Investigation of the steps involved in the difference of susceptibility of Ephestia kuehniella and Spodoptera littoralis to the Bacillus thuringiensis Vip3Aa16 toxin

Bacterial Vegetative Insecticidal Proteins (Vip) from Entomopathogenic Bacteria

“…The behavioral symptoms observed in susceptible insects after ingestion of the Vip3Aa protein resemble the ones observed after Cry intoxication: feeding cessation, loss of gut peristalsis, and overall paralysis of the insect (65). Analysis of gut cross sections of susceptible insects after ingestion of the Vip3Aa protein shows extensive damage in the midgut, with disrupted, swollen, and/or lysed epithelial cells and leakage of cellular material into the lumen (65,68,69,(88)(89)(90)(91).…”

“…In vitro proteolysis of full-length Vip3Aa proteins using insect midgut juice showed that they are processed to several major proteolytic products, generally of ϳ62 to 66, 45, 33, and 22 kDa (65,68,69,88,89). The 22-kDa fragment corresponds to the N terminus of the protein (amino acids 1 to 198 in Vip3Aa1), the 66-kDa fragment corresponds to the remainder of the protein (from amino acid 199 to the end of Vip3Aa1), and the 45-and 33-kDa fragments are thought to be derived from the 66-kDa portion (57).…”

“…Although an early study claimed that the minimal fragment that retained insecticidal activity after proteolysis was the 33-kDa fragment (57), all subsequent studies are in favor of the 62-to 66-kDa fragment being the Vip3Aa toxic core (58,60,68,69,71,72,(87)(88)(89)(92)(93)(94). Interestingly, the 20-kDa fragment produced upon proteolytic processing of the Vip3Aa16 protoxin copurifies with the 62-kDa fragment, suggesting that after activation of the full-length protein, the two fragments remain together (89).…”

“…Compared to Cry proteins, the 62-to 66-kDa toxic core of the Vip3A proteins is more susceptible to the action of proteases. Incubation of either Vip3Aa or Vip3Ae with commercial serine proteases or insect midgut juice showed the unstable nature of the 62-kDa fragment, which started to break down even before all the protoxin was processed (65,69,88,89,94). Partial purification of peptidases from S. frugiperda midgut juice showed that cationic trypsin-like and anionic chymotrypsin-like peptidases are involved in the formation of the Vip3A 62-kDa fragment, whereas cationic chymotrypsin-like peptidases participated in its further processing (94).…”

“…It has been shown that the midgut juice of a nonsusceptible insect (O. nubilalis) could process Vip3A in vitro into a 65-kDa fragment that was fully toxic when fed to susceptible insects (65). However, in some cases, the rate of processing of the full-length protein was proposed to account for differences in the toxicity of a given Vip3A protein to different insect species (71,88,94). Indeed, some studies have shown that differences in mortality disappeared when the trypsin-activated protein was used instead of the fulllength protein (71,72).…”