Protein engineering of d-endotoxins of Bacillus thuringiensis Nachimuthu
Saraswathy Polumetla
Ananda Kumar* *Corresponding author Keywords: brush border membrane vesicles, cry proteins/d-endotoxin, ion channel conductance, planar lipid bilayers, protein engineering. Abbreviations:
Bacillus thuringiensis
(Bt) is a valuable environment-friendly biopesticide, which occupies
90% of the world biopesticide market. Its insecticidal properties
are attributed to the presence of d-endotoxins
which are synthesized during the sporulation phase of the bacterium.
d-endotoxin or crystal toxin is a multi-domain
protein molecule comprising of three distinct domains. Domain I is
made of seven a-helices, domain II comprises
three antiparallel b sheets, which are
folded into loops and domain III is made of a b
sandwich of two antiparallel b strands.
Molecular studies on the structure and functional properties of different
d-endotoxins revealed that the domain I
by virtue of its membrane spanning hydrophobic and amphipathic a-helices
is capable of forming pores in the cell membranes of the larval midgut.
Domain II being hyper variable in nature determines the insecticidal
specificity of a toxin and domain III is involved in varied functions
like structural stability, ion channel gating, binding to Brush Border
Membrane Vesicles and insecticidal specificity. Recent studies on
toxin aggregation and interaction revealed that the three domains
interact closely to bring about the insecticidal activity of Bt. In
this review we describe the protein engineering studies conducted
on different d-endotoxins which led to
an understanding of their molecular mode of action and construction
of novel toxins with enhanced insecticidal activity and specificity. |
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