Bioassays with insecticidal crystal proteins (ICPs) from Bacillus thuringiensis have demonstrated that Cry1Aa, Cry1Ac, and Cry1Ba are the most active toxins on larvae of the Anticarsia gemmatalis. The toxins Cry1Da and Cry1Ea are less toxic, and toxins Cry2Aa are not active. Binding of these ICPs to midgut sections of the A. gemmatalis larvae was studied using streptavidin-mediated detection. The observed staining patterns showed that Cry1Aa and Cry1Ac bound to the brush border throughout the whole length of the midgut. However, the binding sites of Cry1Ba were not evenly distributed in the midgut microvilli. The in vivo assays against larvae of 2nd instar A. gemmatalis confirmed the results from the in vitro binding studies. These binding data correspond well with the bioassay results, demonstrating a correlation between receptors binding and toxicity of the tested ICPs in this insect. 1. Introduction Bacillus thuringiensis is one of the most widely used microorganisms for the biological control of insects [1–3]. This gram-positive spore-forming bacterium characteristically produces crystals containing one or several insecticidal crystal proteins—ICPs [4–7]. A novel nomenclature has been proposed based exclusively on amino acid identity. Currently, more than 560 cry genes have been identified and classified into 68 classes based on the homology of their proteins (http://www.lifesci.sussex.ac.uk/home/Neil_Crickmore/Bt). The cry genes code for proteins with a range of molecular masses from 50 to 140？kDa [8, 9]. Lepidopteran-specific ICPs are typically produced as bipyramidal crystals that are solubilized in the often alkaline environment (pH 10 to 12) of the larval midgut [10–13]. These proteins, belonging to the Cry1 class of ICPs, are protoxins with a range of molecular masses from 130 to 140？kDa [9, 14] that are proteolytically activated by midgut proteases to toxic protease-resistant fragments (55 to 70？kDa) corresponding to the N-terminal half of the protoxin [14–16]. The delta-endotoxins bind with high affinity to proteins located in the midgut brush border membrane of susceptible insects [17–21]. Following the binding, the toxic fragment or a part of it inserts in to the membrane forming pores [22–24]. The formation of pores in the plasmatic membrane of the cells causes an ionic unbalance between the cytoplasm and the outside environment of the cell. The first effects are the stoppage of feeding and the paralysis of the gut, which causes the insect to die [5, 14, 23, 24]. Traditionally, the binding has been studied using native or biotinylated
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