Importance of Thr(328) and Thr(369) for functional maintenance of two receptor-binding β-hairpins of the Bacillus thuringiensis Cry4Ba toxin: Implications for synergistic interactions with Cyt2Aa2

• Published in: Biochemical and biophysical research communications 2016-01, Vol.469 (3), p.698
• Main Authors: Kaikaew, Autaipohn, Promptmas, Chamras, Angsuthanasombat, Chanan
• Format: Article
• Language: English
• Subjects: Aedes - drug effects; Aedes - physiology; Animals; Bacterial Proteins - administration & dosage; Bacterial Proteins - chemistry; Bacterial Proteins - ultrastructure; Binding Sites; Drug Synergism; Endotoxins - administration & dosage; Endotoxins - chemistry; Hemolysin Proteins - administration & dosage; Hemolysin Proteins - chemistry; Hemolysin Proteins - ultrastructure; Models, Biological; Models, Chemical; Molecular Docking Simulation; Mosquito Control - methods; Protein Binding; Protein Conformation; Structure-Activity Relationship; Survival Rate; Threonine - chemistry
• ISSN: 1090-2104
• DOI: 10.1016/j.bbrc.2015.11.115

Bacillus thuringiensis Cry4Ba mosquito-active toxin was previously shown to utilize two critical loop-residues, Tyr(332) and Phe(364) which are respectively located in β2-β3 and β4-β5 loops, for synergistic interactions with its alternative receptor-Cyt2Aa2. Here, structural analysis of the Cry4Ba-receptor-binding domain revealed that its N-terminal subdomain encompasses β2-β3 and β4-β5 hairpins which are stabilized by inter-hairpin hydrogen bonding between Thr(328) in β2 and Thr(369) in β5. Functional importance of these two side-chains was demonstrated by single-Ala substitutions (T328A and T369A), adversely affecting toxin activity against Aedes aegypti larvae. Unlike toxicity restoration of the inactive E417A/Y455A toxin mutated within another receptor-binding subdomain, defective bioactivity of T328A and T369A mutants cannot be restored by Cyt2Aa2 as also observed for β2-β3 (Y332A) and β4-β5 (F364A) loop-mutants. ELISA-based analysis further verified a loss in binding of all four bio-inactive mutants (T328A, Y332A, T369A and F364A) to the immobilized Cyt2Aa2. Protein-protein docking suggested that the two critical loop-residues (Tyr(332) and Phe(364)) correspondingly located at β2-β3 and β4-β5 loops can clearly interact with four counterpart surface-exposed residues of Cyt2Aa2. Altogether, our present data demonstrate structural importance of Thr(328) and Thr(369) toward hydrogen-bonded stabilization of two receptor-binding hairpins (β2-β3 and β4-β5) for synergistic toxicity of Cry4Ba with Cyt2Aa2.