Recombinant shiga toxin-2 subunit B [His]

Shiga-like toxin 2 (SLT2, Stx2) is a toxigenic protein that some Escherichia coli strains secrete and belong to the bacterial AB5 family. SLT2 has two components, A and B that are cytotoxic in their own right. The A subunit acts as a ribosome-breaking protein, breaking adenine chains from 28S rRNA, inhibiting protein production and leading to cell death. It is the B subunit (which has five subunit molecules) that binds to the Gb3 receptor (globotriaosylceramide) on the surface of host cells. This interaction mediates toxin endocytosis, allowing the A subunit to enter the cytoplasm and exert its toxic effect. The N-glycosidase activity of SLT2 resides within the A subunit. Once SLT2 binds to the host cell through the B subunit, the B subunit forms a pentamer, directing the toxin into the cell, where it undergoes retrograde transport to the endoplasmic reticulum, releasing the A subunit to perform its cytotoxic action. Notably, the A subunit of SLT2 has a longer N-terminal than that of SLT1, affecting its affinity for the Gb3 receptor. The extended N-terminal sequence in SLT2's A subunit structurally distinguishes it from SLT1, potentially contributing to its reduced toxicity in Vero cells and other mammalian cells. Yet in target cells that are pathogenic in conditions such as hemolytic uremic syndrome (HUS), SLT2 is more so, meaning that its toxicity is localized to different cell types, depending on membrane elements and receptor composition.

Figure 1. Histopathology of Kidney Injury in Non-Human Primates Following Stx1 or Stx2 Attack. (Source: Mayer CL, et al., 2012)

SLT2 is a prominent condition among children with hemolytic uremic syndrome (HUS), a debilitating condition that includes thrombocytopenia, microangiopathy and acute kidney failure. SLT2 is spread by Shiga toxin-producing E. coli (STEC), the most prevalent of which is O157.When STEC is introduced into the body by food or water, SLT2 is digested by intestinal epithelial cells and moves through the bloodstream, killing endothelial cells, particularly glomerular endothelial cells. In kidneys, the cytotoxic and inflammatory actions of SLT2 cause severe destruction of glomerular endothelial cells, eventually resulting in HUS. This high affinity for SLT2's B subunit allows it to target the heavily expressed Gb3 receptor on glomerular endothelial cells, entering the cytoplasm via retrograde transport. This selective binding mechanism is thought to be the key factor in SLT2 accumulation in the kidneys and the resultant severe renal failure in HUS patients. Studies have shown that SLT2 is significantly more toxic than SLT1, with its binding capacity to endothelial cells playing a decisive role in HUS pathogenesis. SLT2 has several subtypes, including SLT2a, SLT2c, and SLT2d, which differ in cytotoxicity and immune response. For example, SLT2c exhibits lower toxicity in Vero cells, while SLT2d shows enhanced cytotoxicity after activation by elastase, closely associated with severe HUS symptoms. The toxicity of SLT2d increases significantly upon exposure to elastase in host intestinal mucus, suggesting its unique pathogenic role in severe STEC infections. The high toxicity of SLT2a and SLT2d is linked to differences in the structure of their B subunits, affecting the binding strength and internalization efficiency of the toxin. Additionally, SLT2 toxicity is also related to the host immune response. SLT2 induces higher levels of cytokine release in human cells than SLT1, especially during later stages of infection, causing a more intense inflammatory response. Literature indicates that SLT2a has a much greater cytotoxic effect on renal tubular epithelial cells than SLT1a. SLT2 also shows a strong immune-activating effect by inducing macrophage inflammatory protein MIP-1α and MIP-1β expression, potentially associated with SLT2's affinity for specific lipid rafts and internalization pathways. SLT2 tends to bind to cholesterol and phosphatidylcholine complexes in cell membranes, showing a distinct cytotoxicity profile, which may explain toxicity differences observed across various animal models.

Currently, the structural characteristics and toxicity mechanisms of SLT2 provide a foundation for vaccine development, antibody testing, and functional studies. The non-toxic B subunit of SLT2, with its receptor-specific binding ability, is an ideal target for antigen design in vaccine research. By constructing non-toxic variants or recombinant proteins of the SLT2 B subunit, researchers aim to elicit a protective immune response against STEC infection, thereby reducing the incidence of HUS. Moreover, the high specificity of SLT2 for certain receptors provides insights for targeted drug development, with approaches to prevent SLT2's binding to Gb3 receptors to avoid toxin internalization and toxicity. In summary, Shiga-like toxin 2 (SLT2) poses a significant threat to human health, playing a crucial role in the pathogenesis of HUS. Through its specific A and B subunit structures, SLT2 selectively binds to host cell receptors and blocks protein synthesis, leading to cell death and tissue damage. Differences in the toxicity and immune activity of SLT2's various subtypes deepen our understanding of its pathogenic mechanism and offer potential applications for treating and preventing HUS-related diseases. Future research will focus on further elucidating the structure-function relationships of SLT2, as well as developing vaccines and targeted therapies based on its B subunit, with the goal of effectively controlling SLT2-related diseases caused by STEC.

Recombinant E. coli Shiga Toxin-2 Subunit B
Shiga Toxin 2 Recombinant Protein