Insect Control: Biological and Synthetic Agents - Index of

258 7: Bacillus thuringiensis: Mechanisms and Use
Figure 6 Receptor molecules of Cry1A proteins. Kd values are average affinity values of Cry1A toxins to aminopeptidase-N and
cadherin receptors.
reduced Cry1Ac pore formation activity (Lorence
et al., 1997). These reports suggest that APN binding
is also an important step in the mode of action of
Cry1 toxins.
In the case of the dipteran specific Cry11A and
Cry4B toxins, two binding proteins of 62 and
65 kDa were identified on brush border membrane
vesicles from Aedes aegypti larvae (Buzdin et al.,
2002). The identity of these binding molecules and
their role as receptors of Cry11A and Cry4B toxins
still remains to be analyzed.
One interesting feature of the Cry1 receptor molecules
identified so far is that all these proteins are
glycosylated. As mentioned before, it is interesting
to note that domain II and domain III of Cry toxins
have structural homology to several protein domains
that interact with carbohydrates. In the case of
Cry1Ac, Cry5, and Cry14 toxins different experimental
evidence suggest a role of carbohydrate
recognition in the mode of action of these toxins.
The domain III of Cry1Ac interacts with a sugar
N-acetylgalactosamine on the aminopeptidase
receptor (Masson et al., 1995). In Caenorhabditis
elegans resistance to the Cry5B and Cry14 toxins
arises because of changes in the expression
of enzymes involved in glycosylation (Griffits
et al., 2001). However, the structural similarities of
domain II and III with carbohydrate binding proteins
does not exclude the possibility that these
domains may have protein–protein interactions
with the receptor molecules. In fact some loop
regions of domain II of Cry1Ab and the Bt-R1 receptor
do have protein–protein interaction (Gómez
et al., 2002a). The interaction between the toxin
and its receptors can be complex involving multiple
interactions with different toxin–receptor epitopes
or carbohydrate molecules. Interestingly, the binding
of Cry1Ac to two sites on the purified APN from
M. sexta has been reported, and N-acetylgalactosamine
could inhibit 90% of Cry1Ac binding, which
does not inhibit the binding of Cry1Aa and Cry1Ab
to the same APN receptor (Masson et al., 1995).
Only one of the Cry1Ac binding sites on APN
is shared and recognized by Cry1Aa and Cry1Ab
toxins (Masson et al., 1995).
7.4.4. Toxin Binding Epitopes
The identification of epitopes involved in Cry toxin–
receptor interaction will provide insights into the
molecular basis of insect specificity and could help
in the characterization of insect resistant populations
in nature. Such studies would also aid in
developing strategies to design toxins that could
overcome receptor point mutations leading to Cry
toxin resistance.
The toxin binding epitopes have been mapped for
several Cry toxins. Domains II and III are the most
variable regions of Cry toxins and had, therefore,