Mutagenesis Targeting the S153 Residue Within the Transmembrane β-Hairpin of Mosquito-Larvicidal Mpp46Ab Affects Its Toxicity and the Synergistic Toxicity with Cry4Aa

We constructed a library of Mpp46Ab mutants, in which S153 within the transmembrane β-hairpin was randomly replaced by other amino acids. Mutagenesis and subsequent primary screening yielded 10 different Mpp46Ab mutants in addition to the wild type. Remarkably, S153 was replaced with a more hydrophobic amino acid in most of the mutants, and the S153I mutant in particular exhibited significantly increased toxicity. Electrophysiologic analysis using artificial lipid bilayers revealed that the single-channel conductance and PK/PCl permeability ratio were significantly increased for S153I pores. This suggests that the formation of highly ion-permeable and highly cation-selective toxin pores increases the influx of cations and water into cells, thereby facilitating osmotic shock. In addition, the S153F, S153L, and S153I mutants exhibited significantly reduced synergistic toxicity with Cry4Aa. Electrophysiologic analysis showed that the S153F, S153L, and S153I mutants form toxin pores with a significantly reduced PK/PNa permeability ratio and a significantly increased PK/PCa permeability ratio compared to wild-type pores. Thus, our results suggest that pore formation is central to the insecticidal activity of Mpp46Ab and that the ion permeability of toxin pores is a potential indicator correlated with both toxicity and synergistic toxicity with other toxins.