Enhanced exocytosis of the receptor BT-R1 induced by the Cry1Ab toxin of Bacillus thuringiensis directly correlates to the execution of cell death

Cry1Ab toxin produced by Bacillus thuringiensis exerts insecticidal action upon binding to BT-R1, a cadherin receptor localized in the midgut epithelium of the tobacco hornworm Manduca sexta. The univalent binding of toxin to receptor transmits a death signal into the cell and turns on a multi-step signal transduction pathway involving adenylyl cyclase (AC) and protein kinase A (PKA), which drives the biochemical events that culminate in oncotic cell death. Here, we report that cell killing by the Cry1Ab toxin is a dynamic episode in which the toxin promotes exocytotic transport of BT-R1 from intracellular membrane vesicles to the plasma membrane. The resultant dramatic increase in BT-R1 displayed on the surface of toxin-treated cells effects the recruitment and concomitant binding of additional toxin monomers which, in turn, amplifies the original signal in a cascade-like manner. Blocking the activation of AC/PKA signal transduction by either EDTA or PKAi inhibits exocytotic trafficking of BT-R1 and prevents cell death. Moreover, the exocytosis inhibitor Exo1 blocks translocation of receptor and progression of cell death alike. Obviously, movement of BT-R1 is mediated by toxin-induced signal transduction and amplification of this signaling apparently is critical to the execution of cell death.