TcdA1-Lewis X complex reveals that the glycan interacts with the receptor-binding domain D of the toxin. Our results suggest a two-step association mechanism of Tc toxins involving glycans on the surface of host cells.

molecular biology↗

bioRxiv2019

Conserved architecture of Tc toxins from human and insect pathogenic bacteria

Tc toxin complexes use a syringe-like mechanism to penetrate the membrane and translocate a toxic enzyme into the host cytosol. They are composed of three components: TcA, TcB and TcC. Until recently, low-resolution structures of TcA from different bacteria suggested that Tc toxins differ considerably in their architecture and possibly in their mechanism of action. Here, we present high-resolution structures and functional studies of five TcAs from different insect and human pathogenic bacteria. Contrary to previous expectations, their overall composition and domain organization is almost identical. The TcAs assemble as a pentamer with a central -helical channel surrounded by a shell composed of conserved -helical domains and variable {beta}-sheet domains. Essential structural features, including a conserved trefoil protein knot, are present in all five TcAs, suggesting a common mechanism of action. All TcAs form functional pores and can be combined with TcB-TcC subunits from other species resulting in chimeric holotoxins. We have identified a conserved ionic pair that stabilizes the shell, likely operating as a strong latch that only springs open after the destabilization of other regions. Our results lead to new insights into the architecture and host specificity of the Tc toxin family.

molecular biology↗