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Abstract
Bacillus thuringiensis Crystal (Cry) and Cytolitic (Cyt) protein families are a diverse
group of proteins with activity against insects of different orders--Lepidoptera,
Coleoptera, Diptera and also against other invertebrates such as nematodes. Their
primary action is to lyse midgut epithelial cells by inserting into the target membrane
and forming pores. Among this group of proteins, members of the 3-Domain Cry family
are used worldwide for insect control, and their mode of action has been characterized
in some detail. Phylogenetic analyses established that the diversity of the 3-Domain
Cry family evolved by the independent evolution of the three domains and by swapping
of domain III among toxins. Like other pore-forming toxins (PFT) that affect mammals,
Cry toxins interact with specific receptors located on the host cell surface and are
activated by host proteases following receptor binding resulting in the formation
of a pre-pore oligomeric structure that is insertion competent. In contrast, Cyt toxins
directly interact with membrane lipids and insert into the membrane. Recent evidence
suggests that Cyt synergize or overcome resistance to mosquitocidal-Cry proteins by
functioning as a Cry-membrane bound receptor. In this review we summarize recent findings
on the mode of action of Cry and Cyt toxins, and compare them to the mode of action
of other bacterial PFT. Also, we discuss their use in the control of agricultural
insect pests and insect vectors of human diseases.