WhyBacillus thuringiensisinsecticidal toxins are so effective: unique features of their mode of action

Abstract

The spore-forming bacterium Bacillus thuringiensis produces intracellular inclusions comprised of protoxins active on several orders of insects. These highly effective and specific toxins have great potential in agriculture and for the control of disease-related insect vectors. Inclusions ingested by larvae are solubilized and converted to active toxins in the midgut. There are two major classes, the cytolytic toxins and the delta-endotoxins. The former are produced by B. thuringiensis subspecies active on Diptera. The latter, which will be the focus of this review, are more prevalent and active on at least three orders of insects. They have a three-domain structure with extensive functional interactions among the domains. The initial reversible binding to receptors on larval midgut cells is largely dependent upon domains II and III. Subsequent steps involve toxin insertion into the membrane and aggregation, leading to the formation of gated, cation-selective channels. The channels are comprised of certain amphipathic helices in domain I, but the three processes of insertion, aggregation and the formation of functional channels are probably dependent upon all three domains. Lethality is believed to be due to destruction of the transmembrane potential, with the subsequent osmotic lysis of cells lining the midgut. In this review, the mode of action of these delta-endotoxins will be discussed with emphasis on unique features.

Heliothis virescens and Manduca sextaLipid Rafts Are Involved in Cry1A Toxin Binding to the Midgut Epithelium and Subsequent Pore Formation

“…Cry toxin pores are likely to be composed of four to six toxin molecules (52), which suggests that Cry toxin aggregation occurs before the membrane pore is formed, but it is not clear how the Cry toxin aggregates. However, it is clearly established that many of the processes that lead to toxic action of Cry proteins involve the insect midgut epithelium, including activation, toxin insertion, aggregation, and pore formation (17). In this report we show that detergent-insoluble lipid rafts are present in the midgut epithelium of insects susceptible to the Cry1A toxins, and several of the Cry1Ac-binding proteins, including the 120-and 170-kDa aminopeptidases from H. virescens and the 120-kDa aminopeptidase from M. sexta, were preferentially associated with lipid rafts.…”

“…M␤CD Disrupts Cry1Ab Pore Formation Activity-Cry toxins have been shown to alter K ϩ permeability in liposomes and BBMV (17). We thus tested whether lipid rafts have a role in pore formation.…”

“…However, the process by which Cry toxin-receptor binding leads to membrane pore formation remains ambiguous. Previous studies suggested that Cry toxin aggregates on the midgut epithelial membrane (16,17), but the mechanism of toxin aggregation is unknown. With some mammalian pore-forming toxins, lipid rafts play an essential role in toxin aggregation (18 -20).…”

Single Molecule Fluorescence Study of the Bacillus thuringiensis Toxin Cry1Aa Reveals Tetramerization

“…before the toxin partitions into the membrane, are scarce and variable, ranging from monomers to oligomers with 8 -10 subunits (17)(18)(19)54). It was suggested that the toxin assembles into stable oligomer barrels (prepores) before inserting into the bilayer (43) but also that monomers insert into the bilayer first and subsequently diffuse laterally in the membrane to assemble into multimers to form pores (46,55,56). In this study, we established that the oligomerization process may be described by a kinetic model as shown in Fig.…”

The Role of Bacillus thuringiensis Cry1C and Cry1E Separate Structural Domains in the Interaction with Spodoptera littoralis Gut Epithelial Cells

“…It is assumed that ␣ 4 -␣ 5 pairs from at least 4 toxin molecules are assembled to form a lethal nonselective ion channel (5)(6)(7)(8). Insertion is also linked to the spreading of the other domain I ␣-helices over the membrane (6,9,10). Oligomer formation may occur prior to the insertion of ␣-helices assisted by an earlier interaction with a membrane receptor as was recently shown for the interaction of cadherin-like Bt-R1 receptor with Cry1Ab (11).…”