, 1993). After ingestion of the crystal toxins by the susceptible larvae, crystalline inclusions are dissolved due to
the alkaline pH of the larval midgut. Then the 51- and 42-kDa protoxins are activated by midgut proteases to form the active proteins, of approximately 43 and 39 kDa, respectively (Broadwell & Baumann, 1987; Nicolas et al., 1990). This is then followed by the binding of the activated binary toxin to a specific receptor presented on the surface of midgut epithelium cells of susceptible larvae (Davidson, 1988; Silva-Filha et al., 1997). The binary toxin receptor has been identified as a 60-kDa α-glucosidase (Cpm1), which is attached to the cell membrane by a glycosyl-phosphatidyl inositol anchor (Silva-Filha et al., 1999; Darboux et LGK-974 price al., 2001). Using N- and C-terminal deletion
constructs of both BinA and BinB in in vivo gut binding studies, it has been proposed that the C-terminus of BinA is important for larvicidal toxicity, whereas both N- and C-terminal fragments of BinA are required for interaction with BinB. In addition, it has been proposed that the N-terminus of BinB is crucial for binding to the receptor in gut epithelial cells (Oei et al., 1992). Even though BinB has been shown to play a role in receptor recognition, its binding mechanism is still unknown. Because of the lack of structural information for the binary toxin, selleck inhibitor functional studies have been based mainly on its primary amino acid sequence and Methane monooxygenase secondary structure prediction (Broadwell et al., 1990; Berry et al., 1993; Shanmugavelu et al., 1998; Elangovan et al., 2000; Yuan et al., 2001; Promdonkoy et al., 2008; Sanitt et al., 2008). Interestingly, the amino acid sequences of BinA or BinB are not similar to other bacterial toxins. They
are, however, homologous to each other, with a 25% amino acid identity and a 40% similarity, which suggests a similar 3D structure (Promdonkoy et al., 2008). Despite their homology, the two proteins have distinct functions: BinB is responsible for receptor binding, whereas BinA acts as a toxic component (Oei et al., 1992; Charles et al., 1997; Shanmugavelu et al., 1998; Elangovan et al., 2000). It is thus possible that the different functions of these two proteins are contributed by the nonhomologous segments. For example, an amino acid sequence alignment shows that two regions in BinB are absent in BinA (Fig. 1). These regions are located in the N-terminal part of BinB. It is possible that some amino acids in these regions confer distinct functionality to BinB. To identify these possible functional elements, we have performed amino acid substitutions at residues spanning positions 111–117 and 143–150. Our results demonstrate that the aromaticity of F149 and Y150 plays a crucial role in larvicidal activity, with these residues possibly being involved in interaction with the epithelial membrane and receptor. Escherichia coli K-12 JM109 was used as a host strain for mutagenesis.