Receptor affinity, stability and binding mode of Shiga toxins are determinants of toxicity

The closely related Shiga toxins, Shiga toxin 1 (Stx1) and Shiga toxin 2 (Stx2), can bind to Gb3 receptors. However, Stx2-producing enterohemorrhagic Escherichia coli (EHEC) strains are more commonly associated with serious human disease (viz., hemolytic–uremic syndrome) than Stx1-producing strains. To clarify the relationship between properties and toxicity of these toxins, we constructed and analyzed a hybrid holotoxin composed of Stx2A and Stx1B, designated as Stx2A1B, and a B subunit chimeric holotoxin composed of Stx2A and Stx2B (III V), designated as Stx2A2B (III V). The affinity of Stx2A1B to Gb3 was lower than that of Stx1, higher than that of Stx2 and identical to that of Stx2A2B (III V). On the other hand, the 50% lethal dose (LD50) for mice of Stx2A1B was lower than that of Stx1, but higher than that of Stx2. These results suggested that pathogenicity in mice was inversely related to the receptor affinity of the holotoxins. However, LD50 of Stx2A1B was not identical to that of Stx2A2B (III V). Gel filtration analysis indicated that Stx2A2B (III V) was relatively less stable than Stx2A1B. Moreover, cross-linking experiments demonstrated that the modes of cell surface binding of Stx2A2B (III V) and Stx2A1B were different. These results indicated that the receptor affinity, stability and binding mode of Shiga toxins might be important determinants for toxicity in mice.