Structural implications for the transformation of the Bacillus thuringiensis δ-endotoxins from water-soluble to membrane-inserted forms

Abstract

Crystal structures combined with biochemical data show that the delta-endotoxins from Bacillus thuringiensis are structurally poised towards large-scale, irreversible conformational changes that transform them from the soluble protein bound at the cell surface into a membrane-embedded form causing lysis of susceptible insect cells. Cry delta-endotoxins are made of a helix bundle, a beta-prism and a beta-sandwich. The conformational change involves an umbrella-like opening between the helix-4,5-hairpin and the remaining helices, and between the helical domain and the two sheet domains. Comparison of Cry1Ac structures with and without the bound receptor ligand GalNAc associates occupation of the high-affinity site on the beta-sandwich with an increase of temperature factors in the helical, pore-forming domain, which may indicate how receptor binding could trigger the required major conformational change. The structure of Cyt delta-endotoxins indicates that the surface helix hairpins must peel away to expose the beta-strands for membrane attack. Single amino acid substitutions in hinge residues or the core can restore activity following an inhibitory mutation.

Role of Receptors in Bacillus thuringiensis Crystal Toxin Activity

“…Domain III shows less structural variability than domain II, and the main differences are found in the lengths, orientations, and sequences of the loops (11). The importance of these differences is particularly evident with Cry1Aa and Cry1Ac, where a loop extension in Cry1Ac creates a unique N-acetylgalactosamine (GalNAc) binding pocket implicated in receptor binding (21,32,108). Domain III has been compared to a number of different proteins (28), but its similarity to carbohydrate-binding modules (CBMs) found in microbial glycoside hydrolases, lyases, and esterases is particularly striking.…”

“…To date, seven structures have been solved by X-ray crystallography: Cry1Aa (64), Cry1Ac (32,108), Cry2Aa (127), Cry3Aa (109), Cry3Ba (45), Cry4Aa (12), and Cry4Ba (11). These toxins show considerable differences in their amino acid sequences and insect specificity but, remarkably, they all have highly similar three domain structures (Table 1; Fig.…”

The N-Terminal Domain of Bcl-xL Reversibly Binds Membranes in a pH-Dependent Manner

“…Structurally, Bcl-x L ΔTM is most similar to the translocation domain of diphtheria toxin (23). For this toxin, as well as others, membrane association is mediated in part by the so-called hydrophobic, helical hairpin, which corresponds to α-helices 5 and 6 in Bcl-x L ΔTM (Figure 5a) (38)(39)(40)(41)(42)(43)(44)(45)(46). Despite the structural similarity, the sequences of these proteins are less than 19% identical, and they possibly are an example of convergent evolution toward a versatile protein fold.…”

Cooperation and bacterial pathogenicity: an approach to social evolution

“…The proteins of which they are composed are called δ-endotoxins and there are two main types: Cry toxins and Cyt toxins. δ-endotoxins are pore-forming proteins specifically active toward membranes from insects (Lepidoptera, Coleoptera, and Diptera), acari, nematodes, flat worms, and protozoa (Schnepf et al 1998;Lightwood et al 2000;Li et al 2001).…”