Structure and function of a fungal AB toxin-like chimerolectin involved in anti-nematode defense
Fungal defense against predators largely relies on protein toxins, many of which are lectins. We previously showed that the production of the nematotoxin CCTX2 is upregulated in the Agaricomycete Coprinopsis cinerea upon predation by nematodes. Here, we classify CCTX2 as the founding member of a family of fungal chimerolectins. Cryo-EM analysis to 3.2 [A] resolution reveals five domains. The four N-terminal {beta}-trefoil fold (BTF) domains cradle a C-terminal domain, which exhibits a novel +{beta} protein fold. Mutational analysis shows that both N-terminal and C-terminal domains are required for nematotoxicity. While the biochemical function of the C-terminal domain remains unclear, the first two BTF domains enable CCTX2 to bind to glycosphingolipids with LacNAc or LacdiNAc glycoepitopes on nematode intestinal epithelial cells. Experiments in the model nematode Caenorhabditis elegans demonstrate that the chimerolectin CCTX2 exploits the endocytic and retrograde trafficking machinery of the target cell to exert its toxicity and obtain access to the yet-to-be-identified intracellular target of the non-lectin domain. The structure and mode of action of CCTX2 is reminiscent of bacterial and plant AB toxins.