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Appl. Environ. Microbiol. doi:10.1128/AEM.02844-07
Copyright (c)2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Investigating the Properties of Bacillus thuringiensis Cry Proteins with Novel Loop Replacements Created Using Combinatorial Molecular Biology

Department of Biochemistry, University of Cambridge, 80 Tennis Court Road, Cambridge CB2 1GA, U.K.

^* To whom correspondence should be addressed. Email: cpigott{at}interchange.ubc.ca.

	Abstract

Cry proteins are a large family of crystalline toxins produced by Bacillus thuringiensis. Individually, family members are highly specific but collectively they target a diverse range of insects and nematodes. Domain II of the toxin is important for target specificity and three loops at its apex have been studied extensively. There is considerable interest in determining whether modifications in this region may lead to toxins with novel specificity or potency. In this work we studied the effect of loop substitution on toxin stability and specificity. For this purpose, sequences derived from antibody complementarity determining regions (CDR) were used to replace native domain II apical loops to create "Crybodies". Each apical loop was substituted either individually or in combination with a library of CDR-H3 sequences to create seven distinct Crybody types. An analysis of variants from each library indicated that the Cry1Aa framework can tolerate considerable sequence diversity at all loop positions but that some sequence combinations negatively affect structural stability and protease sensitivity. CDR-H3 substitution showed that loop position was an important determinant of insect toxicity: loop 2 was essential for activity whereas the effect of substitutions at loop 1 and loop 3 was sequence dependent. Unexpectedly, differences in toxicity did not correlate with binding to cadherin—a major class of toxin receptor—since all Crybodies retained binding specificity. Collectively, these results serve to better define the role of the domain II apical loops as determinants of specificity and establish guidelines for their modification.