Cry proteins become toxic after:
Bt protoxins are inert in plant tissues because solubilization and cleavage conditions absent in neutral pH plant cells. Upon ingestion by susceptible larvae, midgut lumen alkalinity dissociates crystal lattice, releasing soluble protoxin. Midgut serine proteases like trypsin and chymotrypsin cleave N-terminus 28 and C-terminus half, exposing hydrophobic helix alpha-1 for membrane insertion. Resulting 60 to 65 kDa toxin undergoes conformational change induced by cadherin receptor binding, leading to oligomerization into pre-pore tetramer that inserts into apical microvilli causing osmotic imbalance. This proteolytic activation step provides species specificity, as only insects possessing appropriate pH and protease pattern generate active toxin. Mammals lacking alkaline gut and specific receptors do not activate protoxin efficiently, contributing to safety profile of Bt crops. Hence gut protease processing is pivotal step converting inactive inclusion into membrane-perforating toxin. In vitro activation assays use midgut juice from susceptible larvae incubated with solubilized crystal to generate active core, confirming requirement for specific proteases. Inhibitors of serine proteases block activation and toxicity, linking proteolysis to insecticidal activity. Engineered Cry toxins with introduced chymotrypsin cleavage sites broaden activation range, demonstrating importance of gut protease processing for host range determination and biosafety considerations in non-target organisms.
Ref: Bravo Biochim Biophys Acta 2007 activation; NCBI NBK24601; Pardo-Lopez Peptides 2013 cleavage; PubMed 16885436.