Study of the Basis for Toxicity and Specificity of Bacillus thuringiensis d-Endotoxins

Abstract

The report contains three parts which summarizes the three years achievements of the three participating research groups; The Weizmann group, Tel-Aviv group and Purdue group. The firs part describes the achievements obtained by Shai's group toward the elucidation of the mechanism of membrane insertion and the structural organization of the pores formed by the Cry3A and Cry1Ac B. thuringiensis d-endotoxins. For that purpose Shai's group synthesized, fluorescently labeled and structurally and functionally characterized peptides corresponding to the seven helices that compose the pore-forming domain of Cry3A toxin, including mutants peptides and the hairpin a4G-a5 of both Cry3A and Cry 1Ac toxins composed of a4, a5 and the loop connecting a4-a5. Among the synthesized peptides were three mutated a4 helices based on site directed mutagenesis done at Aronson's group that decreased or increased Cry 1Ac toxicity. The results of these studies are consistent with a situation in which only helices a4 anda5 insert into the membrane as a helical hairpin in an antiparallel manner, while the other helices lie on the membrane surface like ribs of an umbrella (the "umbrella model"). In order to test this model Shai's group synthesized the helical hairpin a4<-->a5 of both Cry3A and Cry 1 Ac toxins, as well. Initial functional and structural studies showed direct correlation between the properties of the mutated helices and the mutated Cry1Ac. Based on Shai's findings that a4 is the second helix besides a5 that insert into the membrane, Aronson and colleagues performed extensive mutation on this helix in the CrylAc toxin, as well as in the loop connecting helices 4 and 5, and helix 3 (part two of the report). In addition, Aronson performed studies on the effect of mutations or type of insect which influence the oligomerization either the Cry 1Ab or Cry 1Ac toxins with vesicles prepared from BBMV. In the third part of the report Zilberstein's and Sneh's groups describe their studies on the three domains of Cry 1C, Cry 1E and crylAc and their interaction with the epithelial membrane of the larval midgut. In these studies they cloned all three domains and combinations of two domains, as well as cloning of the pore forming domain alone and studying its interaction with BBMV. In addition they investigated binding of Cry1E toxin and Cry1E domains to BBMV prepared from resistant (R) or sensitive larvae. Finally they initiated expression of the loop a4G<-->a5 Cry3A in E. coli to be compared with the synthetic one done by Shai's group as a basis to develop a system to express all possible pairs for structural and functional studies by Shai's group (together with Y. Shai).