Recombinant Cholera Toxin B subunit

Acute secretory diarrheal disease caused by toxigenic strains of the gram-negative bacterium Vibrio cholerae is called cholera. Based on the O-antigen of the surface lipopolysaccharide, Vibrio cholerae can be divided into more than two hundred serogroups, only two of which are capable of secreting cholera toxin (CT) and causing disease. The O1 serogroup has been subdivided into the "classical" and "El Tor" biotypes.

CT contains two functional components, the enzymatic A fragment and the cell-bound B fragment. These toxins are released into the extracellular space but attack targets inside the host cell. Thus, the toxin must cross a membrane barrier to reach its intracellular target. Only after the surface-bound toxin is endocytosed is it translocated to the cell membrane. In vivo activity of the toxin requires separation of the A chain from the B subunit. In addition, the A chain must reach an unfolded state to enter the cytoplasm. The transported toxin then returns to the folded conformation to modify its cytoplasmic target. The A fragment of CT is divided into A1 and A2 subunits, which are connected by disulfide bonds. Catalytic activity is present in the A1 subunit, while the A2 subunit acts as a bridging CTA1 anchor to the B portion. The monomers of the B fragment form a cyclic pentamer, with the C-terminal region of the A2 subunit occupying the center pore of the structure.

Figure 1. CT structure
(Source: White C, et al. 2022)

Vibrio cholerae virulence is determined by two key factors: CT and toxin coregulated pilus (TCP). These factors are controlled by a signaling cascade of ToxR regulators, which respond to environmental stimuli to regulate the production of host environmental virulence factors. TCP helps bacteria colonize the gut, and CT is critical for enterotoxicity. CT binds to GM1 ganglioside receptors on the surface of intestinal epithelial cells and enters the cells by endocytosis. CT is subsequently processed in the endoplasmic reticulum to stimulate cyclic AMP production via the adenylate cyclase complex, and the activated kinase triggers chloride exocytosis and inhibits sodium uptake. The resulting osmotic potential draws water away from the surrounding tissues, leading to diarrhea.

Figure 2. Cholera toxin mechanism of action
(Source: Gabutti G, et al. 2020)

Recombinant CT B subunit