Adsorption and binding of the insecticidal proteins from Bacillus thuringiensis subsp. kurstaki and subsp. tenebrionis on clay minerals

Abstract

The equilibrium adsorption and binding of the toxins from Bacillus thuringiensis subsp. kurstaki ( Btk ) (66 kDa), toxic to lepidopteran larvae, and from subsp. tenebrionis ( Btt ) (68 kDa), toxic to coleopteran larvae, on the clay minerals, montmorillonite (M) and kaolinite (K), homoionic to various cations (‘clean’ clays) or coated with two types of polymeric oxyhydroxides of Fe(III) (‘dirty’ clays) were studied. Adsorption of the toxins on a constant amount of the clays increased with toxin concentration and then reached a plateau. Larger amounts of the toxins from Btk than from Btt were adsorbed. Adsorption of the toxins was rapid (<30 min for maximal adsorption of the toxins from Btk', <30 min for 70% of maximal adsorption of the toxins from Btt, which was complete at 3 h), and maximal between pH 6 and 8 onto clean clays and between pH 5 and 9 onto dirty clays. Adsorption of the toxins from Btk or Btt on clean clays was affected by the type of cation to which the clays were homoionic. The adsorption of the toxins from Btk was greater on M homoionic to monovalent than to polyvalent cations, and adsorption decreased as the valency of the charge-compensating cation increased, with the exception of M homoionic to La, which adsorbed more than M homoionic to divalent cations or to Al. The amounts of toxins from Btt adsorbed were also greater on M homoionic to monovalent than to di- and trivalent cations, with the exception of M homoionic to Mg, which adsorbed the most. Adsorption of the toxins from both Btt and Btk on K was significantly lower than on M, and the valency of the charge-compensating cations on K had little effect on adsorption. Smaller amounts of the toxins from Btk and Btt were adsorbed on dirty clays than on clean clays. Only ca 10 and 30% of the toxins from Btk and Btt, respectively, adsorbed at equilibrium were desorbed by one or two washes with water. Additional washings desorbed no more toxins, indicating that the toxins were tightly bound on the clays. The formation of complexes between the toxins and the clays did not appear to alter significantly the structure of the toxins, as indicated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of the equi- librium supernatants and desorption washes and by dot-blot enzyme-linked immunosorbent assays and Fourier-transform infrared analyses of the bound toxins. The toxins partially intercalated M, with more intercalation by the toxins from Btt. However, the entire proteins did not appear to penetrate M. There was no intercalation of K.