Liposome-Based Study Provides Insight into Cellular Internalization Mechanism of Mosquito-Larvicidal BinAB Toxin

Glycosylphosphatidylinositols (GPIs) anchored proteins are commonly localized onto lipid rafts. These extracellular proteins participate in a variety of cellular functions, including as receptors for viruses and toxins. Intracellular trafficking of World Health Organization recognized mosquito-larvicidal BinAB toxin is mediated via GPI-anchored Cqm1 receptor protein in Culex mosquitoes. We confirmed conformational change in Cqm1 dimer on interaction with BinA/BinB proteins by dynamic light scattering, modelling of hydrodynamic parameters using the atomic structures, and synchrotron Small Angle solution X-ray scattering (SAXS). A reliable model of the receptor-BinB complex was also constructed from joint SAXS/SANS refinement. We confirmed electrostatic interactions of the Cqm1 ectodomain with lipid rafts reconstituted in model membranes and report receptor-dependent impairment of model liposomes by BinA/B proteins. Liposomal disruption was toxin concentration-dependent as monitored by the release of encapsulated carboxyfluorescein dye. Interestingly, BinA alone, without BinB, showed efficient efflux of the fluorescent dye in agreement with the reported high larvicidal activity of BinA variants. The study provides insight into BinA/B toxin internalization mechanism in the membrane model that is toxin internalization is mediated via receptor-dependent pore formation mechanism. It also suggests a tangible and environmentally safe strategy for control of mosquito population.