Insect Control: Biological and Synthetic Agents - Index of

264 7: Bacillus thuringiensis: Mechanisms and Use
Figure 8 Mode of action of Cry toxins. (a) Crystals are solubilized and activated give rise to the monomeric toxin. (b) The toxin
monomer binds the cadherin receptor, followed by proteolytic cleavage of helix a-1. (c) The tetramer is formed by intermonomeric
contacts. (d) The toxin oligomer binds to the APN receptor. (e) The APN receptor and oligomeric Cry toxin localize to lipid rafts.
(f) Following a conformational change the oligomer inserts into membrane forming a tetrameric pore.
molten globule state of the monomer exposing
hydrophobic regions.
3. Formation of a tetramer by intermonomeric
contacts.
4. Binding of the toxin oligomer to the APN
receptor.
5. Mobilization of the APN receptor and Cry toxin
to membrane microdomains.
6. A second conformational change of the oligomer,
resulting in the insertion of the toxin into the
membrane and formation of the membrane
active tetrameric pore.
7.5. Synergism of Mosquitocidal
Toxins
As previously mentioned (see Section 7.3.1) Cyt
toxins synergizes the insecticidal effect of some Cry
toxins. Bacillus thuringiensis subsp. israelensis is
highly toxic to different mosquito species like
Aedes spp., Culex spp., and blackfly, and also to
Anopheles spp. but with lower toxicity (Margalith
and Ben-Dov, 2000). This bacterium produces a
crystal inclusion composed of at least four toxins:
Cry4A, Cry4B, Cry11A, and Cyt1A (Huber and
Luthy , 1981; Guerchicoff et al., 1997). The toxicity
of the crystal inclusion is greater, by far, than the
toxicity of the isolated Cry and Cyt components.
These Cry toxins are more toxic against mosquito
larvae than Cyt1A with a difference in toxicity of
at least 25-fold (Wirth et al., 1997). However, the
Cyt1A toxin synergizes the effect of the other Cry
toxins. Toxicity of different combinations of Cry
toxins with CytA1 was higher than the addition of
expected toxicities of the isolated components
(Crickmore et al., 1995; Khasdan et al., 2001). Also,
the Cyt toxins synergize the toxicity of the binary
toxins produced by Bacillus sphaericus. A ratio of
10 : 1 of B. sphaericus toxins to Cyt1A was 3600fold
more toxic to A. aegypti than B. sphaericus
alone (Wirth et al., 2001), and the presence of Cyt
toxin can overcome the resistance of Culex quinquefasciatus
to the binary toxins (Wirth et al., 2001).
The mode of action of Cyt toxins includes the
following steps: solubilization of the toxin under
the alkaline pH and reducing conditions of the midgut,
proteolytic processing of the protoxin, binding
of the toxin to the epithelium surface of midgut
cells, and pore formation leading to cell lysis (Li
et al., 1996, 2001). An important difference between
Cyt and Cry toxins is the lack of a protein
receptor for the Cyt toxin. Cyt toxins bind phospholipids
and are capable of forming pores in cell lines
of different origin (Gill et al., 1987; Chow et al.,
1989; Li et al., 1996, 2001). There are currently two
proposed mechanisms of insertion and membrane
perturbation by Cyt toxins. The first mechanism
proposes that multimers of four Cyt toxin subunits
form a b-barrel structured pore within the membrane,
where the transmembrane region involves