Combined molecular dynamics and continuum solvent studies of the pre-pore Cry4Aa trimer suggest its stability in solution and how it may form pore

Cry4Aa toxin is one of the highly specific mosquito-larvicidal proteins produced by the bacterium Bacillus thuringiensis subspecies israelensis. It is thought to form pores in the larval midgut membrane that cause membrane leakage and subsequent insect death. Therefore, Cry4Aa and other Cry toxins have been used as efficient and safe bacterial insecticides to control the disease-carrying mosquitoes such as Aedes, Anopheles, and Culex. However, we still do not clearly understand how Cry toxins kill mosquito-larvae at molecular details. Recent electron crystallographic images of Cry4Ba toxin, another toxin closely related to Cry4Aa toxin, have suggested that the protein forms trimer in aqueous solution and in lipid monolayer. Moreover, the unit cell of X-ray crystal structure of Cry4Ba toxin has been shown to be trimeric. In this study, we constructed the first full-atom structural model of Cry4Aa trimer using the trimeric unit cell structure of Cry4Ba toxin as a template and then used the methods of molecular dynamics (MD) and molecular mechanics combined with Poisson-Boltzmann and surface area (MM-PBSA) to show that the trimeric structure of Cry4Aa toxin is stable in 150 mM KCl solution on 10 ns timescale. The results reveal that Cry4Aa toxins use polar amino acid residues on α-helices 3, 4, and 6 to form trimer and suggest that the proteins form trimer to reduce their non-polar interactions with surrounding water. Based on the obtained trimeric structure of Cry4Aa toxins, we propose that pore formation of Cry toxins may involve a 90°-hairpin rotation during the insertion of their three α4-α5 hairpins into the membrane. This process may be mediated by water and ions. PACS Codes: 87.15.ap, 87.15.bk, 87.14.ep