It displays high activity

against mosquito larvae and Culex and Anopheles genera are its major targets (Lacey, 2007). The Bin toxin has a selective mode of action that depends on successive steps comprising ingestion of crystals by the larvae; midgut processing of protoxin into toxin; and binding to specific receptors within the midgut epithelium. These in turn lead to cytopathological effects on midgut cells and other unknown events ALK mutation that cause the death of larvae (Charles, 1987; de Melo et al., 2008). The protoxin is a heterodimer formed by the BinA (42 kDa) and BinB (51 kDa) subunits, which are proteolytically processed to generate the 39- and 43-kDa toxic fragments (Broadwell & Baumann, 1987; Nicolas et al., 1993). Bcl-2 inhibitor The BinA and BinB proteins are related in sequence and, when aligned, display 25% identity and 40% similarity (Charles et al., 1996). They are not related to better described insecticidal proteins and, although crystallography studies have been performed (Smith et al., 2004), three-dimensional

structures or models based on similar proteins are not available to date. For its activity, the two subunits act in synergy, because neither BinA nor BinB individually displays larval toxicity, except BinA in high concentrations (Broadwell et al., 1990; Nicolas et al., 1993). Interaction between the subunits is essential to achieve full toxicity against larvae and the toxin seems to form oligomers (Charles et al., 1997; Smith et al., 2005). Because of the lack of structural data, the roles of individual subunits and/or functional domains have been investigated using different approaches through their action on Culex larvae. Generally, the BinB component is recognized as being responsible for receptor binding, while BinA seems to play a role in toxicity (Oei et al., 1990; Nicolas et al., 1993; Charles et al., 1997; Shanmugavelu et al., 1998; Elangovan et al., 2000).

Mutagenesis studies have identified amino acids from the Cell press BinA that are critical for the interaction with BinB and for inducing mortality in target larvae (Elangovan et al., 2000; Promdonkoy et al., 2008). Other residues, such as the charged amino acids R97, E98 and E114, might play a proper role in toxicity and do not interfere with subunit interaction (Sanitt et al., 2008). Previous investigations have shown the BinB ability to recognize and bind to midgut receptors through its N-terminal region, while the C-terminal segment seems to contain regions responsible for binding to BinA (Clark & Baumann, 1990; Elangovan et al., 2000). The BinB confers specificity to the Bin toxin because it recognizes and binds to GPI-anchored midgut α-glucosidases, Cqm1 and Agm3, characterized as specific receptors in Culex quinquefasciatus and Anopheles gambiae larvae, respectively (Romão et al., 2006; Opota et al., 2008).