Two distinct binding sites for globotriaosyl ceramide on verotoxins: Identification by molecular modelling and confirmation using deoxy analogues and a new glycolipid receptor for all verotoxins

Background: The Escherichia coli verotoxins (VTs) can initiate human vascular disease via the specific recognition of globotriaosyl-ceramide (Gb(3)) on target endothelial cells. To explore the structural basis for receptor recognition by different VTs we used molecular modelling based on the crystal structure of VT1, mutational data and binding data for deoxy galabiosyl receptors. Results: We propose a model for the verotoxin 'cleft-site complex' with Gb(3). Energy minimizations of Gb(3) within the 'cleft site' of verotoxins VT1, VT2, VT2c and VT2e resulted in stable complexes with hydrogen-bonding systems that were in agreement with binding data obtained for mono-deoxy analogues of Gb(3). N-deacetylated globoside (aminoGb(4)), which was found to be a new, efficient receptor for all verotoxins, can be favorably accommodated in the cleft site of the VTs by formation of a salt bridge between the galactosamine and a cluster of aspartates in the site. The model is further extended to explain the binding of globoside by VT2e. Docking data support the possibility of an additional binding site for Gb(3) on VT1. Conclusions: The proposed models for the complexes of verotoxins with their globoglycolipid receptors are consistent with receptor analogue binding data and explain previously published mutational studies. The results provide a first approach to the design of specific inhibitors of VT-receptor binding.