Crystallization of the Bacillus thuringiensis toxin Cry1Ac and its complex with the receptor ligand N-acetyl-D-galactosamine.

Cry1Ac from Bacillus thuringiensis ssp. kurstaki HD-73 is a pore-forming protein specifically toxic to lepidopteran insect larvae. It binds to the cell-surface receptor aminopeptidase N in Manduca sexta midgut via the sugar N-acetyl-D-galactosamine (GalNAc). By using 1,3-diaminopropane (DAP) as the buffer throughout protoxin activation and chromatography on Q-Sepharose at pH 10.3, trypsin-activated Cry1Ac has been purified in a monomeric state, which was crucial to obtaining single crystals of Cry1Ac and of the Cry1Ac-GalNAc complex. Crystals of Cry1Ac alone are triclinic, with unit-cell parameters a = 51.78, b = 113.23, c = 123.41 A, alpha = 113.11, beta = 91.49, gamma = 100.46 degrees; those of the Cry1Ac-GalNAc complex show P2(1) symmetry, with unit-cell parameters a = 121.36, b = 51.19, c = 210.56 A, beta = 105.75 degrees. Data sets collected to 2.36 and 2.95 A resolution, respectively, show that both crystal forms contain four molecules of the 66 kDa toxin in the asymmetric unit and have related packing arrangements. The deaggregating effect of DAP may be explained by its capacity for bivalent hydrogen bonding and hydrophobic interactions at protein interfaces.

NMR trial models: experiences with the colicin immunity protein Im7 and the p85alpha C-terminal SH2-peptide complex.

Two cases of successful molecular replacement using NMR trial models are presented. One is the crystal structure of the Escherichia coli colicin immunity protein Im7; the other is a heretofore unreported crystal structure of a specific PDGF receptor-derived peptide complex of the carboxy-terminal SH2 domain from the p85alpha subunit of human phosphatidylinositol 3-OH kinase. In both cases, molecular replacement was non-trivial. Success was achieved using trial models that consisted of an ensemble of NMR structures from which the more flexible portions had been excised. Use of maximum-likelihood refinement proved critical to be able to refine the poor starting models. The challenges typical of the use of NMR trial models in molecular replacement are discussed.

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

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.

N-acetylgalactosamine on the putative insect receptor aminopeptidase N is recognised by a site on the domain III lectin-like fold of a Bacillus thuringiensis insecticidal toxin.

Binding of the insecticidal Bacillus thuringiensis Cry1Ac toxin to the putative receptor aminopeptidase N is specifically inhibited by N-acetylgalactosamine (GalNAc), suggesting that this toxin recognises GalNAc on the receptor. A possible structural basis for involvement of domain III of the toxin in carbohydrate-mediated receptor recognition was noted in the similarity between the domain III fold of the related toxin Cry3A and a carbohydrate-binding domain in the 1,4-beta-glucanase from Cellulomonas fimi. This possibility was investigated by making selected mutations in domain III of the Cry1Ac delta-endotoxin. Mutagenesis of residues Asn506, Gln509 or Tyr513 resulted in toxins with reduced binding and a slower rate of pore formation in Manduca sexta midgut membrane vesicles compared to the wild-type Cry1Ac. These mutants also showed reduced binding to the 120 kDa Cry1Ac putative receptor aminopeptidase N. Unlike the wild-type toxin, binding of the triple mutant N506D,Q509E,Y513A (Tmut) to M. sexta midgut membrane vesicles could not be inhibited by GalNAc. These data indicate that GalNAc binding is located on domain III of Cry1Ac and therefore support a lectin-like role for this domain. A preliminary analysis of the Cry1Ac crystal structure locates Asn506, Gln509 and Tyr513 in a region on and adjacent to beta-16 in domain III, which has a unique conformation compared to the other known Cry structures. These residues are in a favourable position to interact with either soluble or protein-bound carbohydrate.

X-ray structure of recombinant ricin A-chain at 1.8 A resolution.

Ricin is a potent plant toxin which acts by removing a specific adenine residue from the ribosome. The X-ray crystal structure of a new, tetragonal crystal form of the recombinant ricin A-chain diffracting to 1.8 A resolution has been determined via molecular replacement methods and refined to a crystallographic R-factor of 18.6%. The higher resolution electron density allowed improvements to be made upon previously published models, resulting in an increase in the assigned secondary structure of the protein. The enzyme adopts the same global conformation in this crystal form with differences in detail due only partly to crystal packing. The active site superimposes closely with those of previously published models but the locations of the active-site water molecules differ in this structure. To address the current mechanistic model, an additional two structures are presented: recombinant ricin A-chain complexed with the substrate analogue formycin monophosphate as well as with adenosine monophosphate, which is cleaved by the crystalline enzyme. The formycin monophosphate displaces a putative catalytic water molecule. This supports the notion that the analogue does not bind in a transition state conformation and that contacts from other elements of the 28 S RNA natural substrate are required to achieve full reactivity. The structure of the adenosine monophosphate complex suggests a mechanism for the release of the adenine product via of the side-chain Tyr80. The structures suggest that Glu177 is better positioned for the activation of the catalytic water molecule than Arg180.