Sarinporn VisitsattapongseSomsri SakdeeSomphob LeetacheewaChanan AngsuthanasombatMahidol UniversityBiophysics Institute for Research and Development (BIRD)2018-11-092018-11-092014-07-25Biochemical and Biophysical Research Communications. Vol.450, No.2 (2014), 948-952109021040006291X2-s2.0-84905092360https://repository.li.mahidol.ac.th/handle/20.500.14594/33238Bacillus thuringiensis Cry4Aa toxin was previously shown to be much more toxic to Culex mosquito-larvae than its closely related toxin - Cry4Ba, conceivably due to their sequence differences within the β10-β11 receptor-binding loop. Here, single-Ala substitutions of five residues (Pro510, Thr512, Tyr513, Lys514and Thr515) within the Cry4Aa β10-β11 loop revealed that only Lys514corresponding to the relative position of Cry4Ba-Asp454is crucial for toxicity against Culex quinquefasciatus larvae. Interestingly, charge-reversal mutations at Cry4Ba-Asp454(D454R and D454K) revealed a marked increase in toxicity against such less-susceptible larvae. In situ binding analyses revealed that both Cry4Ba-D454R and D454K mutants exhibited a significant increase in binding to apical microvilli of Culex larval midguts, albeit at lower-binding activity when compared with Cry4Aa. Altogether, our present data suggest that a positively charged side-chain near the tip of the β10-β11 loop plays a critical role in determining target specificity of Cry4Aa against Culex spp., and hence a great increase in the Culex larval toxicity of Cry4Ba was obtained toward an opposite-charge conversion of the corresponding Asp454. © 2014 Elsevier Inc. All rights reserved.Mahidol UniversityBiochemistry, Genetics and Molecular BiologyArticleSCOPUS10.1016/j.bbrc.2014.06.090