Bacillus thuringiensis (Bt) is a unique bacterium in that it shares a common place with a number of chemical compounds which are used commercially to control insects important to agriculture and public health. Although other bacteria, including B. popilliae and B. sphaericus, are used as microbial insecticides, their spectrum of insecticidal activity is quite limited compared to Bt. Importantly, Bt is safe for humans and is the most widely used environmentally compatible biopesticide worldwide. Furthermore, insecticidal Bt genes have been incorporated into several major crops, rendering them insect resistant, and thus providing a model for genetic engineering in agriculture.This review highlights what the authors consider the most relevant issues and topics pertaining to the genomics and proteomics of Bt. At least one of the authors (L.A.B.) has spent most of his professional life studying different aspects of this bacterium with the goal in mind of determining the mechanism(s) by which it kills insects. The other authors have a much shorter experience with Bt but their intellect and personal insight have greatly enriched our understanding of what makes Bt distinctive in the microbial world. Obviously, there is personal interest and bias reflected in this article notwithstanding oversight of a number of published studies. This review contains some material not published elsewhere although several ideas and concepts were developed from a broad base of scientific literature up to 2010.

Endonexin (annexin IV) is a member of the annexin family of homologous proteins that share the ability to bind to pure lipid membranes and to aggregate vesicles in a Ca(2+)-dependent fashion. Endonexin appears to preferentially interact with certain types of lipids such as phosphatidylglycerol (PG) in PG/phosphatidylcholine (PC) mixed lipid membranes. Such preferential binding should result in localization of PG lipids to membrane regions where endonexin is bound. This was tested using a PG derivative containing the fluorophore pyrene, which exhibits fluorescence sensitive to molecular collision frequency. Motional restriction of pyrene-PG upon endonexin-membrane binding was evident from decreased ratios of excimer-to-monomer (E/M) pyrene fluorescence with endonexin binding to 97% PG/3% pyrene-PG vesicles. A maximum decrease of 30% suggests a 30% decrease in the average diffusion constant of pyrene-PG molecules or a 53% decrease assuming that only outer-monolayer lipid molecules interact with endonexin. In vesicles containing 5% and 10% pyrene-PG in PC, segregation of lipids was evident from observed increases in E/M of 14.2 +/- 1.8% and 6.8 +/- 0.1%, respectively, in the presence of endonexin and either 10 mM (5% pyrene-PG) or 2 mM (10% pyrene-PG) free Ca2+. At higher concentrations of Ca2+ (> 10 mM for 5% pyrene-PG and > 2 mM for 10% pyrene-PG), smaller endonexin-dependent increases in E/M are observed as endonexin molecules at high surface densities compete for the limited pool of pyrene-PG. The nature of these interactions of endonexin with mixed lipid systems has implications for the way annexins may modulate membrane structure in cells.

The Cry1Ab toxin produced by Bacillus thuringiensis (Bt) exerts insecticidal action upon binding to BT-R1, a cadherin receptor localized in the midgut epithelium of the tobacco hornworm Manduca sexta [Dorsch, J. A., Candas, M., Griko, N. B., Maaty, W. S., Midboe, E. G., Vadlamudi, R. K., and Bulla, L. A., Jr. (2002) Cry1A toxins of Bacillus thuringiensis bind specifically to a region adjacent to the membrane-proximal extracellular domain of BT-R1 in Manduca sexta: involvement of a cadherin in the entomopathogenicity of Bacillus thuringiensis, Insect Biochem. Mol. Biol. 32, 1025-1036]. BT-R1 represents a family of invertebrate cadherins whose ectodomains (ECs) are composed of multiple cadherin repeats (EC1 through EC12). In the present work, we determined the Cry1Ab toxin binding site in BT-R1 in the context of cadherin structural determinants. Our studies revealed a conserved structural motif for toxin binding that includes two distinct regions within the N- and C-termini of EC12. These regions are characterized by unique sequence signatures that mark the toxin-binding function in BT-R1 as well as in homologous lepidopteran cadherins. Structure modeling of EC12 discloses the conserved motif as a single broad interface that holds the N- and C-termini in close proximity. Binding of toxin to BT-R1, which is univalent, and the subsequent downstream molecular events responsible for cell death depend on the conserved motif in EC12.