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.