Insecticidal Protein Crystals of Bacillus thuringiensis

Abstract

The spore-forming bacterium Bacillus thuringiensis bears plasmids encoding genes for insecticidal proteins typically synthesized during sporulation. These proteins crystallize forming large polyhedral parasporal inclusions that make up as much as 30% of the cellular dry weight. When ingested by insects and certain other arthropods, these inclusions dissolve and the proteins are cleaved by proteases releasing active toxins that bind to specific receptors on the host’s midgut membrane. Activated toxins then oligomerize, inserting into this membrane where they form cation-selective channels that cause cell lysis and host death. Two types of crystal proteins are recognized, Cry proteins, the most common, and Cyt proteins. Cry proteins are generally either 60–80 or 130–150 kDa, the former being truncated versions of the latter. Cyt proteins are approximately 28 kDa, and are unrelated to Cry proteins, having an affinity for midgut membrane lipids. Cry and Cyt proteins are the active ingredients of many commercial insecticides. More importantly, several Cry proteins are the basis of transgenic insecticidal crops such as Bt cotton and Bt corn, now a multibillion dollar global industry. In this chapter, we summarize the role Cry and Cyt proteins play in the biology of B. thuringiensis, and then focus on the synthesis and structure of the crystals they form. We also show how their synthesis can be manipulated with recombinant DNA techniques to increase crystal size and improve insecticidal activity.