Role of Receptors in Bacillus thuringiensis Crystal Toxin Activity

Abstract

SUMMARY Bacillus thuringiensis produces crystalline protein inclusions with insecticidal or nematocidal properties. These crystal (Cry) proteins determine a particular strain's toxicity profile. Transgenic crops expressing one or more recombinant Cry toxins have become agriculturally important. Individual Cry toxins are usually toxic to only a few species within an order, and receptors on midgut epithelial cells have been shown to be critical determinants of Cry specificity. The best characterized of these receptors have been identified for lepidopterans, and two major receptor classes have emerged: the aminopeptidase N (APN) receptors and the cadherin-like receptors. Currently, 38 different APNs have been reported for 12 different lepidopterans. Each APN belongs to one of five groups that have unique structural features and Cry-binding properties. While 17 different APNs have been reported to bind to Cry toxins, only 2 have been shown to mediate toxin susceptibly in vivo. In contrast, several cadherin-like proteins bind to Cry toxins and confer toxin susceptibility in vitro, and disruption of the cadherin gene has been associated with toxin resistance. Nonetheless, only a small subset of the lepidopteran-specific Cry toxins has been shown to interact with cadherin-like proteins. This review analyzes the interactions between Cry toxins and their receptors, focusing on the identification and validation of receptors, the molecular basis for receptor recognition, the role of the receptor in resistant insects, and proposed models to explain the sequence of events at the cell surface by which receptor binding leads to cell death.

Diversity of aminopeptidases, derived from four lepidopteran gene duplications, and polycalins expressed in the midgut of Helicoverpa armigera: Identification of proteins binding the δ-endotoxin, Cry1Ac of Bacillus thuringiensis

“…Of these, five formed a lepidopteran-specific radiation comprising the clades termed Classes I, II, III, IV and VI. Classes I, III, IV and V correspond with four of the lepidopteran clades reported previously (Nakanishi et al, 2002;Herrero et al, 2005;Wang et al, 2005;Pigott and Ellar, 2007). Classes II, VI and VII are noteworthy because they each contain a single APN from H. armigera and B. mori with only one other sequence from P. xylostella in class II.…”

“…All four sequences predict motifs present in other lepidopteran APNs (Pigott and Ellar, 2007): secretion signal peptides, a GPI cleavage site and a hydrophobic tail, indicating that they would be retained on the cell membrane, the zincbinding/gluzincin motif, HEX 2 HX 18 E, required by APN proteins for enzymatic function, and the gluzincin APN motif GAMENWG, also believed to form part of the active site. HaAPN4 differed from the other three by the lack a polythreonine (mucin) region upstream of the GPI prepeptide anchor.…”

“…Glycosylation is generally regarded as an important determinant of Cry1Ac-binding (Pigott and Ellar, 2007). Therefore it is likely the Cry1Ac-binding HaAPNs all share the minimum amount of necessary GalNAc-containing glycan for interaction with activated Cry1Ac toxin since the toxin is known to bind to GalNAc residues and the proteins were eluted with GalNAc.…”

“…Current theories of the mode of action of the Cry1Ac suggest that the toxin binds to GPIanchored proteins prior to insertion into the membrane but it is unclear whether the anchor has any role in that binding Gómez et al, 2007;Pigott and Ellar, 2007). In at least one case the GPI-anchor is not required for binding since an APN from M. sexta was still able to bind Cry1Ac after cleavage of its anchor (Lu and Adang, 1996), however, that particular APN (from class I) has the C-terminal, mucin-like sequence with demonstrated GalNAc content (Knight et al, 2004).…”

“…The mode of action of Bt Cry toxins was recently reviewed by Pigott and Ellar (2007). Multiple steps lead to cell lysis through the formation of pores in the brush border membrane of the midgut of susceptible insects.…”