Preliminary X-ray diffraction analysis of crystals of Bacillus thuringiensis toxin, a cell membrane disrupting protein

Penetration of dyes into protein crystals

Experiments were carried out on 15 different protein crystals with the objective of estimating the rates of penetration of dye molecules into the crystals. The dyes were in the molecular-weight range 250-1000 Da and the protein crystals were of dimensions of 0.7 mm or greater. Experiments were also conducted on protein crystals grown between glass cover slips (separation 200 µm) that restricted the direction of diffusion. The rate of penetration of dyes into protein crystals depends very much on the degree of association between the dye and protein molecules. Dye penetration was not consistent with pure diffusion when the affinity of the protein for the dye was significant, and this was frequent. Penetration rates were less dependent on factors such as the molecular weight of the dye or the diffusion direction. For weakly interacting protein crystal/dye combinations, penetration was a fair measure of diffusivity and the observed rates were in the range 60-100 µm h-1. For strongly interacting combinations, the rates of penetration were of the order of 15-30 µm h-1.

Cyt1Aa toxin: crystal structure reveals implications for its membrane-perforating function

During sporulation, Bacillus thuringiensis subsp. israelensis produces a mosquito larvicidal protein complex containing several crystalline and cytolytic (Cyt) toxins. Here, the activated monomeric form of Cyt1Aa, the most toxic Cyt family member, was isolated and crystallized, and its structure was determined for the first time at 2.2 Å resolution. Cyt1Aa adopts a typical cytolysin fold containing a β-sheet held by two surrounding α-helical layers. The absence of a β-strand (between residues V26 and I37) in the dimeric structure of Cyt2Aa led us to deduce that this is the only essential segment for dimer formation and that activation of the toxin occurs by proteolytic processing of its N-terminus. Based on the Cyt1Aa structure, we suggest that the toxicity of Cyt1Aa and other nonrelated proteins, all sharing a cytolysin fold, is correlated with their ability to undergo conformational changes that are necessary prior to their membrane insertion and perforation. This fold allows the α-helical layers to swing away, exposing the β-sheet to insert into the membrane. The identification of a putative lipid binding pocket between the β-sheet and the helical layer of Cyt1Aa supports this mechanism. Sequence-based structural analysis of Cyt1Aa revealed that the lack of activity of Cyt1Ca may be related to the latter's inability to undergo this conformational change due to its lack of flexibility. The pattern of the hemolytic activity of Cyt1Aa presented here (resembling that of pore-forming agents), while differing from that imposed by ionic and nonionic detergents, further supports the pore-forming model by which conformational changes occur prior to membrane insertion and perforation.

Crystallization of a membrane pore-forming protein with mosquitocidal activity from Bacillus thuringiensis subspecies kyushuensis

CytB, a membrane pore-forming toxin from Bacillus thuringiensis subspecies kyushuensis, is specifically toxic to dipteran insect larvae but broadly cytolytic in vitro. It has been purified in the protoxin form from a recombinant Escherichia coli source and crystals have been obtained which diffract X-rays to at least 2.6 A resolution. The tendency for CytB to aggregate in solution was overcome by including 50 mM of urea or 8 mM of ethanolamine during crystallization. Mutants designed to add or subtract single cysteine residues for the purpose of heavy atom derivative preparation were similarly purified and crystallized. The crystals are hexagonal bipyramids. They belong to space group P6(1)22 (or P6(5)22) with lattice constants a = b = 67.34 A, c = 170.96 A, and contain one molecule of the CytB protoxin (MW 29235) per asymmetric unit and 27% solvent by volume.

Binding and aggregation of the 25-kilodalton toxin of Bacillus thuringiensis subsp. israelensis to cell membranes and alteration by monoclonal antibodies and amino acid modifiers

The 25-kilodalton toxin of Bacillus thuringiensis subsp. israelensis binds irreversibly to Aedes albopictus cells, Choristoneura fumiferana cells, and erythrocytes. The binding to cells increased with both toxin concentration and time and when the cells were first preincubated with unlabeled toxin. Binding data indicated a two- to threefold increase in the rate of binding after the amount of the membrane-bound toxin reached approximately 3.5 fmol/3 x 10(5) A. albopictus cells or 3.3. fmol/2 x 10(5) C. fumiferana cells. When this level of bound toxin was reached, the toxins also began forming aggregates at the cell membrane. The toxin aggregates were extracted with 10% Triton X-100 and separated from the monomers with a 5 to 20% sucrose density gradient. The toxin aggregates isolated from A. albopictus and C. fumiferana cell membranes were ca. 400 kilodaltons, while those isolated from human erythrocytes were significantly smaller. The proportion of the toxin found in aggregate form increased rapidly with the amount of toxin bound; however, the molecular size of the aggregates remained constant. Eleven monoclonal antibodies raised against the native form of the toxin blocked 80 to 97% of the toxin binding to cells. The epitope of one of these monoclonal antibodies was mapped to a domain which included the cysteine, suggesting the importance of the domain around this amino acid to binding. Toxin binding and cell lysis were also inhibited by treating the toxin with HgCl2, further indicating the importance of the C-terminal hydrophobic cysteine-containing domain in cytolytic activity of the 25-kilodalton protein.

Single amino acid changes in the Bacillus thuringiensis var. israelensis delta-endotoxin affect the toxicity and expression of the protein

Site-directed mutagenesis has been used to change individual amino acids of the larvicidal 27,000 Mr delta-endotoxin of Bacillus thuringiensis var. israelensis. Basic and acidic residues have been systematically replaced by alanine, and the resulting mutant polypeptides analysed for cytolytic and larvicidal activity, and binding to phosphatidyl choline liposomes. Replacement of residues at positions 154, 163, 164, 213 and 225 results in proteins which accumulate as inclusions in recombinant Bacillus subtilis cells similar to the wild-type, but have considerably reduced in-vitro and in-vivo toxicity. One mutant (Glu45 to Ala45) results in a protein that has reduced activity in vitro, but retains wild-type larvicidal toxicity. In addition, seven other mutations of charged residues result in proteins which form small or no inclusions in recombinant cells, despite being produced at levels similar to the wild-type in six out of seven cases. In most instances, the toxicity of these aberrantly expressed proteins is considerably less than the wild-type, although one (Lys124 to Ala124) results in a polypeptide with approximately threefold increased activity in vitro. A secondary structural model is proposed to explain these observations.